Polypropylene and its application, method and system for continuously preparing polypropylene

Abstract

The present invention relates to the technical field of polymer production, and discloses a polypropylene and its application, a method and a system for continuously preparing polypropylene. The method is carried out in a system containing at least two reactors connected in series, and the method comprises: (1) mixing a catalyst and a diene monomer in the presence of a solvent to obtain a suspension; the diene monomer is selected from C6-C 20 (1) preparing at least one symmetrical non-conjugated α-dienes; (2) introducing the suspension, propylene monomer I, and ethylene monomer I into a reactor I to carry out polymerization reaction I, thereby obtaining a first stream; and (3) introducing the first stream, propylene monomer II, and ethylene monomer II into a reactor II to carry out polymerization reaction II. The method of the present invention can effectively solve the problem of sag in polypropylene materials, and the resulting polypropylene material has the advantages of anti-sag and good dimensional stability.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer production, and in particular to polypropylene and applications thereof, and a method and system for continuously preparing polypropylene. Background Art

[0002] Ordinary polypropylene chains have a linear structure and a relatively narrow molecular weight distribution, making them susceptible to a sharp drop in melt viscosity during the thermoforming process, known as sag. Due to this sag, polypropylene can become dimensionalally non-uniform during extrusion and subsequent processing.

[0003] To overcome these drawbacks, existing technologies often improve the melt strength of polypropylene by increasing relative molecular weight, broadening relative molecular weight distribution, introducing long-chain branching, and crosslinking. These methods all improve the sag of polypropylene by strengthening the forces between polypropylene chains. Therefore, existing technologies often use reactive extrusion to introduce long-chain branching. By incorporating functional monomers, such as pentaerythritol triacrylate, this structure can, to some extent, overcome the dimensional non-uniformity that occurs during polypropylene processing.

[0004] However, the reactive extrusion method is a modification based on the traditional method of preparing polypropylene. During the subsequent processing, polypropylene will degrade inside the screw. Moreover, the post-processing process increases the overall time cost and processing cost of the material, resulting in limited processing capacity. In addition, small molecules may remain inside the material after processing using this method, which can easily cause the material to break during the long-term aging process.

[0005] Therefore, developing a method for continuously preparing polypropylene to obtain polypropylene materials with anti-sag properties is crucial for the industrial production of polypropylene materials. Summary of the Invention

[0006] The purpose of the present invention is to solve the problem that the polypropylene obtained in the prior art is prone to sag.

[0007] In order to achieve one of the above objectives, the present invention provides a method for continuously preparing polypropylene in a system comprising at least two reactors connected in series, the method comprising:

[0008] (1) In the presence of a solvent, a catalyst and a diene monomer are contacted and mixed to obtain a suspension; the diene monomer is selected from C6-C 20 At least one of the symmetrical non-conjugated α-diolefins;

[0009] (2) introducing the suspension, propylene monomer I, and ethylene monomer I into a reactor I to carry out a polymerization reaction I to obtain a first stream; the molar ratio of the propylene monomer I to the ethylene monomer I is 1:0.1-0.6;

[0010] (3) introducing the first stream, propylene monomer II and ethylene monomer II into a reactor II to carry out polymerization reaction II; the molar ratio of the propylene monomer II to the ethylene monomer II is 1:0.01-0.3.

[0011] The second aspect of the present invention provides polypropylene produced by the method described in the first aspect.

[0012] The third aspect of the present invention provides use of the polypropylene described in the second aspect in a foaming material.

[0013] A fourth aspect of the present invention provides a system for continuously preparing polypropylene, comprising a catalyst reactor, a reactor I and a reactor II connected in series, wherein the catalyst reactor is provided with a solvent inlet, a catalyst inlet, a diene monomer inlet and a suspension outlet; the reactor I is provided with inlets for propylene monomer I and ethylene monomer I, a first logistics outlet, and a suspension inlet connected to the suspension outlet; and the reactor II is provided with inlets for propylene monomer II and ethylene monomer II, and a first logistics inlet connected to the first logistics outlet.

[0014] The method of the present invention is used to prepare polypropylene, which can effectively solve the problem that polypropylene materials are prone to sag. The prepared polypropylene material has the advantages of anti-sag and good dimensional stability. At the same time, the polypropylene also has good aging resistance.

[0015] In particular, the present invention prepares polypropylene in a system containing at least two reactors connected in series, which is not limited by the output of the screw extruder, effectively solving the problem of limited processing capacity in traditional methods. In addition, the preparation process is simple, no by-products are generated throughout the entire process, the cleanliness level is high, and the production cost can be effectively reduced, making large-scale industrial production possible. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a process flow chart of a preferred specific embodiment of the method of the present invention.

[0017] Description of Reference Numerals

[0018] 1. Solvent 2. Catalyst

[0019] 3. Diolefin monomer 4. Catalyst reactor

[0020] 5. Suspension 6. Propylene monomer I and ethylene monomer I

[0021] 7. Reactor I 8. First Logistics

[0022] 9. Propylene monomer II and ethylene monomer II 10. Reactor II DETAILED DESCRIPTION

[0023] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0024] In the present invention, unless otherwise stated, the material level refers to the height occupied by the solid in the reactor.

[0025] In the present invention, unless otherwise stated, the pressures mentioned are gauge pressures.

[0026] As mentioned above, the first aspect of the present invention provides a method for continuously preparing polypropylene, which is carried out in a system comprising at least two reactors connected in series, and the method comprises:

[0027] (1) In the presence of a solvent, a catalyst and a diene monomer are contacted and mixed to obtain a suspension; the diene monomer is selected from C6-C 20 At least one of the symmetrical non-conjugated α-diolefins;

[0028] (2) introducing the suspension, propylene monomer I, and ethylene monomer I into a reactor I to carry out a polymerization reaction I to obtain a first stream; the molar ratio of the propylene monomer I to the ethylene monomer I is 1:0.1-0.6;

[0029] (3) introducing the first stream, propylene monomer II and ethylene monomer II into a reactor II to carry out polymerization reaction II; the molar ratio of the propylene monomer II to the ethylene monomer II is 1:0.01-0.3.

[0030] Preferably, the diene monomer is selected from C8-C 12 At least one of the symmetrical non-conjugated alpha dienes.

[0031] According to a particularly preferred embodiment, in step (1), the diene monomer is selected from at least one of 1,7-octadiene, 1,8-nonadiene, 1,9-decadiene, and 1,11-dodecadiene.

[0032] The present invention has no particular limitation on the contact mixing in step (1), as long as all components of the catalyst can be uniformly dispersed in the solvent. Stirring, ultrasound, mass transfer, or oscillation can be used. Preferably, in step (1), the contact mixing conditions include at least: a stirring speed of 5-50 rpm, a temperature of 10-90° C., and an average residence time of 2-200 min. More preferably, in step (1), the contact mixing conditions include at least: a stirring speed of 20-50 rpm, a temperature of 20-60° C., and an average residence time of 100-150 min.

[0033] Preferably, in step (1), the weight ratio of the catalyst, the diene monomer, and the solvent is 0.0001-0.005:0.1-5:1, more preferably 0.0005-0.002:0.5-3:1.

[0034] Preferably, in step (1), the solvent is a combination of white oil and vaseline in a weight ratio of 0.1-5:1. More preferably, the weight ratio of the white oil to the vaseline in the solvent is 0.5-2:1. In this preferred embodiment, the solution of the present invention can produce polypropylene with a higher content of long-chain branches.

[0035] In the present invention, the catalyst can be selected from Ziegler-Natta type catalysts known in the art. For example, the catalyst can adopt a MgCl2 supported catalytic system or a VOCl3-AlEt2Cl catalytic system.

[0036] Preferably, the catalyst contains an organoaluminum compound, an external electron donor compound, an internal electron donor compound, and a titanium-containing solid active center component.

[0037] According to a particularly preferred embodiment, in the catalyst, the molar ratio of the titanium-containing solid active center component calculated as titanium element to the organoaluminum compound calculated as aluminum element is 1:1-10.

[0038] Preferably, the internal electron donor compound is a phthalate and / or ether compound.

[0039] Preferably, the organoaluminum compound is at least one selected from triethylaluminum, triisobutylaluminum, tri-n-butylaluminum and trihexylaluminum.

[0040] Preferably, the external electron donor compound is at least one selected from 1,3-dialkoxypropane, 2-ethyl-2-butyl-1,3-dimethoxypropane, cyclohexylmethyldimethoxysilane, diisobutyldimethoxysilane, and 9,9-bis(methoxymethyl)fluorene.

[0041] Preferably, in step (2), the molar ratio of the propylene monomer I to the ethylene monomer I is 1:0.2-0.5.

[0042] Preferably, in step (2), the conditions of the polymerization reaction I include: pressure of 2.0-2.4 MPa, temperature of 55-75° C., average residence time of 0.2-1.2 h, and material level of 55-87%.

[0043] Preferably, in step (2), the conditions of the polymerization reaction I include: a pressure of 2.2-2.4 MPa, a temperature of 60-75° C., an average residence time of 0.8-1.2 h, and a material level of 60-87%. In this preferred embodiment, polypropylene with better dimensional stability and better aging resistance can be obtained.

[0044] Preferably, in step (3), the molar ratio of the propylene monomer II to the ethylene monomer II is 1:0.01-0.1.

[0045] Preferably, in step (3), the conditions of the polymerization reaction II include: pressure of 2.0-2.4 MPa, temperature of 55-85° C., average residence time of 0.4-1.4 h, and material level of 55-87%.

[0046] Preferably, in step (3), the conditions of the polymerization reaction II include: a pressure of 2.1-2.4 MPa, a temperature of 70-85° C., an average residence time of 1-1.4 hours, and a material level of 80-87%. In this preferred embodiment, polypropylene with better dimensional stability and better aging resistance can be obtained.

[0047] The following combination Figure 1 The following is a process flow diagram of a preferred embodiment of the method for continuously preparing polypropylene according to the present invention:

[0048] (1) In the presence of solvent 1, catalyst 2 and diene monomer 3 are introduced into catalyst reactor 4 and mixed to obtain suspension 5;

[0049] (2) introducing the suspension 5, propylene monomer 1, and ethylene monomer 16 into a reactor 17 to carry out a polymerization reaction 1 to obtain a first stream 8;

[0050] (3) The first stream 8, propylene monomer II and ethylene monomer II 9 are introduced into the reactor II 10 to carry out polymerization reaction II.

[0051] As mentioned above, the second aspect of the present invention provides a polypropylene produced by the method described in the first aspect.

[0052] As mentioned above, the third aspect of the present invention provides the use of the polypropylene described in the second aspect in a foamed material.

[0053] As described above, the fourth aspect of the present invention provides a system for continuously preparing polypropylene, comprising a catalyst reactor, a reactor I and a reactor II connected in series, wherein the catalyst reactor is provided with a solvent inlet, a catalyst inlet, a diene monomer inlet and a suspension outlet; the reactor I is provided with inlets for propylene monomer I and ethylene monomer I, a first logistics outlet, and a suspension inlet connected to the suspension outlet; the reactor II is provided with inlets for propylene monomer II and ethylene monomer II, and a first logistics inlet connected to the first logistics outlet.

[0054] Preferably, the system further comprises:

[0055] A connector is provided between the reactor I and the reactor II, and is used to introduce the first logistics into the reactor II.

[0056] According to a particularly preferred embodiment, the reactor I and the reactor II are each independently selected from horizontal reactors.

[0057] The present invention will be described in detail below by way of examples. In the following examples, unless otherwise specified, all raw materials used are commercially available.

[0058] The polymerization reaction is carried out in a horizontal continuous gas phase polymerization device;

[0059] Propylene: Purity is 99.95%, produced by Beijing Yanshan Branch of Sinopec;

[0060] Ethylene: Purity is 99.95%, produced by Beijing Yanshan Branch of Sinopec;

[0061] Diolefin: 1,9-decadiene, brand ZS720, purchased from Jiuding Chemical Company;

[0062] Diolefins: 1,7-octadiene, purity 95%, purchased from Beijing Bailingwei Technology Co., Ltd.;

[0063] Diolefins: 1,8-nonadiene, purity 98%, purchased from Beijing Bailingwei Technology Co., Ltd.;

[0064] Diolefin: 1,11-dodecadiene, purity 99%, purchased from Beijing Bailingwei Technology Co., Ltd.;

[0065] Catalyst-1: The internal electron donor is di-n-butyl phthalate, the external electron donor compound is 2-ethyl-2-butyl-1,3-dimethoxypropane, the organoaluminum compound is triethylaluminum, and the molar ratio of the titanium-containing solid active center component calculated as titanium element to the organoaluminum compound calculated as aluminum element is 1:3;

[0066] Catalyst-2: Similar to Catalyst-1, except that the external electron donor compound is cyclohexylmethyldimethoxysilane;

[0067] Catalyst-3: Similar to Catalyst-1, except that the external electron donor compound is diisobutyldimethoxysilane.

[0068] The LCB f / 1000TC represents the number of long-chain branching points contained in 1000 carbon atoms and is determined by gel permeation chromatography-multi-angle laser scattering (GPC-MALLS) method;

[0069] The EPDM content is tested using GB / T 24282-2009;

[0070] The melt flow index MFR is measured using the method of ASTM-D1238.

[0071] Example 1

[0072] The catalyst used in the method of this embodiment is catalyst-1.

[0073] This embodiment provides a method for continuously preparing polypropylene, the method comprising:

[0074] (1) 500 mg of the catalyst and 1000 g of 1,7-octadiene were dispersed in 1000 g of a solvent at a stirring speed of 20 rpm and a temperature of 60°C for an average residence time of 100 min to obtain a suspension; wherein the solvent was a mixed solution of white oil and vaseline in a weight ratio of 2:1;

[0075] (2) The suspension obtained above was pumped into a reactor I using a pressure pump, and propylene monomer and ethylene monomer at a molar ratio of 1:0.2 were introduced into the reactor I at a temperature of 75° C. and a pressure of 2.4 MPa to carry out polymerization reaction I, wherein the amount of propylene monomer added was 1000 g, the material level was 87%, and the average polymerization residence time was 1.2 h, to obtain a first stream;

[0076] (3) The first stream obtained above and a propylene monomer and an ethylene monomer in a molar ratio of 1:0.1 were introduced into reactor II to carry out polymerization reaction II at a temperature of 85° C. and a pressure of 2.4 MPa. The amount of propylene monomer added was 1000 g, the material level was 87%, and the average polymerization residence time was 1.4 h, thereby obtaining polypropylene S1. The properties of the obtained product are shown in Table 1.

[0077] Example 2

[0078] The catalyst used in the method of this embodiment is Catalyst-2.

[0079] This embodiment provides a method for continuously preparing polypropylene, the method comprising:

[0080] (1) Under conditions of a stirring speed of 30 rpm and a temperature of 20° C., 300 mg of a catalyst and 900 g of 1,8-nonadiene monomer were dispersed in 300 g of a solvent for an average residence time of 150 min to obtain a suspension; wherein the solvent was a mixed solution of white oil and vaseline in a weight ratio of 1:1;

[0081] (2) The suspension obtained above was pumped into a reactor I using a pressure pump, and propylene monomer and ethylene monomer at a molar ratio of 1:0.4 were introduced into the reactor I at a temperature of 70° C. and a pressure of 2.2 MPa to carry out polymerization reaction I, wherein the amount of propylene monomer added was 1000 g, the material level was 60%, and the average polymerization residence time was 0.8 h, to obtain a first stream;

[0082] (3) The first stream obtained above and a propylene monomer and an ethylene monomer in a molar ratio of 1:0.05 were introduced into reactor II to carry out polymerization reaction II at a temperature of 80° C. and a pressure of 2.2 MPa. The amount of propylene monomer added was 1000 g, the material level was 80%, and the average polymerization residence time was 1 hour, thereby obtaining polypropylene S2. The properties of the obtained product are shown in Table 1.

[0083] Example 3

[0084] The catalyst used in the method of this embodiment is Catalyst-3.

[0085] This embodiment provides a method for continuously preparing polypropylene, the method comprising:

[0086] (1) 6000 mg of catalyst and 1000 g of 1,11-dodecadiene monomer were dispersed in 3000 g of solvent at a stirring speed of 50 rpm and a temperature of 45° C., with an average residence time of 100 min, to obtain a suspension; wherein the solvent was a mixed solution of white oil and vaseline at a weight ratio of 0.8:1;

[0087] (2) The suspension obtained above was pumped into a reactor I using a pressure pump, and propylene monomer and ethylene monomer at a molar ratio of 1:0.5 were introduced into the reactor I at a temperature of 73° C. and a pressure of 2.3 MPa to carry out polymerization reaction I, wherein the amount of propylene monomer added was 1000 g, the material level was 75%, and the average polymerization residence time was 1 h, to obtain a first stream;

[0088] (2) The first stream obtained above and a propylene monomer and an ethylene monomer in a molar ratio of 1:0.01 were introduced into reactor II to carry out polymerization reaction II at a temperature of 70° C. and a pressure of 2.1 MPa. The amount of propylene monomer added was 1000 g, the material level was 85%, and the average polymerization residence time was 1.2 h, thereby obtaining polypropylene S3. The properties of the obtained product are shown in Table 1.

[0089] Example 4

[0090] This example is similar to the method of Example 3, except that in step (1), 1,11-dodecadiene is replaced with an equal mass of 1,9-decadiene.

[0091] The remaining steps are the same as those in Example 3.

[0092] Polypropylene S4 was obtained, and the properties of the obtained product are shown in Table 1.

[0093] Example 5

[0094] This embodiment refers to the method of embodiment 3, except that in step (1), the polymerization temperature in the reactor I is 55°C.

[0095] The remaining steps are the same as those in Example 3.

[0096] Polypropylene S5 was obtained, and the properties of the obtained product are shown in Table 1.

[0097] Example 6

[0098] This embodiment refers to the method of embodiment 3, except that in step (2), the polymerization temperature in reactor II is 65°C.

[0099] The remaining steps are the same as those in Example 3.

[0100] Polypropylene S6 was obtained, and the properties of the obtained product are shown in Table 1.

[0101] Example 7

[0102] This embodiment refers to the method of Example 3, except that in step (1), the weight ratio of the white oil to the vaseline is 0.1:1.

[0103] The remaining steps are the same as those in Example 3.

[0104] Polypropylene S7 was obtained, and the properties of the obtained product are shown in Table 1.

[0105] Example 8

[0106] This embodiment refers to the method of Example 3, except that in step (1), the weight ratio of the white oil to the vaseline is 5:1.

[0107] The remaining steps are the same as those in Example 3.

[0108] Polypropylene S8 was obtained, and the properties of the obtained product are shown in Table 1.

[0109] Comparative Example 1

[0110] This comparative example adopts the screw extrusion method to prepare polypropylene, and the specific steps include:

[0111] (1) Dissolve 0.1 g of initiator dicumyl peroxide, 1 g of grafting monomer pentaerythritol triacrylate, and 0.1 g of free radical activity regulator zinc dithiocarbamate in an appropriate amount of acetone to obtain a mixed solution:

[0112] (2) The resulting mixture was then evenly sprayed onto the surface of 100 g of polypropylene pellets. Antioxidant B215 was added and mixed thoroughly. The pellets were then placed in a vacuum oven at 80°C for 2 hours and then loaded into a twin-screw extruder with a screw length-to-diameter ratio of 35. The extruder section temperatures were 160°C, 170°C, 175°C, 170°C, and 165°C, respectively, and the screw speed was 70 r / min, to obtain polypropylene DS1. The properties of the resulting product are shown in Table 1.

[0113] Comparative Example 2

[0114] This comparative example refers to the method of Example 3, except that the catalyst, diene monomer, propylene monomer and ethylene monomer are directly introduced into the reactor I to carry out polymerization reaction I, wherein the molar ratio of the propylene monomer to the ethylene monomer is 1:0.5.

[0115] The rest are the same as in Example 3.

[0116] Polypropylene DS2 was obtained, and the properties of the obtained product are shown in Table 1.

[0117] Comparative Example 3

[0118] This comparative example refers to the method of Example 3, except that in step (2), the molar ratio of the propylene monomer to the ethylene monomer is 1:0.8.

[0119] Comparative Example 4

[0120] This comparative example refers to the method of Example 3, except that in step (3), the molar ratio of the propylene monomer to the ethylene monomer is 1:0.5.

[0121] Table 1

[0122]

[0123]

[0124] The results in Table 1 show that the polypropylene produced by the method of the present invention has a high long chain branch content and melt tensile tension. At the same time, the melt index of the polypropylene produced in the example of the present invention does not change substantially before and after screw extrusion, while the melt index of the comparative example is greatly improved, indicating that the polypropylene produced by the method of the present invention has excellent dimensional stability.

[0125] Test Example 1

[0126] The polymer products prepared in the examples and comparative examples were compression molded and subjected to mechanical property tests. They were then subjected to accelerated aging in air at 80°C. The mechanical properties after 120 hours were tested. The mechanical properties included tensile stress at break, flexural modulus, and Charpy notched impact strength. The test results are shown in Table 2.

[0127] Among them, the tensile breaking stress is tested according to GB / T 1040.2-2006 standard.

[0128] The flexural modulus is tested according to GB / T 9341-2008.

[0129] The simply supported beam notched impact strength is tested according to the GB / T 1043.1-2008 method.

[0130] Table 2

[0131]

[0132] Table 2 (continued 1)

[0133]

[0134] It can be seen from the results in Table 2 that the polypropylene prepared by the method of the present invention has excellent aging resistance.

[0135] Test Example 2

[0136] The polypropylenes produced in the Examples and Comparative Examples were used to prepare foamed materials. Each polypropylene produced in the Examples and Comparative Examples was added to a single-screw extruder for foaming. Butane was used as the blowing agent, the pressure was 4.5 MPa, and the average extruder temperature was 170°C. The density of the foamed materials was measured. The specific test results are shown in Table 3.

[0137] Table 3

[0138] <![CDATA[Density, g / cm 3 > Example 1 0.072 Example 2 0.083 Example 3 0.078 Example 4 0.105 Example 5 0.088 Example 6 0.077 Example 7 0.075 Example 8 0.071 Comparative Example 1 0.602 Comparative Example 2 0.710 Comparative Example 3 0.431 Comparative Example 4 0.426

[0139] It can be seen from Table 3 that the polypropylene obtained by the method of the present invention can obtain a foamed material with a higher foaming ratio and a lower density.

[0140] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A method for continuously preparing polypropylene, characterized in that: The method is carried out in a system containing at least two reactors connected in series, and the method comprises: (1) In the presence of a solvent, a catalyst and a diene monomer are contacted and mixed to obtain a suspension; the diene monomer is selected from C6-C 20 At least one of the symmetrical non-conjugated α-dienes; the solvent is a combination of white oil and vaseline in a weight ratio of 0.1-5:1; (2) introducing the suspension, propylene monomer I and ethylene monomer I into a reactor I to carry out a polymerization reaction I to obtain a first stream; the molar ratio of the propylene monomer I to the ethylene monomer I is 1:0.1-0.6; (3) Introducing the first stream, propylene monomer II and ethylene monomer II into a reactor II to carry out polymerization reaction II; the molar ratio of the propylene monomer II to the ethylene monomer II is 1:0.01-0.

1.

2. The method according to claim 1, wherein In step (1), the diene monomer is selected from C8-C 12 At least one of the symmetrical non-conjugated alpha dienes.

3. The method according to claim 2, wherein: In step (1), the diene monomer is selected from at least one of 1,7-octadiene, 1,8-nonadiene, 1,9-decadiene, and 1,11-dodecadiene.

4. The method according to claim 1 or 2, wherein: In step (1), the contact mixing conditions include at least: a stirring speed of 5-50 rpm, a temperature of 10-90° C., and an average residence time of 2-200 min.

5. The method according to any one of claims 1 to 3, wherein: In step (1), the weight ratio of the catalyst, the diene monomer, and the solvent is 0.0001-0.005:0.1-5:

1.

6. The method according to claim 5, wherein: In step (1), the weight ratio of the catalyst, the diene monomer, and the solvent is 0.0005-0.002:0.5-3:

1.

7. The method according to any one of claims 1 to 3, wherein: In step (1), in the solvent, the weight ratio of the white oil to the vaseline is 0.5-2:

1.

8. The method according to any one of claims 1 to 3, wherein: In step (1), the catalyst is a Ziegler-Natta type catalyst, which contains an organic aluminum compound, an external electron donor compound, an internal electron donor compound, and a titanium-containing solid active center component.

9. The method according to claim 8, wherein In the catalyst, the molar ratio of the titanium-containing solid active center component calculated as titanium element to the organic aluminum compound calculated as aluminum element is 1:1-10.

10. The method according to claim 8, wherein The internal electron donor compound is a phthalate ester and / or ether compound.

11. The method according to claim 8, wherein The organoaluminum compound is at least one selected from triethylaluminum, triisobutylaluminum, tri-n-butylaluminum and trihexylaluminum.

12. The method according to claim 8, wherein The external electron donor compound is selected from at least one of 1,3-dialkoxypropane, 2-ethyl-2-butyl-1,3-dimethoxypropane, cyclohexylmethyldimethoxysilane, diisobutyldimethoxysilane, and 9,9-bis(methoxymethyl)fluorene.

13. The method according to any one of claims 1 to 3, wherein: In step (2), the molar ratio of the propylene monomer I to the ethylene monomer I is 1:0.2-0.

5.

14. The method according to any one of claims 1 to 3, wherein: In step (2), the conditions of the polymerization reaction I include: pressure of 2.0-2.4 MPa, temperature of 55-75° C., average residence time of 0.2-1.2 h, and material level of 55-87%.

15. The method according to claim 14, wherein In step (2), the conditions of the polymerization reaction I include: pressure of 2.2-2.4 MPa, temperature of 60-75° C., average residence time of 0.8-1.2 h, and material level of 60-87%.

16. The method according to any one of claims 1 to 3, wherein: In step (3), the conditions of the polymerization reaction II include: pressure of 2.0-2.4 MPa, temperature of 55-85° C., average residence time of 0.4-1.4 h, and material level of 55-87%.

17. The method according to claim 16, wherein In step (3), the conditions of the polymerization reaction II include: pressure of 2.1-2.4 MPa, temperature of 70-85° C., average residence time of 1-1.4 h, and material level of 80-87%.

18. Polypropylene obtained by the process according to any one of claims 1 to 17.

19. Use of the polypropylene according to claim 18 in foaming materials.

20. A system for continuously preparing polypropylene, characterized in that: The system is used to implement the method for continuously preparing polypropylene according to any one of claims 1 to 17, and the system comprises a catalyst reactor, a reactor I, and a reactor II connected in series, wherein the catalyst reactor is provided with a solvent inlet, a catalyst inlet, a diene monomer inlet, and a suspension outlet; the reactor I is provided with inlets for propylene monomer I and ethylene monomer I, a first logistics outlet, and a suspension inlet connected to the suspension outlet; and the reactor II is provided with inlets for propylene monomer II and ethylene monomer II, and a first logistics inlet connected to the first logistics outlet.

21. The system of claim 20, wherein: The system also includes: A connector is provided between the reactor I and the reactor II, and is used to introduce the first logistics into the reactor II.

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