A polypropylene, its preparation method and application

Abstract

The present invention provides a polypropylene, a preparation method thereof and an application thereof. The preparation method includes: taking propylene, hydrogen, a catalyst, a cocatalyst and an external electron donor in a first loop reactor for a first polymerization reaction, adding the obtained material after the first polymerization reaction, additional propylene and additional hydrogen into a second loop reactor for a second polymerization reaction, and then successively carrying out steam distillation, drying and pelletizing on the obtained material after the second polymerization reaction to obtain polypropylene; the polypropylene obtained by the present invention has a melt flow rate of 0.2 g / 10 min to 0.4 g / 10 min, an isotacticity of 98.5% to 99.0%, an ash content of 0.0015% to 0.003%, a molecular weight distribution index of 6 to 9, and a weight-average molecular weight of 500,000 to 700,000. When it is used for preparing a 10-μm lithium battery separator, it shows strong puncture strength and high breakdown voltage resistance, and has excellent properties.

Description

Technical Field

[0001] The present application relates to the technical field of polyolefin resins, and particularly relates to a polypropylene, a preparation method thereof, and an application thereof. Background Art

[0002] With the development of the new energy industry, the comprehensive indicators such as the capacity, energy density, performance, and safety of the downstream lithium battery industry are continuously improving. The demand for the thinning and functionalization of separator products will also be further enhanced. The thinning separator products have always been one of the key points in the development of the lithium battery separator industry.

[0003] Currently, as a general lithium battery separator, the materials of polyolefin separators mainly include polyethylene and polypropylene. Among them, the production process of polypropylene lithium battery separators is a dry process, which can be further divided into unidirectional stretching and biaxial stretching according to different stretching directions. The dry unidirectional stretching process (dry single stretching process) first forms defects such as crazes at low temperature, and then opens the defects at high temperature to form an oblong microporous structure. The principle of the dry biaxial stretching process (dry double stretching process) is to add a β-crystalline form modifier with a nucleating effect to polypropylene. It mainly forms a microporous structure by changing the crystal form of polypropylene under the action of thermal stress during the stretching process due to the density difference between different phases of polypropylene. However, at present, more than 90% of the domestic dry processes for producing lithium battery separators are dry single stretching processes.

[0004] The Chinese patent document with the publication number CN115322271A discloses a special material for polypropylene lithium battery separator and its preparation method. This method uses a liquid-phase bulk gas-phase polymerization process to prepare a special material for polypropylene lithium battery separator with the characteristics of high isotactic index and low ash content. Among them, the isotactic index can reach 99.3%, and the ash content can be reduced to 0.006%. Moreover, the production process of the precursor film prepared by it is stable, has high safety, and strong controllability, so that the catalyst cost can be reduced, and the precursor film has good tensile properties. The product performance indicators prepared by this invention can simultaneously meet the melt flow rate (2.16 kg) of 2.00 ± 0.10 g / 10 min, isotactic index ≥ 98.0%, ash content ≥ 0.0100%, and crystallinity ≥ 48.5%. The Chinese patent document with the authorization announcement number CN115160463B discloses a method for producing polypropylene with high purity and low exudates. The preparation method includes: mixing propylene monomer with a main catalyst, an external electron donor, and an activator, and then adding them into a prepolymerization reactor, and then successively entering a first loop reactor and a second loop reactor for polymerization reaction; the product is flash-vaporized to remove liquid-phase propylene, and an antistatic agent is added during flash-vaporization, and then further subjected to degassing, steam distillation, and drying treatments to obtain homopolypropylene powder. Finally, a compounding additive is added and mixed, and melt granulation is carried out by an extruder to obtain homopolypropylene resin. This invention uses a Ziegler-Natta catalyst and is directly produced by a one-step method. It is a high-purity polypropylene material with a high isotactic index, a wide molecular weight distribution, and a relatively small content of small molecules (ash content less than 30 mg / kg, ultra-high-purity polypropylene), and has the characteristics of high withstand voltage at high temperatures, excellent dielectric breakdown properties, good film-forming processing performance, less smoke and less oligomer exudation, and is mainly applied to films for capacitors and lithium battery separators.

[0005] The Chinese patent document with the publication number CN112876591A discloses an industrial production method of polypropylene resin for battery separators. In a twin-loop reactor, polypropylene powder is produced using a synergistic hydrogen transfer catalyst (HCAT). The polypropylene powder is melt-extruded and pelletized with resin additives. The obtained polypropylene resin product has the characteristics of high isotacticity and low ash content. The product is mainly used as a raw material for battery separators and is applied to the lithium-ion battery industry. The Chinese patent document with the invention publication number CN111019026A discloses a preparation method of polypropylene resin for high-isotacticity lithium battery separators, which includes the following steps: adding propylene into a polymerization kettle, stirring and heating to 35-40 °C, then adding a cocatalyst, an external electron donor and a supported main catalyst thereto, and introducing hydrogen. Controlling the polymerization reaction temperature at 75-80 °C, the polymerization reaction pressure at 0.6-0.8 MPa, and the polymerization reaction time at 120-240 min, high-isotacticity polypropylene resin for lithium battery separators is obtained. The polymerization process of this invention adopts the batch bulk method. By controlling parameters such as the melt index of the product during the polymerization reaction at 0.3-2.1 g / 10 min, the polymerization reaction temperature at 75-80 °C, the polymerization reaction pressure at 0.6-0.8 MPa, the polymerization reaction time at 120-240 min, and the dosage of the external electron donor at 10-18% of the total mass of the catalyst, polypropylene resin particles with an isotacticity of more than 98.8% and an elastic recovery rate of more than 90% are obtained, which are used to produce high-performance and high-value-added lithium battery separators, and the quality and film-forming rate of the lithium battery separators are well improved.

[0006] The Chinese patent document with the publication number CN106397643A discloses a production method for a special material for lithium battery membranes using polypropylene resin particles. The specific steps include: in four series-connected reaction vessels with polymer-grade propylene, by controlling different residence times and different melt flow rates of the products, a lithium battery membrane material is finally prepared. By adjusting the process parameters of each reaction vessel and strictly controlling the melt flow rate of each reaction vessel, the problem that the special material for polypropylene lithium battery separators requires a relatively wide molecular weight distribution and good film-forming properties is solved; by adjusting the polymerization temperature of each reaction vessel and strictly controlling it within the specified range, the problem that the lithium battery membrane material should have good mechanical properties, including puncture strength and tensile strength, is solved. The Chinese patent document with the authorization announcement number CN103540020B discloses a production method for a special homopolypropylene material for lithium ion battery barrier membranes. This invention uses a propylene polymerization catalyst with high activity and moderate hydrogen regulation sensitivity. The propylene polymerization activator is triethylaluminum, and its addition amount is controlled below 7.0 L / hour. The external electron donor uses dicyclopentyldimethoxysilane and tetraethoxysilane. In the granulation section of this invention, a stabilizer system composed of a main antioxidant, a secondary antioxidant, an acid remover, etc. is added. Finally, a special BOPP film material for lithium ion barrier membrane production with an isotacticity greater than 98%, a melt flow rate (MFR) controlled at 2.8 ± 0.2 g / 10 min, an ash content controlled at 50 - 100 ppm, a volatile content less than 100 ppm, an oxidation induction period greater than 15 min, and a molecular weight distribution controlled at 7.0 - 10.0 is made.

[0007] Therefore, the above patents are all for preparing polypropylene materials for lithium ion separators. However, all the above polypropylene products are used for lithium battery membranes with a thickness of more than 10 microns. When used for lithium battery membranes with a thickness of less than 10 microns, there are situations of low puncture strength and low breakdown voltage resistance, thus affecting their application in lithium batteries.

[0008] Based on this, in the face of the problems of low puncture strength or low breakdown voltage resistance in the application of polypropylene products in the prior art to lithium battery membranes, there is an urgent need to provide a polypropylene and its preparation method to improve the above problems. Summary of the Invention

[0009] The main purpose of the present invention is to provide a polypropylene, its preparation method and application, so as to solve the problems of low puncture strength or low breakdown voltage resistance in the application of polypropylene products in the prior art to lithium battery membranes.

[0010] To achieve the above object, according to one aspect of the present invention, a method for preparing polypropylene is provided. The preparation method includes: Step S1, taking propylene, hydrogen, a catalyst, a cocatalyst and an external electron donor and carrying out a first polymerization reaction in the first loop reactor of a double-loop reactor to obtain a material after the first polymerization reaction;

[0011] Step S2, taking the material after the first polymerization reaction, additional propylene and additional hydrogen and carrying out a second polymerization reaction in the second loop reactor of the double-loop reactor to obtain a material after the second polymerization reaction;

[0012] Step S3, taking the material after the second polymerization reaction and successively carrying out steam distillation, drying and pelletizing treatments to obtain polypropylene;

[0013] Wherein, the double-loop reactor includes a first loop reactor and a second loop reactor. The molar amount of hydrogen added in the first loop reactor accounts for 0.007% to 0.015% of propylene, and the molar amount of additional hydrogen added in the second loop reactor accounts for 0.4% to 0.6% of additional propylene; the cocatalyst is triethylaluminum, and the addition amount of the cocatalyst accounts for 0.0018% to 0.0025% of the weight of propylene; the external electron donor is cyclohexylmethyldimethoxysilane and / or dicyclopentyldimethoxysilane, and the weight ratio of the addition amount of the external electron donor to the cocatalyst is 1:1 to 3; in Step S3, the temperature of the steam distillation treatment is 100°C to 130°C, the time is 0.2 h to 0.5 h, the temperature of the drying treatment is 70°C to 100°C, and the time is 0.1 h to 0.3 h.

[0014] Further, the molar amount of hydrogen added in the first loop reactor accounts for 0.007% to 0.012% of propylene, and the molar amount of additional hydrogen added in the second loop reactor accounts for 0.5% to 0.6% of additional propylene.

[0015] Further, in Step S2, the weight ratio of the amount of additional propylene to the weight of the material after the first polymerization reaction is (0.5~1):1.

[0016] Further, the catalyst is a titanium-based catalyst, the model of the titanium-based catalyst is HA catalyst and / or HA-R catalyst, and the activity of the titanium-based catalyst is 100000 gPP / g catalyst to 200000 gPP / g catalyst.

[0017] Further, the reaction temperature of the double-loop reactor is 67°C to 73°C, the reaction time is 1.5 h to 3.0 h, and the reaction pressure is 3.6 MPa to 4.0 MPa.

[0018] To achieve the above object, according to one aspect of the present invention, a polypropylene is provided, and the polypropylene is a polypropylene product obtained by the above preparation method.

[0019] Furthermore, the melt flow rate of the polypropylene is 0.2 g / 10 min to 0.4 g / 10 min, the isotacticity is 98.5% to 99.0%, the ash content is 0.0015% to 0.0030%, the molecular weight distribution index is 6 to 9, and the weight average molecular weight is 500,000 to 700,000.

[0020] According to another aspect of the present invention, there is provided an application of polypropylene in the field of lithium battery separators.

[0021] When the polypropylene product obtained by applying the technical solution of the present invention is used to prepare a 10-micron lithium battery separator, it exhibits strong puncture strength and high breakdown voltage resistance, and has excellent performance. Detailed Embodiments

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application are described clearly and completely. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the scope of protection of the present application.

[0023] As described in the background art section of the present invention, the polypropylene products in the prior art have problems of low puncture strength or low breakdown voltage resistance when applied to lithium battery separators. Based on this, the present invention provides a preparation method of polypropylene, which includes: Step S1, taking propylene, hydrogen, a catalyst, a cocatalyst, and an external electron donor into the first loop reactor of a double-loop reactor to carry out a first polymerization reaction to obtain a material after the first polymerization reaction; Step S2, taking the material after the first polymerization reaction, supplementary propylene, and supplementary hydrogen into the second loop reactor of the double-loop reactor to carry out a second polymerization reaction to obtain a material after the second polymerization reaction; Step S3, taking the material after the second polymerization reaction and successively carrying out steam distillation, drying, and pelletizing treatments to obtain polypropylene; wherein, the double-loop reactor includes a first loop reactor and a second loop reactor, the molar amount of hydrogen added in the first loop reactor accounts for 0.007% to 0.015% of propylene, and the molar amount of supplementary hydrogen added in the second loop reactor accounts for 0.4% to 0.6% of supplementary propylene; the cocatalyst is triethylaluminum, and the addition amount of the cocatalyst accounts for 0.0018% to 0.0025% of the weight of propylene; the external electron donor is cyclohexylmethyldimethoxysilane and / or dicyclopentyldimethoxysilane, and the weight ratio of the addition amount of the external electron donor to the cocatalyst is 1:1 to 3; in Step S3, the temperature of the steam distillation treatment is 100°C to 130°C, the time is 0.2 h to 0.5 h, the temperature of the drying treatment is 70°C to 100°C, and the time is 0.1 h to 0.3 h.

[0024] Based on the polypropylene materials prepared in the prior art for lithium-ion diaphragms, these polypropylene products are all used for lithium battery diaphragms with a thickness of more than 10 microns. When used for lithium battery diaphragms with a thickness of less than 10 microns, they have low puncture strength and low breakdown voltage resistance, thus affecting their application in lithium batteries. The present invention uses a new preparation method to prepare polypropylene. This preparation method includes: Step S1, taking propylene, hydrogen, a catalyst, a cocatalyst, and an external electron donor and carrying out a first polymerization reaction in the first loop reactor of a double-loop reactor to obtain a material after the first polymerization reaction; Step S2, taking the material after the first polymerization reaction, adding additional propylene and additional hydrogen, and carrying out a second polymerization reaction in the second loop reactor of the double-loop reactor to obtain a material after the second polymerization reaction; Step S3, taking the material after the second polymerization reaction and successively carrying out steam distillation, drying, and pelletizing treatments to obtain polypropylene; wherein, the double-loop reactor includes a first loop reactor and a second loop reactor. In particular, the molar amount of hydrogen added to the first loop reactor accounts for 0.007% to 0.015% of propylene, and the molar amount of additional hydrogen added to the second loop reactor accounts for 0.4% to 0.6% of the additional propylene. This asymmetric hydrogenation method can maximize the broadening of the molecular weight distribution of the product, so that there are both small molecule parts in the product, which can ensure its good processing performance, and large molecule parts, which can ensure the mechanical properties of the product. Thus, the polypropylene product has both excellent processing performance and good mechanical properties, and further improves the puncture strength and breakdown voltage resistance of the polypropylene product. On the other hand, a cocatalyst and an external electron donor are also added in the preparation process of the present invention. Among them, the cocatalyst is triethylaluminum, and the addition amount of the cocatalyst accounts for 0.0018% to 0.0025% of the weight of the propylene; the external electron donor is cyclohexylmethyldimethoxysilane and / or dicyclopentyldimethoxysilane, and the weight ratio of the addition amount of the external electron donor to the cocatalyst is 1:1 to 3. By adding the above-mentioned cocatalyst, it is more conducive to the activation of the catalyst activity, thus meeting the technical effect of low ash content of the product. At the same time, adding an external electron donor can play a role in increasing the isotactic index of the product, so as to achieve a good pore-forming performance when processing the lithium battery diaphragm.

[0025] Finally, the material after the second polymerization reaction is successively subjected to steam distillation, drying, and pelletizing treatments to obtain a polypropylene product; wherein, the temperature of the steam distillation treatment is 100°C to 130°C, and the time is 0.2 h to 0.5 h. The temperature of the drying treatment is 70°C to 100°C, and the time is 0.1 h to 0.3 h. Optimizing the above steam distillation treatment and drying treatment parameters within the above ranges can make the product have a low volatile content, enabling it to better reduce the smoke generation amount during the polyolefin processing process, thereby improving the processing environment during the preparation process.

[0026] In a preferred embodiment, the molar amount of hydrogen added to the first loop reactor accounts for 0.007% to 0.012% of propylene, and the additional molar amount of hydrogen added to the second loop reactor accounts for 0.5% to 0.6% of the additional propylene, so as to further improve the puncture strength and breakdown voltage resistance of polypropylene.

[0027] In order to further improve the polymerization reaction activity and catalyze the full progress of the polymerization reaction, the preferred catalyst is a titanium-based catalyst. This titanium-based catalyst is a Ziegler-Natta catalyst, and the active component is titanium (Ti). Its models are HA catalyst and / or HA-R catalyst. The activity of this catalyst is 100,000 gPP / g catalyst to 200,000 gPP / g catalyst, so as to further improve the activity and stability of the catalyst.

[0028] In a preferred embodiment, in step S2, the weight ratio of the amount of additional propylene to the material after the first polymerization reaction is (0.5~1):1, so as to further improve the activity of the catalyst, improve the performance of polypropylene, and optimize the polymerization process.

[0029] In order to further improve the performance of the polypropylene product, the reaction temperature of the double loop reactor is preferably 67°C to 73°C, the reaction time is 1.5 h to 3.0 h, and the reaction pressure is 3.6 MPa to 4.0 MPa, so that the polypropylene product has a wider molecular weight distribution, and the product has better processing performance at a low melt flow rate.

[0030] On the other hand, the present invention also provides a kind of polypropylene, which is the polypropylene product prepared by the above-mentioned polypropylene preparation method. And the melt flow rate of this polypropylene product is 0.2 g / 10 min to 0.4 g / 10 min, the isotacticity is 98.5% to 99.0%, the ash content is 0.0015% to 0.0030%, the molecular weight distribution index is 6 to 9, and the weight average molecular weight is 500,000 to 700,000.

[0031] On the other hand, the present invention also provides an application of polypropylene in the field of lithium battery separators. When it is applied to improve lithium battery separators, it has the characteristics of large puncture strength and high breakdown voltage resistance, and can ultimately be applied to the batch preparation of lithium battery separators with a thickness of 10 microns and below.

[0032] The following further describes the present application in detail with specific examples, and these examples should not be construed as limiting the scope claimed by the present application.

[0033] Example 1

[0034] Propylene, hydrogen, a catalyst, a cocatalyst, and an external electron donor are taken into the first loop reactor of a double-loop reactor for a first polymerization reaction to obtain a material after the first polymerization reaction. Among them, the catalyst is a titanium-based catalyst with a model of HA catalyst and an activity of 180,000 gPP / g catalyst. The cocatalyst is triethylaluminum, and its addition amount accounts for 0.0018% of the weight of propylene. The external electron donor is cyclohexylmethyldimethoxysilane, and its weight ratio to the cocatalyst is 1:1. The material after the first polymerization reaction, additional propylene, and additional hydrogen are taken into the second loop reactor of the double-loop reactor for a second polymerization reaction to obtain a material after the second polymerization reaction. Among them, the molar amount of hydrogen added in the first loop reactor accounts for 0.007% of propylene, and the addition amount of hydrogen in the second loop reactor accounts for 0.6% of the additional propylene. The weight ratio of the additional propylene to the material after the first polymerization reaction is 0.6:1. The reaction temperature of the double-loop reactor is 70 °C, the reaction time is 2 h, and the reaction pressure is 4 MPa. Then, the material after the second polymerization reaction is successively subjected to steam distillation, drying, and pelletizing treatments. Among them, the steam distillation temperature is 120 °C and the time is 0.5 h, and the drying temperature is 90 °C and the time is 0.2 h. A polypropylene product is obtained. The melt flow rate of this polypropylene is 0.3 g / 10 min, the isotacticity is 98.8%, the ash content is 0.002%, the molecular weight distribution index is 7.5, and the weight-average molecular weight is 580,000.

[0035] Example 2

[0036] The difference from Example 1 is only that: the catalyst activity is 120,000 gPP / g catalyst, the cocatalyst is triethylaluminum, and its addition amount accounts for 0.0025% of the weight of propylene. The external electron donor is cyclohexylmethyldimethoxysilane, and its weight ratio to the cocatalyst is 1:2. The addition amount of hydrogen in the second loop reactor accounts for 0.4% of the additional propylene, and the reaction pressure is 3.8 MPa. A polypropylene product is obtained. The melt flow rate of this polypropylene is 0.2 g / 10 min, the isotacticity is 98.6%, the ash content is 0.0025%, the molecular weight distribution index is 6.5, and the weight-average molecular weight is 650,000.

[0037] Example 3

[0038] The difference from Example 1 is only that: the catalyst activity is 150,000 g PP / g catalyst, the cocatalyst is triethylaluminum, and its addition amount accounts for 0.0025% of the weight of propylene; the external electron donor is cyclohexylmethyldimethoxysilane, and its weight ratio to the cocatalyst is 1:3; the molar amount of hydrogen added in the first loop reactor accounts for 0.015% of propylene; the molar amount of supplementary hydrogen added in the second loop reactor accounts for 0.5% of supplementary propylene; the reaction temperature is 73 °C, and the reaction pressure is 4 MPa, obtaining a polypropylene product. The melt flow rate of this polypropylene is 0.3 g / 10 min, the isotacticity is 98.5%, the ash content is 0.0025%, the molecular weight distribution index is 6.5, and the weight-average molecular weight is 580,000.

[0039] Example 4

[0040] The difference from Example 1 is only that: the catalyst activity is 100,000 g PP / g catalyst, the cocatalyst is triethylaluminum, and its addition amount accounts for 0.0025% of the weight of propylene; the external electron donor is cyclohexylmethyldimethoxysilane, and its weight ratio to the cocatalyst is 1:3; the molar amount of hydrogen added in the first loop reactor accounts for 0.015% of propylene; the molar amount of hydrogen added in the second loop reactor accounts for 0.6% of supplementary propylene; the reaction temperature is 67 °C, and the reaction pressure is 4 MPa, obtaining a polypropylene product. The melt flow rate of this polypropylene is 0.4 g / 10 min, the isotacticity is 98.5%, the ash content is 0.003%, the molecular weight distribution index is 7, and the weight-average molecular weight is 500,000.

[0041] Example 5

[0042] The difference from Example 1 is only that: the cocatalyst used is triethylaluminum, and its addition amount accounts for 0.002% of the weight of propylene; the external electron donor is dicyclopentyldimethoxysilane, and its weight ratio to the cocatalyst is 1:2. The melt flow rate of this polypropylene is 0.3 g / 10 min, the isotacticity is 98.6%, the ash content is 0.0024%, the molecular weight distribution index is 7.5, and the weight-average molecular weight is 580,000.

[0043] Comparative Example 1

[0044] A polypropylene product prepared by a conventional traditional method, the difference from Example 1 is only that: a symmetric hydrogenation method is adopted, the molar amount of hydrogen added in the first loop reactor accounts for 0.032% of propylene, the molar amount of hydrogen added in the second loop reactor accounts for 0.032% of propylene, the reaction temperature is 70 °C, the reaction pressure is 3.4 MPa, and the catalyst activity is 100,000 g PP / g catalyst, obtaining a polypropylene product. The melt flow rate of this polypropylene is 1.0 g / 10 min, the isotacticity is 98.6%, the ash content is 0.003%, the molecular weight distribution index is 4, and the weight-average molecular weight is 380,000.

[0045] Comparative Example 2

[0046] The difference from Example 1 is only that: no external electron donor was added. The melt flow rate of this polypropylene is 0.3 g / 10 min, the isotacticity is 97.8%, the ash content is 0.002%, the molecular weight distribution index is 7.5, and the weight-average molecular weight is 580,000.

[0047] Comparative Example 3

[0048] The difference from Example 1 is only that: the weight ratio of the added amount of the external electron donor to the cocatalyst is 1:5. The melt flow rate of this polypropylene is 0.3 g / 10 min, the isotacticity is 98.1%, the ash content is 0.002%, the molecular weight distribution index is 7.5, and the weight-average molecular weight is 580,000.

[0049] Comparative Example 4

[0050] The difference from Example 1 is only that: the weight ratio of the added amount of the external electron donor to the cocatalyst is 1:0.5. The melt flow rate of this polypropylene is 0.3 g / 10 min, the isotacticity is 99.0%, the ash content is 0.003%, the molecular weight distribution index is 7.5, and the weight-average molecular weight is 580,000.

[0051] Comparative Example 5

[0052] The difference from Example 1 is only that: no cocatalyst was added. The melt flow rate of this polypropylene is 3.0 g / 10 min, the isotacticity is 98.8%, the ash content is 0.005%, the molecular weight distribution index is 7.5, and the weight-average molecular weight is 300,000.

[0053] Comparative Example 6

[0054] The difference from Example 1 is only that: the added amount of the cocatalyst accounts for 0.005% of the weight of propylene. The melt flow rate of this polypropylene is 0.3 g / 10 min, the isotacticity is 98.8%, the ash content is 0.005%, the molecular weight distribution index is 7.5, and the weight-average molecular weight is 580,000.

[0055] Comparative Example 7

[0056] The difference from Example 1 is only that: during the post-treatment process of the polymerization reaction, the temperature of the steam treatment is 80 °C and the time is 0.5 h. The melt flow rate of this polypropylene is 0.3 g / 10 min, the isotacticity is 98.8%, the ash content is 0.002%, the molecular weight distribution index is 7.5, and the weight-average molecular weight is 580,000.

[0057] Comparative Example 8

[0058] The difference from Example 1 is only that: during the post-treatment process of the polymerization reaction, the temperature of the steam treatment is 150 °C and the time is 0.2 h. The melt flow rate of this polypropylene is 0.3 g / 10 min, the isotacticity is 98.8%, the ash content is 0.002%, the molecular weight distribution index is 7.5, and the weight average molecular weight is 580,000.

[0059] Performance Test

[0060] (1)Puncture Strength

[0061] The test was carried out using a tensile machine, specifically including: using the polypropylene products prepared in Examples 1 to 4 and Comparative Example 1 above, and preparing lithium battery separators by a dry method unidirectional stretching process (that is, first forming defects such as silver streaks by stretching at a low temperature, and then pulling the defects apart at a high temperature). Then, use a scalpel to cut the lithium battery separator into strips about 10 cm wide in the transverse direction. The first test point is 5 cm from the edge, and three points are tested along the transverse direction of the separator. Determine the test point positions according to the membrane width to ensure that the distance between each test point is consistent. The speed of the puncture flat needle is 120 mm / min, the speed of the round needle is 50 mm / min, and the speed of the return needle is 200 mm / min.

[0062] (2)Breakdown Voltage Resistance

[0063] The test was carried out using a withstand voltage tester, specifically including: selecting a flat film sample with a width of about 20 cm, placing it on the lower sheet, gently placing the upper sheet in the middle position of the test sample, using the positive probe to contact the upper sheet, and pressing the start button to start the test.

[0064] Take the polypropylene prepared in the above examples and comparative examples, prepare lithium battery separators with a thickness of 10 μm by the dry method single stretching process, and conduct performance tests. The test results are shown in Table 1.

[0065] Table 1

[0066] 。

[0067] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0068] It can be found from the test results in Table 1 above that when the melt flow rate of the product is lower, the isotacticity is higher, and the weight average molecular weight is larger, the puncture strength of the prepared lithium battery separator is higher; when the melt flow rate of the product is lower, the isotacticity is higher, the ash content is lower, and the weight average molecular weight is larger, the prepared lithium battery separator has better breakdown voltage resistance.

[0069] The polypropylene product obtained by the preparation method of the present invention has higher puncture strength and higher breakdown voltage resistance compared to the polypropylene obtained by the traditional method when it is used to prepare a 10-micron lithium battery separator, and its comprehensive performance is better.

[0070] Although this specification contains many specific implementation details, these should not be construed as limiting the scope of any invention or the scope of what is claimed, but are mainly used to describe the features of specific embodiments of a particular invention. Certain features described in multiple embodiments in this specification can also be implemented in combination in a single embodiment. On the other hand, the various features described in a single embodiment can also be implemented separately in multiple embodiments or in any suitable sub-combination. In addition, although the features may function in certain combinations as described above and are even initially claimed as such, one or more features from the claimed combination can in some cases be removed from the combination, and the claimed combination can refer to a sub-combination or a variant of the sub-combination.

[0071] Thus, specific embodiments of the subject matter have been described. Other embodiments are within the scope of the appended claims. In some cases, the acts recited in the claims can be performed in a different order and still achieve the desired result.

[0072] It should be noted that, in this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprises", "comprising" or any other variant thereof are intended to cover a non-exclusive inclusion, such that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising a..." does not exclude the existence of additional identical elements in the process, method, article or device comprising the element.

[0073] The above description is only the specific implementation manners of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing polypropylene, characterized in that, The preparation method includes the following steps: Step S1: Propylene, hydrogen, a catalyst, a cocatalyst, and an external electron donor are taken and subjected to a first polymerization reaction in the first loop reactor of a double-loop reactor to obtain a material after the first polymerization reaction. The catalyst is a titanium-based catalyst, and the model of the titanium-based catalyst is HA catalyst and / or HA-R catalyst. The activity of the titanium-based catalyst is 100,000 g PP / g catalyst to 200,000 g PP / g catalyst. Step S2: The material after the first polymerization reaction, additional propylene, and additional hydrogen are taken and subjected to a second polymerization reaction in the second loop reactor of the double-loop reactor to obtain a material after the second polymerization reaction. Step S3: The material after the second polymerization reaction is successively subjected to steam distillation, drying, and pelletizing treatments to obtain polypropylene. Among them, The double-loop reactor includes a first loop reactor and a second loop reactor. The molar amount of hydrogen added in the first loop reactor accounts for 0.007% to 0.015% of the propylene, and the molar amount of the additional hydrogen added in the second loop reactor accounts for 0.4% to 0.6% of the additional propylene. The cocatalyst is triethylaluminum, and the addition amount of the cocatalyst accounts for 0.0018% to 0.0025% of the weight of the propylene. The external electron donor is cyclohexylmethyldimethoxysilane and / or dicyclopentyldimethoxysilane, and the weight ratio of the addition amount of the external electron donor to the cocatalyst is 1:1 to 3. In step S3, the temperature of the steam distillation treatment is 100~130 °C, the time is 0.5 h, the temperature of the drying treatment is 70~100 °C, and the time is 0.1~0.3 h. The reaction temperature of the double-loop reactor is 67 °C to 73 °C, the reaction time is 1.5 h to 3.0 h, and the reaction pressure is 3.6 MPa to 4.0 MPa.

2. The preparation method of polypropylene according to claim 1, characterized in that, The molar amount of hydrogen added in the first loop reactor accounts for 0.007% to 0.012% of the propylene, and the molar amount of the additional hydrogen added in the second loop reactor accounts for 0.5% to 0.6% of the additional propylene.

3. The method for preparing polypropylene according to claim 1 or 2, characterized in that, In step S2, the weight ratio of the amount of the additional propylene to the material after the first polymerization reaction is 0.5 to 1:

1.

4. A polypropylene, characterized in that, The polypropylene is obtained by the polypropylene preparation method according to any one of claims 1 to 3.

5. Application of the polypropylene according to claim 4 in the field of lithium battery separators.