Water pipe single kettle bimodal polyethylene and preparation method thereof

Abstract

The application belongs to the technical field of polyethylene, and particularly relates to single-kettle bimodal polyethylene for water supply pipes and a preparation method thereof. The single-kettle bimodal polyethylene for water supply pipes is prepared by using ethylene as raw material, hydrogen as molecular weight regulator, a main catalyst and a cocatalyst, and a gas phase fluidized bed process to perform copolymerization with 1-hexene in a single reactor. The main catalyst is a supported metallocene bimetallic catalyst which is prepared by loading bis(cyclopentadienyl)zirconium dichloride, dichlorotitanocene and bis-arene imidazole pyridine iron complex on a silica carrier. The length of tie molecules and the lamella thickness are increased by increasing the polyethylene molecular weight. Meanwhile, the content of the comonomer in the polyethylene molecular chain is increased, which further increases the entanglement effect of the tie molecules between the lamellas, so that the lamellas are not easy to slip, thereby improving the slow crack growth resistance of the material.

Description

Technical Field

[0001] This invention belongs to the field of polyethylene technology, specifically relating to single-reactor bimodal polyethylene for water supply pipes and its preparation method. Background Technology

[0002] Polyethylene pipes are widely used in the water supply industry due to their excellent physical properties, chemical stability, and cost-effectiveness, one of their main applications being the transportation of urban drinking water. To ensure water quality safety and a continuous supply, industry regulations stipulate that the service life of polyethylene water supply pipes must not be less than 50 years. In early urban construction, the laying of polyethylene pipes mainly relied on the traditional open-cut method, which involved completely excavating the ground, laying the pipes, and then backfilling with soil. Although this method was relatively simple and direct, it caused significant disruption and inconvenience to the urban environment and residents' daily travel.

[0003] With the acceleration of urbanization and the increasing awareness of environmental protection, trenchless pipe laying technology has emerged and gradually become the mainstream. Trenchless pipe laying uses underground drilling or pipe jacking to lay pipelines, avoiding large-scale ground excavation and significantly reducing the impact on the urban environment and residents' lives. However, during the laying process, the outer surface of the pipe is easily scratched by stones and other hard foreign objects in the soil, resulting in scratches. Over long-term use, these scratches may gradually develop into defects, eventually leading to pipe failure. This damage caused by scratches is commonly referred to as slow crack growth.

[0004] Slow crack growth is a failure phenomenon that occurs under relatively low stress, manifesting as brittle fracture in hydrostatic tests of pipes. In-depth research has revealed that the main cause of slow crack growth is the low content or poor entanglement of tie molecules in the pipe material, leading to easy slippage between lamellar grains. Tie molecules are the long-chain segments of the polyethylene molecular chain; they connect different molecular chains together through entanglement, thereby enhancing the strength and toughness of the pipe. Therefore, improving the content and entanglement effect of tie molecules has become an important research direction in the development and production of pipes.

[0005] To improve the length and entanglement effect of tylosing molecules, bimodal catalysts are typically used in the polymerization process. Bimodal catalysts catalyze the formation of different molecular weight fractions in the polyethylene molecular chains at different reaction stages, resulting in polyethylene with a bimodal molecular weight distribution. This type of polyethylene not only has a higher molecular weight but also a relatively higher content of comonomers in the high molecular weight polyethylene chains, which is beneficial for the formation and entanglement of tylosing molecules. However, traditional polyethylene preparation methods and catalyst systems often fail to simultaneously meet these requirements. Therefore, developing a novel polyethylene preparation method and catalyst system to improve the tylosing molecule content and entanglement effect of polyethylene pipes has become an urgent problem to be solved. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a single-reactor bimodal polyethylene for water supply pipes. By increasing the molecular weight of polyethylene, the length of the ligand molecules and the thickness of the lamellar crystals are increased. Simultaneously, by increasing the content of copolymer monomers in the polyethylene molecular chain, the entanglement effect of the ligand molecules between the lamellar crystals is further enhanced, making it less prone to slippage between the lamellar crystals, thereby improving the material's resistance to slow crack growth.

[0007] Another objective of this invention is to provide a method for preparing single-reactor bimodal polyethylene for water supply pipes, which is simple in process and easy to scale up for production.

[0008] The technical solution adopted in this invention is as follows:

[0009] The method for preparing single-reactor bimodal polyethylene for water supply pipes includes: using ethylene as raw material, hydrogen as molecular weight regulator, employing a main catalyst and a co-catalyst, and using a gas-phase fluidized bed process to copolymerize with 1-hexene in a single reactor to prepare single-reactor bimodal polyethylene for water supply pipes.

[0010] The main catalyst is a supported metallocene dual-center catalyst, in which both active centers are metallocene catalysts. It is prepared by supporting bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl), dichlorotitanium dicene (Cp2TiCl), and a bisaromatic imine pyridine iron complex with silica as the support.

[0011] The concentration of ethylene is 60-68 vol%, and the concentration of hydrogen is 1200-1650 ppm.

[0012] The molar ratio of 1-hexene to ethylene is (0.0030-0.0040):1.

[0013] The co-catalyst is methylaluminoxane (MAO).

[0014] The amount of the main catalyst used is 28-50 g / h, and the amount of the co-catalyst used is 35-45 g / h.

[0015] The copolymerization reaction is carried out at a temperature of 96-100℃ and a pressure of 1.8-2.3MPa.

[0016] The aforementioned single-reactor bimodal polyethylene for water supply pipes is prepared using the above-described preparation method for single-reactor bimodal polyethylene for water supply pipes.

[0017] The density of the polyethylene is 0.946-0.953 g / cm³. 3 The melt flow rate for 5 kg is 0.2-0.5 g / 10 min.

[0018] The polyethylene has a weight-average molecular weight of 280,000-320,000 and a molecular weight distribution of 18-25.

[0019] The content of the copolymer monomer 1-hexene in the polyethylene is 0.4-0.6 mol%.

[0020] The structure of polyethylene was analyzed using TREF (Temperature Wash Fraction). The weight average molecular weight of the fractions at 60℃ was 70,000-90,000, the weight average molecular weight of the fractions at 70℃ was 120,000-150,000, and the weight average molecular weight of the fractions at 96℃ was 350,000-400,000. The content of comonomers in the fractions at 96℃ was 0.25-0.35 mol.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) This invention utilizes a single-reactor bimodal catalyst and a gas-phase fluidized bed process to produce bimodal polyethylene, greatly simplifying the production process and reducing production costs. Simultaneously, by precisely controlling process parameters, this invention successfully increases the molecular weight of polyethylene, increases the length of the ligature molecules and the thickness of the lamellar crystals, thereby significantly improving the resistance of polyethylene pipes to slow crack growth. This is of great significance for ensuring the long-term stability and service life of polyethylene pipes.

[0023] (2) This invention optimizes the distribution of comonomers, increases the content of comonomers in the high molecular weight polyethylene chain, further enhances the entanglement effect of ligase molecules between lamellae, and effectively prevents slippage between lamellae. This not only improves the mechanical properties of polyethylene pipes but also makes them more resistant to damage when faced with various external impacts and scratches that may be encountered in trenchless laying processes.

[0024] (3) Generally, theoretically, the higher the content of polyethylene comonomers with similar properties, the higher the mechanical properties. This is because a higher content of comonomers increases the intermolecular entanglement, thereby improving mechanical properties. However, in reality, comonomers can only effectively improve mechanical properties when polymerized on high molecular weight polyethylene chains. This invention, by precisely controlling the amount of comonomers inserted into high molecular weight polyethylene chains, successfully improves the overall performance of polyethylene while reducing the total comonomer content, and further reduces the production cost of polyethylene. Detailed Implementation

[0025] The present invention will be further described below with reference to the embodiments, but these embodiments do not limit the implementation of the present invention.

[0026] Unless otherwise specified, the raw materials used in the examples and comparative examples are all commercially available materials, and the process methods used in the examples and comparative examples are all conventional methods in the art.

[0027] The supported metallocene dual-center catalyst TH-6 was purchased from Zibo Xinsuo Chemical Co., Ltd.

[0028] The 3490LS twin-reactor twin-peak tube material was purchased from Borouge Trading (Shanghai) Co., Ltd.

[0029] The single-reactor twin-peak tube material 4808 was purchased from China National Petroleum Corporation.

[0030] Example 1

[0031] The method for preparing single-reactor bimodal polyethylene for water supply pipes includes the following steps:

[0032] 35 g / h of supported metallocene dual-center catalyst TH-6 and 40 g / h of MAO were added to a gas-phase fluidized bed reactor. The temperature of the gas-phase fluidized bed reactor was set to 98 °C, the pressure was set to 2.1 MPa, the ethylene concentration was adjusted to 66%, the hydrogen concentration was adjusted to 1350 ppm, and the molar ratio of 1-hexene to ethylene was controlled to be 0.0034, thus preparing single-reactor bimodal polyethylene for water supply pipes.

[0033] The structure of polyethylene was analyzed using TREF (temperature wash fractionation). The weight average molecular weight of the fraction at 60°C was 83,000, the weight average molecular weight of the fraction at 70°C was 140,000, and the weight average molecular weight of the fraction at 96°C was 380,000. The content of comonomer in the fraction at 96°C was 0.3 mol.

[0034] Example 2

[0035] The method for preparing single-reactor bimodal polyethylene for water supply pipes includes the following steps:

[0036] 30 g / h of supported metallocene dual-center catalyst TH-6 and 42 g / h of MAO were added to a gas-phase fluidized bed reactor. The temperature of the gas-phase fluidized bed reactor was set to 99 °C, the pressure was set to 2.0 MPa, the ethylene concentration was adjusted to 68%, the hydrogen concentration was adjusted to 1500 ppm, and the molar ratio of 1-hexene to ethylene was controlled to be 0.0032, thus preparing single-reactor bimodal polyethylene for water supply pipes.

[0037] The structure of polyethylene was analyzed using TREF (temperature wash fractionation). The weight average molecular weight of the fraction at 60°C was 76,000, the weight average molecular weight of the fraction at 70°C was 128,000, and the weight average molecular weight of the fraction at 96°C was 360,000. The content of comonomer in the fraction at 96°C was 0.26 mol.

[0038] Example 3

[0039] The method for preparing single-reactor bimodal polyethylene for water supply pipes includes the following steps:

[0040] 47 g / h of supported metallocene dual-center catalyst TH-6 and 36 g / h of MAO were added to a gas-phase fluidized bed reactor. The temperature of the gas-phase fluidized bed reactor was set to 97 °C, the pressure was set to 1.9 MPa, the ethylene concentration was adjusted to 65%, the hydrogen concentration was adjusted to 1460 ppm, and the molar ratio of 1-hexene to ethylene was controlled to be 0.0038, thus preparing single-reactor bimodal polyethylene for water supply pipes.

[0041] The structure of polyethylene was analyzed using TREF (temperature wash fractionation). The weight average molecular weight of the fraction at 60°C was 85,000, the weight average molecular weight of the fraction at 70°C was 146,000, and the weight average molecular weight of the fraction at 96°C was 352,000. The content of comonomer in the fraction at 96°C was 0.33 mol.

[0042] Example 4

[0043] The method for preparing single-reactor bimodal polyethylene for water supply pipes includes the following steps:

[0044] 50 g / h of supported metallocene dual-center catalyst TH-6 and 44 g / h of MAO were added to a gas-phase fluidized bed reactor. The temperature of the gas-phase fluidized bed reactor was set to 98 °C, the pressure was set to 2.3 MPa, the ethylene concentration was adjusted to 62%, the hydrogen concentration was adjusted to 1650 ppm, and the molar ratio of 1-hexene to ethylene was controlled to be 0.0030, thus preparing single-reactor bimodal polyethylene for water supply pipes.

[0045] The structure of polyethylene was analyzed using TREF (temperature wash fractionation). The weight average molecular weight of the fraction at 60°C was 88,000, the weight average molecular weight of the fraction at 70°C was 13,800, and the weight average molecular weight of the fraction at 96°C was 395,000. The content of comonomer in the fraction at 96°C was 0.27 mol.

[0046] Comparative Example 1

[0047] The method for preparing single-reactor bimodal polyethylene for water supply pipes includes the following steps:

[0048] 25 g / h of supported metallocene dual-center catalyst TH-6 and 35 g / h of MAO were added to a gas-phase fluidized bed reactor. The temperature of the gas-phase fluidized bed reactor was set to 95℃, the pressure to 2.0 MPa, the ethylene concentration to 65%, the hydrogen concentration to 1150 ppm, and the molar ratio of 1-hexene to ethylene to 0.0030, to prepare single-reactor bimodal polyethylene for water supply pipes.

[0049] The structure of polyethylene was analyzed using TREF (temperature wash fractionation). The weight average molecular weight of the fraction at 60°C was 55,000, the weight average molecular weight of the fraction at 70°C was 103,000, and the weight average molecular weight of the fraction at 96°C was 282,000. The content of comonomer in the fraction at 96°C was 0.07 mol.

[0050] Comparative Example 2

[0051] The method for preparing single-reactor bimodal polyethylene for water supply pipes includes the following steps:

[0052] 44 g / h of supported metallocene dual-center catalyst TH-6 and 48 g / h of MAO were added to a gas-phase fluidized bed reactor. The temperature of the gas-phase fluidized bed reactor was set to 102 °C, the pressure was set to 1.7 MPa, the ethylene concentration was adjusted to 58%, the hydrogen concentration was adjusted to 800 ppm, and the molar ratio of 1-hexene to ethylene was controlled to be 0.0030. Single-reactor bimodal polyethylene for water supply pipes was prepared.

[0053] The structure of polyethylene was analyzed using TREF (temperature wash fractionation). The weight average molecular weight of the fraction at 60°C was 35,000, the weight average molecular weight of the fraction at 70°C was 76,000, and the weight average molecular weight of the fraction at 96°C was 361,000. The content of comonomer in the fraction at 96°C was 0.05 mol.

[0054] Comparative Example 3

[0055] The dual-reactor, dual-peak tube material 3490LS has a titanium-based catalyst as its active center.

[0056] The structure of polyethylene was analyzed using TREF (temperature wash fractionation). The weight average molecular weight of the fraction at 60°C was 82,000, the weight average molecular weight of the fraction at 70°C was 126,000, and the weight average molecular weight of the fraction at 96°C was 408,000. The content of comonomer in the fraction at 96°C was 0.20 mol.

[0057] Comparative Example 4

[0058] The single-reactor bimodal tube material is 4808, and the active center is a metallocene catalyst.

[0059] The structure of polyethylene was analyzed using TREF (temperature wash fractionation). The weight average molecular weight of the fraction at 60°C was 46,000, the weight average molecular weight of the fraction at 70°C was 95,000, and the weight average molecular weight of the fraction at 96°C was 316,000. The content of comonomer in the fraction at 96°C was 0.10 mol.

[0060] The performance of polyethylene in Examples 1-4 and Comparative Examples 1-4 was tested, and the test methods are as follows:

[0061] Density (g / cm³) 3 The test was conducted according to GB / T 1033.2-2010, using method D, and the test was performed after boiling for 30 minutes.

[0062] Melt mass flow rate (MFR, g / 10min): Tested according to GB / T 3682-2018;

[0063] Comonomer content (mol%): First, 75 mg of sample was placed in a 5 mm sample tube, and 0.5 mL of deuterated o-dichlorobenzene solvent was added. The sample tube was kept in a constant temperature bath at 140℃ for 3.5 ± 0.5 h to ensure uniform dispersion. Then, the prepared sample tube was placed in a nuclear magnetic resonance spectrometer and stabilized at the experimental temperature of 125℃ for 30 min before scanning the sample (pulse angle 90°, pulse interval 5 s, spectral width 220 ppm, combined pulse decoupling). After scanning, the spectrum was processed, and the peaks in the range of 5-50 ppm were accurately integrated (calibrated to 30 ppm with the isolated -CH2- peak in the polymer).

[0064] Weight-average molecular weight (%) and molecular weight distribution: Gel permeation chromatography (GPC) was used with two columns in series. The solvent and mobile phase were both dibutoxymethane. The sample weight was 2.25 ± 0.75 mg. The column temperature was 150 ℃ and the solution was dissolved for 3 h. Narrow distribution polystyrene standard was used for universal standardization.

[0065] Tensile yield stress (MPa) and nominal strain at tensile fracture (%): Tested according to GB / T 13022-1991, using type 5 spline, with a tensile speed of 200 mm / min;

[0066] Impact strength of simply supported beam (kJ / m) 2 Tested according to GB / T 1043.1-2008, type A notch, 23℃;

[0067] 20℃, 11.2MPa hydrostatic pressure (h): Tested according to GB / T 6111-2003;

[0068] 80℃, 5.4MPa hydrostatic pressure (h): Tested according to GB / T 6111-2003;

[0069] Inflection point time (h): Tested according to GB / T 6111-2003.

[0070] The test results are shown in Table 1.

[0071] Table 1 Performance Test Results

[0072]

[0073] Note: The nominal tensile fracture strain in Table 1 is >713 because the tensile testing machine fixture has reached its maximum stroke and the spline has not broken.

[0074] As shown in Table 1, the polyethylene prepared in Examples 1-4 exhibits good mechanical properties and long-term pressure resistance, with no inflection point observed in the long-term hydrostatic test. Although Comparative Example 1 has a higher comonomer content, it shows earlier brittle fracture. Comparative Examples 2 and 4 show inflection points in the long-term hydrostatic test due to their lower comonomer content. While Comparative Example 3 demonstrates good mechanical properties and long-term hydrostatic performance, its higher comonomer content is a significant factor.

[0075] Based on the structural analysis data in Table 1 and the aforementioned TREF analysis results, the molecular weight of the low-temperature fraction and the content of comonomers in the high-temperature fraction of the examples are both higher than those of Comparative Examples 1, 2, and 4, indicating that more comonomers have polymerized onto the high molecular weight polyethylene. When highly branched high molecular weight polyethylene molecules act as ligands connecting lamellar crystals, they exhibit better entanglement compared to low-branched high molecular weight polyethylene molecules, making them less prone to detachment from the lamellar crystals under external force, thus preventing slow crack growth. This demonstrates that when comonomers polymerize onto high molecular weight polyethylene molecules, the material's resistance to slow crack growth is significantly improved compared to when comonomers polymerize onto low molecular weight polyethylene molecules. Comparative Example 1, although the comonomer content is similar to that of the examples, the poor resistance to slow crack growth and earlier brittle fracture of the pipes are due to the majority of comonomers polymerizing onto low molecular weight polyethylene. Compared to Comparative Example 3, the examples, by better controlling the polymerization of comonomers onto the high molecular weight polyethylene molecular chains, can reduce the amount of comonomers used and lower production costs.

Claims

1. A method for preparing bimodal polyethylene for water supply pipes using a single-reactor process, characterized in that, include: Using ethylene as raw material and hydrogen as molecular weight regulator, and employing a main catalyst and co-catalyst, a gas-phase fluidized bed process is used to copolymerize ethylene with 1-hexene in a single reactor to prepare single-reactor bimodal polyethylene for water supply pipes. The main catalyst is a supported metallocene dual-center catalyst, which is prepared by supporting silica and loading bis(cyclopentadienyl)zirconium dichloride, dichlorotitanium cadmium and bisaromatic imine pyridine iron complex.

2. The method for preparing bimodal polyethylene for water supply pipes according to claim 1, characterized in that, The concentration of ethylene is 60-68 vol%, and the concentration of hydrogen is 1200-1650 ppm.

3. The method for preparing bimodal polyethylene for water supply pipes according to claim 1, characterized in that, The molar ratio of 1-hexene to ethylene is (0.0030-0.0040):

1.

4. The method for preparing bimodal polyethylene for water supply pipes according to claim 1, characterized in that, The cocatalyst is methylaluminoxane.

5. The method for preparing bimodal polyethylene for water supply pipes according to claim 1, characterized in that, The amount of the main catalyst used is 28-50 g / h, and the amount of the co-catalyst used is 35-45 g / h.

6. The method for preparing bimodal polyethylene for water supply pipes according to claim 1, characterized in that, The copolymerization reaction is carried out at a temperature of 96-100℃ and a pressure of 1.8-2.3MPa.

7. A single-reactor bimodal polyethylene for water supply pipes, characterized in that, It is prepared using the single-reactor bimodal polyethylene preparation method for water supply pipes as described in any one of claims 1-6.

8. The single-reactor bimodal polyethylene for water supply pipes according to claim 7, characterized in that, The density of the polyethylene is 0.946-0.953 g / cm³. 3 The melt flow rate for 5 kg is 0.2-0.5 g / 10 min.

9. The single-reactor bimodal polyethylene for water supply pipes according to claim 7, characterized in that, The polyethylene has a weight-average molecular weight of 280,000-320,000 and a molecular weight distribution of 18-25.

10. The single-reactor bimodal polyethylene for water supply pipes according to claim 7, characterized in that, The content of the copolymer monomer 1-hexene in the polyethylene is 0.4-0.6 mol%.

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