Surface tack-reduced impact polypropylene and process for its preparation

Abstract

The application discloses a kind of surface tack-reducing impact polypropylene and its preparation method.The impact polypropylene of the present application is distributed in the radial gradient distribution of ethylene-propylene copolymer dispersed phase in polypropylene particles, and the rubber phase area proportion of particle center is ≥40%, and the surface area proportion is ≤20%.The preparation method of the present application forms a multistage channel structure with an average pore size of 30-120 nanometers, a porosity of 20-50%, and a nano-pore (<20 nanometers) proportion of ≥60% by precisely regulating the propylene homopolymerization stage.In the ethylene-propylene gas phase copolymerization stage, inert condensing medium is introduced to cause capillary condensation in the internal channel of polypropylene particles, promote the enrichment of ethylene-propylene copolymer in the internal particles and form a rubber-poor surface layer;when the xylene soluble content is not higher than 45 wt%, the surface tackiness index is less than 2.The impact polypropylene obtained by the present application has low surface tackiness and good rigid-flexible balance performance.

Description

Technical Field

[0001] This invention relates to the field of impact-resistant polypropylene technology, and more specifically, to an impact-resistant polypropylene with reduced surface tack and its preparation method. Background Technology

[0002] Multiphase polypropylene is a multiphase composite material obtained by copolymerization or blending, and it is widely used in automobiles, home appliances and other fields. However, in the industrial preparation of ethylene-propylene copolymerization, the heat of reaction is difficult to remove effectively, causing the rubber phase inside the particles to migrate to the surface, resulting in particle melting and bonding, which seriously restricts the development and performance improvement of polypropylene with high rubber content.

[0003] Currently, ultrafine powders (such as silica) are commonly used in industry for temporary viscosity reduction, but this method easily clogs the active sites of the catalyst and interferes with the copolymerization reaction. As the rubber phase content increases, the problem of particle stickiness becomes increasingly prominent. Coupled with the low activity of gas-phase copolymerization, it is difficult for ordinary industrial equipment to stably produce high-rubber-content polypropylene. Existing viscosity reduction technologies, such as selective deactivation of surface active sites or physical barriers, have limited effectiveness and are generally accompanied by problems such as reduced copolymerization activity and poor product homogeneity. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a surface-reducing, impact-resistant polypropylene and its preparation method.

[0005] The technical solution of the present invention is as follows: According to a first aspect of the present invention, the present invention provides a surface-tack-reducing impact-resistant polypropylene, the impact-resistant polypropylene comprising an isotactic polypropylene (iPP) matrix and an ethylene-propylene copolymer dispersed phase, wherein the ethylene-propylene copolymer dispersed phase is distributed in a radial gradient decreasing from the central region to the surface region within the cross-section of the polypropylene product particles, the central region being a circular region with a radius ranging from r = 0 to 0.5R; the surface region being an annular region with a radius ranging from r = 0.5R to R, where R is the radius of the polypropylene particle; the rubber phase area ratio in the central region of the particle is ≥40%, preferably ≥60%; the rubber phase area ratio in the surface region is ≤20%, and the difference between the rubber phase area ratios in the central region and the surface region is ≥20%, preferably ≥50%; the rubber phase is a xylene-soluble substance in the polypropylene product particles.

[0006] In a preferred embodiment of the present invention, the area ratio of the rubber phase is obtained by AFM-IR infrared chemical distribution mapping. Specifically, the area ratio of the rubber phase is obtained by the following method: cryosectioning of nascent polypropylene particles, collecting particle cross-sections at 1378 cm⁻¹. -1The AFM-IR infrared chemical distribution map is used to identify and count the area of ​​the corresponding rubber phase in the infrared chemical distribution map, and then the area ratio of the rubber phase is calculated after dividing the region into different regions along the particle radius direction.

[0007] In a preferred embodiment of the present invention, the ethylene-propylene copolymer dispersed phase comprises ethylene-propylene random copolymer (EPR) and ethylene-propylene multiblock copolymer (EbP); the EPR is the 50°C fraction in a P-TREF temperature rinsing fraction, and the EbP is the 100°C fraction in a P-TREF temperature rinsing fraction. The EbP accounts for 10-40% of the total mass of the ethylene-propylene copolymer dispersed phase. Preferably, the EbP accounts for 32% of the total mass of the ethylene-propylene copolymer dispersed phase.

[0008] The number-average sequence length of ethylene units in EbP n E The value is 10–60, and the ethylene molar content of EbP is ≥30%; the number-average sequence length of the ethylene units is the average length of consecutive ethylene units calculated from the NMR characterization results. n E In a preferred embodiment of the present invention, the number-average sequence length of ethylene units in the EbP is preferably 50.24, and the molar content of ethylene in the EbP is preferably 49.4%.

[0009] In a preferred embodiment of the present invention, the flexural modulus of polypropylene is preferably greater than 800 MPa, and the impact strength is preferably greater than 45 kJ / m. 2 .

[0010] In a preferred embodiment of the present invention, when the total content of the rubber phase, based on the mass percentage of xylene-soluble matter, is not higher than 45 wt%, the surface viscosity index of the polypropylene product is less than 2.

[0011] The surface viscosity index is determined by the following method: at room temperature, 100 grams of polypropylene product particles are allowed to fall freely through a stainless steel funnel with an inner diameter of 8 mm, and the falling time is measured; the surface viscosity index is the ratio of this falling time to the falling time of isotactic polypropylene homopolymer measured under the same conditions.

[0012] A prominent feature of the impact-resistant polypropylene provided by the present invention is that the inert condensing medium enriches olefin monomers in the internal channels of polypropylene particles through capillary condensation effect and enhances ethylene mass transfer, so that the ethylene-propylene copolymer dispersed phase is preferentially generated and enriched inside the particles, thereby forming a rubber-rich core and a rubber-poor surface layer, thereby achieving surface viscosity reduction of multiphase polypropylene particles.

[0013] According to a second aspect of the present invention, the present invention provides a method for preparing the above-mentioned surface-reduced, impact-resistant polypropylene, comprising the following steps: The preparation method includes a propylene prepolymerization stage, a propylene homopolymerization stage, a solvent removal stage, and an ethylene-propylene gas-phase copolymerization stage. By controlling the reaction time of the propylene homopolymerization stage, the resulting polypropylene particles form a hierarchical porous structure. The pore size distribution of the hierarchical porous structure has a pore size distribution of ≥60% smaller than 20 nm, an average pore size of 30–120 nm, and a porosity of 20–50%. Furthermore, an inert condensing medium is introduced in the ethylene-propylene gas-phase copolymerization stage, causing capillary condensation of the inert condensing medium within the pores of the polypropylene particles, thereby preferentially generating and enriching the ethylene-propylene copolymer dispersed phase inside the particles.

[0014] Compared with existing technologies, the beneficial effects of this invention are as follows: By precisely controlling the propylene homopolymerization stage, this invention forms a multi-level pore structure with an average pore size of 30-120 nm, a porosity of 20-50%, and a nanopore (<20 nm) ratio of ≥60%. This allows the induced condenser to undergo capillary condensation in the subsequent ethylene-propylene copolymerization stage, thereby improving the mass transfer efficiency and reactivity of the monomer and suppressing particle agglomeration and adhesion during the copolymerization process. Furthermore, combined with AFM-IR infrared chemical distribution maps, it can be seen that the rubber phase of the particles in the example is more distributed inside the particles than on the surface, which can reduce the surface stickiness of the product. Attached Figure Description

[0015] Figure 1 This is a simplified flow chart of the production of impact-resistant polypropylene by adding a condensing medium in a gas-phase copolymerization reactor according to one embodiment of the present invention; Figure 2 The diagram shows the overall morphology of the particle cross-section and the location of AFM-IR sampling points, where (a) is Comparative Example 1 and (b) is Example 1. Figure 3 1378 cm² of the cross-section of the sample particles in Comparative Example 1 -1 Infrared chemical distribution maps at the locations; (a), (b), (c), (d), (e), and (f) correspond to... Figure 2 The location of the point in (a); Figure 4 , Figure 5 Example 1: 1378 cm² of the cross-section of the sample particles -1 Infrared chemical distribution maps at the locations; (a), (b), (c), (d), (e), (f), (g), and (h) correspond to... Figure 2 The location of the point in (b).

[0016] The reactor includes: reactor 1 for propylene prepolymerization; reactor 2 for propylene slurry homopolymerization; flash tank 3 for vacuum removal of solvent from polypropylene particles; reactor 4 for ethylene-propylene gas-phase copolymerization; solvent pipeline 5; cocatalyst pipeline 6; external electron donor pipeline 7; main catalyst pipeline 8; propylene gas pipeline 9; molecular weight regulating gas pipeline 10; pipeline for introducing a certain composition of ethylene-propylene mixed gas 11; heat exchanger 12 for cooling the circulating gas flow; circulating gas flow pipeline at the outlet of the gas-phase reactor 13; inert condensate medium pipeline 14; circulating gas flow pipeline at the inlet of the gas-phase reactor 15; and pipeline 16 for extracting impact-resistant polypropylene products. Detailed Implementation

[0017] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0018] The following implementation process is an optional embodiment of the present invention, which is a four-step method for preparing impact-resistant polypropylene with surface de-tack reduction. It should be noted that propylene prepolymerization is an optional step and can be flexibly selected to be retained or removed as needed. This optional embodiment is as follows: Step 1: Propylene prepolymerization The catalyst, external electron donor, and co-catalyst are premixed and introduced into the propylene prepolymerization reactor 1 via a solvent (e.g., n-heptane). Propylene and hydrogen are introduced into the reactor 1 through the propylene gas pipeline 9 and the molecular weight regulating gas pipeline 10. The reaction temperature is 10–30 °C, the reaction pressure is 1–2 bar, and the prepolymerization time is 1–30 minutes.

[0019] Step 2: Homogenization of propylene slurry The slurry in reactor 1 is pumped into propylene homopolymer reactor 2. Propylene and hydrogen are introduced into reactor 2 via propylene gas line 9 and molecular weight regulating gas line 10. The reaction temperature is 70–75 °C, and the reaction pressure is 4–8 bar. By controlling the reaction time of the propylene homopolymerization stage, the resulting polypropylene particles form a hierarchical porous structure. The pore size distribution of this hierarchical porous structure has a pore size distribution of ≥60% smaller than 20 nm, an average pore size of 30–120 nm, and a porosity of 20–50%. The slurry in reactor 2 is then pumped into flash tank 3. The homopolymerization stage time is controlled to achieve the desired hierarchical porous structure. For example, but not necessarily, the homopolymerization stage time is selected between 15 and 60 minutes.

[0020] The third step is to evacuate the reactor. After solvent removal in flash tank 3, a highly active reaction substrate is obtained, which is then transported into gas-phase copolymerization reactor 4.

[0021] Step 4: Qi of Yi and Bing combine In reactor 4, an ethylene-propylene mixture and hydrogen are introduced through pipeline 11 and molecular weight regulating gas pipeline 10. Additionally, a condenser (e.g., n-hexane) is added through pipeline 14. The reaction temperature is 65–75°C, and the reaction pressure is 3–6 bar. After heat exchange in the heat exchanger 12 (cooling the circulating gas flow) via pipeline 13 at the outlet of the gas phase reactor, the gas flows into the gas phase reactor 10 through pipeline 15 (replenishing the inert condenser lost with the product; for continuous industrial reactions, replenishment of lost inert condenser is necessary, while batch reactions do not). The copolymerization reaction lasts 20–70 minutes. The impact-resistant polypropylene product is obtained through pipeline 16, and after devolatilization or granulation, it enters the silo.

[0022] The present invention will be described below with reference to specific embodiments.

[0023] Example 1 exist Figure 1The polymerization method shown produces high-impact polypropylene. Under vacuum conditions in the prepolymerization stirred tank reactor 1 at an initial temperature of 30 °C, catalyst is added to the 1.1 L stirred tank reactor 1 via catalyst line 8. The catalyst is a TiCl4 / MgCl2 Ziegler-Natta spherical catalyst system, with a dosage of 42–46 mg each time. Simultaneously, 0.10–0.13 mL of external electron donor (D-donor) is added via external electron donor line 7, and 0.60–0.75 mL of co-catalyst triethylaluminum (TEA) is added via co-catalyst line 6. 150 mL of n-heptane is added via solvent line 5. Stirring is started and maintained at 500 rpm. To ensure sufficient turbulence in the reactor, hydrogen is added via line 10, and propylene monomer is added via line 9, maintaining a system pressure of 1 bar. The residence time in the prepolymerization stage is maintained at 15 min, and a small amount of polymer particles are observed to be generated in the reactor. The prepolymerized product is transferred to homopolymer reactor 2, where the temperature is maintained at 70–75 °C. Propylene monomer is introduced through propylene pipeline 9 to maintain the system pressure at 6 bar, and the temperature is controlled at 70 °C. After homopolymerization, the polypropylene matrix in reactor 2 is transferred to flash tank 3. After the solvent n-heptane is removed in flash tank 3, the product enters gas-phase copolymerization reactor 4. A 1:1 molar ratio mixture of ethylene and propylene is introduced into reactor 4 through ethylene-propylene mixed gas pipeline 11. Additionally, 12 mL of n-hexane condenser is added through inert condenser pipeline 14. The reaction temperature is 65–75 °C, and the reaction pressure is 6 bar. The copolymerization reaction is carried out for 30 minutes, and the impact-resistant polypropylene product is obtained through pipeline 16.

[0024] Example 2 exist Figure 1This invention illustrates a gas-phase polymerization method for producing high-impact polypropylene. Under vacuum conditions in a prepolymerized stirred tank reactor 1 at an initial temperature of 30 °C, catalyst is added to a 1.1 L stirred tank reactor 1 via catalyst line 8. The catalyst is a Ziegler-Natta catalyst, with a dosage of 42–46 mg per addition. Simultaneously, 0.10–0.13 mL of the external electron donor dicyclopentyldimethoxysilane (D-donor) is added via external electron donor line 7, and 0.60–0.75 mL of the co-catalyst triethylaluminum (TEA) is added via co-catalyst line 6. 150 mL of n-heptane is added via solvent line 5. The stirrer is started and maintained at 500 rpm. To ensure sufficient turbulence in the reactor, hydrogen is added via line 10, and propylene monomer is added via line 9, maintaining a system pressure of 1 bar. The prepolymerization stage residence time is maintained at 15 min, and a small amount of polymer particles are observed to be generated in the reactor. The prepolymer product is transferred to homopolymer reactor 2, where the temperature is maintained at 70–75 °C. Propylene monomer is introduced through propylene pipeline 9 to maintain the system pressure at 6 bar, and the temperature is controlled at 70 °C. After homopolymerization, the polypropylene matrix in reactor 2 is transferred to flash tank 3. After the solvent n-heptane is removed in flash tank 3, the product enters gas-phase copolymerization reactor 4. A mixture of ethylene and propylene with a molar ratio of 1.5:1 is introduced into reactor 4 through ethylene-propylene mixed gas pipeline 11. Additionally, 12 mL of n-hexane condenser is added through inert condenser pipeline 14. The reaction temperature is 65–75 °C, and the reaction pressure is 6 bar. The copolymerization reaction is carried out for 30 minutes, and the impact-resistant polypropylene product is obtained through pipeline 16.

[0025] Comparative Example 1 exist Figure 1This invention illustrates a gas-phase polymerization method for producing high-impact polypropylene. Under vacuum conditions in a prepolymerized stirred tank reactor 1 at an initial temperature of 30 °C, catalyst is added to a 1.1 L stirred tank reactor 1 via catalyst line 8. The catalyst is a Ziegler-Natta catalyst, with a dosage of 42–46 mg per addition. Simultaneously, 0.10–0.13 mL of the external electron donor dicyclopentyldimethoxysilane (D-donor) is added via external electron donor line 7, and 0.60–0.75 mL of the co-catalyst triethylaluminum (TEA) is added via co-catalyst line 6. 150 mL of n-heptane is added via solvent line 5. The stirrer is started and maintained at 500 rpm. To ensure sufficient turbulence in the reactor, hydrogen is added via line 10, and propylene monomer is added via line 9, maintaining a system pressure of 1 bar. The prepolymerization stage residence time is maintained at 15 min, and a small amount of polymer particles are observed to be generated in the reactor. The prepolymer product is transferred to homopolymer reactor 2, where the temperature is maintained at 70–75 °C. Propylene monomer is introduced through propylene pipeline 9 to maintain the system pressure at 6 bar, and the temperature is controlled at 70 °C. After homopolymerization, the polypropylene matrix in reactor 2 is transferred to flash tank 3. After the solvent n-heptane is removed in flash tank 3, the product enters gas-phase copolymerization reactor 4. A 1:1 molar ratio of ethylene to propylene is introduced into reactor 4 through ethylene-propylene mixed gas pipeline 11. The reaction temperature is 65–75 °C, and the reaction pressure is 6 bar. The copolymerization reaction is carried out for 30 minutes, and the impact-resistant polypropylene product is obtained through pipeline 16.

[0026] Comparative Example 2 exist Figure 1This invention illustrates a gas-phase polymerization method for producing high-impact polypropylene. Under vacuum conditions in a prepolymerized stirred tank reactor 1 at an initial temperature of 30 °C, catalyst is added to a 1.1 L stirred tank reactor 1 via catalyst line 8. The catalyst is a Ziegler-Natta catalyst, with a dosage of 42–46 mg per addition. Simultaneously, 0.10–0.13 mL of the external electron donor dicyclopentyldimethoxysilane (D-donor) is added via external electron donor line 7, and 0.60–0.75 mL of the co-catalyst triethylaluminum (TEA) is added via co-catalyst line 6. 150 mL of n-heptane is added via solvent line 5. The stirrer is started and maintained at 500 rpm. To ensure sufficient turbulence in the reactor, hydrogen is added via line 10, and propylene monomer is added via line 9, maintaining a system pressure of 1 bar. The prepolymerization stage residence time is maintained at 15 min, and a small amount of polymer particles are observed to be generated in the reactor. The prepolymer product is transferred to homopolymer reactor 2, where the temperature is maintained at 70–75 °C. Propylene monomer is introduced through propylene pipeline 9 to maintain the system pressure at 6 bar, and the temperature is controlled at 70 °C. After homopolymerization, the polypropylene matrix in reactor 2 is transferred to flash tank 3. After the solvent n-heptane is removed in flash tank 3, the product enters gas-phase copolymerization reactor 4. A mixed gas with an ethylene-propylene molar ratio of 1.5:1 is introduced into reactor 4 through ethylene-propylene mixed gas pipeline 11. The reaction temperature is 65–75 °C, and the reaction pressure is 6 bar. The copolymerization reaction is carried out for 30 minutes, and the impact-resistant polypropylene product is obtained through pipeline 16.

[0027] The homopolymer polypropylene obtained in the examples and comparative examples was subjected to mercury intrusion porosimetry to determine its pore structure. The high-impact copolymer polypropylene was tested for molecular weight distribution, copolymer content, mechanical properties, and infrared chemical distribution of particle cross sections.

[0028] The pore structure of polypropylene particles was determined by mercury intrusion porosimetry. The average pore size, porosity, and pore distribution results were used to characterize whether the polypropylene particles obtained in the propylene homopolymerization stage have a nanoporous structure suitable for capillary condensation in an inert condensing medium. The total content of the rubber phase was characterized by the content of xylene-soluble matter. The fractions of ethylene-propylene copolymer were obtained by P-TREF elution at 50°C. The fractions of ethylene-propylene random copolymer EPR were obtained by P-TREF elution at 100°C.

[0029] The area ratio of the rubber phase in the particle cross-section was obtained by the following method: cryosectioning of nascent polypropylene particles and collecting particle cross-sections at 1378 cm⁻¹. -1The AFM-IR infrared chemical distribution map is used to identify and statistically analyze the regions corresponding to the rubber phase in the infrared chemical distribution map. Then, different annular regions are divided along the particle radius direction, and the area ratio occupied by the rubber phase is calculated. Specifically, in this embodiment, the AFM-IR infrared chemical distribution map is statistically analyzed. During testing, the area is first obtained as shown in the figure. Figure 2 The particle cross-section and sampling points shown are then collected as follows: Figures 3 to 5 The 1378 cm shown -1 The infrared chemical distribution map was obtained, and the rubber phase area ratio of the central region (circular region with radius r=0 to 0.5R) and the surface region (annular region with radius range r=0.5R to R) was calculated accordingly.

[0030] The surface viscosity index was determined by the following method: at room temperature, 100 grams of polypropylene product particles were allowed to fall freely through a stainless steel funnel with an inner diameter of 8 mm, and the falling time was measured; the surface viscosity index is the ratio of this falling time to the falling time of isotactic polypropylene homopolymer measured under the same conditions.

[0031] The product performance results are shown in Tables 1, 2, and 3: Table 1: Comparison of pore structures between homopolymer polypropylene examples and comparative examples

[0032] Table 2: Performance Comparison of High-Impact Copolymer Polypropylene Examples and Comparative Samples

[0033] Table 3: Performance Comparison of High-Impact Copolymer Polypropylene Examples and Comparative Samples

[0034] The test results from Tables 1, 2, and 3, and Figures 2-5 The AFM-IR infrared chemical distribution diagram shows that the average pore size, porosity, and pore distribution of the polypropylene particles obtained in the propylene homopolymerization stage provide a pore foundation for capillary condensation of the inert condensing medium inside the particles, thereby improving the effective concentration and mass transfer efficiency of olefin monomers inside the particles. After introducing the inert condensing medium in the ethylene-propylene gas-phase copolymerization stage, the rubber phase area ratio of the particle surface region of the example product is significantly lower than that of the comparative example, while the rubber phase area ratio in the central region is higher than that in the surface region, exhibiting a radial gradient distribution of a rubber-rich core and a rubber-poor surface layer. The surface viscosity index of the example product is lower than that of the comparative example, while maintaining good impact strength and flexural modulus. Therefore, the present invention has significant technical effects.

[0035] While the specific embodiments of the present invention have been described in detail above, it should be noted that the scope of protection of the present invention is not limited to these specific embodiments, but is determined by the claims. Those skilled in the art can make appropriate modifications to these embodiments without departing from the technical concept and spirit of the present invention, and these modified embodiments are obviously also included within the scope of protection of the present invention.

Claims

1. A surface-tack-reducing, impact-resistant polypropylene product, characterized in that, The product comprises an isotactic polypropylene (iPP) matrix and an ethylene-propylene copolymer dispersed phase. The ethylene-propylene copolymer dispersed phase exhibits a radial gradient distribution within the cross-section of the polypropylene product particles, decreasing from the central region to the surface region. The central region refers to a circular region with a radius ranging from r=0 to 0.5R; the surface region refers to an annular region with a radius ranging from r=0.5R to R, where R is the radius of the polypropylene particle. The rubber phase accounts for ≥40% of the area in the central region and ≤20% of the area in the surface region. The rubber phase is a xylene-soluble component in the polypropylene product particles.

2. The impact-resistant polypropylene product according to claim 1, characterized in that, The rubber phase area ratio was obtained by the following method: cryo-slicing of nascent polypropylene particles, collecting particle cross-sections at 1378 cm⁻¹. -1 The AFM-IR infrared chemical distribution map is used to identify and count the area of ​​the corresponding rubber phase in the infrared chemical distribution map, and then the area ratio of the rubber phase is calculated after dividing the region into different regions along the particle radius direction.

3. The impact-resistant polypropylene product according to claim 1, characterized in that, The ethylene-propylene copolymer dispersion phase comprises ethylene-propylene random copolymer (EPR) and ethylene-propylene block copolymer (EbP); the EbP accounts for 10-40% of the total mass of the ethylene-propylene copolymer dispersion phase. The EPR is a fraction eluted at 50°C by P-TREF warming rinsing and fractionation, and the EbP is a fraction eluted at 100°C by P-TREF warming rinsing and fractionation.

4. The impact-resistant polypropylene product according to claim 3, characterized in that, The number-average sequence length of ethylene units in EbP n E The value is 10–60, and the ethylene molar content of EbP is ≥30%; the number-average sequence length of the ethylene units is the average length of consecutive ethylene units calculated from the NMR characterization results. n E .

5. The impact-resistant polypropylene product according to claim 1, characterized in that, When the total content of the rubber phase, based on the mass percentage of xylene-soluble matter, is not higher than 45 wt%, the surface tack index of the polypropylene product is less than 2.

6. The impact-resistant polypropylene product according to claim 5, characterized in that, The surface viscosity index is determined by the following method: at room temperature, 100 grams of polypropylene product particles are allowed to fall freely through a stainless steel funnel with an inner diameter of 8 mm, and the falling time is measured; the surface viscosity index is the ratio of this falling time to the falling time of isotactic polypropylene homopolymer measured under the same conditions.

7. A method for preparing the impact-resistant polypropylene product with reduced surface tack as described in any one of claims 1-6, characterized in that: The method includes a propylene prepolymerization stage, a propylene homopolymerization stage, a solvent removal stage, and an ethylene-propylene vapor-phase copolymerization stage. By controlling the reaction time of the propylene homopolymerization stage, the resulting polypropylene particles form a hierarchical pore structure. The pore size distribution of the hierarchical pore structure has a pore size distribution of ≥60% smaller than 20 nm, an average pore size of 30–120 nm, and a porosity of 20–50%. Furthermore, an inert condensing medium is introduced in the ethylene-propylene vapor-phase copolymerization stage, causing capillary condensation of the inert condensing medium in the pores inside the polypropylene particles, thereby preferentially generating and enriching the ethylene-propylene copolymer dispersed phase inside the particles.

8. The method according to claim 7, characterized in that, The multi-level porous system induces capillary condensation of the condenser in the subsequent ethylene-propylene copolymerization stage, thereby improving the mass transfer efficiency and reactivity of the monomers and inhibiting particle agglomeration and adhesion during the copolymerization process.

9. The method according to claim 7, characterized in that, In the ethylene-propylene gas-phase copolymerization stage, the molar ratio of ethylene to propylene is 1:2 to 2:1, the reaction temperature is 60 to 80 °C, the reaction pressure is 1 to 9 bar, and the reaction time is 10 to 80 min.

10. The method according to claim 7, characterized in that, The reaction temperature of the propylene homopolymerization stage is 70–75°C, and the reaction pressure is 4–8 bar.

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