Preparation method of low-density polyethylene, preparation method of polyolefin composition and its application

Abstract

The present invention provides a method for preparing low-density polyethylene, a polyolefin composition and a method for preparing the same, and its application. Ethylene monomers are polymerized in the presence of an initiator, and then a mixture of propylene, propionaldehyde, 1,8-nonadiene, and 1,9-decadiene is added to terminate the polymerization reaction to prepare a low-density polyethylene material. After the ultra-clean raw materials produced by the low-density polyethylene (LDPE) polymerization device are pelletized by extrusion with a specific additive 1, they are passed through a positive-pressure ultra-clean conveying system, and then melt-blended and filtered ultra-cleanly with additive 2, and then flexibly mixed with additive 3 through a static adsorption system, pelletized, and extruded to prepare an extra-high voltage cable. This method solves the technical problem of ultra-cleanliness in the preparation process of extra-high voltage cables, avoids the problem of generating impurities due to high-temperature decomposition in the extrusion pelletizing system, and also reduces the requirements for mixing equipment.

Description

Technical Field

[0001] The present invention relates to the field of polyolefins, and particularly relates to a method for preparing low-density polyethylene, a polyolefin composition, a method for preparing the same, and its applications. Background Art

[0002] Ultra-pure insulating material is an essential part of high-voltage cables. At present, in the field of medium- and low-voltage cable insulating materials below 35 kV in China, due to the relatively low voltage level, after improving ordinary LDPE, it can be applied to 35 kV cables. However, the ultra-high voltage cable insulating materials above 220 kV still have not been broken through, resulting in a high risk of breakdown failure of the cable insulating layer, which urgently needs to be solved.

[0003] Volume resistivity

[0004] Volume resistivity is the impedance of a material to current per unit volume and is used to characterize the electrical properties of the material. Generally, the higher the volume resistivity, the higher the efficiency of using the material as an electrical insulation component. Therefore, high-voltage cable insulating materials are required to have higher insulation characteristics.

[0005] Electrical breakdown reliability

[0006] The Weibull distribution is widely used in reliability engineering, especially suitable for the distribution form of cumulative wear failures of electromechanical products. Since it can easily infer its distribution parameters using probability values, it is widely used in the data processing of various life tests. Cable breakdown is weak-point breakdown, which is reflected in the weakest point and has a certain probability. Therefore, it is particularly necessary to improve the system reliability.

[0007] Patent CN102382222A discloses an LDPE resin for cable insulating materials with a withstand voltage level of 110 kV and above and a preparation method thereof. The molecular weight is regulated by the content of peroxide composition in different zones and propionaldehyde, propylene, acetone, pentane or any combination thereof to prepare LDPE, and then polymer granulation, pneumatic conveying, dust removal and packaging are carried out. The disadvantages of the above method are as follows: ① How to reduce the impurity content of XLPE, not generate large crystal points and oxidized particles during the granulation process, ensure that it is not contaminated during the transportation, storage and shipping processes, and improve the treeing voltage of the material; ② How to adjust the molecular structure of the LDPE resin and reduce the addition amount of peroxide in the product, and reduce the fluctuations of key parameters (such as density, melt index, etc.) during processing.

[0008] However, regarding the above difficulties, domestic and foreign companies have carried out a lot of research and development. However, due to the requirement of upstream and downstream integration characteristics for the production and preparation of ultra-high voltage cable materials, there is no domestic company that can integrally prepare special polyolefin materials meeting the high voltage levels of 220 kV and above. Summary of the Invention

[0009] An object of the present invention is to provide a method for preparing low-density polyethylene, a method for preparing a polyolefin composition, and their applications in the cladding of power cables of 220 kV and above.

[0010] A method for preparing low-density polyethylene includes the following steps:

[0011] Polymerize ethylene monomer in the presence of an initiator, and then add a mixture of propylene, propionaldehyde, 1,8-nonadiene, and 1,9-decadiene to terminate the polymerization reaction to prepare a low-density polyethylene material.

[0012] Preferably, the total mass of the propylene, propionaldehyde, 1,8-nonadiene, and 1,9-decadiene accounts for 0.05%-0.8% of the mass of ethylene.

[0013] Preferably, the ratio of the total mass of the propylene and propionaldehyde to the total mass of the 1,8-nonadiene and 1,9-decadiene is (1:9) to (9:1).

[0014] Preferably, the mass ratio of the propylene to the propionaldehyde is: (1:9) to (9:1);

[0015] Preferably, the mass ratio of the 1,8-nonadiene to the 1,9-decadiene is: (1:9) to (9:1);

[0016] Preferably, a compressor is used for pressurization during the polymerization of the ethylene monomer, and lubricating oil is added to the compressor. The lubricating oil is a mixture of mineral oil and synthetic oil. Preferably, the mass ratio of the mineral oil to the synthetic oil is 1:9 to 9:1.

[0017] Preferably, the mineral oil is Shell Corena 100, and the synthetic oil is a macromolecular base oil obtained by the polymerization catalytic reaction of ethylene and propylene, preferably Total 270DS or Clariant PE-K270;

[0018] Preferably, the added mass of the lubricating oil accounts for 0.1 wt% to 0.8 wt% of the mass of ethylene;

[0019] Preferably, the initiator is a peroxide initiator, and the peroxide is preferably tert-butyl perpivalate or 2,5-di(tert-butylperoxy)-2,5-dimethylhexane.

[0020] Preferably, the added amount of the initiator accounts for 0.08 wt% - 0.15 wt% of the mass of ethylene;

[0021] Preferably, during the polymerization of the ethylene monomer, the reaction temperature is 100 - 320 °C, and the pressure range is: 100 - 350 MPa.

[0022] Preferably, after adding a mixture of propylene, propionaldehyde, 1,8-nonadiene, and 1,9-decadiene, the reaction temperature is 200 - 300 °C.

[0023] Preferably, in some preferred embodiments of the present invention, the method for preparing the low-density polyethylene comprises the following steps:

[0024] a1) Ethylene compression: The ethylene monomer is boosted to 1800 - 2500 bar via a compressor, the temperature is controlled at 130 - 150 °C, and it enters the reactor; the lubricating oil used in the compressor is mineral oil / synthetic oil;

[0025] a2) Initiator injection: By controlling the temperature in the first zone of the reactor at about 160 - 180 °C, and then injecting the peroxide solution through the injection port with an independent pump (generally, the injection port consists of 2 - 5 injection ports) to initiate the polymerization reaction. Since the polymerization reaction is an exothermic reaction, continuous cooling (by water or steam) is required, and it is carried out under the conditions of the peak temperature and the initiation temperature, with the temperature range: 100 - 320 °C and the pressure range: 100 - 350 MPa (controlled by the end valve);

[0026] a3) Reaction adjustment and termination: The regulators (CTA), propylene, propionaldehyde, 1,8-nonadiene, and 1,9-decadiene, are added to the reactor synchronously with the ethylene monomer. The temperature of each zone of the reactor is controlled at 200 - 300 °C through the balanced reactor. When the target performance is reached, the required LDPE melt is prepared by adjusting the jacket cooling water and reducing the pressure and cooling through the valve.

[0027] Preferably, the melt flow rate of the low-density polyethylene prepared by the preparation method of the present invention at 190 °C and under a load of 2.16 kg is 1.0 - 3.0 g / 10 min; the density is 910 - 930 kg / m 3 ; the molecular weight distribution measured by GPC is Mw / Mn of 4 - 6; the elongation at break is 700 - 1200%; the dielectric constant is less than 2.3; the volume resistivity is greater than 1.0×10 16 Ω·cm; the gel size is less than 200 μm;

[0028] The present invention also provides a method for preparing a polyolefin composition, comprising the following steps:

[0029] b1) Mixing the low-density polyethylene prepared by the present invention with an additive 1 through an extruder and then pelletizing, wherein the additive 1 is a thioester antioxidant; preferably one or two of dilauryl thiodipropionate (DLTDP) and distearyl thiodipropionate (DSTDP);

[0030] b2) The pellets obtained in step (b1) are melt-blended with Auxiliary Agent 2 through an extruder, filtered, and then pelletized. Auxiliary Agent 2 is one or more of a hindered phenol antioxidant, a phosphate antioxidant, or a molecular weight regulator;

[0031] b3) The pellets prepared in step b2 are melt-blended with Auxiliary Agent 3 by a low-temperature static adsorption method. Auxiliary Agent 3 is an organic peroxide;

[0032] b4) The pellets prepared in step b3 are cooled by clean air cooling and then packaged in a gravity-fed manner;

[0033] Preferably, the extruder is a reciprocating single-screw extruder or a single-screw extrusion granulator, and the extrusion temperature is 80 - 200 °C;

[0034] Preferably, the hindered phenol antioxidants in step (b2) include 4,4'-thiobis(6-tert-butyl-3-methylphenol), pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], n-octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 2,2'-thiobis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]; the phosphate antioxidants include pentaerythritol bisphosphite, bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, and the molecular weight regulators include dodecyl mercaptan, 2,4-diphenyl-4-methyl-1-pentene, 2,4-dimethyl-1,3-pentadiene. Preferably, liquid antioxidants and molecular weight regulators are used;

[0035] Preferably, the organic peroxides include one or two of dicumyl peroxide, triallyl isocyanurate, bis(tert-butylperoxyisopropyl)benzene, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane;

[0036] Preferably, step b3) is carried out by a low-temperature static adsorption method for mixing, and the temperature range is 55 - 75 °C, preferably 60 - 70 °C;

[0037] Preferably, based on the mass of low-density polyethylene being 100%, the addition amount of Auxiliary Agent 1 is 0.05 wt% - 0.2 wt%, preferably 0.08 - 0.15 wt%, the addition amount of Auxiliary Agent 2 is 0.15 - 0.3 wt%, preferably 0.18 - 0.25 wt%; the addition amount of Auxiliary Agent 3 is 1.0 - 2.0 wt%, preferably 1.5 - 1.7 wt%.

[0038] Preferably, the polyolefin composition prepared by the preparation method of the present invention

[0039] The electrical breakdown reliability is 63.2% which is greater than 130 kV / mm, and the thickness of the test piece is 0.2 mm; cleanliness: the number of gel content sizes (100 - 150) um / kg is less than 10, the number of those greater than 150 um / kg is 0, the number of impurity content sizes greater than 50 um / kg is less than 10, and more preferably, the number of impurity content sizes greater than 50 um / kg is 0; other properties all meet the requirements of GB / T 22078.

[0040] When using the polyolefin composition of the present invention to manufacture high-voltage and extra-high-voltage cables, the electrical properties can pass GB / T11017, GB / T22078, and GB / T18890.

[0041] Beneficial Effects

[0042] According to the first aspect of the present invention, the present invention provides a high-performance LDPE melting method, which includes introducing unsaturated comonomers, adjusting the composition ratio of compressor lubricating oil, and adjusting the composition ratio of chain transfer agents, so that the ratio of carbon-carbon double bonds to carbon atoms in the prepared low-density polyethylene reaches more than 0.4 / 1000. This provides a basis for the realization of the polyolefin composition. At the same time, when preparing the polyolefin composition, the entire preparation process is improved. Creatively, by adding different types of additives and controlling the addition sequence of each additive, the super-purity of LDPE melt is achieved, the electrical breakdown reliability of LDPE is significantly improved, and the performance is significantly better than that of the existing LDPE on the market.

[0043] In the present invention, by introducing Additive 1, the risk of carbonylation of LDPE pellets is reduced, and the purity is significantly improved.

[0044] After preparing the polyolefin composition from the low-density polyethylene prepared by the present invention, it is suitable for preparing the insulating layer of extra-high-voltage cables. Specifically, this polyolefin composition is preferably an insulating sheath composition for manufacturing alternating current (AC) power cables, and more specifically, it is a composition and its preparation method suitable for cable insulating materials with a withstand voltage level of 35 kV and above.

[0045] The above object can be achieved through the polyolefin composition of the present invention and the preparation process route. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a schematic diagram of the preparation process of the polyolefin composition of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0047] The present invention will be described below through specific implementation examples. Unless otherwise specified, the technical means used in the present invention are all methods well known to those skilled in the art; the materials used are all commercially available materials. In addition, the implementation examples should be understood as illustrative and not limiting the scope of the present invention. The essence and scope of the present invention are only defined by the claims. For those skilled in the art, various changes or modifications to the material components and dosages in these implementation examples also fall within the protection scope of the present invention without departing from the essence and scope of the present invention.

[0048] Through in-depth research and improvement by the inventors of the present invention, such as Figure 1 , in the improved technical route, first, the structure of the LDPE melt is optimized and improved to prepare unsaturated LDPE with a carbon-carbon double bond content of 0.4 / 1000 carbon atoms or more, providing a basis for the realization of the polyolefin composition. At the same time, the entire preparation process is improved. Creatively, through the introduction of additive 1, the super-purity of the LDPE melt is achieved, significantly improving the electrical breakdown reliability of the LDPE, and the performance is significantly better than that of the existing LDPE on the market. Finally, through integrated modification, the present invention is completed on this basis.

[0049] a) The LDPE melt has the following properties: melt flow rate at 190 °C and 2.16 kg load is 1.0 - 3.0 g / 10 min; density is 910 - 930 kg / m3; the molecular weight distribution by GPC test is Mw / Mn of 4 - 6; elongation at break is 700 - 1200%; dielectric constant is less than 2.3; volume resistivity is greater than 1.0×1016 Ω·cm; gel size is less than 200 μm; carbon-carbon double bond is 0.4 / 1000 carbon atoms or more;

[0050] The preparation method of the above LDPE melt has the following steps:

[0051] a1) Ethylene compression: The ethylene monomer is pressurized to 1800 - 2500 bar via a compressor, the temperature is controlled at 130 - 150 °C, and it enters the reactor; the lubricating oil used in the compressor is mineral oil / synthetic oil;

[0052] a2) Initiator injection: By controlling the temperature of the first zone of the reactor at about 160 - 180 °C, then the peroxide solution is injected through the injection port with an independent pump (generally, the injection port consists of 2 - 5 injection ports) to initiate the polymerization reaction. Since the polymerization reaction is an exothermic reaction, continuous cooling (by water or steam) is required and it is carried out under the conditions of peak temperature and initiation temperature, with a temperature range of 100 - 320 °C and a pressure range of 100 - 350 MPa (controlled by the end valve);

[0053] a3) Reaction regulation and termination: The regulators (CTA), propylene, propionaldehyde, 1,8-nonadiene, and 1,9-decadiene are added to the reactor synchronously with the ethylene monomer. The temperature of each zone is controlled at 200 - 300 °C through the balanced reactor. After the target performance is achieved, the required LDPE melt is prepared by adjusting the jacket cooling water, reducing pressure through the valve, and cooling.

[0054] Preferably, the melt flow rate of the low-density polyethylene prepared by the preparation method of the present invention at 190 °C and under a load of 2.16 kg is 1.0 - 3.0 g / 10 min; the density is 910 - 930 kg / m 3 ; The molecular weight distribution tested by GPC is Mw / Mn of 4 - 6; the elongation at break is 700 - 1200%; the dielectric constant is less than 2.3; the volume resistivity is greater than 1.0×10 16 Ω·cm; the gel size is less than 200 um;

[0055] b) For the polyolefin composition, the electrical breakdown reliability is greater than 130 kV / mm at 63.2%, the test piece thickness is 0.2 mm, cleanliness: the number of gel content sizes (100 - 150) um / kg is less than 10, the number of those greater than 150 um / kg is 0, the number of impurity content sizes greater than 50 um / kg is less than 10, and more preferably, the number of impurity content sizes greater than 50 um / kg is 0; other properties all meet the requirements of GB / T 22078.

[0056] The preparation steps of the above polyolefin composition are as follows:

[0057] b1) The low-density polyethylene prepared by the present invention and the auxiliary agent 1 are mixed through an extruder and then pelletized. The auxiliary agent 1 is a thioester antioxidant; preferably one or both of dilauryl thiodipropionate (DLTDP) and distearyl thiodipropionate (DSTDP);

[0058] b2) The pellet material from step (b1) and the auxiliary agent 2 are blended, filtered through an extruder, and then pelletized. The auxiliary agent 2 is one or more of a hindered phenol antioxidant, a phosphate antioxidant, or a molecular weight regulator;

[0059] b3) The pellet material prepared in step b2 is blended with the auxiliary agent 3 by the method of low-temperature static adsorption. The auxiliary agent 3 is an organic peroxide;

[0060] b4) The pellet material prepared in step b3 is cooled by clean air cooling and packaged in the form of gravity feeding;

[0061] c) For high-voltage and extra-high-voltage cables, the electrical properties can comply with GB / T22017, GB / T22078, and GB / T18890.

[0062] Table 1, Ethylene Raw Materials

[0063] Component Specification Unit Ethylene Min. 99.98 Vol.% Methane + Ethane Max. 200 Vol byppm H2 Max. 5 Vol by ppm C3 and above Max. 10 Vol by ppm Acetylene Max. 2 Vol by ppm CO2 Max. 5 Vol by ppm other Max. 10 Vol by ppm

[0064] Regulators: propionaldehyde, propylene, with purity above 99.99% after refining: 1,8-nonadiene and 1,9-decadiene, with purity > 99.99% after refining;

[0065] Lubricants: mineral oil, Shell Corena 100, purchased from Shell, Total 270DS, purchased from Total, Clariant PE-K270, purchased from Clariant;

[0066] Peroxide initiators: tert-butyl perpivalate, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane, purchased from Arkema, with purity of 99.99%; where the solvent is isododecane, commercially available, with purity > 99.9%.

[0067] Additive 1 is thioether antioxidant dilauryl thiodipropionate (DLTDP) and distearyl thiodipropionate (DSTDP);

[0068] Additive 2 is a hindered phenol antioxidant or a phosphate ester antioxidant or a molecular weight regulator, preferably a liquid antioxidant or a molecular weight regulator; preferably one or two of antioxidant 1035, antioxidant 1076, antioxidant 1520, dodecyl mercaptan, 2,4-diphenyl-4-methyl-1-pentene, 2,4-dimethyl-1,3-pentadiene; commercially available, with purity > 99.9%.

[0069] Additive 3, dicumyl peroxide, triallyl isocyanurate, di-tert-butyl peroxyisopropylbenzene, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, preferably one or more of dicumyl peroxide, triallyl isocyanurate, di-tert-butyl peroxyisopropylbenzene; commercially available, with purity > 99.9%.

[0070] Example 1

[0071] The refined ethylene raw material is sucked in at a flow rate of 35 t / h by a compressor. The lubricating oil used in the compressor is mineral oil / synthetic oil = 5:5. The flow rate is controlled at 30 kg / h and the pressure is increased to 1800 bar, and the temperature is 150 °C. The mineral oil is Shell Corena 100 and the synthetic oil is Total 270DS;

[0072] The ratio of tert-butyl peroxy pivalate, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane to isododecane solvent is 1:4, which are added separately in four reaction zones; the concentration is controlled at 1000 ppm (35 kg), and the flow rates are designed as (15 kg / h, 10.0 kg / h, 5.0 kg / h, 5.0 kg / h) respectively. The pressure is raised to 1800 bar and added to the reactor to initiate the polymerization reaction. Cooling is continuously carried out through water or steam, and it is controlled under the peak temperature and initiation temperature conditions, with the temperature being about 240 °C and the pressure being 1900 bar (controlled by the end valve);

[0073] The regulators: propylene, propionaldehyde, 1,8-nonadiene, 1,9-decadiene, (mass ratio 5:1:1:5) are added to the reactor synchronously with the ethylene monomer. The total concentration is controlled at 800 ppm (28 kg / h), and the flow rates at the four ports are 8 kg / h, 8 kg / h, 6 kg / h, and 6 kg / h respectively. When the pressure is raised to 1800 bar;

[0074] The temperature is controlled by the water flow rate of the reactor jacket, which needs to be controlled between 180 and 190 °C. The reaction peak temperature is controlled between 220 and 280 °C. The pressure is controlled by the pressure-building valve, and the melt index is adjusted to 1.7 - 2.3 to obtain the LDPE melt;

[0075] The LDPE melt and additive 1 are extruded and pelletized by an extruder to obtain low-density polyethylene. Additive 1 is DLTDP, and the addition amount is 0.05 wt% of the mass of the LDPE melt. It is transported to the ultra-clean filtration equipment through positive pressure in a pipeline and blended with additive 2, and filtered through a multi-layer composite filter screen with more than 500 meshes to obtain an intermediate product. Additive 2 is antioxidant 1035, and the addition amount is 0.15 wt% of the mass of the LDPE melt;

[0076] Finally, after extrusion and pelletization, continuous on-line detection is carried out by an optical detector with a resolution of more than 25 μm, and the detection capacity is greater than 300 kg / h. The qualified materials are classified according to cleanliness through a three-way valve. Finally, the qualified materials are subjected to low-temperature static adsorption with additive 3, and the adsorption amount is 1.6 wt%, at 60 °C for 16 h to obtain the polyolefin blend. Additive 3 is diisopropylbenzene peroxide;

[0077] After classification and packaging, it is transported to the cable factory in a clean manner and extruded through the VCV production line to prepare high-voltage or extra-high-voltage cables, which are inspected in accordance with GB / T 22078;

[0078] The test results are shown in Table 2:

[0079] Example 2

[0080] The refined ethylene raw material is sucked in by a compressor at a flow rate of 35 t / h. The lubricating oil used in the compressor is mineral oil / synthetic oil = 8:2, with a controlled flow rate of 28 kg / h, boosted to 1800 bar, and the temperature is 150 °C. The mineral oil is Shell Corena 100, and the synthetic oil is Clariant PE-K270;

[0081] The ratio of tert-butyl peroxy pivalate, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane to isododecane is 1:4 and is added separately in four reaction zones; the concentration is controlled at 1000 ppm (35 kg / h), and the flow rates are designed as (15 kg / h, 10.0 kg / h, 5.0 kg / h, 5.0 kg / h) respectively. The pressure is raised to 1800 bar and added to the reactor to initiate the polymerization reaction, and continuous cooling is carried out through water or steam, controlled under peak temperature and initiation temperature conditions, with a temperature of about 250 °C and a pressure range: 2600 bar (controlled by the end valve);

[0082] The regulators: propylene, propionaldehyde, 1,8-nonadiene, 1,9-decadiene, (mass ratio 10:2:1:5) are added synchronously with the ethylene monomer to the reactor, the total concentration is controlled at 1000 ppm (35 kg / h), and the flow rates at the four ports are 10 kg / h, 10 kg / h, 7.5 kg / h, 7.5 kg / h respectively when the pressure is raised to 1800 bar;

[0083] The temperature is controlled by the water flow rate in the reactor jacket, which needs to be controlled at 180 to 190 °C, the reaction peak temperature is controlled at 220 to 280 °C, the pressure is controlled by the pressure-building valve, and the melt index is adjusted to 1.7 - 2.3 to obtain the LDPE melt;

[0084] The LDPE melt and additive 1 are extruded and pelletized by an extruder to obtain low-density polyethylene. Additive 1 is distearyl thiodipropionate, and the addition amount is 0.06 wt% of the LDPE melt mass. It is transported to the ultra-clean filtration equipment by positive pressure through a pipeline and blended with additive 2, and filtered through a multi-layer composite filter screen with more than 500 meshes to obtain an intermediate product. Additive 2 is antioxidant 1035, and the addition amount is 0.25 wt% of the LDPE melt mass;

[0085] Finally, after extrusion and pelletization, continuous on-line detection is carried out by an optical detector with a resolution of more than 25 μm, and the detection capacity is greater than 300 kg / h. The qualified materials are classified according to cleanliness through a three-way valve. Finally, the qualified materials are subjected to low-temperature static adsorption with additive 3, and the adsorption amount is 1.6 wt%, at 65 °C for 16 h to obtain the polyolefin blend. Additive 3 is diisopropylbenzene peroxide;

[0086] After being classified and packaged, it is transported cleanly to the cable factory, produced and extruded through a VCV production line to prepare high-voltage or extra-high-voltage cables, and inspected in accordance with GB / T 22078;

[0087] The test results are shown in Table 2:

[0088] Example 3

[0089] The refined ethylene raw material is sucked in by a compressor at a flow rate of 35 t / h. The lubricating oil used in the compressor is mineral oil / synthetic oil = 5:5. The flow rate is controlled at 28 kg / h, the pressure is increased to 1800 bar, and the temperature is 150 °C. The mineral oil is Shell Corena 100, and the synthetic oil is Total 270DS;

[0090] The ratio of tert-butyl perpivalate, 2,5-di(tert-butylperoxy)-2,5-dimethylhexane to isododecane solvent is 1:4, and they are added separately in four reaction zones; the concentration is controlled at 1000 ppm (35 kg / h), and the flow rates are designed as (15 kg / h, 8.0 kg / h, 7.0 kg / h, 5.0 kg / h) respectively. The pressure is increased to 1800 bar and added to the reactor to initiate the polymerization reaction. Cooling is continuously carried out through water or steam, and it is controlled under the peak temperature and initiation temperature conditions, with a temperature of 250 °C and a pressure of 2000 bar (controlled by the end valve);

[0091] The regulators: propylene, propionaldehyde, 1,8-nonadiene, 1,9-decadiene, (mass ratio 2:1:5:10) are added synchronously with the ethylene monomer to the reactor. The total concentration is controlled at 600 ppm (21 kg / h), and the flow rates at the four ports are 7 kg / h, 6 kg / h, 4 kg / h, 4 kg / h respectively. When the pressure is increased to 1800 bar;

[0092] The temperature is controlled by the water flow rate in the reactor jacket, and it needs to be controlled at 180 to 190 °C. The reaction peak temperature is controlled at 220 to 280 °C. The pressure is controlled by the pressure-building valve, and the melt index is regulated to 1.7 - 2.3 to obtain the LDPE melt;

[0093] The LDPE melt and Additive 1 are extruded and pelletized by an extruder to obtain low-density polyethylene. Additive 1 is DLTDP, and the addition amount is 0.07 wt% of the mass of the LDPE melt. It is transported to the ultra-clean filtration equipment by positive pressure through a pipeline and blended with Additive 2, and filtered through a multi-layer composite filter screen with more than 500 meshes to obtain the intermediate product. Additive 2 is antioxidant 300, and the addition amount is 0.17 wt% of the mass of the LDPE melt;

[0094] Finally, after extrusion granulation, continuous on-line detection is carried out with an optical detector with a resolution of more than 25 μm, and the detection capacity is greater than 300 kg / h. The qualified materials detected are classified according to cleanliness through a three-way valve. Finally, the qualified materials are subjected to low-temperature static adsorption with additive 3, and the adsorption capacity is 1.65 wt%. After 16 hours at 70 °C, a polyolefin blend is obtained, where additive 3 is dicumyl peroxide;

[0095] After grading and packaging, it is cleanly transported to the cable factory, and is extruded through the VCV production line to prepare high-voltage or extra-high-voltage cables, and inspected in accordance with GB / T 22078;

[0096] The test results are shown in Table 2:

[0097] Example 4

[0098] Low-density polyethylene was prepared by the same method as in Example 3;

[0099] After the LDPE melt and additive 1 are extruded and granulated through an extruder, low-density polyethylene is obtained, where additive 1 is DSTDP, and the addition amount is 0.07 wt% of the mass of the LDPE melt. It is transported to a super-clean filtration device by positive pressure through a pipeline and blended with additive 2, and filtered through a multi-layer composite filter screen with more than 500 meshes to obtain an intermediate product, where additive 2 is antioxidant 300 and 2,4-dimethyl-1,3-pentadiene (mass ratio 1:1), and the addition amount is 0.17 wt% of the mass of the LDPE melt;

[0100] Finally, after extrusion granulation, continuous on-line detection is carried out with an optical detector with a resolution of more than 25 μm, and the detection capacity is greater than 300 kg / h. The qualified materials detected are classified according to cleanliness through a three-way valve. Finally, the qualified materials are subjected to low-temperature static adsorption with additive 3, and the adsorption capacity is 1.5 wt%. After 16 hours at 70 °C, a polyolefin blend is obtained, where additive 3 is dicumyl peroxide;

[0101] After grading and packaging, it is cleanly transported to the cable factory, and is extruded through the VCV production line to prepare high-voltage or extra-high-voltage cables, and inspected in accordance with GB / T 22078;

[0102] The test results are shown in Table 2:

[0103] Comparative Example 1

[0104] The refined ethylene raw material is sucked in at a flow rate of 35 t / h by a compressor. The lubricating oil used in the compressor is mineral oil, the flow rate is controlled at 28 kg / h, the pressure is raised to 1800 bar, and the temperature is 150 °C, where the mineral oil is Shell Corena 100;

[0105] tert-Butyl peroxy pivalate, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane and isododecane are proportioned at 1:4 and added separately in four reaction zones; the concentration is controlled at 1000 ppm (35 kg / h), and the flow rates are designed as (15 kg / h, 8.0 kg / h, 7.0 kg / h, 5.0 kg / h) respectively. The pressure is increased to 1800 bar and then added to the reactor to initiate the polymerization reaction. Cooling is continuously carried out through water or steam, and it is controlled under the peak temperature and initiation temperature conditions, with the temperature being about 240 °C and the pressure being 1900 bar (controlled by the end valve);

[0106] The regulator: propylene, is added to the reactor synchronously with the ethylene monomer. The total concentration is controlled at 600 ppm (21 kg / h), and the flow rates are 7 kg / h, 6 kg / h, 4 kg / h, 4 kg / h respectively. When the pressure is increased to 1800 bar;

[0107] The temperature is controlled by the water flow rate in the reactor jacket and needs to be controlled at 180 to 190 °C. The reaction peak temperature is controlled at 220 to 280 °C. The pressure is controlled by the pressure building valve, and the melt index is regulated to be 1.7 - 2.3 to obtain the LDPE melt;

[0108] The LDPE melt is extruded and pelletized by an extruder to obtain low-density polyethylene, which is transported to a super-clean filtration device by positive pressure through a pipeline and blended with additive 2. It is filtered through a multi-layer composite filter screen with more than 500 meshes to obtain an intermediate product. Additive 2 is antioxidant 300, and the addition amount is 0.15 wt% of the mass of the LDPE melt;

[0109] Finally, after extrusion and pelletization, continuous on-line detection is carried out by an optical detector with a resolution of more than 25 μm, and the detection capacity is greater than 300 kg / h. The qualified materials are classified according to cleanliness through a three-way valve. Finally, the qualified materials are subjected to low-temperature static adsorption with additive 3, and the adsorption amount is 2 wt%, at 65 °C for 16 h to obtain a polyolefin blend. Additive 3 is diisopropylbenzene peroxide;

[0110] After classification and packaging, it is cleanly transported to a cable factory and extruded through a VCV production line to prepare high-voltage or extra-high-voltage cables, which are inspected in accordance with GB / T 22078;

[0111] The test results are shown in Table 2:

[0112] Comparative Example 2

[0113] The refined ethylene raw material is sucked in by a compressor at a flow rate of 35 t / h. The lubricating oil used in the compressor is synthetic oil, and the flow rate is controlled at 28 kg / h. The pressure is increased to 1800 bar, and the temperature is 150 °C. The synthetic oil is Total 270DS;

[0114] The ratio of tert-butyl peroxy pivalate, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane to isododecane is 1:4, which are added separately in four reaction zones; the concentration is controlled at 1000 ppm (35 kg / h), and the flow rates are designed as (15 kg / h, 10.0 kg / h, 5.0 kg / h, 5.0 kg / h) respectively. The pressure is increased to 1800 bar and then added to the reactor to initiate the polymerization reaction. Cooling is continuously carried out through water or steam, and it is controlled under the peak temperature and initiation temperature conditions. Temperature: 250 °C, Pressure: 2000 bar (controlled by the end valve);

[0115] The regulators: propylene and propionaldehyde (mass ratio 1:1) are added to the reactor synchronously with the ethylene monomer. The total concentration is controlled at 600 ppm (21 kg / h), and the flow rates are 7 kg / h, 6 kg / h, 4 kg / h, and 3 kg / h respectively. When the pressure is increased to 1800 bar;

[0116] The temperature is controlled by the water flow rate in the reactor jacket, which needs to be controlled at 180 to 190 °C. The reaction peak temperature is controlled at 220 to 280 °C. The pressure is controlled by the pressure-building valve, and the melt index is adjusted to 1.7 - 2.3 to obtain the LDPE melt;

[0117] The LDPE melt is extruded and pelletized by an extruder and then transported to a super-clean filtration device under positive pressure through a pipeline. It is blended with Additive 2 and filtered through a multi-layer composite filter screen with more than 500 meshes to obtain an intermediate product. Additive 2 is antioxidant 300, and the addition amount is 0.16 wt% of the mass of the LDPE melt;

[0118] Finally, after extrusion and pelletization, continuous on-line detection is carried out by an optical detector with a resolution of more than 25 μm, and the detection capacity is greater than 300 kg / h. The qualified materials are classified according to cleanliness through a three-way valve. Finally, the qualified materials are subjected to low-temperature static adsorption with Additive 3, and the adsorption amount is 1.9 wt%, at 70 °C for 16 h to obtain the polyolefin blend. Additive 3 is diisopropyl peroxide;

[0119] After classification and packaging, it is cleanly transported to a cable factory and extruded through a VCV production line to prepare high-voltage or extra-high-voltage cables, which are inspected according to GB / T 22078;

[0120] The test results are shown in Table 2:

[0121] Comparative Example 3

[0122] Low-density polyethylene is prepared by the same method as in Example 3.

[0123] After extruding and pelletizing the LDPE melt, it is positively transported through a pipeline to a super-clean filtration device, where it is blended with Auxiliary Agent 2 and filtered through a multi-layer composite filter screen with a mesh size of over 500 meshes to obtain an intermediate product. Auxiliary Agent 2 is antioxidant 1035, and the addition amount is 0.17 wt%, where the wt% is the mass fraction based on the LDPE melt;

[0124] Finally, after extrusion and pelletizing, continuous on-line detection is carried out using an optical detector with a resolution of 25 μm or more, and the detection capacity is greater than 300 kg / h. The qualified materials are classified according to cleanliness through a three-way valve. Finally, the qualified materials are subjected to low-temperature static adsorption with Auxiliary Agent 3, and the adsorption amount is 1.65 wt%. After 16 hours at 70 °C, a polyolefin blend is obtained. Auxiliary Agent 3 is dicumyl peroxide;

[0125] After grading and packaging, it is cleanly transported to a cable factory and extruded through a VCV production line to prepare high-voltage or extra-high-voltage cables, which are inspected in accordance with GB / T 22078;

[0126] The test results are shown in Table 2:

[0127] Table 2. Test Results

[0128]

[0129] It can be seen from the comparison between the examples and the comparative examples that by using the method of the present invention, the production of high-voltage resins of 220 kV and above is achieved. Through the regulation of the LDPE molecular structure, the carbon-carbon double bonds can be increased, the addition of peroxides can be greatly reduced, the gel content and the number of impurities can be significantly reduced, and the volume resistivity and breakdown reliability can be improved. The blend of the present invention fully meets the relevant requirements of GB / T 22078.

[0130] The above examples are only used to describe the preferred embodiments of the present invention, rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that various modifications or equivalent replacements made to the technical solutions of the present invention should fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing low-density polyethylene, characterized in that, The following steps are involved: The ethylene monomer is polymerized in the presence of an initiator, and then a mixture of propylene, propionaldehyde, 1,8-nonadiene and 1,9-decadiene is added to terminate the polymerization reaction to prepare a low-density polyethylene material; The ratio of the total mass of propylene and propionaldehyde to the total mass of 1,8-nonadiene and 1,9-decadiene is (1:9) to (9:1); During the polymerization of ethylene monomer, a compressor is used for pressurization, and the compressor is added with lubricating oil, and the lubricating oil is a mixture of mineral oil and synthetic oil, and the mass ratio of the mineral oil to the synthetic oil is 1:9 to 9:1; Synthetic oil is a macromolecular base oil obtained through the catalytic polymerization reaction of ethylene and propylene.

2. The preparation method according to claim 1, characterized in that, The total mass of the propylene, propionaldehyde, 1,8-nonadiene and 1,9-decadiene accounts for 0.05%-0.8% of the mass of ethylene.

3. The preparation method according to claim 1, characterized in that, The mass ratio of propylene to propionaldehyde is: (1:9) to (9:1).

4. The preparation method according to claim 1, wherein The mass ratio of the 1,8-nonadiene to 1,9-decadiene is (1:9) to (9:1).

5. The preparation method according to claim 1, characterized in that, The mineral oil is Shell Corena 100.

6. The preparation method according to claim 1, characterized in that, The added mass of lubricating oil accounts for 0.1wt% to 0.8wt% of the mass of ethylene.

7. The preparation method according to claim 1, wherein The initiator is a peroxide initiator.

8. The preparation method according to claim 7, characterized in that, The peroxide initiator is tert-butyl peroxypivalate and di-2,5-tert-butyl peroxide.

9. The preparation method according to claim 1, wherein The amount of the initiator added is 0.08wt%-0.15wt% of the mass of ethylene.

10. The preparation method according to claim 1, wherein, When ethylene monomer is polymerized, the reaction temperature is 100-320°C and the pressure range is 100-350MPa.

11. The preparation method according to claim 1 or 10, characterized in that, After adding a mixture of propylene, propionaldehyde, 1,8-nonadiene and 1,9-decadiene, the reaction temperature is 200-300°C.

12. The preparation method according to any one of claims 1-10, characterized in that, The preparation method of the low-density polyethylene comprises the following steps: a1) Ethylene compression: Ethylene monomer is pressurized to 1800-2500 bar by a compressor, and the temperature is controlled at 130-150°C before entering the reactor; the lubricating oil used in the compressor is a mixture of mineral oil and synthetic oil; a2) Initiator injection: The temperature of the first zone of the reactor is controlled at 160-180°C, and then the peroxide solution is injected through the injection port with an independent pump to initiate the polymerization reaction. Since the polymerization reaction is an exothermic reaction, it needs to be continuously cooled and controlled under the conditions of peak temperature and initiation temperature. The temperature range is 100-320°C and the pressure range is 100-350MPa. a3) Reaction regulation and termination: The regulators propylene, propionaldehyde, 1,8-nonadiene and 1,9-decadiene are added to the reactor simultaneously with the ethylene monomer, and the temperature of each zone is controlled at 200-300°C by balancing the reactor. When the target performance is achieved, the required LDPE melt is prepared by adjusting the jacket cooling water and reducing the pressure and cooling by the valve.

13. The preparation method according to claim 12, characterized in that, The melt flow rate of the low-density polyethylene prepared by this preparation method at 190 °C and under a load of 2.16 kg is 1.0 - 3.0 g / 10 min; the density is 910 - 930 kg / m 3 ; the molecular weight distribution tested by GPC is Mw / Mn of 4 - 6; the elongation at break is 700 - 1200%; the dielectric constant is less than 2.3; the volume resistivity is greater than 1.0×10 16 Ω·cm; the gel size is less than 200 μm.

14. A method for preparing a polyolefin composition, comprising the following steps: b1) mixing the low-density polyethylene prepared by the preparation method according to any one of claims 1 to 13 with the auxiliary agent 1 through an extruder and then granulating, wherein the auxiliary agent 1 is a thioester antioxidant; b2) The pellet material from step b1) and auxiliary agent 2 are melt-blended and filtered through an extruder and then pelletized, where the auxiliary agent 2 is one or more of a hindered phenol antioxidant, a phosphate ester antioxidant, or a molecular weight regulator; b3) The pellet material prepared in step b2) is melt-blended with auxiliary agent 3 by a low-temperature static adsorption method, where the auxiliary agent 3 is an organic peroxide.

15. The preparation method according to claim 14, characterized in that, b4) The pellet material prepared in step b3) is cooled by clean air cooling and packaged in a form of gravity feeding.

16. The preparation method according to claim 14, wherein The extruder is a reciprocating single-screw extruder or a single-screw extrusion granulator, where the extrusion temperature is 80 - 200 °C.

17. The preparation method according to claim 14, characterized in that, The thioester antioxidant is selected from one or two of dilauryl thiodipropionate and distearoyl thiodipropionate.

18. The preparation method according to claim 14, characterized in that, In step b2), the hindered phenol antioxidants include 4,4'-thiobis(6-tert-butyl-3-methylphenol), pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], n-octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 2,2'-thiobis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]; the phosphate ester antioxidants include pentaerythritol bisphosphite, bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, and the molecular weight regulators include dodecyl mercaptan, 2,4-diphenyl-4-methyl-1-pentene, 2,4-dimethyl-1,3-pentadiene.

19. The preparation method according to claim 14, wherein, The organic peroxides include one or two of dicumyl peroxide, triallyl isocyanurate, bis(tert-butylperoxyisopropyl)benzene, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.

20. The preparation method according to claim 14, wherein, In step b3), the mixing is carried out by a low-temperature static adsorption method, and the temperature range is 55 - 75 °C.

21. The preparation method according to claim 20, characterized in that, In step b3), the mixing is carried out by a low-temperature static adsorption method, and the temperature range is 60 - 70 °C.

22. The preparation method according to claim 14, characterized in that, Based on the mass of low-density polyethylene being 100%, the addition amount of the auxiliary agent 1 is 0.05 wt% - 0.2 wt%, the addition amount of the auxiliary agent 2 is 0.15 - 0.3 wt%; the addition amount of the auxiliary agent 3 is 1.0 - 2.0 wt%.

23. The preparation method according to claim 22, characterized in that, Based on the mass of low-density polyethylene being 100%, the addition amount of the auxiliary agent 1 is 0.08 - 0.15 wt%, the addition amount of the auxiliary agent 2 is 0.18 - 0.25 wt%; the addition amount of the auxiliary agent 3 is 1.5 - 1.7 wt%.

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