Water-tree-resistant long-focal-length crosslinked polyethylene insulating material and its preparation method

Abstract

The application relates to a water-treeing-resistant long-focus-burning crosslinked polyethylene insulating material and a preparation method thereof. The insulating material is prepared from components including the following components in parts by weight: 100 parts of bimodal low-density polyethylene, 2-6 parts of an ethylene-butyl acrylate copolymer, 0.19-0.35 parts of a compounded antioxidant, 1.6-2.0 parts of a composite crosslinking agent and 0.2-1.2 parts of a water-treeing polarity inhibitor; the water-treeing polarity inhibitor is a vinyl trimethoxysilane grafted polyethylene glycol, and the composite crosslinking agent includes a main crosslinking agent, an auxiliary crosslinking agent and a vinyl siloxane grafted auxiliary agent. The preparation method includes preparing the water-treeing polarity inhibitor, mixing and granulating the water-treeing polarity inhibitor with the bimodal low-density polyethylene, the ethylene-butyl acrylate copolymer and the compounded antioxidant, and then adsorbing and absorbing the composite crosslinking agent. The obtained insulating material has the advantages of water-treeing resistance, low dielectric loss and long focus-burning processing window, and is suitable for the insulating layers of 66 kV and below medium-high voltage cables.

Description

Technical Field

[0001] This invention relates to the field of cable insulation materials technology, specifically to a water-tree resistant, long-burnt cross-linked polyethylene insulation material and its preparation method, and to the application of this insulation material in the insulation layer of medium and high voltage cables. Background Technology

[0002] Cross-linked polyethylene (XLPE) possesses high electrical insulation properties, low dielectric loss, good thermomechanical properties, and a mature extrusion cross-linking process, and is widely used in the insulation layers of medium and high voltage power cables. However, for submarine cables and medium-voltage cables operating in humid environments, moisture near insulation defects, impurities, or areas of concentrated local electric fields can induce water tree growth, thereby reducing the long-term dielectric strength of the insulation layer.

[0003] For submarine cables with voltage levels of 110 kV and above, the insulation system usually relies more on structures such as metal sheaths to achieve radial water blocking; while for medium and high voltage submarine cables of 66 kV and below, the insulation layer is thinner and the working field strength is not low. The material itself needs to meet the requirements of water tree resistance, low dielectric loss, mechanical properties and long-term stable extrusion capability at the same time.

[0004] Existing water-tree resistant cross-linked polyethylene insulation materials typically suppress water treeing by physically blending polyethylene glycol, polar ethylene copolymers, or other hydrophilic / polar additives. While this approach can improve initial water-tree resistance, the compatibility of hydrophilic small molecules with polyethylene is limited, posing a risk of migration or precipitation during long-term service. Furthermore, increasing the amount of peroxide cross-linking agent to improve cross-linking efficiency shortens the scorch time, affecting the continuous extrusion stability of long-length cables.

[0005] Therefore, there is a need for a cross-linked polyethylene insulation material and its preparation method that can improve water tree stability and extend the scorch processing window while maintaining electrical insulation and mechanical properties. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a cross-linked polyethylene insulation material that can take into account water tree resistance, electrical insulation performance and long scorch processing performance in the application of medium and high voltage cable insulation layers, and reduce the risk of migration and precipitation of water tree resistance additives.

[0007] To solve the above-mentioned technical problems, the present invention provides a water-tree resistant long-burn crosslinked polyethylene insulation material, which is prepared from the following components in parts by weight: 100 parts of bimodal low-density polyethylene, 2-6 parts of ethylene-butyl acrylate copolymer, 0.19-0.35 parts of compound antioxidant, 1.6-2.0 parts of composite crosslinking agent, and 0.2-1.2 parts of water tree polarity inhibitor.

[0008] The density of the bimodal low-density polyethylene is 0.910-0.925 g / cm³.3 The melt flow index at 190℃ and 2.16 kg is 2-6 g / 10 min, with high molecular weight components accounting for 35%-45% and low molecular weight components accounting for 55%-65%. The proportions of high and low molecular weight components can be calculated based on the integral area of ​​the gel permeation chromatography curve. High molecular weight components are beneficial for improving resistance to mechanical stress and creep, while low molecular weight components are beneficial for improving melt processing and additive dispersion.

[0009] The butyl acrylate structural unit in the ethylene-butyl acrylate copolymer is preferably 8%-20% by mass, and the addition amount is preferably 2-6 parts, more preferably 2-4 parts. The butyl acrylate structural unit can improve the dispersion and binding ability of polar microregions for moisture; however, if its content or addition amount is too high, it will lead to increased dielectric loss or decreased breakdown strength, so it needs to be controlled within the above range.

[0010] The compound antioxidant comprises sulfur-containing antioxidants and hindered phenolic antioxidants. The sulfur-containing antioxidant can be selected from one or both of antioxidant 300 and antioxidant 736, and the hindered phenolic antioxidant can be selected from one or both of antioxidant 1010 and antioxidant 1076. The mass ratio of sulfur-containing antioxidant to hindered phenolic antioxidant is 5.3:1 to 1:2.5 to balance free radical control during peroxide processing, hydroperoxide decomposition, and long-term thermo-oxidative stability.

[0011] The composite crosslinking agent includes a primary crosslinking agent, a secondary crosslinking agent, and a vinylsiloxane grafting aid. The primary crosslinking agent may be selected from one or more of dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl peroxide (2-ethylhexanoate), bis(tert-butylisopropyl) peroxide, and dodecanoyl peroxide, and preferably includes 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane; the secondary crosslinking agent may be selected from one or more of triallyl isocyanurate, N,N'-m-phenylenebismaleimide, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, and propoxylated trimethylolpropane triacrylate; the vinylsiloxane grafting aid is preferably vinyltrimethoxysilane.

[0012] The water tree polar inhibitor is vinyltrimethoxysilane-grafted modified polyethylene glycol (PEG), wherein the PEG has a molecular weight of 600-2000 and a grafting rate of 8%-15%. By modifying PEG with silane grafting, the water-capturing and binding effect of the PEG hydrophilic segments can be maintained, while improving its binding stability with the polyethylene / EBA system and cross-linked network, thereby reducing the risk of migration or precipitation during long-term service.

[0013] The present invention also provides a method for preparing the above-mentioned insulating material, comprising: first preparing vinyltrimethoxysilane graft-modified polyethylene glycol; then mixing bimodal low-density polyethylene, ethylene-butyl acrylate copolymer, compound antioxidant and the water tree polar inhibitor and extruding and granulating to obtain an intermediate material; finally mixing the intermediate material with a composite crosslinking agent, and after adsorption and post-absorption, obtaining the insulating material.

[0014] Beneficial effects Compared with the prior art, the present invention has at least the following beneficial effects: First, by using bimodal low-density polyethylene and an appropriate amount of ethylene-butyl acrylate copolymer in synergy, the dispersion of additives and the ability to homogenize moisture in micro-regions are improved, while maintaining low dielectric loss and high breakdown strength.

[0015] Second, by grafting polyethylene glycol with vinyltrimethoxysilane, the fixation ability of anti-water-tree additives in the polyethylene system is improved, and the risk of polyethylene glycol migration or precipitation is reduced.

[0016] Third, by combining sulfur-containing antioxidants with hindered phenolic antioxidants, uncontrolled premature cross-linking caused by free radicals in the early stages of processing can be suppressed within a specified ratio range, while also taking into account thermo-oxidative stability.

[0017] Fourth, by combining the main crosslinking agent, the co-crosslinking agent, and the vinylsiloxane grafting aid, the amount of the main crosslinking agent can be controlled while maintaining the degree of crosslinking to meet the requirements for thermal elongation, thereby extending the scorching processing window. Attached Figure Description

[0018] Figure 1 This is a schematic diagram illustrating the preparation of vinyltrimethoxysilane grafted modified polyethylene glycol.

[0019] Figure 2 This is a schematic diagram comparing the Mooney viscosity-time curves of Example 1 with those of Control Sample 1 and Control Sample 2 at 140°C.

[0020] Figure 3 This is a schematic diagram comparing the Mooney viscosity-time curves of Example 1 with those of Control Sample 1 and Control Sample 2 at 150°C. Detailed Implementation

[0021] The present invention will be further described below with reference to the embodiments. Unless otherwise stated, "parts" in this specification refers to parts by weight; numerical ranges include endpoints; "EBA" refers to ethylene-butyl acrylate copolymer; "PEG" refers to polyethylene glycol; "bis25" refers to 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane; "A171" refers to vinyltrimethoxysilane; "TAIC" refers to triallyl isocyanurate; "DBTDL" refers to dibutyltin dilaurate.

[0022] In this manual, "scorch time t5" refers to the time taken by using a Mooney tester at a specified temperature with a large rotor from the start of the test until the sample viscosity drops to its minimum value and then rises by 5 Mooney values.

[0023] In this specification, "grafting rate" refers to the degree of grafting of vinyltrimethoxysilane onto PEG molecules, expressed as a percentage by mass. The grafting rate can be determined by ICP-OES of the mass fraction of silicon in the grafted sample after washing and vacuum drying, and expressed as G = (w / ( ...(w / (w / ((w / (w / ((w / ((w / ((w / ((w / ((w / ((w / ((w / (w / ((w / ((w / ((w / ((w / (( Si / M Si) ×M A171 Calculate the silane grafting rate by multiplying by 100%; where G is the grafting rate, and w Si M represents the mass fraction of silicon in the grafted sample after washing and vacuum drying. Si The molar mass of silicon atoms is 28.09 g / mol, and the molar mass of vinyltrimethoxysilane is 148.23 g / mol. In a group of grafted samples, the silane grafting rate was approximately 10% as determined by the above method.

[0024] raw material Table 1 Raw Material Information

[0025] Table 1 lists the main raw materials used in the examples and their functional assignments. All raw materials can be obtained from conventional industrial channels. The parameters in Table 1 are used to illustrate the selection of raw materials in the examples and do not constitute a limitation on the sources of similar alternative raw materials.

[0026] Preparation of water tree polarity inhibitors Polyethylene glycol with a molecular weight of 600-2000 was placed in a reaction vessel and heated to 70-90℃ under nitrogen protection. Vinyltrimethoxysilane, an organotin catalyst, and deionized water were added, with the mass ratio of vinyltrimethoxysilane to polyethylene glycol being 1:4 to 1:10. The mixture was stirred for 2-4 hours. After the reaction, unreacted monomers were removed by holding the mixture at 100-110℃ and a vacuum of 4 kPa for 2 hours. The mixture was then cooled and discharged to obtain vinyltrimethoxysilane-grafted modified PEG. A schematic diagram of this preparation process is shown below. Figure 1 .

[0027] In a preferred embodiment, the PEG is PEG1400, and the mass ratio of vinyltrimethoxysilane to PEG is 1:4 to 1:10; the amount of dibutyltin dilaurate is 0.1%-0.5% of the total mass of PEG and vinyltrimethoxysilane, and the amount of deionized water is 0.5%-2% of the total mass of PEG and vinyltrimethoxysilane; the grafting rate is controlled at 8%-15%, more preferably 10%-12%.

[0028] More preferably, for preparing a silane-grafted PEG with a grafting rate of approximately 10%, the composition is: 90 parts PEG1400, 15 parts vinyltrimethoxysilane, 1.1 parts deionized water, and 0.26 parts dibutyltin dilaurate; the mass ratio of vinyltrimethoxysilane to PEG in this formulation is 1:6. Following the above grafting rate determination method, the silane grafting rate of this group of grafted samples is approximately 10%.

[0029] Preparation of insulating materials Weigh out the bimodal low-density polyethylene, EBA, compound antioxidants, and water tree polar inhibitors according to the formula, and mix, extrude, and granulate them in a twin-screw extruder to obtain the intermediate material. The mixing temperature is 160-200℃, and the extruder speed is 200-400 r / min.

[0030] The obtained intermediate material was mixed with a composite crosslinking agent, and the crosslinking agent was adsorbed in an adsorption tank. The adsorption temperature was 60-80℃, and the adsorption time was 0.5 h. After adsorption, the particles entered an insulated silo and were further absorbed under insulated conditions for 15 h. After cooling and packaging, water-tree resistant long-burn crosslinked polyethylene insulation material was obtained.

[0031] Examples and Comparative Examples Table 2. Formulations of Examples and Comparative Examples

[0032] Table 2 illustrates the effects of changes in BA content, EBA dosage, grafted modified PEG dosage, PEG molecular weight, antioxidant dosage, and composite crosslinking agent composition on performance. Example 1 is the baseline formulation, and Examples 2-13 modify it by changing one or a few variables; Comparative Examples 1-7 are used to verify performance changes when key limitations are exceeded or missing.

[0033] Detection methods Table 3 Detection Methods

[0034] Table 3 lists the test criteria and key conditions for scorch performance, water tree performance, electrical performance, mechanical performance, thermal aging and thermal elongation performance, which are used to define the evaluation criteria for the performance results listed in Tables 4 to 7.

[0035] Performance Results Table 4 Comparison of scorch performance between Example 1 and Control Samples 1 and 2

[0036] The control samples 1 and 2 in Table 4 are cross-linked polyethylene insulation samples of the same type, and their scorch performance was evaluated under the same test conditions. Figure 2 and Figure 3The Mooney viscosity-time curves for Example 1 and Control Samples 1 and 2 are shown at 140℃ and 150℃, respectively. Figure 2 As can be seen, under 140℃ conditions, the Mooney viscosity of Example 1 did not show a rapid increase to reach the t5 criterion during the test period, while Control Samples 1 and 2 entered the viscosity recovery stage earlier. Figure 3 It can be seen that, under the condition of 150℃, the viscosity recovery of Example 1 is significantly lagging behind that of Control Sample 1 and Control Sample 2, which corresponds to the scorch time of 54.17 min in Table 4, indicating that the synergistic design of composite crosslinking agent and compound antioxidant is beneficial to extending the high-temperature processing window.

[0037] Table 5 Comparison of water tree performance between Example 1 and Control Samples 3 and 4

[0038] Table 6 Mechanical, thermal aging and thermal elongation properties of each sample

[0039] Table 7 Electrical properties, scorch properties, and water tree resistance of each sample

[0040] As shown in Table 4, the scorching time of Example 1 at 150℃ was 54.17 min, and it did not reach t5 within a 60-minute test period at 140℃, indicating that Example 1 has a longer scorching processing window. As shown in Table 5, the longitudinal length, transverse length, and tree lifting rate of the water tree in Example 1 were at similar levels to those of Control Samples 3 and 4.

[0041] As shown in Table 6, Examples 1-13 maintained a good overall balance in terms of tensile strength, elongation at break, thermal aging, and thermal extension. In Comparative Example 3, when the total amount of antioxidant was less than 0.19 parts, the rate of change in tensile strength under thermal aging increased. In Comparative Example 4, when the total amount of composite crosslinking agent was less than 1.6 parts, the elongation under thermal extension load and the permanent deformation rate after cooling were both higher than those of Example 1. In Comparative Example 5, when the total amount of composite crosslinking agent was more than 2.0 parts, the tensile strength and elongation at break decreased.

[0042] As shown in Table 7, when Comparative Example 1 used EBA with a BA content of 24%, the tree-setting rate decreased, but the breakdown strength dropped to 26 kV / mm, and the dielectric loss factor increased to 0.000654, indicating that excessive BA content in EBA reduces electrical insulation performance. In Comparative Example 2, when an equal amount of ungrafted modified PEG1400 was used, the tree-setting rate increased to 55.34%, indicating that grafted modified PEG plays an important role in resisting water treeing.

[0043] As can be seen from Table 7, when Comparative Example 6 did not add A171 and Comparative Example 7 did not add TAIC, the scorching time at 150°C was 23.17 min and 25.16 min, respectively, both lower than the 54.17 min of Example 1. This indicates that the composite crosslinking system composed of the main crosslinking agent, the co-crosslinking agent and the vinylsiloxane grafting agent is beneficial to prolong the high-temperature scorching time while satisfying the crosslinking performance.

[0044] Industrial applicability The insulating material described in this invention can be prepared by conventional cable material mixing, granulation, adsorption and extrusion crosslinking processes. The raw materials are industrially available, the process parameters are clear, and it can be used to prepare medium and high voltage cable insulation layers of 66 kV and below. It is also suitable for cables that have requirements for continuous extrusion length, water tree resistance and electrical insulation performance.

Claims

1. A water-tree resistant, long-burning cross-linked polyethylene insulating material, characterized in that, The insulating material is prepared from the following components in parts by weight: 100 parts of bimodal low-density polyethylene, 2-6 parts of ethylene-butyl acrylate copolymer, 0.19-0.35 parts of compounded antioxidant, 1.6-2.0 parts of composite crosslinking agent, and 0.2-1.2 parts of water tree polarity inhibitor; wherein the density of the bimodal low-density polyethylene is 0.910-0.925 g / cm³. 3 The melt flow index is 2-6 g / 10 min at 190℃ and 2.16 kg, with high molecular weight components accounting for 35%-45% and low molecular weight components accounting for 55%-65%. The compound antioxidant includes sulfur-containing antioxidants and hindered phenolic antioxidants. The composite crosslinking agent includes a main crosslinking agent, a co-crosslinking agent, and a vinylsiloxane grafting aid. The water tree polar inhibitor is vinyltrimethoxysilane grafted modified polyethylene glycol, with a molecular weight of 600-2000 and a grafting rate of 8%-15%.

2. The water-tree resistant, long-burnt cross-linked polyethylene insulation material according to claim 1, characterized in that, The butyl acrylate structural unit in the ethylene-butyl acrylate copolymer has a mass content of 8%-20%.

3. The water-tree resistant, long-burnt cross-linked polyethylene insulating material according to claim 1, characterized in that, The mass ratio of the sulfur-containing antioxidant to the hindered phenolic antioxidant is 5.3:1 to 1:2.5; the sulfur-containing antioxidant is selected from one or two of antioxidant 300 and antioxidant 736, and the hindered phenolic antioxidant is selected from one or two of antioxidant 1010 and antioxidant 1076.

4. The water-tree resistant, long-burning, cross-linked polyethylene insulation material according to claim 1, characterized in that, The primary crosslinking agent is selected from one or more of dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl peroxide (2-ethylhexanoate), bis(tert-butylisopropyl) peroxide, and dodecanoyl peroxide; the secondary crosslinking agent is selected from one or more of triallyl isocyanurate, N,N'-m-phenylenebismaleimide, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, and propoxylated trimethylolpropane triacrylate; the vinylsiloxane grafting aid is vinyltrimethoxysilane, and the primary crosslinking agent includes 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.

5. The water-tree resistant, long-burnt cross-linked polyethylene insulating material according to claim 1, characterized in that, The water tree polar inhibitor is prepared by reacting polyethylene glycol, vinyltrimethoxysilane, organotin catalyst, and deionized water under nitrogen protection; wherein the mass ratio of vinyltrimethoxysilane to polyethylene glycol is 1:4 to 1:10, the amount of organotin catalyst is 0.1%-0.5% of the total mass of polyethylene glycol and vinyltrimethoxysilane, and the amount of deionized water is 0.5%-2% of the total mass of polyethylene glycol and vinyltrimethoxysilane.

6. The water-tree resistant, long-burnt cross-linked polyethylene insulating material according to claim 1, characterized in that, In the composite crosslinking agent, the main crosslinking agent is 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, the co-crosslinking agent is triallyl isocyanurate, and the vinylsiloxane grafting aid is vinyltrimethoxysilane; based on 100 parts by weight of bimodal low-density polyethylene, the 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane is 0.8-1.2 parts, the triallyl isocyanurate is 0.3 parts, and the vinyltrimethoxysilane is 0.5 parts.

7. A method for preparing the water-tree resistant, long-burning cross-linked polyethylene insulating material according to any one of claims 1-6, characterized in that, The process includes the following steps: S1, Preparation of water tree polar inhibitor: Polyethylene glycol with a molecular weight of 600-2000 is placed in a reaction vessel, heated to 70-90℃ and protected with nitrogen gas. Vinyltrimethoxysilane, organotin catalyst, and deionized water are added, wherein the mass ratio of vinyltrimethoxysilane to polyethylene glycol is 1:4 to 1:10, the amount of organotin catalyst is 0.1%-0.5% of the total mass of polyethylene glycol and vinyltrimethoxysilane, and the amount of deionized water is 0.5%-2% of the total mass of polyethylene glycol and vinyltrimethoxysilane. The reaction is stirred for 2-4 hours. h. After the reaction is complete, unreacted monomers are removed under vacuum, and the material is cooled and discharged to obtain vinyltrimethoxysilane grafted modified polyethylene glycol; S2. Bimodal low-density polyethylene, ethylene-butyl acrylate copolymer, compound antioxidant and the water tree polar inhibitor are mixed and extruded into granules to obtain intermediate material; S3. The intermediate material is mixed with a composite crosslinking agent, and after adsorption and post-absorption, the water tree resistant long scorch crosslinked polyethylene insulation material is obtained.

8. The preparation method according to claim 7, characterized in that, In step S2, a twin-screw extruder is used for mixing, extrusion and granulation. The mixing temperature is 160-200℃ and the extruder speed is 200-400 r / min. In step S3, the adsorption is carried out in an adsorption tank at an adsorption temperature of 60-80℃ for 0.5 h. After adsorption, the particles are placed in an insulated silo for 15 h for post-absorption.

9. The application of the water-tree resistant, long-burning cross-linked polyethylene insulating material according to any one of claims 1-6 in the preparation of insulation layers for medium and high voltage cables of 66 kV and below, wherein the medium and high voltage cable is a submarine cable.

10. A cable comprising a conductor, a conductor shielding layer covering the conductor, an insulation layer, and an insulation shielding layer, characterized in that, The insulating layer is formed by extrusion and cross-linking of the water-tree resistant, long-burnt cross-linked polyethylene insulating material as described in any one of claims 1-6.

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