A method for temperature-switching production of high-performance polypropylene

Abstract

The application discloses a method for preparing high-performance polypropylene by temperature switching, which realizes significant improvement of polypropylene performance by dynamically regulating ethylene-propylene copolymerization temperature. The method comprises the following optional step combinations: 1) propylene prepolymerization at 1-30 bar and 10-50 DEG C; 2) propylene homopolymerization at 1-100 bar and 40-90 DEG C; 3) key ethylene-propylene copolymerization, which is periodically switched at 60-90 DEG C and 1-100 bar, the switching frequency is 0.1-10 minutes / time, and the ethylene / propylene molar ratio is 1:99-50:50. The method reduces the average particle size of an ethylene-propylene random copolymer (EPR) dispersed phase to 0.1-1.0 microns under the condition that the EPR content is maintained at 5-30 wt%. When the switching frequency is increased to 5-10 times / hour, the impact strength of the obtained polypropylene at-20 DEG C reaches more than 85% of the impact strength at 23 DEG C.

Description

Technical Field

[0001] This invention relates to the field of polypropylene resin technology, and more specifically to a method for preparing high-performance polypropylene by temperature switching. Background Technology

[0002] Polypropylene, as an important general-purpose plastic, is characterized by low production cost, light weight, high product transparency, good chemical stability and heat resistance, and ease of processing. It is widely used in various sectors of national production, including chemical, construction, automotive, home appliance, packaging, and medical industries. Due to its high cost-effectiveness, polypropylene has become the fastest-growing and most developed of the five major general-purpose thermoplastic resins, second only to polyethylene in terms of production volume.

[0003] Since the 1980s, the emergence of spherical TiCl4 / MgCl2 catalysts based on Ziegler-Natta catalysts has enabled the continuous in-situ copolymerization of ethylene and propylene gases in a reactor to obtain a mixture known as impact-resistant copolymerized polypropylene. Compared with simple blends, the copolymerized product exhibits superior overall performance, especially significantly improved low-temperature toughness. Due to its excellent impact resistance and good overall performance, impact-resistant polypropylene has become one of the main varieties of polypropylene materials, attracting considerable attention in both basic and applied research, and leading to rapid development of impact-resistant polypropylene materials in recent years.

[0004] Currently, the main polypropylene production processes in China include the Innovene process, Novlen process, Unipol process, Horizone process, Spheripol process, Spherzone process, and the Sinopec loop process. Tianjin Petrochemical Company primarily uses the Spherzone process. This process has relatively unique production conditions and a wide range of applications. It can directly produce bimodal products, has excellent resin processing performance, and exhibits high processing efficiency. The comonomers produced through this process have good optical and mechanical properties.

[0005] The Spherizone process combines the advantages of gas-phase and liquid-phase methods, utilizing a single multi-zone circulating reactor (MZCR) to achieve integrated production of propylene homopolymerization and olefin copolymerization. The MZCR is divided into an ascending section and a descending section. In the ascending section (high hydrogen concentration, high temperature zone), propylene homopolymerization occurs to produce highly crystalline iPP; in the descending section (low hydrogen concentration, low temperature zone), ethylene or other α-olefins are introduced for copolymerization. The particles repeatedly experience different reaction conditions during circulation within the reactor, allowing for precise control of the multiphase polypropylene molecular chain structure.

[0006] To develop multiphase polypropylene with excellent mechanical properties, a temperature switching method is proposed. An inert additive is introduced during the ethylene-propylene gas-phase copolymerization stage to adjust the copolymerization temperature to fluctuate around the dew point temperature of the polymerization reaction system. Under the action of capillary induced condensation, the temperature field and concentration field are rapidly switched, thereby realizing the periodic switching of active particles between high-temperature and low-temperature environments and achieving stable production of multiphase polypropylene. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing high-performance polypropylene by temperature switching and the high-performance polypropylene obtained therefrom.

[0008] According to a first aspect of the present invention, the present invention provides a method for preparing high-performance polypropylene by temperature switching, the method comprising steps 1, 2, and 3, or steps 2 and 3, or steps 1 and 3, or step 3:

[0009] 1) Prepolymerize propylene by controlling the pressure at 1~30 bar and the temperature at 10~50℃;

[0010] 2) The pressure is controlled at 1~100 bar and the temperature is controlled at 40~90℃ for propylene homopolymerization;

[0011] 3) The copolymerization temperature is controlled at 60~90℃, and the pressure is controlled at 1~100 bar. The temperature is periodically switched between two target temperatures (high target temperature and low target temperature, such as 65℃ and 85℃) to periodically change the temperature of the reaction environment for ethylene-propylene copolymerization. Introducing inert additives at this stage can enhance the temperature switching effect.

[0012] According to a preferred embodiment of the present invention, the ethylene / propylene molar ratio in the ethylene-propylene copolymerization stage in step 3) is 1:99~50:50; preferably 30:70~50:50.

[0013] According to a preferred embodiment of the present invention, the difference between the high target temperature and the low target temperature in step 3) is ≤20℃.

[0014] According to a preferred embodiment of the present invention, in step 3), the temperature transition time required for one switch between the high target temperature and the low target temperature in the copolymerization temperature is ≤5 min. The frequency of the temperature change in the periodic reaction environment is 5~90 ​​times / hour, preferably 24~60 times / hour.

[0015] According to a preferred embodiment of the present invention, an inert additive is introduced in step 3), and the dosage of the added inert additive is such that the dew point of the ethylene-propylene copolymer system is 60~90°C.

[0016] According to a preferred embodiment of the present invention, in step 3), the copolymerization temperature is periodically switched between a high target temperature and a low target temperature within the dew point range of the ethylene-propylene copolymer system, and the temperature transition time required for one switch between the high target temperature and the low target temperature is 0.1 to 10 minutes.

[0017] According to a preferred embodiment of the present invention, the inert additive is one of C5-C8 alkanes, preferably any one of n-hexane, n-heptane, and isopentane, and more preferably n-hexane.

[0018] According to a preferred embodiment of the present invention, the reaction temperature of propylene prepolymerization in step 1) is 10~30℃, the partial pressure of propylene is 0.1~0.2 MPa, and the partial pressure of hydrogen is 0.01~0.1 MPa.

[0019] According to a preferred embodiment of the present invention, in step 2), the polymerization reaction temperature is 60~70℃, the propylene pressure is 0.5~0.6 MPa, the reaction time is 10~60 min, and the stirring speed is 450~600 r / min.

[0020] According to a preferred embodiment of the present invention, after step 2), the solvent in the reactor is dried using a vacuum pump for 10 to 30 minutes.

[0021] According to a preferred embodiment of the present invention, the reaction time in step 3) is 5-30 min, preferably 5-20 min, and more preferably 10-20 min. The reaction temperature is preferably 50-70°C, and the stirring speed is 350-650 r / min.

[0022] According to a second aspect of the present invention, the present invention provides a high-performance polypropylene prepared by a temperature switching process, wherein the high-performance polypropylene is prepared by the temperature switching process; the temperature switching process includes periodic switching of at least two target temperatures; when the frequency of temperature switching is ≥5 times / h, the impact strength of the high-performance polypropylene at -20°C is more than 85% of the impact strength of the high-performance polypropylene at 23°C.

[0023] According to a preferred embodiment of the present invention, the high-performance polypropylene contains 5-30 wt% of an ethylene-propylene random copolymer dispersion phase; the average particle size of the ethylene-propylene random copolymer dispersion phase is 0.1-1.0 μm, and the particle size distribution index of the ethylene-propylene random copolymer dispersion phase is ≤1.5.

[0024] According to a second aspect of the present invention, the present invention provides a method for preparing high-performance polypropylene by temperature switching after introducing an inert additive, wherein the high-performance polypropylene has an average particle size of 0.07~0.9 μm in the ethylene-propylene random copolymer dispersed phase and the particle size distribution index of the ethylene-propylene random copolymer dispersed phase is ≤1.3.

[0025] The polypropylene resin produced according to a preferred embodiment of the present invention comprises 10-100% by mass of ethylene-propylene copolymer, wherein the xylene-soluble component in the ethylene-propylene copolymer accounts for 40-90 wt% by mass.

[0026] The glass transition temperature of the copolymer of polypropylene resin produced according to a preferred embodiment of the present invention is -15°C to -50°C.

[0027] The polypropylene resin produced according to a preferred embodiment of the present invention has the following properties: the content of the fraction below 50°C obtained by temperature rinsing and grading is in the range of 5 to 40 wt%; the content of the fraction at 80°C is in the range of 1 to 10 wt%; and the content of the fraction at 100°C is in the range of 5 to 25 wt%.

[0028] According to a preferred embodiment of the present invention, the melt index of the polypropylene resin is 0.3 g / 10 min to 200 g / 10 min, preferably 0.3 g / 10 min to 60 g / 10 min; and the isotacticity is 95 to 99.9%.

[0029] Beneficial effects:

[0030] This invention innovatively employs a temperature dynamic oscillation process. By switching temperatures, while maintaining the content of ethylene propylene random copolymer (EPR) at 5-30 wt%, the average particle size of the EPR dispersed phase is reduced to 0.1-1.0 μm. When the switching frequency is increased to 5-10 times / hour, the resulting polypropylene achieves an impact strength at -20℃ that is more than 85% of its impact strength at 23℃. The produced polypropylene resin exhibits a 15% increase in impact strength at room temperature and a 30% increase in impact strength at low temperatures.

[0031] A key feature of the polypropylene resin produced according to this invention is that the introduction of inert additives and temperature switching results in the formation of more long-block ethylene-propylene copolymers. The microphase-separated structure of the long blocks endows the material with excellent mechanical properties (such as high elasticity and impact resistance) and thermal stability, making it widely applicable in elastomers, plastic toughening, packaging films, and adhesives. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 This is a schematic diagram of the process for preparing polypropylene resin by temperature switching according to the present invention;

[0034] Figure 2 SEM images of the brittle fracture surfaces of samples (a) Example 1, (b) Example 2, (c) Example 3 and (d) Example 4 after being etched with xylene, showing different temperature switching frequencies. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0036] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0037] Polymer preparation

[0038] The polymerization process is divided into three parts: propylene prepolymerization, propylene homopolymerization, and ethylene-propylene copolymerization. First, propylene prepolymerization is carried out. In the presence of a solvent, the dissolved propylene content in the reaction system (catalyst, cocatalyst, electron donor, propylene monomer) is 2%–10% molar. A high propylene concentration is beneficial because it increases catalyst activity per unit time. The polymerization temperature is 20–30°C, and the pressure is 1–10 bar. Then, propylene homopolymerization is carried out. In the presence of a solvent, the dissolved propylene content in the reaction system (propylene prepolymer product, propylene monomer) is 2%–10% molar. The polymerization temperature is 65–85°C, and the pressure is 1–40 bar. Propylene prepolymerization and homopolymerization can be carried out in any reactor known for slurry polymerization, including continuously stirred tank reactors and circulating reactors. The slurry can be continuously or intermittently removed from the reactor. Hydrogen is introduced into the polymerization (propylene prepolymerization and propylene homopolymerization) stages to control the melt index of polypropylene. The amount of hydrogen required to achieve the desired melt index depends on the catalyst used and the polymerization conditions. The average residence time for propylene prepolymerization is 15-20 min, and the average residence time for propylene homopolymerization is 30-80 min. After the propylene homopolymerization stage, the reactor temperature is adjusted to 80°C, and a vacuum pump is used to extract the solvent from the reactor until the pressure inside the reactor reaches -1.0 bar, at which point the extraction is stopped.

[0039] The ethylene-propylene copolymerization process then involves periodically switching the reaction temperature between two preset target temperatures. Specifically, the copolymerization temperature cyclically changes between the higher and lower target temperatures according to a pre-defined switching frequency. This is carried out in a gas-phase fluidized bed reactor. The copolymerization reaction temperature is operated within the range of 65–85°C; the pressure is 1–30 bar; and the average residence time during the copolymerization stage is 10–100 min.

[0040] This strategy can be implemented alone without introducing any inert additives to regulate polymer properties; alternatively, a specific amount of inert additive (an inert hydrocarbon, one of the C5-C8 alkanes) can be selectively injected to enhance the temperature-controlled effect. The introduction of inert additives can significantly regulate the effects of large temperature changes on the molecular weight distribution, comonomer composition, crystallinity, and final product properties (such as impact resistance and rigidity).

[0041] Example 1

[0042] like Figure 1As shown, the initial temperature was 30℃. The catalyst, co-catalyst, and electron donor were added to a 1.5 L stirred tank R1. The catalyst, co-catalyst, and electron donor were flushed into the tank from the catalyst tube using n-heptane solvent. The stir bar was turned on, and the rotation speed was set to 500 rpm. 0.4 bar of hydrogen was added through the hydrogen line, and propylene monomer was added through the propylene line, maintaining a system pressure of 1 bar. Prepolymerization began for 15 minutes, with the tank temperature controlled between 15 and 30℃. After prepolymerization, the tank temperature was raised to 60℃, and propylene monomer was added through the propylene line, pressurizing the tank to 6 bar. The temperature was controlled at 70℃, and the reaction was allowed to proceed for 30 minutes. After the polypropylene homopolymerization stage, the water bath temperature in stirred tank R1 was adjusted to 80℃. Simultaneously, a vacuum pump was used to remove the n-heptane solvent from the tank until the pressure inside the tank reached -1.0 bar, at which point the vacuuming process was stopped.

[0043] Then, in the ethylene-propylene copolymerization stage with temperature switching, a target ratio of ethylene / propylene mixture (ethylene to propylene ratio of 1:1) and 12 mL of n-hexane are added to the bottom of the stirred tank R1 as an inert additive. During the ethylene-propylene copolymerization step, two different temperatures are alternated, with the two target temperatures in the stirred tank R1 set at 70℃ and 80℃. The switching frequency is set to 12 times / h (i.e., one temperature switching operation every 5 minutes, with two switching operations constituting a complete switching cycle, where one switching cycle refers to the cycle from 70℃ to 80℃ and then back to 70℃), the total reaction residence time is 30 minutes, and the reaction pressure is 6 bar. After the ethylene-propylene copolymerization stage ends, the product is obtained after cyclone separation and granulation.

[0044] Example 2

[0045] Example 2 is the same as Example 1, except that in the ethylene-propylene copolymerization stage, the temperature cycle switching frequency is set to 20 times / h (i.e., a temperature switching operation is performed every 3 minutes).

[0046] Example 3

[0047] Example 3 is the same as Example 1 in general, except that in the ethylene-propylene copolymerization stage, the temperature cycle switching frequency is set to 32 times / h (that is, a temperature switching operation is performed once every 1.875 minutes).

[0048] Example 4

[0049] Example 4 is the same as Example 1, except that in the ethylene-propylene copolymerization stage, the temperature cycle switching frequency is set to 40 times / h (i.e., a temperature switching operation is performed once every 1.5 minutes).

[0050] Example 5

[0051] Example 5 is the same as Example 4 in general, except that 12 mL of n-hexane is not added as an inert additive during the ethylene-propylene copolymerization stage.

[0052] Example 6

[0053] Example 6 is the same as Example 1 in general, except that it only includes the propylene prepolymerization and ethylene-propylene copolymerization stages, and does not include the propylene homopolymerization stage.

[0054] Example 7

[0055] Example 7 is the same as Example 1 in general, except that it only includes the propylene homopolymerization and ethylene-propylene copolymerization stages, and does not include the propylene prepolymerization stage.

[0056] Comparative Example 1

[0057] Comparative Example 1 was largely the same as Example 1, except for the ethylene-propylene copolymerization stage: a target ratio of ethylene / propylene mixture (ethylene to propylene ratio of 1:1) was added to the bottom of the stirred tank R1. The temperature-switching polymerization stage involved alternating between two different temperatures: 70°C and 80°C. The total reaction time was 30 min, and the reaction pressure was 6 bar. First, ethylene-propylene copolymerization was carried out for 15 min at 70°C, followed by another 15 min at 80°C. After the temperature polymerization stage, the product was obtained after cyclone separation and granulation.

[0058] Comparative Example 2

[0059] Comparative Example 2 was largely the same as Example 1, except that during the ethylene-propylene copolymerization stage, a target ratio of ethylene / propylene mixture (ethylene to propylene ratio of 1:1) and 12 mL of n-hexane were added to the bottom of the stirred tank R1 as an inert additive. No temperature cycle changes were performed; the polymerization reaction was directly carried out at 75°C for 30 min. After the temperature polymerization stage ended, the product was obtained after cyclone separation and granulation.

[0060] The high-impact copolymer polypropylene prepared in the examples and comparative examples was tested for ethylene content, rubber content, and molecular weight, and the results are shown in Table 1.

[0061] Table 1. Test results of polypropylene obtained by different preparation methods

[0062]

[0063] As shown in Table 1, the mechanical properties of the polypropylene composition prepared by the olefin polymerization method involving temperature switching polymerization during the impact-resistant polypropylene polymerization process of this invention are significantly better than those of the comparative example. With the increase of the switching frequency, the low-temperature toughness (-20℃) is significantly improved, from 37.35 kJ / m... 2Increased to 50.79 kJ / m 2 Meanwhile, the flexural modulus only decreased from 379 MPa to 368 MPa. Therefore, this invention has significant technical advantages. When the temperature switching frequency is ≥5 times / h, the impact strength of polypropylene at -20℃ is more than 85% of its impact strength at 23℃. Simultaneously, while ensuring the formation of 5~30 wt% ethylene-propylene random copolymer (EPR) dispersed phase, the particle size of the dispersed phase is effectively reduced. The average particle size of the EPR dispersed phase is 0.1~1.0 μm, and its particle size distribution index (PDI) is ≤1.5. The addition of inert additives can enhance the temperature switching effect, resulting in a high-performance polypropylene ethylene-propylene random copolymer dispersed phase with an average particle size of 0.07~0.9 μm and a particle size distribution index (PDI) ≤1.3. Figure 2 SEM images of multiphase polypropylene samples with different temperature switching frequencies during the copolymerization stage after liquid nitrogen embrittlement and xylene etching are shown. The images reveal numerous pores on the surface of the multiphase polypropylene, formed after the rubber phase in the samples was removed by xylene etching. This allows direct observation of the distribution of the rubber phase within the polypropylene matrix. Images a, b, c, and d in the figures are SEM images of the surfaces of Examples 1, 2, 3, and 4 after xylene etching, respectively. It can be seen that as the temperature switching frequency during the copolymerization stage increases, the size of the dispersed phase gradually decreases, and its distribution within the polypropylene matrix becomes more dense. This is the main reason for the improved impact strength of Example 4.

[0064] 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 method for preparing high-performance polypropylene by temperature switching, characterized in that: The method includes steps 1), 2), and 3), or includes steps 2) and 3), or includes steps 1) and 3). 1) Prepolymerize propylene by controlling the pressure at 1~30 bar and the temperature at 10~50℃; 2) The pressure is controlled at 1~100 bar and the temperature is controlled at 40~90℃ for propylene homopolymerization; 3) The copolymerization temperature is controlled at 60~90℃, and the high target temperature and the low target temperature are periodically switched within the temperature range of 60~90℃. The difference between the high target temperature and the low target temperature is ≤20℃, and the temperature transition time required for one switch between the high target temperature and the low target temperature is ≤5min; the copolymerization pressure is controlled at 1~100bar; ethylene-propylene copolymerization is carried out.

2. The method for preparing high-performance polypropylene by temperature switching according to claim 1, characterized in that, In step 3), the molar ratio of ethylene to propylene in the ethylene-propylene copolymerization stage is 1:99 to 50:

50.

3. The method for preparing high-performance polypropylene by temperature switching according to claim 1, characterized in that, In step 3), an inert additive is introduced, and the dosage of the added inert additive is such that the dew point of the ethylene-propylene copolymer system is 60~90℃.

4. The method for preparing high-performance polypropylene by temperature switching according to claim 3, characterized in that, After adding the inert additive, the copolymerization temperature in step 3) is periodically switched between the high target temperature and the low target temperature within the dew point range of the ethylene-propylene copolymer system; the temperature transition time required for one switch between the high target temperature and the low target temperature is 0.1~5 min.

5. The method for preparing high-performance polypropylene by temperature switching according to claim 3, characterized in that, The inert additive is one of the C5-C8 alkanes.

6. The high-performance polypropylene prepared by the temperature-switching method for preparing high-performance polypropylene according to any one of claims 3-5, characterized in that, The average particle size of the ethylene-propylene random copolymer dispersion phase of the high-performance polypropylene is 0.07~0.9 μm, and the particle size distribution index of the ethylene-propylene random copolymer dispersion phase is ≤1.

3.

7. A high-performance polypropylene prepared by a temperature switching process, characterized in that: The high-performance polypropylene is prepared by a temperature-switching method according to any one of claims 1-5, wherein the temperature-switching process includes periodic switching of at least two target temperatures; when the frequency of temperature switching is ≥5 times / h, the impact strength of the high-performance polypropylene at -20℃ is more than 85% of the impact strength of the high-performance polypropylene at 23℃.

8. A high-performance polypropylene prepared by a temperature switching process according to claim 7, characterized in that, The high-performance polypropylene contains 5-30 wt% of an ethylene-propylene random copolymer dispersion phase; the average particle size of the ethylene-propylene random copolymer dispersion phase is 0.1-1.0 μm, and the particle size distribution index of the ethylene-propylene random copolymer dispersion phase is ≤1.5.

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