Methods for preparing polypropylene

Abstract

This invention relates to a method for preparing polypropylene. More specifically, it relates to a method for efficiently preparing high-strength isotactic polypropylene with high crystallinity even under short-time polymerization. More specifically, it relates to a method for preparing polypropylene containing less than 3% by weight of xylene-soluble substances, the method comprising the step of polymerizing propylene in the presence of a catalyst composition comprising a Ziegler-Natta catalyst, an exogenous electron donor, dialkyl aluminum hydride, and trialkyl aluminum.

Description

Technical Field

[0001] This invention relates to a method for preparing polypropylene. More specifically, it relates to a method for efficiently preparing high-strength isotactic polypropylene with high crystallinity even under short-time polymerization. Background Technology

[0002] Polypropylene has been used as a general-purpose resin in various existing fields due to its low specific gravity, high heat resistance, excellent processability, and chemical resistance.

[0003] Many efforts have been made to improve the mechanical strength of propylene polymers without compromising their inherent properties. It is known that the properties described above can be improved by adjusting the crystallinity and molecular weight of the propylene polymer.

[0004] Therefore, it is necessary to investigate a catalyst composition that can prepare polypropylene with excellent mechanical strength and high crystallinity and superior physical properties in a shorter time compared with the existing general Ziegler-Natta catalyst for preparing homopolymer polypropylene. Summary of the Invention

[0005] Technical problems to be solved

[0006] One objective of this invention is to provide a catalyst composition that, compared to existing methods for preparing homopolymer polypropylene using general-purpose Ziegler-Natta catalysts, can achieve above-average yields in a shorter time and can produce homopolymer polypropylene with excellent physical properties, high mechanical and physical properties, and crystallinity, and a method for preparing polypropylene using the catalyst to manufacture high-strength products.

[0007] Technical solution

[0008] As a result of research conducted to achieve the above objectives, the inventors of this invention discovered that, compared with the use of existing Ziegler-Natta catalysts, homopolymer polypropylene with equivalent or better physical properties can be prepared in a shorter time by using a specific combination of catalyst compositions, thus completing this invention.

[0009] One embodiment of the present invention is a method for preparing polypropylene, which is a method for manufacturing polypropylene with a xylene cold soluble content of less than 3% by weight as measured by a CRYSTEX apparatus (CRYSTEX 42 Model manufactured by Polymer Char), the method comprising the step of polymerizing propylene in the presence of a catalyst composition comprising a Ziegler-Natta catalyst, an exogenous electron donor, dialkyl aluminum hydride and trialkyl aluminum.

[0010] As one embodiment, the catalyst composition may contain 0.005-0.1 wt% of Ziegler-Natta catalyst, 5-20 wt% of exogenous electron donor, 0.05-3 wt% of dialkylaluminum hydride and 10-94 wt% of trialkylaluminum, and is not limited thereto.

[0011] As one embodiment, the molar ratio of titanium in the trialkylaluminum / Ziegler-Natta catalyst of the catalyst composition can be 500-5000.

[0012] As one embodiment, the Ziegler-Natta catalyst may contain 10-30 wt% magnesium (Mg) and 0.5-5 wt% titanium (Ti).

[0013] As one embodiment, the molar ratio of titanium in the dialkyl aluminum hydride / Ziegler-Natta catalyst can be 1-30.

[0014] As one embodiment, the dialkyl aluminum hydride may comprise diisobutyl aluminum hydride (DIBAL-H).

[0015] As one embodiment, the external electron donor may be any one or a mixture of two or more selected from diphenyldimethoxysilane, phenyltrimethoxysilane, phenethyldimethoxysilane, benzyldimethoxysilane, N,N-diethylaminotriethoxysilane, bis(ethylamino)dicyclopentylsilane, trimethoxypropylsilane, methoxytrimethylsilane, isobutyltrimethoxysilane, diisobutyldimethoxysilane, diisopropyldimethoxysilane, di-tert-butyldimethoxysilane, dicyclopentyldimethoxysilane, cyclohexylmethyldimethoxysilane, and dicyclohexyldimethoxysilane.

[0016] As one embodiment, the preparation method may include the following steps: a) adding dialkylaluminum hydride and trialkylaluminum to a reactor, followed by adding a Ziegler-Natta catalyst and an external electron donor to prepare a catalyst composition; and b) adding propylene and hydrogen to the catalyst composition and polymerizing it.

[0017] As one implementation, in step b), the polymerization can be carried out at 50-80°C for 10-120 minutes.

[0018] As one implementation, in step b), polymerization can be carried out at a pressure of 20-50 bar.

[0019] As one implementation, the polypropylene may be a propylene homopolymer.

[0020] As one embodiment, the melt index of the polypropylene, measured according to ASTM D1238 at 230°C with a load of 2.16 kg, can be 30-60 g / 10 min, and the weight-average molecular weight can be 330,000-500,000 g / mol.

[0021] As one implementation, when the polypropylene is measured using successive self-nucleation and annealing (SSA)-differential scanning calorimeter (DSC) for the endothermic peak, the sum of the peak value (DH4) observed at 170°C and the peak value (DH5) observed at 176°C can be more than 75% of the entire peak.

[0022] As one implementation, the peak value (DH5) observed at 176°C can be more than 25% of the entire peak.

[0023] As one embodiment, the Ziegler-Natta catalyst can be used at a content of 0.0001-0.002 parts by weight relative to 100 parts by weight of the propylene.

[0024] As one embodiment, the dialkyl aluminum hydride can be used in an amount of 0.00001-0.1 parts by weight relative to 100 parts by weight of propylene.

[0025] Beneficial effects

[0026] Compared to the use of existing Ziegler-Natta catalysts, the use of the catalyst composition of the present invention with a specific combination has the effect of producing homopolymer polypropylene with equivalent or better physical properties in a shorter time.

[0027] In addition, it has the effect of preparing homopolymer polypropylene with excellent mechanical and physical properties and high crystallinity. Detailed Implementation

[0028] The present invention will now be described in more detail. However, the specific embodiments or implementation schemes described below are merely for the purpose of illustrating the invention in detail, and the invention is not limited thereto, and can be implemented in various ways.

[0029] Furthermore, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this invention is merely for the purpose of effectively describing specific embodiments and is not intended to limit the invention.

[0030] In addition, unless otherwise specified, the singular form used in the specification and claims may also include the plural form.

[0031] Furthermore, unless otherwise specifically stated to the contrary, describing a part as "containing" or "including" a constituent element means that it may also include other constituent elements, rather than excluding other constituent elements.

[0032] The various components of the present invention will now be described in more detail.

[0033] The present invention is characterized by using a catalyst composition comprising a Ziegler-Natta catalyst, an exogenous electron donor, diisobutylaluminum hydride (DIBAL-H), and trialkylaluminum as a catalyst composition for polymerizing polypropylene. Compared to using only existing Ziegler-Natta catalysts and exogenous electron donors, the use of said catalyst composition allows for the provision of homopolymer polypropylene polymers with equivalent or superior physical properties in a shorter time.

[0034] More specifically, one embodiment of the preparation method of the present invention is characterized by the order and proportion of catalyst addition during the preparation of the catalyst composition, such that, within the range of simultaneously satisfying these conditions, polypropylene with a xylene low-temperature soluble content of less than 3% by weight, as measured by a CRYSTEX apparatus (CRYSTEX 42 model manufactured by Polymer Char), can be prepared.

[0035] In one embodiment of the invention, the catalyst composition may contain 0.005-0.1 wt% of Ziegler-Natta catalyst, 5-20 wt% of exogenous electron donor, 0.05-3 wt% of dialkylaluminum hydride, and 10-94 wt% of trialkylaluminum, but is not limited thereto. As one embodiment, the molar ratio of dialkylaluminum hydride to titanium in the Ziegler-Natta catalyst composition may be 1-30, and the molar ratio of trialkylaluminum to titanium in the Ziegler-Natta catalyst may be 500-5000. As one embodiment, the Ziegler-Natta catalyst may contain 10-30 wt% magnesium (Mg) and 0.5-5 wt% titanium (Ti).

[0036] Within the specified content range, highly crystalline propylene polymers can be prepared in a shorter time. More specifically, within a polymerization time of 10-60 minutes, they can exhibit physical properties equivalent to or better than those of polypropylene polymers prepared by polymerization of more than 60 minutes.

[0037] The Ziegler-Natta catalyst comprises transition metal compounds containing elements belonging to Group 4, 5, or 6 of the periodic table and organometallic compounds containing elements belonging to Group 13 of the periodic table.

[0038] As one embodiment, the molar ratio of the organometallic compound to the transition metal compound can be 5-50.

[0039] As the transition metal compound, a solid titanium catalyst containing magnesium, titanium, halogen elements, and an endogenous electron donor can be used. The Ziegler-Natta catalyst used in the prepolymerization process is dispersed as uniform particles, and then high molecular weight monomers are polymerized on the catalyst surface. The Ziegler-Natta catalysts described above can be used without particular limitation as long as they are commonly used for olefin polymerization, but preferably catalysts containing transition metal compounds containing elements belonging to Groups 4, 5, or 6 of the periodic table; and organometallic compounds containing elements belonging to Group 13 of the periodic table.

[0040] The transition metal compound is used as the main catalyst in the Ziegler-Natta catalyst. Preferably, a solid titanium catalyst containing magnesium, titanium, halogen elements, and an endogenous electron donor can be used. The endogenous electron donor can be, for example, a diether-based compound, a phthalate-based compound, or a mixture thereof, specifically diisobutyl phthalate, etc.

[0041] More preferably, the transition metal compound may contain magnesium and titanium, and may contain 10-30% by weight of magnesium (Mg) and 0.5-5% by weight of titanium (Ti).

[0042] In one embodiment of the invention, the organometallic compound may be used without limitation as long as it is an organoaluminum compound commonly used in the art.

[0043] In one embodiment of the invention, the Ziegler-Natta catalyst may be used in an amount of 0.0001-0.01 parts by weight relative to 100 parts by weight of the propylene, and is not limited thereto.

[0044] In one embodiment of the invention, the exogenous electron donor may be an organosilane compound, specifically, it may be any one or a mixture of two or more selected from, for example, diphenyldimethoxysilane, phenyltrimethoxysilane, phenethyldimethoxysilane, benzyldimethoxysilane, N,N-diethylaminotriethoxysilane, bis(ethylamino)dicyclopentylsilane, trimethoxypropylsilane, methoxytrimethylsilane, isobutyltrimethoxysilane, diisobutyldimethoxysilane, diisopropyldimethoxysilane, di-tert-butyldimethoxysilane, dicyclopentyldimethoxysilane, cyclohexylmethyldimethoxysilane, and dicyclohexyldimethoxysilane, and is not limited thereto. More preferably, any one or more selected from cyclohexylmethyldimethoxysilane and dicyclohexyldimethoxysilane may be used.

[0045] The trialkylaluminum may be any one or a mixture of two or more selected from triethylaluminum, trimethylaluminum, tri(isopropyl)aluminum, tri(n-butyl)aluminum, tri(isobutyl)aluminum, tri(tert-butyl)aluminum, tri(n-hexyl)aluminum, and tri(n-octyl)aluminum, and is not limited thereto. More preferably, it may be any one or more selected from triethylaluminum and trimethylaluminum.

[0046] The alkyl group in the dialkyl aluminum hydride can be C3-C10, and more preferably includes diisobutyl aluminum hydride (DIBAL-H).

[0047] The content of the dialkyl aluminum hydride relative to 100 parts by weight of propylene can be used in the range of 0.0001-0.01 parts by weight, and is not limited thereto.

[0048] The preferred order of addition of the catalyst composition is to add diisobutylaluminum hydride (DIBAL-H) ​​and trialkylaluminum to the reactor, followed by the addition of a Ziegler-Natta catalyst and an external electron donor to prepare the catalyst composition. At this time, the temperature of the reactor is preferably below 10°C, more specifically, for example, 1-10°C, and more preferably 3-7°C.

[0049] More specifically, diisobutylaluminum hydride (DIBAL-H) ​​and trialkylaluminum can be added in the presence of dissolved organic solvents, more specifically in the presence of dissolved organic solvents such as toluene, followed by the addition of an external electron donor, and the addition of a Ziegler-Natta catalyst dissolved in an organic solvent such as methylcyclohexane.

[0050] In one embodiment of the present invention, propylene is preferably added after the preparation of the catalyst composition, and the molecular weight can be controlled by adding hydrogen gas when adding propylene.

[0051] As one embodiment, it is preferable to increase the reactor reaction temperature and carry out polymerization after adding the propylene and hydrogen. The polymerization can be carried out at 50-80°C for 10-60 minutes. Furthermore, the polymerization can be carried out at a pressure of 20-50 bar.

[0052] More specifically, a method for preparing polypropylene according to one embodiment of the present invention includes the following steps: a) adding dialkylaluminum hydride and trialkylaluminum to a reactor, followed by adding a Ziegler-Natta catalyst and an exogenous electron donor to prepare a catalyst composition; and b) adding propylene and hydrogen to the catalyst composition and polymerizing it.

[0053] The xylene cryosoluble content of the polypropylene polymer prepared by the preparation method of one embodiment of the present invention, as measured by a CRYSTEX apparatus (CRYSTEX 42 model manufactured by PolymerChar), can be less than 3% by weight, more preferably 0.5-3% by weight. More specifically, it can be 2-2.8% by weight. The lower the content of xylene cryosoluble content, the higher the crystallinity. In the present invention, the above range can be satisfied by using a mixture of four catalyst compositions, namely, a mixture of dialkylaluminum hydride, trialkylaluminum, Ziegler-Natta catalyst, and an external electron donor.

[0054] Furthermore, specifically, the polypropylene polymer of one embodiment of the present invention may, for example, have a melt index of 30-60 g / 10 min and a weight-average molecular weight of 330,000-500,000 g / mol, as measured according to ASTM D1238 at 230°C with a load of 2.16 kg, but is not limited thereto.

[0055] Furthermore, in one embodiment of the present invention, a polypropylene polymer using a four-catalyst composition was demonstrated to exhibit no decrease in room temperature / low temperature impact strength and elongation, compared to preparation by removing any one of the components. This can be attributed to the improvement in tensile strength resulting from a reduction in the content of xylene low-temperature solubles that cause a decrease in mechanical and physical properties and an increase in highly crystalline regions that enhance mechanical and physical properties. Moreover, the uniformity of crystal distribution and the tie molecules between crystals is also excellent, thus exhibiting equivalent or higher values ​​for room temperature / low temperature impact strength and elongation.

[0056] Specifically, for example, the polymer can simultaneously satisfy a tensile strength of 200 kgf / cm. 2 The physical properties include the above-mentioned properties and an elongation of 10% or more. More specifically, it can simultaneously meet the requirement of tensile strength of 200-500 kgf / cm². 2 It also has physical properties with an elongation of 10%-30%.

[0057] Furthermore, when the polypropylene is measured using continuous self-nucleation and annealing thermal fractionation (SSA)-differential scanning calorimetry (DSC), and five peaks are observed every 6°C within the 152-176°C range, the sum of the peak value (DH4) observed at 170°C and the peak value (DH5) observed at 176°C can be more than 90% of the entire peak. That is, compared to the case where the sum of the peak value (DH4) observed at 170°C and the peak value (DH5) observed at 176°C is only 72% of the entire peak when using only the Ziegler-Natta catalyst and an external electron donor, it can be confirmed that the polypropylene prepared using the four catalyst compositions of the present invention has a purity of 75% or more, more preferably 75-80%, and its crystallinity is further increased. In addition, the peak value (DH5) observed at 176°C can be more than 25% of the entire peak, more preferably 25-45%.

[0058] Furthermore, compared to the case where a polymerization reaction is carried out for 50 minutes using only Ziegler-Natta catalyst and exogenous electron donor, it has been confirmed that polypropylene prepared using the four catalyst compositions of the present invention can also meet the above physical properties even when a polymerization reaction is carried out for 10-15 minutes.

[0059] The present invention will now be described in more detail based on embodiments and comparative examples. However, the embodiments and comparative examples described below are merely examples for illustrating the present invention in more detail, and the present invention is not limited to the embodiments and comparative examples described below.

[0060] 1) Content of xylene solubles at low temperatures (XCS%)

[0061] Measurements were performed using a CRYSTEX (CRYSTEX 42 model manufactured by Polymer Char).

[0062] Trichlorobenzene was added to 160 mg of polypropylene resin samples prepared according to the examples and comparative examples, and the samples were heated at 160 °C for 1 hour for pretreatment. The solvent was flowed at a rate of 3 mL per minute for analysis. Soluble fraction analysis was performed at 35 °C for 40 minutes, followed by crystalline fraction analysis at 165 °C for 25 minutes.

[0063] 2) Melt Flow Index (MI)

[0064] Analysis was performed using a melt indexer (Goettfert MI-4 model).

[0065] According to ASTM D1238, the measurement was performed at 230°C with a load of 2.16 kg and expressed as the weight (g) of polymer dissolved over a period of 10 minutes.

[0066] 3) Flexural Modulus

[0067] Measurements were performed according to ASTM D790. Polymers were granulated to prepare pellets using a Brabender 30mm single-screw extruder at temperatures ranging from 180°C to 230°C, and ASTM test pieces were prepared using a Boy 25-ton injection molding machine at the same temperature range. The prepared test pieces were then placed at 23°C and 50% relative humidity for at least 48 hours before testing. Flexural modulus values ​​were obtained using the secant modulus of 0.01 mm / mm.

[0068] 4) Measurement of Continuous Self-Nucleation and Annealing Thermal Classification (SSA) - Differential Scanning Calorimetry (DSC)

[0069] Measurements were performed using a DSC (TA Instruments Q20 Model). Samples were prepared using 10-15 mg of polypropylene resin according to the examples and comparative examples.

[0070] Heat to 200°C at a rate of 10°C per minute to completely dissolve, then cool to 50°C at a rate of 20°C per minute and hold for 5 minutes. Heat to 164°C at a rate of 20°C per minute. Hold for 10 minutes, then cool to 50°C at a rate of 20°C per minute and hold for 5 minutes. Heat to 159°C at a rate of 20°C per minute. Hold for 10 minutes, then cool to 50°C at a rate of 20°C per minute and hold for 5 minutes. Heat to 154°C at a rate of 20°C per minute. Hold for 10 minutes, then cool to 50°C at a rate of 20°C per minute and hold for 5 minutes. Heat to 149°C at a rate of 20°C per minute. Hold for 10 minutes, then cool to 50°C at a rate of 20°C per minute and hold for 5 minutes. Heat to 144°C at a rate of 20°C per minute. Hold for 10 minutes, then cool to 50°C at a rate of 10°C per minute and hold for 5 minutes. The temperature is increased to 200°C at a rate of 20°C per minute.

[0071] Five peaks were observed every 6℃ within the range of 152-176℃. That is, the peak at 152℃ is denoted as DH1, the peak at 158℃ as DH2, the peak at 164℃ as DH3, the peak at 170℃ as DH4, and the peak at 176℃ as DH5.

[0072] 5) Weight-average molecular weight (Mw)

[0073] Measurements were performed using gel permeation chromatography (GPC) (Agilent PL-GPC 220). The GPC column was connected to PLgel Olexis Guard (7.5 × 50 mm) and PLgel Olexis (7.5 × 300 mm). 1,2,4-trichlorobenzene was used as the solvent, and polystyrene (Mw6870000) was used as the standard. The analysis was conducted at 160 °C.

[0074] 6) Tensile strength and elongation

[0075] Measurements were performed according to ASTM D638. Polymers were granulated to prepare pellets using a 30mm single-screw extruder from Brabender at temperatures ranging from 180°C to 230°C. ASTM test specimens were prepared using a 25-ton injection molding machine from Boy at the same temperature range. The prepared samples were placed at 23°C and 50% relative humidity for at least 48 hours before testing. Tensile strength is expressed as the strength at yield point and fracture point, and elongation is expressed as the total strain elongated at fracture.

[0076] 7) IZOD impact strength

[0077] Measurements were performed according to ASTM D256 at both ambient temperature (23°C) and low temperature (-20°C). Polymers were granulated to prepare pellets using a 30mm single-screw extruder from Brabender at temperatures ranging from 180°C to 230°C. ASTM test pieces were prepared using a 25-ton injection molding machine from Boy at the same temperature range. The prepared samples were then placed at 23°C and 50% relative humidity for at least 48 hours before being used in the experiments.

[0078] [Example 1] Preparation of polypropylene homopolymer

[0079] In a 3L high-pressure reactor, 0.030 mmol of a toluene solution containing 1 M diisobutylaluminum hydride (DIBAL-H, Sigma Aldrich), 6.6 mmol of triethylaluminum (Sigma Aldrich), and 0.66 mmol of cyclohexylmethyldimethoxysilane as an exogenous electron donor were added. 14 mg of Ziegler-Natta catalyst (Mg-supported Ti Ziegler-Natta catalyst, 20 wt% Mg, 3 wt% Ti) was mixed with 10 ml of methylcyclohexane and added to the reactor.

[0080] 1000 g of liquid propylene was added, followed by the addition of hydrogen gas at a rate of 4000 standard mL / min (sccm) for 3 minutes. The polymerization reaction was carried out at an internal temperature of 62 °C for 50 minutes. After the reaction was complete, the pressure was released, and the mixture was purged five times with nitrogen gas to remove residual propylene. The physical properties of the prepared polypropylene homopolymer were measured and recorded in Tables 2 to 4 below.

[0081] [Example 2]

[0082] Except for varying the content of diisobutylaluminum hydride (DIBAL-H) ​​as shown in Table 2 below in Example 1, the polymer was prepared using the same method as in Example 1. The physical properties of the prepared polypropylene homopolymer were measured and are recorded in Tables 2 to 4 below.

[0083] [Example 3]

[0084] Except for varying the content of diisobutylaluminum hydride (DIBAL-H) ​​as shown in Table 2 below, and adjusting the polymerization time to 15 minutes, the polymer was prepared using the same method as in Example 1. The physical properties of the prepared polypropylene homopolymer were measured and recorded in Tables 2 to 4 below.

[0085] [Example 4]

[0086] Except that in Example 1 above, diisobutylaluminum hydride (DIBAL-H) ​​was replaced with dioctylaluminum hydride (DOAL-H, synthesized according to the method described in Table 2 below, Dalton Trans., 2015, 44, 15286-15296, Nandita et al.), the polymer was prepared by the same method as in Example 1. The physical properties of the prepared polypropylene homopolymers were measured and are recorded in Tables 2 to 4 below.

[0087] [Comparative Example 1]

[0088] Except that diisobutylaluminum hydride (DIBAL-H) ​​and triethylaluminum were not used in Example 1 above, the polymer was prepared by the same method as in Example 1. The physical properties of the prepared polypropylene homopolymer were measured and are recorded in Tables 2 to 4 below.

[0089] 0.66 mmol of cyclohexylmethyldimethoxysilane as an exogenous electron donor was added to a 3 L high-pressure reactor. 14 mg of Ziegler-Natta catalyst (magnesium-supported titanium Ziegler-Natta catalyst, Mg 20 wt%, Ti 2 wt%) was mixed with 10 ml of methylcyclohexane and added to the reactor.

[0090] 1000g of liquid propylene was added, followed by the addition of hydrogen gas at 4000 sccm for 3 minutes. The polymerization reaction was carried out at an internal temperature of 62°C for 50 minutes. After the reaction was completed, the pressure was released, and the mixture was purged with nitrogen gas 5 times to remove residual propylene. The physical properties of the prepared polypropylene homopolymer were measured and recorded in Tables 2 to 4 below.

[0091] [Comparative Example 2]

[0092] Except that the polymerization time was adjusted to 15 minutes in Comparative Example 1 above, the polymer was prepared using the same method as in Comparative Example 1. The physical properties of the prepared polypropylene homopolymer were measured and are recorded in Table 2 below.

[0093] Table 1 below shows the molar ratio of the catalysts.

[0094] [Table 1]

[0095]

[0096] [Table 2]

[0097]

[0098] As shown in Table 2 above, it was confirmed that polymers with lower xylene low-temperature soluble content were obtained in Examples 1 to 4 using the four catalysts of the present invention. That is, it was confirmed that polymers with higher crystallinity were obtained. Furthermore, when comparing Example 3 with Comparative Example 2, it was confirmed that the addition of DIBAL-H also showed equivalent or higher activity at a polymerization time of 15 minutes.

[0099] [Table 3]

[0100]

[0101]

[0102] As shown in Table 3 above, it can be seen that although the tensile strength increased in Examples 1 to 4, the room temperature / low temperature impact strength and elongation did not decrease. This can be attributed to the improvement in tensile strength as the content of X / S, which causes a decrease in mechanical and physical properties, decreased and the highly crystalline regions that improve mechanical and physical properties increased. Furthermore, the uniformity of crystal distribution and the binding molecules between crystals were also excellent, resulting in room temperature / low temperature impact strength and elongation exhibiting values ​​of equal or higher.

[0103] [Table 4]

[0104]

[0105] As shown in Table 4 above, the SSA-DSC analysis results confirmed that in Examples 1 to 3 using the four catalysts of the present invention, the proportion of DH4 and DH5 regions corresponding to the highly crystalline regions increased.

[0106] As shown above, the present invention has been described with specific content and limited embodiments, but this is only provided to help to understand the present invention more fully. The present invention is not limited to the above embodiments, and those skilled in the art can make various modifications and variations based on such description.

[0107] Therefore, the concept of this invention should not be limited to the described embodiments, and all contents of the claims of this invention and those equivalent to or having equivalent variations thereof fall within the scope of the concept of this invention.

Claims

1. A method for preparing polypropylene, comprising a method for preparing polypropylene with a xylene low-temperature soluble content of less than 3% by weight, wherein the method includes the step of polymerizing propylene in the presence of a catalyst composition comprising a Ziegler-Natta catalyst, an exogenous electron donor, dialkylaluminum hydride, and trialkylaluminum. The content of xylene low-temperature soluble matter was determined by the following method: using CRYSTEX, trichlorobenzene was added to the sample to be tested, and the sample was heated at 160°C for 1 hour for pretreatment. The solvent was then flowed at a rate of 3 mL per minute for analysis. in, When the polypropylene was measured using continuous self-nucleation and annealing thermal fractionation SSA-differential scanning calorimetry (DSC) to determine the endothermic peak, the sum of the peak value DH4 observed at 170℃ and the peak value DH5 observed at 176℃ accounted for more than 75% of the entire peak. The entire peak refers to the five peaks observed every 6℃ within the range of 152-176℃. The Ziegler-Natta catalyst comprises 10-30 wt% magnesium (Mg) and 0.5-5 wt% titanium (Ti). The exogenous electron donor is any one or a mixture of two or more of the following: diphenyldimethoxysilane, phenyltrimethoxysilane, phenethyldimethoxysilane, benzyldimethoxysilane, N,N-diethylaminotriethoxysilane, bis(ethylamino)dicyclopentylsilane, trimethoxypropylsilane, methoxytrimethylsilane, isobutyltrimethoxysilane, diisobutyldimethoxysilane, diisopropyldimethoxysilane, di-tert-butyldimethoxysilane, dicyclopentyldimethoxysilane, cyclohexylmethyldimethoxysilane, and dicyclohexyldimethoxysilane. The alkyl group of the dialkyl aluminum hydride is C3-C10. The trialkylaluminum is selected from any one or a mixture of two or more of triethylaluminum, trimethylaluminum, tri(isopropyl)aluminum, tri(n-butyl)aluminum, tri(isobutyl)aluminum, tri(tert-butyl)aluminum, tri(n-hexyl)aluminum, and tri(n-octyl)aluminum.

2. The method for preparing polypropylene according to claim 1, wherein, The molar ratio of titanium in the trialkylaluminum / Ziegler-Natta catalyst of the catalyst composition is 500-5000.

3. The method for preparing polypropylene according to claim 1, wherein, The molar ratio of titanium in the dialkyl aluminum hydride / Ziegler-Natta catalyst is 1-30.

4. The method for preparing polypropylene according to claim 1, wherein, The dialkyl aluminum hydride comprises diisobutyl aluminum hydride DIBAL-H.

5. The method for preparing polypropylene according to claim 1, comprising the following steps: a) Dialkylaluminum hydride and trialkylaluminum are added to the reactor, followed by the addition of a Ziegler-Natta catalyst and an external electron donor to prepare the catalyst composition; as well as b) Add propylene and hydrogen to the catalyst composition and polymerize it.

6. The method for preparing polypropylene according to claim 5, wherein, In step b), polymerization is carried out at 50-80°C for 10-120 minutes.

7. The method for preparing polypropylene according to claim 5, wherein, In step b), polymerization is carried out under a pressure of 20-50 bar.

8. The method for preparing polypropylene according to claim 1, wherein, The peak value DH5 observed at 176℃ is more than 25% of the entire peak.

9. The method for preparing polypropylene according to claim 1, wherein, The Ziegler-Natta catalyst is used at a concentration of 0.0001-0.002 parts by weight relative to 100 parts by weight of the propylene.

10. The method for preparing polypropylene according to claim 1, wherein, The dialkyl aluminum hydride is used in an amount of 0.00001-0.01 parts by weight relative to 100 parts by weight of propylene.

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