Bimodal polyethylene composition as well as preparation method and application thereof

Abstract

The invention provides resin for a PE-RT II type pipe as well as a preparation method and application of the resin. The melt index of the resin for the pipe is 0.5-1.0 g / 10min; the density is 0.945 to 0.955 g / cm3; the number-average molecular weight Mn is 10000 to 15000, the weight-average molecular weight Mw is 250000 to 35000, and the molecular weight distribution width index PD is 16 to 35; the double-peak type PE-RT II type pipe resin provided by the invention can be prepared by virtue of double-ring-pipe reactors which are connected in series. The resin has excellent long-term hydrostatic strength, cannot generate brittle failure under various temperature conditions of 110 DEG C or below, cannot generate inflection points on a hydrostatic curve, can bear relatively high internal pressure, and has good oxygen barrier performance, and produced pipes are suitable for the field of secondary heat supply pipe networks.

Description

Technical Field

[0001] The invention belongs to the technical field of polyethylene resins, and in particular relates to a bimodal polyethylene composition and a preparation method and application thereof. Background Art

[0002] At present, in my country's heating network, whether it is the primary network or the secondary network, the working pipes are generally made of ordinary steel pipes. Steel pipes are resistant to heat and high pressure, but have poor corrosion resistance. After a period of operation, the pipeline is prone to corrosion, and the corrosion is accelerated under high temperatures. Corrosion in a short period of time will cause pipeline blockage, resulting in increased transmission energy consumption and reduced heating effect; with the development of rust, pipeline leakage, bubbling, dripping and leakage problems have become the norm, seriously affecting heating safety.

[0003] PE-RTⅡ material is a new type of pipe material developed on the basis of PE-RTⅠ material and is specially used in the field of high-temperature fluid transportation. It has better low-temperature impact performance, crack resistance, and excellent long-term high temperature and pressure resistance. It is currently an ideal material for secondary heating pipe networks. The Ministry of Housing and Urban-Rural Development issued the "PE-RT II Prefabricated Direct Buried Insulated Pipe" standard on April 1, 2016, marking that the development of China's heating pipe technology has entered a new starting point.

[0004] PE-RT II heating pipes are corrosion-resistant, have a long service life, low hydraulic loss, good insulation effect, and are easy and quick to construct. Their thermal conductivity is less than 1% of that of ordinary steel pipes, and the temperature drop is less than 0.1°C per kilometer. If 8,000 kilometers of PE-RT II heating pipes are used to replace traditional steel pipes each year, energy-saving efficiency can be increased by more than 20%, saving about 1 million tons of standard coal each year, and reducing water replenishment in the pipeline network by more than 10 million tons, which will help achieve clean heating.

[0005] Foreign countries have developed special resins for PE-RTⅡ pipes at an early stage, such as Total XRT70, Basel 4731B, South Korea's LG Chemical Company SP988, South Korea's SK Energy Company DX900, and DOW Chemical Company 2388; however, the main components of the resins in the existing technology are ethylene and hexene, without nucleating agents, and therefore have no oxygen barrier properties. With the popularization of floor heating technology in China, PE-RT pipes have gradually become the first choice in the fields of floor heating, thermal pipe networks, etc. with their excellent performance. my country's demand for PE-RT is about 350,000-450,000 tons / year. The raw materials in the domestic thermal pipe network field basically rely on imports, mainly XRT70, and a small amount of 4731B. At present, there is still a blank in the domestic bimodal PE-RTⅡ pipe resin. The distribution of high molecular weight polyethylene and low molecular weight polyethylene in the polymerization process of existing polyethylene pipe resin is uncontrollable, and the molecular weight distribution is unreasonable. The existing technologies mainly include slurry double loop pipe, gas phase method, etc., and metallocene or Ziegler-Natta catalysts are used for production. The production of special materials for bimodal PE-RTⅡ pipes mainly depends on process condition control and additive ratio. At present, there is no successful industrialized product in China, which cannot meet the balance between large diameter and long-term high temperature and pressure resistance. Summary of the invention

[0006] In order to solve the above problems in the prior art, the present invention proposes a bimodal PE-RTⅡ pipe-specific resin for large-diameter pipes and a preparation method thereof.

[0007] The technical problem solved by the first aspect of the present invention is to produce a bimodal polyethylene composition for preparing a bimodal PE-RTⅡ type pipe of large diameter pipe, which has excellent long-term hydrostatic strength, will not undergo brittle failure under various temperature conditions below 110°C, and will not have an inflection point in the hydrostatic curve.

[0008] The technical problem solved by the second aspect of the present invention is to provide a method for preparing the bimodal polyethylene composition as the raw material for the pipe, which method has simple process and convenient operation.

[0009] In a first aspect, the present invention provides a bimodal polyethylene composition, wherein the raw materials for preparing the composition include polyethylene powder, an antioxidant, a nucleating agent, and a halogen absorber;

[0010] The antioxidant is added in an amount of 0.1-0.4% of the total mass of the polyethylene powder;

[0011] The amount of the nucleating agent added is 0.05-0.2% of the total mass of the polyethylene powder;

[0012] The amount of the halogen absorbent added is 0.1-0.2% of the total mass of the polyethylene powder.

[0013] As a specific embodiment of the present invention, the raw materials for preparing the polyethylene powder include comonomer and ethylene, and the molar ratio of the comonomer to ethylene is (0.2-0.6):1; preferably, the comonomer is 1-hexene.

[0014] As a specific embodiment of the present invention, the antioxidant is a phenolic antioxidant. Preferably, the phenolic antioxidant includes a mixture of antioxidant 1076 and antioxidant 626, and the weight ratio of the antioxidant 1076 to the antioxidant 626 is (1-3):1.

[0015] As a specific embodiment of the present invention, the nucleating agent is the nucleating agent 20e.

[0016] As a specific embodiment of the present invention, the halogen absorber is magnesium stearate.

[0017] As a specific embodiment of the present invention, the composition has a melt index (190°C, 5kg) of 0.5-1.0g / 10min and a density of 0.945-0.955g / cm 3 ; The number average molecular weight Mn is 10000-15000, the weight average molecular weight Mw is 250000-350000, and the molecular weight distribution width index is 16-35.

[0018] According to the present invention, bimodal polyethylene refers to a polyethylene resin whose molecular weight distribution curve presents two peaks. Compared with a unimodal polyethylene composition, its molecular weight distribution width index is generally lower than 10, and the molecular weight distribution width index of a bimodal polyethylene composition is generally higher than 15. The molecular weight distribution width index of the bimodal polyethylene composition of the present invention is 16-35, and the low molecular weight portion has fewer side chains, which greatly improves its processing performance; the high molecular weight portion has more side chains, providing the material with a longer service life.

[0019] As a specific embodiment of the present invention, the thermal stability test of the composition under hydrostatic state was conducted at a test temperature of 110° C., a hydrostatic pressure of 2.4 MPa, and a test time of 8760 h, with no cracks or leakage.

[0020] As a specific embodiment of the present invention, the oxygen permeability of the composition is ≤37.46cm 3 / m 2 , 24h, 0.1MPa; preferably, the oxygen permeability is 29-37.46cm 3 / m 2 , 24h, 0.1MPa.

[0021] In a second aspect, the present invention provides a method for preparing the bimodal polyethylene composition, comprising the following steps:

[0022] S1: polymerizing ethylene, hydrogen and comonomer in the presence of a polymerization catalyst to obtain polyethylene powder;

[0023] S2: Granulate the polyethylene powder obtained in step S1, and steadily and continuously add an antioxidant, a nucleating agent, and a halogen absorber during the granulation process to obtain a bimodal polyethylene composition.

[0024] As a specific embodiment of the present invention, in the step S1, the polymerization reaction includes polymerization using a series reactor, preferably, the series reactor is composed of a double loop reactor, the first loop reactor polymerizes to obtain a low molecular weight portion, and the second loop reactor polymerizes to obtain a high molecular weight portion; more preferably, the yield ratio of the first loop reactor output / the second loop reactor output is (45-49): (51-55); the yields of the two reactors are adjusted to adjust the ratio of the high molecular weight portion to the low molecular weight portion.

[0025] As a specific embodiment of the present invention, in the step S1, the molar ratio of ethylene to hydrogen is (0.015-0.03):1; the comonomer is 1-hexene, and the molar ratio of the comonomer to ethylene is (0.2-0.6):1.

[0026] As a specific embodiment of the present invention, the polymerization catalyst is a Ziegler-Natta catalyst. The catalyst is a highly efficient catalyst, can distribute the yield in two reactors, and has excellent hydrogen regulation sensitivity.

[0027] As a specific embodiment of the present invention, in the step S2, the granulation comprises: using a twin-screw extruder for granulation, the twin-screw adopts a length-to-diameter ratio of 40-60; and the granulation temperature is 170-220°C.

[0028] As a specific embodiment of the present invention, in step S2, the antioxidant is a phenolic antioxidant, and the phenolic antioxidant includes a mixture of antioxidant 1076 and antioxidant 626, and the weight ratio of the antioxidant 1076 to the antioxidant 626 is (1-3):1; the amount of the antioxidant added is 0.1-0.4% of the total mass of the polyethylene powder;

[0029] As a specific implementation of the present invention, the nucleating agent is nucleating agent 20e, and the added amount is 0.05-0.2% of the total mass of the polyethylene powder.

[0030] As a specific embodiment of the present invention, in step S2, the halogen absorber is magnesium stearate, and the added amount is 0.1-0.2% of the total mass of the polyethylene powder.

[0031] Exemplarily, the device of the present invention is as follows Figure 1As shown, ethylene, hydrogen and comonomer are polymerized in the presence of a polymerization catalyst through a series reactor to obtain ethylene-1-hexene copolymer powder, wherein the series reactor consists of two loop reactors and a dehydrogenation processor; first, ethylene, hydrogen and catalyst are added to the first reactor to polymerize to obtain a low molecular weight portion; secondly, the product of the first reactor passes through the dehydrogenation processor to remove hydrogen and then enters the second reactor; at the same time, the comonomer and ethylene are added to the second reactor to continue polymerization reaction with the product of the first reactor to obtain polyethylene powder.

[0032] The above raw materials in the present invention can be prepared in-house or commercially available, and the present invention is not particularly limited thereto.

[0033] In a third aspect, the present invention provides application of the bimodal polyethylene composition in the field of preparing PE-RTⅡ type pipes.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The bimodal polyethylene composition of the present invention has controllable distribution of the high molecular weight part and the low molecular weight part, reasonable molecular weight distribution, excellent long-term hydrostatic strength, no brittle failure under various temperature conditions below 110°C, no inflection point in the hydrostatic curve, can withstand higher internal pressure, and the produced pipes are suitable for the field of secondary thermal pipe networks. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Flow chart of the preparation method in the embodiment of the present invention. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with specific embodiments, but they do not constitute any limitation to the present invention.

[0038] In each embodiment of the present invention, the method for preparing the bimodal polyethylene composition comprises the following specific steps:

[0039] S1: Ethylene, hydrogen and comonomer (1-hexene) are polymerized in the presence of a polymerization catalyst through a series reactor method to obtain polyethylene powder. The first loop reactor polymerizes to obtain a low molecular weight portion, and the second loop reactor polymerizes to obtain a high molecular weight portion. The yields of the two reactors are adjusted to adjust the ratio of the high molecular weight portion to the low molecular weight portion. The reaction flow chart of the device is as follows: Figure 1 shown.

[0040] S2: The polyethylene powder obtained in step S1 is granulated by a twin-screw extruder, the twin-screw adopts an aspect ratio of 40-60, and an antioxidant, a nucleating agent, and a halogen absorber are added stably and continuously during the granulation process, and the temperature is controlled at 170-220° C. to obtain a bimodal polyethylene resin for pipes. The specific raw materials and formulations are shown in Examples 1-6.

[0041] Example 1

[0042] S1: 1-hexene and ethylene in a molar ratio of 0.5:1 are introduced with hydrogen, and polyethylene powder is obtained by polymerization through a series reactor method in the presence of a Ziegler-Natta polymerization catalyst; wherein the yield ratio of the output of the first loop reactor to the output of the second loop reactor is controlled at 45:55;

[0043] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder with a length-to-diameter ratio of 55. During the granulation process, 0.05 parts of nucleating agent 20e, 0.1 parts of magnesium stearate, and 0.2 parts of antioxidant 1076 and antioxidant 626 in a weight ratio of 1:1 are stably and continuously added. The temperature is controlled at 195°C to obtain a bimodal polyethylene resin for pipes.

[0044] The bimodal polyethylene resin powder obtained in Example 1 has a melt index (190°C, 5kg) of 0.50 g / 10 min and a density of 0.948 g / cm 3 The number average molecular weight is 10014, the weight average molecular weight is 350000, and the molecular weight distribution width index is 34.95.

[0045] Example 2

[0046] S1: 1-hexene and ethylene in a molar ratio of 0.6:1 are introduced with hydrogen, and polyethylene powder is obtained by polymerization through a series reactor method in the presence of a Ziegler-Natta polymerization catalyst; wherein the yield ratio of the output of the first loop reactor to the output of the second loop reactor is controlled at 46:54;

[0047] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder with a length-to-diameter ratio of 50. During the granulation process, 0.15 parts of nucleating agent 20e, 0.15 parts of magnesium stearate, and 0.4 parts of antioxidant 1076 and antioxidant 626 in a weight ratio of 2:1 are stably and continuously added. The temperature is controlled at 205°C to obtain a bimodal polyethylene resin for pipes.

[0048] The bimodal polyethylene resin powder obtained in Example 2 has a melt index (190°C, 5kg) of 0.80g / 10min and a density of 0.945g / cm 3The number average molecular weight is 13102, the weight average molecular weight is 280650, and the molecular weight distribution width index is 21.42.

[0049] Example 3

[0050] S1: 1-hexene and ethylene in a molar ratio of 0.4:1 are introduced with hydrogen, and polyethylene powder is obtained by polymerization through a series reactor method in the presence of a Ziegler-Natta polymerization catalyst; wherein the yield ratio of the output of the first loop reactor to the output of the second loop reactor is controlled at 47:53;

[0051] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder, the twin-screw adopts a length-to-diameter ratio of 48, 0.1 parts of nucleating agent 20e, 0.2 parts of magnesium stearate, and 0.1 parts of antioxidant 1076 and antioxidant 626 in a weight ratio of 3:1 are stably and continuously added during the granulation process, and the temperature is controlled at 200°C to obtain a bimodal polyethylene resin for pipes.

[0052] The bimodal polyethylene resin powder obtained in Example 3 has a melt index (190°C, 5kg) of 0.56 g / 10 min and a density of 0.950 g / cm 3 The number average molecular weight is 15000, the weight average molecular weight is 250050, and the molecular weight distribution width index is 16.67.

[0053] Example 4

[0054] S1: introducing hydrogen into 1-hexene and ethylene at a molar ratio of 0.2:1, and polymerizing in the presence of a Ziegler-Natta polymerization catalyst through a series reactor method to obtain polyethylene powder; wherein the yield ratio of the first loop reactor to the second loop reactor is controlled at 48:52;

[0055] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder, the twin-screw adopts a length-to-diameter ratio of 48, 0.2 parts of nucleating agent 20e, 0.1 parts of magnesium stearate, and 0.3 parts of antioxidant 1076 and antioxidant 626 in a weight ratio of 2:1 are stably and continuously added during the granulation process, and the temperature is controlled at 200°C to obtain a bimodal polyethylene resin for pipes.

[0056] The bimodal polyethylene resin powder obtained in Example 4 has a melt index (190°C, 5kg) of 0.56 g / 10 min and a density of 0.955 g / cm 3 The number average molecular weight is 11473, the weight average molecular weight is 334560, and the molecular weight distribution width index is 29.16.

[0057] Example 5

[0058] S1: 1-hexene and ethylene in a molar ratio of 0.3:1 are introduced with hydrogen, and polyethylene powder is obtained by polymerization through a series reactor method in the presence of a Ziegler-Natta polymerization catalyst; wherein the yield ratio of the output of the first loop reactor to the output of the second loop reactor is controlled at 49:51;

[0059] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder with a length-to-diameter ratio of 52. During the granulation process, 0.15 parts of nucleating agent 20e, 0.15 parts of magnesium stearate, and 0.25 parts of antioxidant 1076 and antioxidant 626 in a weight ratio of 2:1 are stably and continuously added. The temperature is controlled at 210°C to obtain a bimodal polyethylene resin for pipes.

[0060] The bimodal polyethylene resin powder obtained in Example 5 has a melt index (190°C, 5kg) of 0.62g / 10min and a density of 0.953g / cm 3 The number average molecular weight is 10034, the weight average molecular weight is 314265, and the molecular weight distribution width index is 31.32.

[0061] Example 6

[0062] S1: 1-hexene and ethylene in a molar ratio of 0.45:1 are introduced with hydrogen, and polyethylene powder is obtained by polymerization through a series reactor method in the presence of a Ziegler-Natta polymerization catalyst; wherein the yield ratio of the output of the first loop reactor to the output of the second loop reactor is controlled at 46:54;

[0063] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder with a length-to-diameter ratio of 55. During the granulation process, 0.2 parts of nucleating agent 20e, 0.1 parts of magnesium stearate, and 0.3 parts of antioxidant 1076 and antioxidant 626 in a weight ratio of 3:1 are stably and continuously added. The temperature is controlled at 215°C to obtain a bimodal polyethylene resin for pipes.

[0064] The bimodal polyethylene resin powder obtained in Example 6 has a melt index (190°C, 5kg) of 0.75g / 10min and a density of 0.949g / cm 3 The number average molecular weight is 11176, the weight average molecular weight is 295400, and the molecular weight distribution width index is 26.43.

[0065] Comparative Example 1

[0066] S1: introducing hydrogen into 1-hexene and ethylene at a molar ratio of 0.1:1, and polymerizing in the presence of a Ziegler-Natta polymerization catalyst through a series reactor method to obtain polyethylene powder; wherein the yield ratio of the first loop reactor to the second loop reactor is controlled at 50:50;

[0067] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder with a length-to-diameter ratio of 55. During the granulation process, 0.25 parts of nucleating agent 20e, 0.05 parts of magnesium stearate, and 0.5 parts of antioxidant 1076 and antioxidant 626 in a weight ratio of 4:1 are stably and continuously added. The temperature is controlled at 215°C to obtain a bimodal polyethylene resin for pipes.

[0068] The bimodal polyethylene resin powder obtained in Comparative Example 1 has a melt index (190°C, 5kg) of 0.25g / 10min and a density of 0.958g / cm 3 The number average molecular weight is 10418, the weight average molecular weight is 380500, and the molecular weight distribution width index is 36.52.

[0069] Comparative Example 2

[0070] S1: 1-hexene and ethylene in a molar ratio of 0.65:1 are introduced with hydrogen, and polyethylene powder is obtained by polymerization through a series reactor method in the presence of a Ziegler-Natta polymerization catalyst; wherein the yield ratio of the output of the first loop reactor to the output of the second loop reactor is controlled at 44:56;

[0071] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder with a length-to-diameter ratio of 58. During the granulation process, 0.15 parts of nucleating agent 20e, 0.25 parts of magnesium stearate, and 0.45 parts of antioxidant 1076 and antioxidant 626 in a weight ratio of 2:1 are stably and continuously added. The temperature is controlled at 210°C to obtain a bimodal polyethylene resin for pipes.

[0072] The bimodal polyethylene resin powder obtained in Comparative Example 2 has a melt index (190°C, 5kg) of 1.2g / 10min and a density of 0.944g / cm 3 The number average molecular weight is 9397, the weight average molecular weight is 231650, and the molecular weight distribution width index is 24.65.

[0073] Comparative Example 3

[0074] S1: 1-hexene and ethylene in a molar ratio of 0.4:1 are introduced with hydrogen, and polyethylene powder is obtained by polymerization through a series reactor method in the presence of a Ziegler-Natta polymerization catalyst; wherein the yield ratio of the output of the first loop reactor to the output of the second loop reactor is controlled at 48:52;

[0075] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder with a length-to-diameter ratio of 50. During the granulation process, 0.2 parts of nucleating agent 20e, 0.2 parts of magnesium stearate, and 0.35 parts of antioxidant 1010 and antioxidant 168 in a weight ratio of 1:2 are stably and continuously added. The temperature is controlled at 190°C to obtain a bimodal polyethylene resin for pipes.

[0076] The bimodal polyethylene resin powder obtained in Comparative Example 3 has a melt index (190°C, 5kg) of 0.65g / 10min and a density of 0.950g / cm 3 The number average molecular weight is 10025, the weight average molecular weight is 323315, and the molecular weight distribution width index is 32.25.

[0077] Comparative Example 4

[0078] S1: 1-hexene and ethylene in a molar ratio of 0.45:1 are introduced with hydrogen, and polyethylene powder is obtained by polymerization through a series reactor method in the presence of a Ziegler-Natta polymerization catalyst; wherein the yield ratio of the output of the first loop reactor to the output of the second loop reactor is controlled at 47:53;

[0079] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated using a twin-screw extruder with a length-to-diameter ratio of 60. During the granulation process, 0.25 parts of magnesium stearate and 0.25 parts of antioxidant 1010 and antioxidant 168 in a weight ratio of 2:1 are stably and continuously added. The temperature is controlled at 200°C to obtain a bimodal polyethylene resin for pipes.

[0080] The bimodal polyethylene resin powder obtained in Comparative Example 4 has a melt index (190°C, 5kg) of 0.55g / 10min and a density of 0.949g / cm 3 The number average molecular weight is 11123, the weight average molecular weight is 336580, and the molecular weight distribution width index is 30.26.

[0081] Comparative Example 5

[0082] S1: 1-hexene and ethylene in a molar ratio of 0.45:1 are introduced with hydrogen, and polyethylene powder is obtained by polymerization through a series reactor method in the presence of a Ziegler-Natta polymerization catalyst; wherein the yield ratio of the output of the first loop reactor to the output of the second loop reactor is controlled at 49:51;

[0083] S2: 100 parts by weight of the polyethylene powder obtained in step S1 are granulated by a twin-screw extruder with a length-to-diameter ratio of 55. During the granulation process, 0.15 parts of magnesium stearate and 0.4 parts of antioxidant 1010 and antioxidant 168 in a weight ratio of 1:2 are stably and continuously added. The temperature is controlled at 210°C to obtain a bimodal polyethylene resin for pipes.

[0084] The bimodal polyethylene resin powder obtained in Comparative Example 5 has a melt index (190°C, 5kg) of 0.25g / 10min and a density of 0.949g / cm 3 The number average molecular weight is 12211, the weight average molecular weight is 381600, and the molecular weight distribution width index is 31.35.

[0085] The physical properties of the high-density polyethylene pipe resins prepared in Examples 1-6 and Comparative Examples 1-5 were tested using a melt flow rate tester, a density meter, a universal tensile testing machine, a hydrostatic testing machine, an oxidation induction time tester, and an oxygen permeability tester according to national standards. The test results are shown in Table 1:

[0086] Table 1 Performance test data of bimodal polyethylene compositions obtained from Examples 1 to 6 and Comparative Examples 1 to 5

[0087]

[0088]

[0089] The data in Table 1 show that the test results of Examples 1-6 of the present invention all meet the expected design, wherein the oxygen barrier performance is improved by more than 45%, and the thermal stability test under the test conditions of 110°C and test pressure of 2.4MPa reaches 8760h, which meets the requirements of PE-RTⅡ pipe resin. (Compared with Comparative Examples 4 and 5 without adding nucleating agent)

[0090] In summary, the bimodal polyethylene composition of the present invention has controllable distribution of the high molecular weight part and the low molecular weight part, reasonable molecular weight distribution, excellent long-term hydrostatic strength, no brittle failure will occur under temperature conditions below 110°C, no inflection point will appear in the hydrostatic curve, and can withstand higher internal pressure. The produced pipes are suitable for the field of secondary thermal pipe networks.

[0091] Any numerical value mentioned in the present invention, if there is only an interval of two units between any minimum value and any maximum value, includes all values ​​from the minimum value to the maximum value each time increasing by one unit. For example, if the amount of a component, or the value of a process variable such as temperature, pressure, time, etc. is declared to be 50-90, in this specification it means that 51-89, 52-88... and 69-71 and 70-71 are specifically listed. For non-integer values, 0.1, 0.01, 0.001 or 0.0001 can be appropriately considered as a unit. These are just some specially specified examples. In this application, in a similar manner, all possible combinations of numerical values ​​between the listed minimum and maximum values ​​are considered to have been disclosed.

[0092] It should be noted that the embodiments described above are only used to explain the present invention and do not constitute any limitation to the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words, rather than restrictive words. The present invention may be modified as specified within the scope of the claims of the present invention, and the present invention may be revised without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein, on the contrary, the present invention can be extended to all other methods and applications with the same functions.

Claims

1. A bimodal polyethylene composition, characterized in that The raw materials for preparing the composition include polyethylene powder, antioxidant, nucleating agent and halogen absorbent; The antioxidant is added in an amount of 0.1-0.4% of the total mass of the polyethylene powder; The amount of the nucleating agent added is 0.05-0.2% of the total mass of the polyethylene powder; The amount of the halogen absorbent added is 0.1-0.2% of the total mass of the polyethylene powder.

2. The bimodal polyethylene composition according to claim 1, characterized in that The raw materials for preparing the polyethylene powder include comonomer and ethylene, and the molar ratio of the comonomer to ethylene is (0.2-0.6):1; preferably, the comonomer is 1-hexene; and / or The antioxidant is a phenolic antioxidant, preferably, the phenolic antioxidant comprises a mixture of antioxidant 1076 and antioxidant 626, the weight ratio of the antioxidant 1076 to the antioxidant 626 is (1-3):1; and / or The nucleating agent is a nucleating agent 20e, and / or The halogen absorber is magnesium stearate.

3. The bimodal polyethylene composition according to claim 1 or 2, characterized in that The composition has a melt index (190°C, 5kg) of 0.5-1.0g / 10min and a density of 0.945-0.955g / cm 3 ; The number average molecular weight Mn is 10000-15000, the weight average molecular weight Mw is 250000-350000, and the molecular weight distribution width index is 16-35.

4. The bimodal polyethylene composition according to any one of claims 1 to 3, characterized in that The thermal stability test of the composition under hydrostatic conditions was conducted at a test temperature of 110°C, a hydrostatic pressure of 2.4 MPa, and a test time of 8760 hours. There was no cracking or leakage. The oxygen permeability of the composition was ≤37.46 cm 3 / m 2 , 24h, 0.1MPa.

5. A method for preparing a bimodal polyethylene composition according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1: polymerizing ethylene, hydrogen and comonomer in the presence of a polymerization catalyst to obtain polyethylene powder; S2: Granulate the polyethylene powder obtained in step S1, and steadily and continuously add an antioxidant, a nucleating agent, and a halogen absorber during the granulation process to obtain a bimodal polyethylene composition.

6. The preparation method according to claim 5, characterized in that: In the step S1, the polymerization reaction includes polymerization using a series reactor. Preferably, the series reactor consists of a double loop reactor, the first loop reactor polymerizes to obtain a low molecular weight portion, and the second loop reactor polymerizes to obtain a high molecular weight portion; more preferably, the yield ratio of the first loop reactor output / the second loop reactor output is (45-49): (51-55).

7. The preparation method according to claim 5 or 6, characterized in that: In the step S1, the molar ratio of ethylene to hydrogen is (0.015-0.03):1; and / or, the comonomer is 1-hexene, and the molar ratio of the comonomer to ethylene is (0.2-0.6):1; And / or, the polymerization catalyst is a Ziegler-Natta catalyst.

8. The preparation method according to any one of claims 5 to 7, characterized in that: In the step S2, the granulation includes: using a twin-screw extruder for granulation, the twin-screw adopts a length-to-diameter ratio of 40-60; and the granulation temperature is 170-220°C.

9. The preparation method according to any one of claims 5 to 8, characterized in that: In the step S2, the antioxidant is a phenolic antioxidant, preferably, the phenolic antioxidant comprises a mixture of antioxidant 1076 and antioxidant 626, the weight ratio of the antioxidant 1076 to the antioxidant 626 is (1-3):1; and / or the amount of the antioxidant added is 0.1-0.4% of the total mass of the polyethylene powder; and / or The nucleating agent is nucleating agent 20e, and the amount of the nucleating agent added is 0.05-0.2% of the total mass of the polyethylene powder; and / or The halogen absorbent is magnesium stearate, and the added amount is 0.1-0.2% of the total mass of the polyethylene powder.

10. Use of the bimodal polyethylene composition according to any one of claims 1 to 5 or the bimodal polyethylene composition prepared by the preparation method according to any one of claims 6 to 10 in the field of preparing PE-RTⅡ type pipes.

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