Optimization design method of crosslinked polyethylene formula with high crosslinking degree

Abstract

The invention relates to the technical field of cable insulating materials, and discloses an optimal design method of a crosslinked polyethylene formula with a high crosslinking degree. The preparation method comprises the following steps: firstly, carrying out molecular chain structure characterization on various low-density polyethylene suitable for the formula, then adjusting the dosage of the low-density polyethylene and a cross-linking agent in the formula, and characterizing the gel content and the cross-linking density of the obtained cross-linked polyethylene, so as to establish a gel content (Gel) empirical correlation formula and a cross-linking density (Mc) empirical correlation formula; therefore, the cross-linking agent formula meeting the design target value can be obtained according to the required design target value. The polyethylene crosslinking optimized formula composition provided by the invention has the advantages that the content of byproducts is reduced while the high crosslinking degree is maintained, the purity of crosslinked polyethylene is improved, and the treatment time of a degassing process is shortened.

Description

Technical Field

[0001] The present invention relates to the technical field of cable insulating materials, and discloses an optimization design method for a cross-linked polyethylene formulation with a high degree of cross-linking. Background Art

[0002] Cross-linked polyethylene materials are widely used in power transmission cables, communication cables, cross-linked polyethylene (PEX) pipes, building insulation materials, etc. In particular, cross-linked polyethylene cables are widely used in the field of power transmission due to their good electrical properties, chemical resistance, and heat resistance. However, with the continuous development of the power system, the performance requirements for cable insulating materials are gradually increasing, including higher voltage withstand levels, lower dielectric losses, and longer service lives. Low-density polyethylene (LDPE) has abundant long branches, short branches, and carbon-carbon double bonds, and the cross-linking speed is faster. Although LDPE provides more branching sites and carbon-carbon double bonds, during the cross-linking modification process, it still consumes more cross-linking agents, introducing more small molecule impurities into the LDPE matrix resin, which not only results in a higher content of by-products and a long degassing time, but also affects the electrical properties, mechanical properties, and other service properties of the LDPE cross-linked modified material. Therefore, it is of great significance to develop a cable material with a low addition amount of cross-linking agent and excellent DC breakdown voltage strength.

[0003] However, the relationship between the molecular chain structure and the cross-linking performance is complex and multi-factor coupled. Most studies only give the qualitative relationship of a single variable, lacking a quantitative model that can accurately describe the relationship between the molecular chain structure and the cross-linking performance. The precise theoretical guidance for the design of the molecular chain structure is still insufficient. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art, and provide a low-density polyethylene formulation with a high degree of cross-linking and its optimization design method. By optimizing the polyethylene structure design and the cross-linking formulation combination through the empirical correlation formulas of gel content and cross-linking density, the cross-linking efficiency is improved, the content of by-products is reduced, and thus the insulation performance of the cross-linked polyethylene is enhanced. The polyethylene cross-linking formulation optimized by this method can meet the requirements of the cross-linking performance in practical applications while minimizing the content of by-products and improving the insulation performance to the greatest extent.

[0005] The present invention provides an optimization design method for a cross-linked polyethylene formulation with a high degree of cross-linking. The cross-linked polyethylene formulation includes, by weight: 100 parts of low-density polyethylene, 0.1 - 1 part of antioxidant, and 0.5 - 3 parts of cross-linking agent;

[0006] The optimization method includes the following steps:

[0007] Step 1): Characterize the alternative low-density polyethylene molecular chain structures in the crosslinked polyethylene formulation to obtain the molecular weight and its distribution curve, degree of branching, vinyl and vinylidene concentrations of the low-density polyethylene;

[0008] Step 2): Prepare crosslinked polyethylene with different formulations by changing the low-density polyethylene in the crosslinked polyethylene formulation and changing the type and addition amount of the crosslinking agent, thereby changing the peroxy bond concentration, and characterize the gel content and crosslinking density of the obtained crosslinked polyethylene;

[0009] Step 3): Construct a correlation formula between the crosslinked polyethylene formulation combination and the gel content of the crosslinked polyethylene based on the data obtained in Steps 1) and 2), and construct a correlation formula between the crosslinked polyethylene formulation combination and the crosslinking density of the crosslinked polyethylene;

[0010] Step 4): Use the two correlation formulas obtained in Step 3) to optimize the design of the crosslinked polyethylene formulation according to the given design target values, and obtain a crosslinked polyethylene formulation that meets the design target values.

[0011] According to the preferred embodiment of the present invention, the optimization design described in Step 4) includes one or more of the following methods a-c:

[0012] a: According to the molecular chain structure data of the known low-density polyethylene and the target values of the gel content and crosslinking density in the crosslinked polyethylene formulation, obtain the minimum peroxy bond concentration that meets the target values, and convert it in combination with the type of crosslinking agent to obtain the addition amount of the crosslinking agent;

[0013] b: According to the molecular chain structure data of the known low-density polyethylene, obtain the change curves of the gel content and crosslinking density of the crosslinked polyethylene with the peroxy bond concentration;

[0014] c: According to the molecular weight distribution curve of the low-density polyethylene and the target values of the gel content and crosslinking density of the crosslinked polyethylene, optimize the degree of branching, vinyl and vinylidene concentrations of the low-density polyethylene to achieve the lowest minimum peroxy bond concentration, and convert it in combination with the type of crosslinking agent to obtain the lowest addition amount of the crosslinking agent.

[0015] Through the above technical solutions, the beneficial effects of the present invention are as follows: Optimize the molecular chain structure of polyethylene and the DCP crosslinking formulation process for the gel content and crosslinking density that meet the actual use requirements, improve the crosslinking efficiency, and reduce the by-product content. Description of the Drawings

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the accompanying drawings required for the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and thus should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0017] Figure 1 It is a schematic diagram of the modeling process of the gel content model;

[0018] Figure 2 It is the process of optimizing the DCP addition amount for a specific molecular structure;

[0019] Figure 3 They are the predicted values of gel content and crosslink density for a specific molecular structure at different DCP addition amounts. Detailed implementation manners

[0020] The following further elaborates and explains the present invention in combination with the detailed implementation manners. The embodiments are only demonstrations of the disclosed content and do not delimit the scope. Without conflict, the technical features of each embodiment in the present invention can be combined accordingly.

[0021] In this disclosure, the endpoints and any values of the disclosed ranges are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values and individual point values of each range, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. The following further elaborates on the present invention in combination with embodiments, but the present invention is not limited thereto. For those of ordinary skill in the art, without departing from the principles of the present invention, several improvements and refinements can be made, and these improvements and refinements are also regarded as within the protection scope of the present invention. The content not described in detail in this specification belongs to the prior art well-known to those skilled in the art.

[0022] The embodiment of the present invention is an optimization design method for a highly crosslinked crosslinked polyethylene formulation. Among them, the crosslinked polyethylene formulation includes, by weight: 100 parts of low-density polyethylene, 0.1-1 part of antioxidant, and 0.5-3 parts of crosslinking agent. The purpose of the optimization design method in the embodiment is to obtain a specific crosslinked polyethylene formulation that meets the target design values from the above formulation. In a typical application scenario, the target design values can be the gel content and crosslinking density of the crosslinked polyethylene. On the premise of knowing the molecular chain structure data (molecular weight distribution, degree of branching, vinyl and vinylidene concentrations) of the low-density polyethylene selected in the known formulation, the minimum amount of crosslinking agent added can be obtained through the method of the present invention. After obtaining the amount of crosslinking agent added, the specific crosslinked polyethylene formulation can be obtained.

[0023] Among them, the crosslinking agent referred to in the present invention is mainly one or a mixture of organic peroxides with a boiling point at least 50°C higher than that of low-density polyethylene, preferably one or a mixture of dicumyl peroxide, di-tert-butyl peroxyisopropylbenzene, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane. The antioxidant is one or a mixture of 4,4'-thiobis(6-tert-butyl-o-cresol), 4,4'-thiobis(6-tert-butyl-m-cresol), 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.

[0024] It should be noted that within the formulation range of the present invention, the type and amount of the antioxidant have little effect on the gel content and crosslinking density of the crosslinked polyethylene. Therefore, it is considered that the type and amount of the antioxidant are not parameters that need to be optimized. After obtaining the formulation, crosslinked polyethylene is prepared according to the preset crosslinking process parameters. Among them, the crosslinking process is a common means in the art, and the present invention does not make specific limitations on it. The same crosslinking process parameters are selected in the embodiments and methods of the present invention, that is, the crosslinking process parameters are determined and are not optimization items.

[0025] The optimization method for crosslinked polyethylene with a high degree of crosslinking provided by this embodiment includes the following steps:

[0026] Step 1): Characterize the molecular chain structure of the alternative low-density polyethylene in the crosslinked polyethylene formulation to obtain the molecular weight and its distribution curve, degree of branching, vinyl and vinylidene concentrations of the low-density polyethylene;

[0027] Step 2): By changing the low-density polyethylene in the crosslinked polyethylene formulation and changing the type and amount of the crosslinking agent to change the peroxy bond concentration, prepare crosslinked polyethylene with different formulations, and characterize the gel content and crosslinking density of the obtained crosslinked polyethylene;

[0028] Step 3): Establish the correlation equations between the cross-linked polyethylene formulation combinations and the gel content of cross-linked polyethylene, and between the cross-linked polyethylene formulation combinations and the cross-linking density of cross-linked polyethylene based on the data obtained in Steps 1) and 2).

[0029] The correlation equation between the cross-linked polyethylene formulation combinations and the gel content of cross-linked polyethylene is:

[0030]

[0031] Ni gel = min(Ni, N RO· )(1 - 3)

[0032]

[0033] The amount of cross-linking agent to be added for the required polyethylene structure satisfies the constraint conditions of Equations 1-1 to 1-4. Among them, the gel content in Equation (2-4) being greater than or equal to 80% is only an example, and this target value can be set according to actual application requirements. For example, it can be greater than 60%, 70%, 80% or 90%.

[0034] Where, Gel is the gel content, m gel is the total weight of the molecular chains participating in the cross-linked network, m total is the total weight of the molecular weight distribution, Mi is a certain molecular weight on the molecular weight distribution curve of low-density polyethylene (such as Figure 1 the area ① surrounded by the black curve and Ni = 0), Ni is the amount of substance corresponding to the component with molecular weight Mi on the polyethylene molecular weight distribution curve; Mi gel is a certain molecular weight of the component constituting the gel (such as Figure 1 the blue area ② shown), Ni gel is the amount of substance corresponding to the component with molecular weight Mi gel that constitutes the gel component;

[0035] N RO· is the number of free radicals ( Figure 1 the blue line segment), C peroxy is the addition concentration of peroxy groups (wt%), Pi is the proportion of the total weight of the molecular chains with a certain molecular weight Mi in the molecular weight distribution of low-density polyethylene to the total weight of the molecular weight distribution, C vinyl is the vinyl concentration (1000C), f is the initiator efficiency factor with a value range of (0.3 - 0.5), preferably 0.31, Q is the parameter to be fitted, with a value range of 0.3 - 0.5, and is obtained by substituting the data obtained in Steps 1) and 2) into the correlation equation for fitting.

[0036] The correlation equation between the cross-linked polyethylene formulation combinations and the cross-linking density of cross-linked polyethylene is:

[0037]

[0038] where Gel is the gel content, C peroxy is the concentration of peroxide group addition (wt%), C CH is the degree of branching, C vinyl is the vinyl concentration, C vinylidene is the vinylidene concentration; f, m, s, q, r, e are parameters, which are obtained by fitting the data obtained in steps 1) and 2) into the correlation formula.

[0039] Step 4): Using the two correlation formulas obtained in step 3), according to the given design target values, optimize the crosslinked polyethylene formula to obtain a crosslinked polyethylene formula that meets the design target values, where the optimization design includes one or more of the following methods a - c:

[0040] a: According to the molecular chain structure data of known low - density polyethylene and the target values of gel content and crosslinking density in the crosslinked polyethylene formula, obtain the minimum peroxide bond concentration that meets the target values, and convert it in combination with the type of crosslinking agent to obtain the crosslinking agent addition amount;

[0041] b: According to the molecular chain structure data of known low - density polyethylene, obtain the curves of gel content and crosslinking density of crosslinked polyethylene changing with peroxide bond concentration;

[0042] c: According to the molecular weight distribution curve of low - density polyethylene and the target values of gel content and crosslinking density of crosslinked polyethylene, optimize the degree of branching, vinyl and vinylidene concentrations of low - density polyethylene to achieve the lowest minimum peroxide bond concentration, and convert it in combination with the type of crosslinking agent to obtain the lowest crosslinking agent addition amount.

[0043] In this example, a molecular chain structure database of low - density polyethylene is constructed, and the database contains a variety of low - density polyethylene products with different grades. These low - density polyethylenes are all applicable to the crosslinked polyethylene formula described above in the present invention. The density of these low - density polyethylene resins is 0.910 - 0.929 g / cm 3 , when tested according to the ASTM D1238 - 98 standard method, with a standard test load of 2.16 kg and a temperature of 190 °C, the melt flow index is 0.3 - 10 g / 10 min.

[0044] Furthermore, these low - density polyethylenes also have the following characteristics: the density is 0.915 - 0.925 g / cm 3, the number-average molecular weight is 14,000 - 30,000, preferably 18,000 - 22,000; the molecular weight distribution (PDI) is 3.0 - 8.0, preferably 3.0 - 6.0; the degree of branching is 8 - 25 / 1000C, preferably 8 - 15 / 1000C; the vinyl content is 0.05 - 0.6 / 1000C, and the vinylidene content is 0.2 - 0.6 / 1000C.

[0045] In the present invention, existing characterization means are used to characterize the molecular weight distribution, degree of branching, vinyl and vinylidene concentrations of these low-density polyethylenes, and a molecular chain structure database of low-density polyethylene is constructed.

[0046] Furthermore, the present invention combines these characterized low-density polyethylenes with antioxidants and crosslinking agents with different addition amounts, and crosslinks them to obtain various crosslinked polyethylenes. The gel content (Gel) and crosslink density (Mc) of the crosslinked polyethylenes are characterized, and a database of the gel content (Gel) and crosslink density of crosslinked polyethylene is constructed.

[0047] In the examples of the present invention, the following performance test methods are used for characterization:

[0048] (1) Preparation of specimens

[0049] Mixing: Weigh dry polyethylene particles and DCP according to the ratio, put them into the flask of a rotary evaporator, set the temperature to 80 °C, and heat by rotary evaporation for 8 h to make the molten DCP uniformly distributed inside the polyethylene particles by diffusion. Place it in an oven at 80 °C for heat preservation for 4 h.

[0050] Crosslinking: Preheat a hot press (GT-7014-A50C) to 140 °C. Place the polyethylene particles mixed with DCP in a mold, exhaust three times after preheating for 1 min, increase the pressure to 15 MPa, raise the temperature to 180 °C, and keep it for 30 min.

[0051] (2) Molecular chain structure characterization

[0052] 13 CNMR

[0053] Use a Bruker AV 400 nuclear magnetic resonance spectrometer from Bruker Corporation, Germany, to obtain the total amount of long and short branches of the sample. When preparing the sample, first weigh about 80 mg of the sample and dissolve it in a deuterated o-dichlorobenzene solvent at 130 °C for 48 h. When measuring, set the measurement temperature to 125 °C, the number of scans to 5000 times, and the delay time to 8 s. Calculate the LCB / 1000 of LCBs (equal to or longer than C6) and the SCB / 1000C of SCBs (shorter than C6), and the total degree of branching (1000C) is represented by TCB.

[0054] GPC

[0055] Characterization was carried out using a PL-GPC 220 high-temperature gel permeation chromatograph (HT-GPC, Polymer Laboratories Ltd). The test samples were prepared by a PL-SP 260 high-temperature sample preparation system (PL Ltd). The sample concentration was approximately 2.0 mg / mL, dissolved at 150 °C for 4 h, and filtered for testing after that. The test temperature was 150 °C, the injection volume was 200 μL, the flow rate was 1.0 mL / min, and the mobile phase was a mixture of 1,2,4-trichlorobenzene (TCB) and 0.05 wt% 2,6-di-tert-butyl-p-cresol (BHT). A set of narrow-distribution polystyrene standards was used as the calibration curve to calculate the relative molecular mass and relative molecular mass distribution.

[0056] FT-IR

[0057] The polyethylene particles were pressed into films at a hot press temperature of 140 °C and a pressure of 15 MPa, and characterized using a Fourier transform infrared spectrometer (Nicolet iS50, USA). The number of scans was 32 times, and the resolution was 4 cm -1 , and KBr powder was used to collect the background spectrum.

[0058] The procedure for determining the amount of vinyl (CH2=CH2-CH2-) and vinylidene (CH2=CH-R1R2) per 1000 carbon atoms is based on the ASTM D3124-72 method. The extinction coefficients of vinyl and vinylidene were determined by the procedure described in Part 9 of ASTM-D 3124.

[0059] A baseline was made from 980 cm -1 to approximately 840 cm -1 , and the peak height corresponding to vinyl was measured at approximately 910 cm -1 . The amount of vinyl per 1000 carbon atoms was calculated using the following formula (ASTM D3124-72):

[0060] Vinyl per 1000 carbon atoms = (14 × A) / (13.13 × L × D), where A is the absorbance (peak height), L is the film thickness (mm), and D is the material density (g / cm 3 ).

[0061] A baseline was made from 980 cm -1 to approximately 840 cm -1 , and the peak height corresponding to vinylidene was measured at approximately 888 cm -1 . The amount of vinylidene per 1000 carbon atoms was calculated using the following formula (ASTM D3124-72):

[0062] Vinylidene / 1000 carbon atoms = (14 × A) / (18.24 × L × D), where A is the absorbance (peak height), L is the film thickness (mm), and D is the material density (g / cm 3 ).

[0063] (3) Gel content

[0064] Weigh the dried and crosslinked sample and wrap it in a pre-weighed copper mesh. Record the sample mass as m1. Place the copper mesh with the sample in a flask, add the solvent according to the mass ratio of xylene to sample of 200:1, keep the xylene boiling, and heat continuously for 12 h. Finally, take out the sample and heat it in an oven at 80 °C for 8 h, weigh it, and record the mass as m2. Calculate the gel content according to the following formula:

[0065]

[0066] (4) Crosslink density

[0067] Wrap the dried sample after gel content test in a copper mesh, then place it in a round-bottom flask with boiling xylene to swell for at least 4 h to reach equilibrium; then quickly take out the swollen gel from the copper mesh and put it into a sealed weighing bottle to weigh to get m3. Then dry the sample in vacuum to get its mass as m4. The mass M of the solvent for gel swelling is calculated by the following relationship:

[0068] M = m3 - m4 (1 - 7)

[0069] Then calculate Mc according to the Flory - Rehner formula:

[0070]

[0071] In the formula, Vr is the volume fraction of polymer in the swollen gel, which can be obtained from the following formula:

[0072]

[0073] Among them, ρ p is the density of the polymer, ρ p = 0.806 g / cm 3 ; ρs is the density of the solvent, ρ s = 0.761 g / cm 3 ; χ is the Huggins parameter, χ = 0.31; V0 is the molar volume of xylene, V0 = 139.3 cm 3 .

[0074] Mc is the molecular weight between two uncorrected crosslinks. When the number-average molecular weight Mn is known, it is corrected to M through the following formula c * .

[0075]

[0076] The present invention uses the data in two constructed databases to fit the parameters to be fitted in Formulas 1-4 and 1-5, and the fitting results of each parameter are obtained as follows: Q = 0.45, e is taken as 3996.04, m is 9.3458, s is 0.7323, q is 0.0722, and r is 0.3901.

[0077] Example 1

[0078] This example aims to illustrate the steps of the optimization design method for the low-density polyethylene formula with high crosslinking degree of the present invention:

[0079] (1) Through the characterization of the molecular chain structure and crosslinking properties (gel content, crosslinking density), the fitting parameters of the empirical correlation formulas 1-4 and 1-5 between the crosslinking properties and the molecular chain structure are obtained. Q = 0.45, e is taken as 3996.04, m is 9.3458, s is 0.7323, q is 0.0722, and r is 0.3901.

[0080] (2) Taking the molecular weight distribution, branching degree, vinyl group content, and vinylidene group content of a known low-density polyethylene as input parameters, and using Formulas 1-1 to 1-5 for optimization, with the goals of gel content > 85% and crosslinking density < 6000, and optimizing the lowest peroxide bond concentration, and obtaining the lowest DCP addition amount through conversion (in this example, the crosslinking agent type is selected as dicumyl peroxide DCP). As Figure 2 and Figure 3 respectively show its process and results.

[0081] Example 2

[0082] This example aims to illustrate the steps of the optimization design method for the low-density polyethylene formula with high crosslinking degree of the present invention:

[0083] (1) Through the characterization of the molecular chain structure and crosslinking properties (gel content, crosslinking density), the fitting parameters of the empirical correlation formulas 1-4 and 1-5 between the crosslinking properties and the molecular chain structure are obtained. Q = 0.45, e is taken as 3996.04, m is 9.3458, s is 0.7323, q is 0.0722, and r is 0.3901.

[0084] (2) Taking the molecular weight distribution of low-density polyethylene as an input parameter, and using Formulas 1-1 to 1-5 for optimization, with the goals of gel content > 85% and crosslinking density < 5500, optimizing the molecular chain structure data under the condition of adding the lowest peroxide bond concentration, and converting the lowest peroxide bond concentration into the DCP addition amount. The final optimization results are shown in Table 1.

[0085] Table 1 Optimization of the addition amount of the lowest peroxy bond concentration with specific molecular weight distribution and results of molecular chains

[0086]

[0087] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent for the present invention. For those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention.

Claims

1. A design optimization method for a cross-linked polyethylene formulation with a high degree of cross-linking, characterized in that, The cross-linked polyethylene formulation includes, by weight: 100 parts of low-density polyethylene, 0.1 - 1 part of antioxidant, and 0.5 - 3 parts of cross-linking agent; The optimization method includes the following steps: Step 1): Characterize the molecular chain structure of the alternative low-density polyethylene in the cross-linked polyethylene formulation to obtain the molecular weight and its distribution curve, degree of branching, vinyl and vinylidene concentrations of the low-density polyethylene; Step 2): By changing the low-density polyethylene in the cross-linked polyethylene formulation and changing the type and addition amount of the cross-linking agent to change the concentration of peroxy bonds, prepare cross-linked polyethylene with different formulations, and characterize the gel content and cross-linking density of the obtained cross-linked polyethylene; Step 3): Based on the data obtained in Steps 1) and 2), establish the correlation equations between the cross-linked polyethylene formulation combination and the gel content of cross-linked polyethylene, and establish the correlation equations between the cross-linked polyethylene formulation combination and the cross-linking density of cross-linked polyethylene; Step 4): Using the two correlation equations obtained in Step 3), according to the given design target values, optimize the design of the cross-linked polyethylene formulation to obtain a cross-linked polyethylene formulation that meets the design target values.

2. The design optimization method of the cross-linked polyethylene formulation with a high degree of cross-linking according to claim 1, characterized in that The optimization design in Step 4) includes one or more of the following methods a - c: a: Based on the molecular chain structure data of the known low-density polyethylene and the target values of the gel content and cross-linking density in the cross-linked polyethylene formulation, obtain the minimum peroxy bond concentration that meets the target values, and convert it in combination with the type of cross-linking agent to obtain the addition amount of the cross-linking agent; b: Based on the molecular chain structure data of the known low-density polyethylene, obtain the change curves of the gel content and cross-linking density of the cross-linked polyethylene with the peroxy bond concentration; c: Based on the molecular weight distribution curve of the low-density polyethylene and the target values of the gel content and cross-linking density of the cross-linked polyethylene, optimize the degree of branching, vinyl and vinylidene concentrations of the low-density polyethylene to achieve the lowest minimum peroxy bond concentration, and convert it in combination with the type of cross-linking agent to obtain the lowest addition amount of the cross-linking agent.

3. The design optimization method of the cross-linked polyethylene formulation with a high degree of cross-linking according to claim 2, characterized in that, The molecular chain structure data includes the molecular weight and its distribution curve, degree of branching, vinyl and vinylidene concentrations of the low-density polyethylene.

4. The design optimization method of the cross-linked polyethylene formulation with a high degree of cross-linking according to claim 2, characterized in that, Method c is: Given the molecular weight distribution curve of the low-density polyethylene and the target values of the gel content and cross-linking density of the cross-linked polyethylene, by adjusting one or more of the degree of branching, vinyl and vinylidene concentrations of the low-density polyethylene, obtain the combination of the degree of branching, vinyl and vinylidene concentrations of the low-density polyethylene that minimizes the addition amount of the cross-linking agent, that is, obtain the molecular chain structure data of the low-density polyethylene that meets the conditions, and thus obtain the cross-linked polyethylene formulation.

5. The design optimization method of the cross-linked polyethylene formulation with a high degree of cross-linking according to claim 1, characterized in that The correlation equation between the cross-linked polyethylene formulation combination and the gel content of cross-linked polyethylene in Step 3) is: Among them, Gel is the gel content, m gel is the total weight of the molecular chains participating in the cross-linked network, m total is the total weight of the molecular weight distribution, Mi is a certain molecular weight on the molecular weight distribution curve of low-density polyethylene, and Ni is the amount of substance corresponding to the component with the molecular weight of Mi on the molecular weight distribution curve of polyethylene; Mi gel is a certain molecular weight constituting the gel component, Ni gel is the amount of substance corresponding to the component with the molecular weight of Mi gel constituting the gel component; Ni gel = min(Ni, N RO· ); N RO· is the number of free radicals; C peroxy is the addition concentration of peroxy groups, with the unit of wt%; Pi is the proportion of the total weight of molecular chains with a certain molecular weight Mi in the molecular weight distribution curve of low-density polyethylene to the total weight of the molecular weight distribution, C vinyl is the vinyl concentration, f is the crosslinking agent efficiency factor, the Q parameter ranges from 0.2 to 0.8, and is obtained by fitting the data obtained in steps 1) and 2) into the correlation formula.

6. The design optimization method of the cross-linked polyethylene formulation with a high degree of cross-linking according to claim 1, characterized in that, The correlation equation between the cross-linked polyethylene formulation combination and the cross-linking density of cross-linked polyethylene in Step 3) is: where Gel is the gel content; C peroxy is the addition concentration of peroxy groups, with the unit of wt%; C CH is the degree of branching, C vinyl is the vinyl concentration, C vinylidene is the vinylidene concentration; f, m, s, q, r, e are parameters, which are obtained by fitting the data obtained in steps 1) and 2) into the correlation formula.

7. The design optimization method of the cross-linked polyethylene formulation with a high degree of cross-linking according to claim 6, characterized in that, The parameter f ranges from 0.3 - 0.5, m ranges from 8 - 10, q ranges from 0.06 - 0.07, r ranges from 0.35 - 0.40, and e ranges from 3995 - 4000.

8. The design optimization method of the cross-linked polyethylene formulation with high cross-linking degree according to claim 1, characterized in that, In the cross-linked polyethylene formulation, the boiling point of the cross-linking agent is at least 50 °C higher than the melting point of the low-density polyethylene, preferably one or a mixture of diisopropylbenzene peroxide, di-tert-butyl peroxide cumene, 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, and the antioxidant is one or a mixture of 4,4'-thiobis(6-tert-butyl-o-cresol), 4,4'-thiobis(6-tert-butyl-m-cresol), 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.

9. The design optimization method of the cross-linked polyethylene formulation with high cross-linking degree according to claim 1, characterized in that In the crosslinked polyethylene formulation, the density of the low-density polyethylene resin is 0.910 - 0.929 g / cm 3 , as measured by the standard method of ASTM D1238-98, with a standard test load of 2.16 kg and a temperature of 190 °C, the melt flow index is 0.3 - 10 g / 10 min.

10. The design optimization method of the cross-linked polyethylene formulation with a high degree of cross-linking according to claim 1, characterized in that, In the cross-linked polyethylene formulation, the low-density polyethylene has the following molecular chain structure characteristics: the density is 0.915 - 0.925 g / cm 3 , the number-average molecular weight is 14,000 - 30,000, the molecular weight distribution is 3.0 - 8.0, the degree of branching is 8 - 25 / 1000C, the vinyl content is 0.05 - 0.6 / 1000C, and the vinylidene content is 0.2 - 0.6 / 1000C.

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