A method for manufacturing a conductor shield layer for a crosslinked polyethylene insulated cable

Abstract

The application discloses a preparation method of a conductor shielding layer for a crosslinked polyethylene insulated cable, which sequentially comprises the steps of conductive primer preparation, conductor activation and immersion coating to form an anchoring layer, electrophoretic deposition of a composite conductive film, cleaning and drying, integrated hot pressing and performance detection. The primer is compounded by epoxy resin, a dicyandiamide curing system, gamma-glycidoxypropyltrimethoxysilane and 16-28 parts of conductive carbon black. The anchoring layer is formed by low-temperature semi-curing to fill the conductor twisting gaps, and then a cationic water-based polyurethane-nano carbon black electrophoretic film is formed, and the two-layer structure is integrally cured and formed by controlled hot pressing. Through the cooperation of raw material compounding and step-by-step process, the conductor gap air gap is fully eliminated, and the conductivity and flexibility of the shielding layer are considered. It is proved by experiments that the shielding layer has low resistivity, uniform surface and excellent heat resistance, solves the defects of insufficient filling of traditional co-extrusion and easy cracking of single coating, and is suitable for 3.6kV-35kV crosslinked polyethylene cable production.

Description

Technical Field

[0001] This invention relates to the field of wire and cable manufacturing technology, and in particular to a method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables. Background Technology

[0002] The conductor shield of a cross-linked polyethylene (XLPE) insulated cable is a semi-conductive layer covering the outer surface of the metallic conductor. Its core functions are to uniformly distribute the electric field, eliminate partial discharge in the conductor strand gaps, and serve as a transition interface between the conductor and the insulation layer. The quality of the conductor shield directly affects the partial discharge level and long-term operational reliability of the cable.

[0003] Currently, the industrial production of cable conductor shielding layers mainly employs a double-layer co-extrusion process. This involves simultaneously extruding semi-conductive shielding material and insulating material through a double-layer die, coating the conductor surface, and then performing a cross-linking treatment together. The semi-conductive shielding material used in this process typically uses ethylene-vinyl acetate copolymer or ethylene-ethyl acrylate copolymer as the base resin, with a large amount of conductive carbon black added to achieve semi-conductivity. However, the double-layer co-extrusion process has the following problems: First, the melt's penetration into the conductor strand gaps during extrusion is limited, making it difficult to completely fill the pits and gaps on the conductor surface, easily leaving air gaps that can trigger discharge under the influence of an electric field. Second, to improve the conductivity of the semi-conductive layer, a large amount of carbon black is required, leading to poor material flowability, a rough extruded surface, and insufficient smoothness at the interface between the shielding and insulating layers, which actually increases the risk of partial discharge. Third, the extrusion process requires a dedicated double-layer co-extrusion unit, resulting in large equipment investment, complex process control, and high extrusion temperatures, limiting its adaptability to heat-sensitive conductor materials.

[0004] To address the issue of insufficient extrusion penetration, some studies have explored coating methods. For example, conductive coatings or adhesives can be directly applied to the conductor surface to form a shielding layer. One known method involves impregnating the conductor with an epoxy resin coating containing conductive carbon black, followed by curing to form a conductive coating. This method can achieve penetration and filling of conductor gaps under normal pressure, and the equipment is simple. However, single epoxy resin coatings suffer from high brittleness and insufficient flexibility, easily cracking when the cable is bent, and the surface smoothness of the coating is difficult to control precisely. Other literature reports a technique for preparing conductive thin films using electrophoretic deposition, which utilizes charged particles to migrate directionally under an electric field and deposit on the conductor surface. This method can obtain films with uniform thickness and smooth surfaces, but the thickness of a single electrophoretic deposited film is typically only in the micrometer range, making it difficult to achieve the thickness requirement of over 0.1 mm for cable conductor shielding layers. Furthermore, the adhesion between the deposited film and the metal conductor is weak, making it prone to peeling during subsequent processing and use.

[0005] In addition, existing technologies often employ passivation processes to improve the corrosion resistance of aluminum conductors. However, the passivation film itself is a poor conductor of electricity, which increases the contact resistance between the conductor and the shielding layer, contradicting the requirement for low contact resistance in the shielding layer.

[0006] In summary, existing technologies suffer from several problems, including insufficient filling of stranded gaps by the conductor shielding layer, difficulty in balancing shielding layer flexibility and conductivity, insufficient thickness and poor adhesion of the electrophoretic deposited film, and a mismatch between conductor pretreatment and the conductivity requirements of the shielding layer. Therefore, a method for preparing a conductor shielding layer that can adequately fill the stranded gaps while ensuring shielding layer flexibility and interface smoothness, and which is relatively simple in process, is needed.

[0007] The above background information is provided only to aid in understanding the inventive concept and technical solution of this invention. It does not necessarily belong to the prior art of this patent application. In the absence of clear evidence that the above information was disclosed on the filing date of this patent application, the above background information should not be used to evaluate the novelty and inventiveness of this application. Summary of the Invention

[0008] The main objective of this invention is to propose a method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables, in order to solve the technical problems existing in the prior art, such as insufficient filling of conductor stranding gaps, difficulty in balancing the flexibility and conductivity of the shielding layer, insufficient adhesion of electrophoretic deposition film, and mismatch between conductor pretreatment and shielding conductivity requirements.

[0009] To achieve the above technical objectives, the present invention adopts the following technical solution:

[0010] A method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables includes the following steps:

[0011] Step 1, Preparation of conductive primer: Prepare conductive primer; the conductive primer is composed of epoxy resin, latent curing agent, conductive carbon black, silane coupling agent and organic solvent; the latent curing agent is dicyandiamide or its derivative; the conductive primer is stable below 80℃ and cured above 120℃, and the volume resistivity after curing is not greater than 100Ω·m;

[0012] Step 2, Dipping and Anchoring Layer Formation: The stranded metal conductor is immersed in the conductive primer for 1-5 minutes, then removed and heat-treated at 60-80℃ for 30-60 minutes until the epoxy resin reaches the B-stage semi-cured state, forming a conductive anchoring layer on the conductor surface and in the stranding gaps; the conductive anchoring layer does not swell or fall off in the electrophoretic deposition solution of Step 3;

[0013] Step 3, Electrophoretic Deposition: The conductor obtained in Step 2 is used as the cathode and immersed in the electrophoretic deposition solution. A stainless steel plate is used as the anode, and a DC voltage is applied to perform electrophoretic deposition, depositing a nano-conductive carbon black / cationic aqueous polyurethane composite deposition film on the surface of the conductive anchoring layer. The DC voltage for electrophoretic deposition is 30-50V, the deposition time is 2-4min, the deposition solution temperature is 25-35℃, and the thickness of the resulting composite deposition film is 15-25μm.

[0014] Step 4, cleaning and drying: Take out the conductor obtained in step 3, clean it with deionized water, and dry it at 80-100℃ for 30-60 minutes;

[0015] Step 5, Hot-pressing integration: The conductor obtained in Step 4 is placed in a hot-pressing mold and hot-pressed at 130-150℃ and 0.5-2MPa for 5-15 minutes; the cationic waterborne polyurethane in the composite deposited film softens and flows at 130-150℃, filling the surface pores; the latent curing agent in the conductive anchoring layer is activated above 130℃, the epoxy resin completes curing, and the anchoring layer and the deposited film layer are integrated into an integrated conductor shielding layer with a total thickness of 0.1-0.3mm;

[0016] Step 6, Online Testing: The conductor with the conductor shielding layer obtained in Step 5 is passed through an eddy current flaw detector and an online resistivity testing device to test the uniformity, continuity and volume resistivity of the shielding layer. The volume resistivity is not greater than 100 Ω·m.

[0017] Preferably, the components of the conductive primer in step one, by weight, are: 100 parts epoxy resin, 5-10 parts latent curing agent dicyandiamide, 0.5-2 parts accelerator, 16-28 parts conductive carbon black, 2-5 parts silane coupling agent, and 50-80 parts organic solvent; the accelerator is a urea accelerator, which sets the curing temperature of dicyandiamide to 130°C; the organic solvent is a mixture of acetone and xylene in a volume ratio of 1:1, in which both epoxy resin and dicyandiamide are dissolved.

[0018] Preferably, the silane coupling agent in step one is γ-glycidoxypropyltrimethoxysilane, wherein the epoxy group of the γ-glycidoxypropyltrimethoxysilane participates in the curing reaction of the epoxy resin, and the siloxane alkyl group condenses with the hydroxyl group on the surface of the metal conductor.

[0019] Preferably, when the metal conductor in step two is an aluminum stranded conductor, a surface activation treatment is performed before immersion in the conductive primer: the aluminum conductor is immersed in a 10% (w / w) mixed acid of phosphoric acid and nitric acid for 20-30 seconds, then rinsed with deionized water and dried with cold air;

[0020] Preferably, the volume ratio of phosphoric acid to nitric acid in the phosphoric acid-nitric acid mixture is 3:1; when the metal conductor is a copper stranded conductor, a surface activation treatment is performed before immersion in the conductive primer: the copper conductor is immersed in 5% dilute sulfuric acid for 15-30 seconds, then rinsed with deionized water and dried with cold air.

[0021] Preferably, the criterion for judging the semi-cured state in stage B in step two is: when the semi-cured conductive anchoring layer is immersed in acetone for 30 minutes, the weight change rate is not greater than 2%.

[0022] Preferably, the electrophoretic deposition solution in step three is composed of deionized water, nano-conductive carbon black, cationic waterborne polyurethane, dispersant and pH adjuster, with a pH value of 4.0-6.0; the softening point of the cationic waterborne polyurethane is 110-130℃ and the glass transition temperature is -30-0℃.

[0023] Preferably, the electrophoretic deposition solution comprises, by weight: 1000 parts deionized water, 8-15 parts nano-conductive carbon black, 20-40 parts cationic waterborne polyurethane, and 1-3 parts dispersant; wherein the dispersant is polyvinylpyrrolidone, and the molecular weight of polyvinylpyrrolidone is 10000-40000.

[0024] Preferably, the inner cavity size of the hot pressing mold in step five is 0.2-0.6 mm larger than the outer diameter of the conductor; a water-soluble release agent polyvinyl alcohol solution is coated on the inner surface of the mold before hot pressing; and the surface of the conductor shielding layer is cleaned with deionized water after hot pressing.

[0025] Preferably, in step six, the eddy current flaw detector detects leaks or areas of uneven thickness in the shielding layer; the volume resistivity is measured online using a coaxial three-electrode system; and the conductor shielding layer shows no melting, flowing, or cracking after being placed at 180°C for 30 minutes.

[0026] The beneficial effects of this invention compared to the prior art include:

[0027] 1. The gaps in the conductor stranding are fully filled.

[0028] Existing double-layer co-extrusion processes rely on melt pressure to force the shielding material into the conductor gaps, but the high melt viscosity limits the penetration depth into the strand gaps. This invention uses a low-viscosity conductive primer to impregnate the conductor. Under capillary action, the primer spontaneously penetrates deep into the strand gaps, forming an anchoring layer after semi-curing, thus eliminating air gap residue at its source. Simultaneously, this anchoring layer is thermally integrated with the subsequent electrophoretic deposition film, avoiding the weak bonding between the coating and conductor interface that occurs when using dip coating alone.

[0029] II. Synergistic effect of flexibility and conductivity

[0030] While existing epoxy resin coatings offer adjustable conductivity, they are brittle after curing, posing a risk of cracking when cables are bent. This invention adds a composite deposition film of cationic waterborne polyurethane and nano-conductive carbon black outside the conductive primer anchoring layer. The inherent flexibility of polyurethane provides the shielding layer with excellent bending performance, while the uniformly dispersed nano-conductive carbon black in the deposition film provides a stable conductive path. After the anchoring layer and the deposition film are hot-pressed together, the shielding layer as a whole possesses both a balanced mechanical property of rigidity and toughness, and uniform semi-conductive properties.

[0031] III. Improved surface smoothness of the shielding layer

[0032] The surface roughness of the shielding layer obtained by extrusion is greatly affected by material flowability and processing parameters, and surface protrusions can easily lead to electric field concentration. This invention uses electrophoretic deposition to form a composite deposition film. The electrophoretic process is based on the electric field-driven directional migration and uniform spreading of charged particles, resulting in a film layer with uniform thickness and a smooth surface. Subsequent hot pressing further shapes the shielding layer, obtaining a smooth and regular outer surface, which is beneficial for improving the interface quality between the shielding layer and the insulating layer.

[0033] IV. Reduced thermal influence on conductors

[0034] Two-layer co-extrusion processes typically require melt extrusion at high temperatures, during which heat-sensitive conductor materials may oxidize or experience mechanical property degradation. The conductive primer of this invention cures at 60 to 80 degrees Celsius, and electrophoretic deposition is performed at room temperature to 35 degrees Celsius. Only the final hot-pressing step involves 130 to 150 degrees Celsius. The overall heat load is lower than that of extrusion processes, resulting in less impact on conductor performance, making it particularly suitable for processing temperature-sensitive aluminum conductors.

[0035] V. Matching Conductor Surface Treatment with Conductivity Requirements

[0036] In existing technologies, passivation treatment is often used for aluminum conductors to improve corrosion resistance. However, the passivation film is an electrically insulating layer, which increases the contact resistance between the conductor and the shielding layer. This invention employs acid pickling and activation treatment on the aluminum conductor to remove the surface oxide layer, exposing a highly conductive metal substrate. A conductive primer is then immediately applied, achieving a continuous connection between conductor surface activation and shielding layer coating. This avoids the formation of a high-resistance layer at the interface and ensures good electrical contact between the conductor and the shielding layer. Detailed Implementation

[0037] The present invention will be further described in detail below with reference to specific embodiments. It should be emphasized that the following description is merely exemplary and is not intended to limit the scope and application of the present invention.

[0038] A method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables includes the following steps:

[0039] Step 1: Preparation of conductive primer

[0040] A conductive primer is prepared, consisting of epoxy resin, latent curing agent, conductive carbon black, silane coupling agent and organic solvent, wherein the latent curing agent is dicyandiamide or its derivative; the conductive primer is stable below 80°C and cured above 120°C, and the volume resistivity after curing is not greater than 100 Ω·m.

[0041] The components are distributed as follows by weight: 100 parts epoxy resin, 5-10 parts latent curing agent dicyandiamide, 0.5-2 parts urea accelerator, 16-28 parts conductive carbon black, 2-5 parts silane coupling agent, and 50-80 parts organic solvent; the urea accelerator sets the curing temperature of dicyandiamide to 130°C; the organic solvent is a mixture of acetone and xylene in a 1:1 volume ratio, in which both epoxy resin and dicyandiamide are dissolved; the silane coupling agent is γ-glycidoxypropyltrimethoxysilane, whose epoxy groups participate in the curing reaction of epoxy resin, and whose siloxane groups condense with the hydroxyl groups on the surface of the metal conductor.

[0042] Step 2: Formation of the coating and anchoring layer

[0043] First, the stranded metal conductors undergo surface activation treatment:

[0044] Copper stranded conductor: Immerse in 5% dilute sulfuric acid for 15-30 seconds, rinse with deionized water and dry with cold air;

[0045] Aluminum stranded conductor: Immerse in a 10% (w / w) mixed acid of phosphoric acid and nitric acid for 20-30 seconds. The volume ratio of phosphoric acid to nitric acid in the mixed acid is 3:1. Rinse with deionized water and then dry with cold air.

[0046] The activated conductor is immersed in the conductive primer for 1-5 minutes, then removed and heat-treated at 60-80℃ for 30-60 minutes to allow the epoxy resin to reach the B-stage semi-cured state, forming a conductive anchoring layer on the conductor surface and in the stranded gaps. The conductive anchoring layer does not swell or fall off in the electrophoretic deposition solution. The criterion for judging the B-stage semi-cured state is: when the semi-cured conductive anchoring layer is immersed in acetone for 30 minutes, the weight change rate is not greater than 2%.

[0047] Step 3, Electrophoretic deposition

[0048] Preparation of the electrophoretic deposition solution: It consists of deionized water, nano-conductive carbon black, cationic waterborne polyurethane, dispersant, and pH adjuster, with the pH value adjusted to 4.0-6.0; the cationic waterborne polyurethane has a softening point of 110-130℃ and a glass transition temperature of -30-0℃; by weight, the ratio is 1000 parts deionized water, 8-15 parts nano-conductive carbon black, 20-40 parts cationic waterborne polyurethane, and 1-3 parts dispersant; the dispersant is polyvinylpyrrolidone with a molecular weight of 10000-40000.

[0049] Using the conductor obtained in step two as the cathode and the stainless steel plate as the anode, an electrophoretic deposition was performed by applying a DC voltage of 30-50V, with the deposition solution temperature at 25-35℃ and the deposition time at 2-4 minutes, to deposit a 15-25μm thick nano-conductive carbon black / cationic aqueous polyurethane composite deposition film on the surface of the conductive anchoring layer.

[0050] Step 4: Clean and dry

[0051] Remove the conductor after electrophoretic deposition, wash it with deionized water, and dry it at 80-100℃ for 30-60 minutes.

[0052] Step 5, Hot Press Integration

[0053] A hot-pressing mold with an inner cavity size 0.2-0.6 mm larger than the outer diameter of the conductor is used. Before hot pressing, a water-soluble release agent, polyvinyl alcohol solution, is coated on the inner surface of the mold. The dried conductor is placed in the mold and hot-pressed for 5-15 minutes at 130-150℃ and 0.5-2MPa. The cationic waterborne polyurethane in the composite deposition film softens and flows to fill the surface pores. The latent curing agent in the conductive anchoring layer is activated above 130℃, and the epoxy resin is completely cured. The anchoring layer and the deposition film are integrated into an integrated conductor shielding layer with a total thickness of 0.1-0.3 mm. After hot pressing, the surface of the conductor shielding layer is cleaned with deionized water.

[0054] Step Six: Online Testing

[0055] The conductor with the conductor shielding layer is passed through an eddy current flaw detector and an online resistivity testing device. The eddy current flaw detector detects leaks or uneven thickness areas in the shielding layer. The volume resistivity is measured online using a coaxial three-electrode system. The measurement method complies with GB / T3048.3-2007 "Electrical Performance Test Methods for Wires and Cables - Part 3: DC Resistance Test of Conductors", ensuring that the volume resistivity is not greater than 100 Ω·m. After the conductor shielding layer is placed at 180℃ for 30 minutes, there is no melting, flowing or cracking.

[0056] Technical principle of the invention:

[0057] This invention is a combination of conductive adhesive impregnation and anchoring, electrophoretic deposition film formation, and hot pressing integrated molding. By matching the functions of raw material components and coordinating the sequence of process steps, it achieves full filling of gaps in the conductor shielding layer, stable conductivity, strong interface bonding, and improved flexibility and surface smoothness. The overall technical effect is better than the simple superposition of individual technical means.

[0058] I. The Role of Each Raw Material Component

[0059] 1. Epoxy resin

[0060] As the main film-forming resin of the conductive primer, it has good wettability and adhesion, and can form a continuous resin matrix on the conductor surface and in the strand gaps, providing a stable dispersion carrier for the conductive filler. After curing, it provides structural strength and dimensional stability for the shielding layer, ensuring the integrity of the overall structure of the shielding layer.

[0061] 2. Latent curing agents: dicyandiamide and urea-based accelerators

[0062] Dicyandiamide remains chemically stable from room temperature to 80°C, preventing premature curing of the conductive primer during preparation, storage, and impregnation application, thus ensuring process operability. It is activated at temperatures above 130°C, undergoing a cross-linking and curing reaction with epoxy resin. With the addition of urea accelerators, the curing temperature can be stably controlled at 130°C, achieving a step-by-step reaction of semi-curing and anchoring followed by complete curing and molding, making it suitable for multi-step molding processes.

[0063] 3. Conductive carbon black

[0064] The conductive filler, used as a conductive primer, is uniformly dispersed in the epoxy resin matrix and overlaps to form a continuous and interconnected conductive network. This enables the conductive anchoring layer to reach a semi-conductive state, reduces the volume resistivity, achieves a uniform distribution of the electric field on the conductor surface, and suppresses the generation of partial discharge.

[0065] 4. γ-glycidoxypropyltrimethoxysilane

[0066] As a bifunctional coupling agent, the epoxy groups in the molecule can participate in the curing and cross-linking reaction of epoxy resin, thereby increasing the cross-linking density and mechanical strength of the conductive primer after curing. The siloxane groups in the molecule can undergo condensation reaction with the hydroxyl groups on the surface of copper and aluminum conductors to form a chemical bond, which significantly improves the interfacial adhesion between the conductive anchoring layer and the metal conductor, and avoids peeling and detachment during use.

[0067] 5. Acetone and xylene mixed solvent

[0068] When mixed at a volume ratio of 1:1, epoxy resin and dicyandiamide can be fully dissolved simultaneously, effectively reducing the viscosity of the conductive base adhesive system. This allows the adhesive to spontaneously penetrate deep into the stranded gaps of the conductors during the impregnation process through capillary action, achieving complete filling of the gaps, eliminating air gap residue, and improving the density of the shielding layer.

[0069] 6. Cationic waterborne polyurethane

[0070] As a film-forming substrate for electrophoretic deposition films, it has a suitable softening point and glass transition temperature. It can soften and flow during the hot pressing stage, filling the micropores on the surface of the electrophoretic deposition film and improving the surface smoothness of the shielding layer. It also has good flexibility, which can improve the problem of epoxy resin being too brittle after curing, making the shielding layer less prone to cracking when the cable is bent or stretched, thus improving the reliability of use.

[0071] 7. Nano-conductive carbon black

[0072] As a conductive component in electrophoretic deposition films, it has a small particle size and a large specific surface area, which can form a dense and uniform conductive path in the deposition film, and form a conductive connection with the underlying conductive anchoring layer, ensuring the overall conductivity uniformity of the shielding layer and avoiding local high resistivity.

[0073] 8. Polyvinylpyrrolidone

[0074] As a dispersant in the electrophoretic deposition solution, it encapsulates the nano-conductive carbon black particles through adsorption, reducing the tendency of particle aggregation and ensuring that the nano-conductive carbon black remains stably dispersed in the aqueous system for a long time. This ensures the stable progress of the electrophoretic deposition process and results in a uniform film thickness and a smooth surface.

[0075] II. Synergistic effects of each component and process step

[0076] 1. Internal synergy of the conductive adhesive system

[0077] The mixed solvent reduces the system viscosity, allowing for full penetration of the adhesive; the epoxy resin provides the foundation for film formation and adhesion; dicyandiamide and accelerators enable stepwise controlled curing; conductive carbon black provides conductivity; and the silane coupling agent achieves chemical bonding between the resin and the conductor. The components work synergistically to give the conductive primer low viscosity wetting properties, stepwise curing characteristics, stable conductivity, and high interfacial adhesion.

[0078] 2. Synergistic effect of conductor activation treatment and conductive anchoring layer

[0079] Copper conductors are activated with dilute sulfuric acid, and aluminum conductors are activated with a mixture of phosphoric acid and nitric acid. This removes the surface oxide layer and contaminants, increases the surface activity and surface energy of the conductors, provides more reaction sites for silane coupling agents, reduces the contact resistance between the conductor and the shielding layer, achieves stable electrical connection, and avoids excessive interface resistance from affecting the electric field distribution.

[0080] 3. Synergistic effect of impregnation semi-curing and electrophoretic deposition

[0081] The conductive base adhesive is impregnated and subjected to low-temperature heat treatment to form a semi-cured conductive anchoring layer. This layer does not swell or fall off in the electrophoretic deposition solution, providing a stable conductive substrate and adhesion interface for electrophoretic deposition. Electrophoretic deposition forms a nano-conductive carbon black / waterborne polyurethane composite film with uniform thickness and smooth surface under the action of an electric field, which makes up for the defects of insufficient flexibility and difficulty in controlling the surface smoothness of a single epoxy resin coating, so that the shielding layer has both a rigid anchoring structure and a flexible surface layer.

[0082] 4. Synergistic effect of hot pressing integration and complete curing

[0083] The hot-pressing temperature simultaneously satisfies the conditions for softening and flowing of cationic waterborne polyurethane and activation and curing by dicyandiamide. After the polyurethane softens, it fills the pores of the deposited film, improving the surface smoothness of the shielding layer. The epoxy resin of the anchoring layer is fully cured, so that the anchoring layer and the electrophoretic deposited film form an integrated structure without delamination or gaps, reducing the interlayer interface resistance and improving the overall structural stability and conductivity continuity of the shielding layer.

[0084] 5. Synergistic effect of process temperature and material properties

[0085] The impregnation, electrophoresis, and drying steps are all completed under medium and low temperature conditions, while only the hot pressing step uses a higher temperature to reduce the thermal damage to the conductor material caused by high temperature, which is especially suitable for the processing of heat-sensitive aluminum conductors; the curing temperature and softening temperature of each raw material are matched with the process temperature to achieve step-by-step molding and layer-by-layer strengthening, so that the final shielding layer simultaneously meets the requirements of thickness, volume resistivity, heat resistance, uniformity and continuity.

[0086] To make the present invention more fully disclosed, more specific embodiments are described below.

[0087] Example 1:

[0088] A method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables includes the following steps:

[0089] Step 1: Preparation of conductive primer

[0090] A conductive primer is prepared, consisting of epoxy resin, latent curing agent, conductive carbon black, silane coupling agent, and organic solvent, wherein the latent curing agent is dicyandiamide; the conductive primer is stable below 80°C and cured above 120°C, with a volume resistivity of no more than 100 Ω·m after curing.

[0091] The components are distributed as follows by weight: 100 parts epoxy resin, 5 parts latent curing agent dicyandiamide, 0.5 parts urea accelerator, 16 parts conductive carbon black, 2 parts silane coupling agent, and 50 parts organic solvent; the urea accelerator sets the curing temperature of dicyandiamide to 130°C; the organic solvent is a mixture of acetone and xylene in a 1:1 volume ratio, in which both epoxy resin and dicyandiamide are dissolved; the silane coupling agent is γ-glycidoxypropyltrimethoxysilane, whose epoxy groups participate in the curing reaction of epoxy resin, and whose siloxane groups condense with the hydroxyl groups on the surface of the metal conductor.

[0092] Step 2: Formation of the coating and anchoring layer

[0093] First, the stranded metal conductors undergo surface activation treatment:

[0094] Aluminum stranded conductor: Immerse in a 10% (w / w) mixed acid of phosphoric acid and nitric acid for 25 seconds. The volume ratio of phosphoric acid to nitric acid in the mixed acid is 3:1. Rinse with deionized water and then dry with cold air.

[0095] The activated conductor is immersed in conductive primer for 3 minutes, then removed and heat-treated at 60°C for 60 minutes to allow the epoxy resin to reach the B-stage semi-cured state, forming a conductive anchoring layer on the conductor surface and in the stranded gaps. The conductive anchoring layer does not swell or fall off in the electrophoretic deposition solution. The criterion for judging the B-stage semi-cured state is: when the semi-cured conductive anchoring layer is immersed in acetone for 30 minutes, the weight change rate is not greater than 2%.

[0096] Step 3, Electrophoretic deposition

[0097] Preparation of electrophoretic deposition solution: Composed of deionized water, nano-conductive carbon black, cationic waterborne polyurethane, dispersant and pH adjuster, with pH value adjusted to 4.0; the cationic waterborne polyurethane has a softening point of 110-130℃ and a glass transition temperature of -30-0℃; by weight, the ratio is 1000 parts deionized water, 8 parts nano-conductive carbon black, 20 parts cationic waterborne polyurethane, and 1 part dispersant; the dispersant is polyvinylpyrrolidone with a molecular weight of 10000.

[0098] Using the conductor obtained in step two as the cathode and the stainless steel plate as the anode, an electrophoretic deposition was performed by applying a DC voltage of 30V, with the deposition solution temperature at 25℃ and the deposition time at 4min. A 15μm thick nano-conductive carbon black / cationic aqueous polyurethane composite deposition film was deposited on the surface of the conductive anchoring layer.

[0099] Step 4: Clean and dry

[0100] The conductor after electrophoretic deposition was removed, washed with deionized water, and dried at 80°C for 60 min.

[0101] Step 5, Hot Press Integration

[0102] A hot-pressing mold with an inner cavity size 0.2 mm larger than the outer diameter of the conductor was used. Before hot pressing, a water-soluble release agent, polyvinyl alcohol solution, was coated on the inner surface of the mold. The dried conductor was placed in the mold and hot-pressed at 130°C and 2 MPa for 15 minutes. The cationic waterborne polyurethane in the composite deposition film softened and flowed to fill the surface pores. The latent curing agent in the conductive anchoring layer was activated above 130°C, and the epoxy resin was completely cured. The anchoring layer and the deposition film were integrated into an integrated conductor shielding layer with a total thickness of 0.1 mm. After hot pressing, the surface of the conductor shielding layer was cleaned with deionized water.

[0103] Step Six: Online Testing

[0104] The conductor with the conductor shielding layer was passed through an eddy current flaw detector and an online resistivity testing device. The eddy current flaw detector detected no leaks or uneven thickness areas in the shielding layer. The volume resistivity was measured online using a coaxial three-electrode system, and the measurement method complied with GB / T3048.3-2007 "Electrical Performance Test Methods for Wires and Cables - Part 3: DC Resistance Test of Conductors". The volume resistivity was 68 Ω·m. After the conductor shielding layer was placed at 180℃ for 30 minutes, there was no melting, flowing or cracking.

[0105] Example 2:

[0106] A method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables includes the following steps:

[0107] Step 1: Preparation of conductive primer

[0108] A conductive primer is prepared, consisting of epoxy resin, latent curing agent, conductive carbon black, silane coupling agent, and organic solvent, wherein the latent curing agent is dicyandiamide; the conductive primer is stable below 80°C and cured above 120°C, with a volume resistivity of no more than 100 Ω·m after curing.

[0109] The components are distributed as follows by weight: 100 parts epoxy resin, 6 parts latent curing agent dicyandiamide, 1 part urea accelerator, 20 parts conductive carbon black, 3 parts silane coupling agent, and 60 parts organic solvent; the urea accelerator sets the curing temperature of dicyandiamide to 130°C; the organic solvent is a mixture of acetone and xylene in a 1:1 volume ratio, in which both epoxy resin and dicyandiamide are dissolved; the silane coupling agent is γ-glycidoxypropyltrimethoxysilane, whose epoxy groups participate in the curing reaction of epoxy resin, and whose siloxane groups condense with the hydroxyl groups on the surface of the metal conductor.

[0110] Step 2: Formation of the coating and anchoring layer

[0111] First, the stranded metal conductors undergo surface activation treatment:

[0112] Copper stranded conductor: Immerse in 5% dilute sulfuric acid for 20 seconds, rinse with deionized water and dry with cold air.

[0113] The activated conductor is immersed in conductive primer for 2 minutes, then removed and heat-treated at 65°C for 50 minutes to allow the epoxy resin to reach the B-stage semi-cured state, forming a conductive anchoring layer on the conductor surface and in the stranded gaps. The conductive anchoring layer does not swell or fall off in the electrophoretic deposition solution. The criterion for judging the B-stage semi-cured state is: when the semi-cured conductive anchoring layer is immersed in acetone for 30 minutes, the weight change rate is not greater than 2%.

[0114] Step 3, Electrophoretic deposition

[0115] Preparation of electrophoretic deposition solution: Composed of deionized water, nano-conductive carbon black, cationic waterborne polyurethane, dispersant and pH adjuster, with pH value adjusted to 4.5; the cationic waterborne polyurethane has a softening point of 110-130℃ and a glass transition temperature of -30-0℃; by weight, the ratio is 1000 parts deionized water, 10 parts nano-conductive carbon black, 25 parts cationic waterborne polyurethane, and 1.5 parts dispersant; the dispersant is polyvinylpyrrolidone with a molecular weight of 20000.

[0116] Using the conductor obtained in step two as the cathode and the stainless steel plate as the anode, an electrophoretic deposition was performed with a DC voltage of 35V, a deposition solution temperature of 28℃, and a deposition time of 3.5min. A nano-conductive carbon black / cationic aqueous polyurethane composite deposition film with a thickness of 18μm was deposited on the surface of the conductive anchoring layer.

[0117] Step 4: Clean and dry

[0118] The conductor after electrophoretic deposition was removed, washed with deionized water, and dried at 85°C for 50 minutes.

[0119] Step 5, Hot Press Integration

[0120] A hot-pressing mold with an inner cavity size 0.3 mm larger than the outer diameter of the conductor was used. Before hot pressing, a water-soluble release agent, polyvinyl alcohol solution, was coated on the inner surface of the mold. The dried conductor was placed in the mold and hot-pressed at 135°C and 1.5 MPa for 12 minutes. The cationic waterborne polyurethane in the composite deposition film softened and flowed to fill the surface pores. The latent curing agent in the conductive anchoring layer was activated above 130°C, and the epoxy resin was completely cured. The anchoring layer and the deposition film were integrated into an integrated conductor shielding layer with a total thickness of 0.15 mm. After hot pressing, the surface of the conductor shielding layer was cleaned with deionized water.

[0121] Step Six: Online Testing

[0122] The conductor with the conductor shielding layer was passed through an eddy current flaw detector and an online resistivity testing device. The eddy current flaw detector detected no leaks or uneven thickness areas in the shielding layer. The volume resistivity was measured online using a coaxial three-electrode system, and the measurement method complied with GB / T3048.3-2007 "Electrical Performance Test Methods for Wires and Cables - Part 3: DC Resistance Test of Conductors". The volume resistivity was 52 Ω·m. After the conductor shielding layer was placed at 180℃ for 30 minutes, there was no melting, flowing or cracking.

[0123] Example 3:

[0124] A method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables includes the following steps:

[0125] Step 1: Preparation of conductive primer

[0126] A conductive primer is prepared, consisting of epoxy resin, latent curing agent, conductive carbon black, silane coupling agent, and organic solvent, wherein the latent curing agent is dicyandiamide; the conductive primer is stable below 80°C and cured above 120°C, with a volume resistivity of no more than 100 Ω·m after curing.

[0127] The components are distributed as follows by weight: 100 parts epoxy resin, 8 parts latent curing agent dicyandiamide, 1 part urea accelerator, 22 parts conductive carbon black, 3.5 parts silane coupling agent, and 65 parts organic solvent; the urea accelerator sets the curing temperature of dicyandiamide to 130°C; the organic solvent is a mixture of acetone and xylene in a 1:1 volume ratio, in which both epoxy resin and dicyandiamide are dissolved; the silane coupling agent is γ-glycidoxypropyltrimethoxysilane, whose epoxy groups participate in the curing reaction of epoxy resin, and whose siloxane groups condense with the hydroxyl groups on the surface of the metal conductor.

[0128] Step 2: Formation of the coating and anchoring layer

[0129] First, the stranded metal conductors undergo surface activation treatment:

[0130] Copper stranded conductor: Immerse in 5% dilute sulfuric acid for 20 seconds, rinse with deionized water and dry with cold air.

[0131] The activated conductor is immersed in conductive primer for 3 minutes, then removed and heat-treated at 70°C for 45 minutes to allow the epoxy resin to reach the B-stage semi-cured state, forming a conductive anchoring layer on the conductor surface and in the stranded gaps. The conductive anchoring layer does not swell or fall off in the electrophoretic deposition solution. The criterion for judging the B-stage semi-cured state is: when the semi-cured conductive anchoring layer is immersed in acetone for 30 minutes, the weight change rate is not greater than 2%.

[0132] Step 3, Electrophoretic deposition

[0133] Preparation of electrophoretic deposition solution: Composed of deionized water, nano-conductive carbon black, cationic waterborne polyurethane, dispersant and pH adjuster, with pH value adjusted to 5.0; the cationic waterborne polyurethane has a softening point of 110-130℃ and a glass transition temperature of -30-0℃; by weight, the ratio is 1000 parts deionized water, 12 parts nano-conductive carbon black, 30 parts cationic waterborne polyurethane, and 2 parts dispersant; the dispersant is polyvinylpyrrolidone with a molecular weight of 20000.

[0134] Using the conductor obtained in step two as the cathode and the stainless steel plate as the anode, an electrophoretic deposition was performed with a DC voltage of 40V, a deposition solution temperature of 30℃, and a deposition time of 3min. A nano-conductive carbon black / cationic aqueous polyurethane composite deposition film with a thickness of 20μm was deposited on the surface of the conductive anchoring layer.

[0135] Step 4: Clean and dry

[0136] The conductor after electrophoretic deposition was removed, washed with deionized water, and dried at 90°C for 45 minutes.

[0137] Step 5, Hot Press Integration

[0138] A hot-pressing mold with an inner cavity size 0.4 mm larger than the outer diameter of the conductor was used. Before hot pressing, a water-soluble release agent, polyvinyl alcohol solution, was coated on the inner surface of the mold. The dried conductor was placed in the mold and hot-pressed at 140°C and 1 MPa for 10 minutes. The cationic waterborne polyurethane in the composite deposition film softened and flowed to fill the surface pores. The latent curing agent in the conductive anchoring layer was activated above 130°C, and the epoxy resin was completely cured. The anchoring layer and the deposition film were integrated into an integrated conductor shielding layer with a total thickness of 0.2 mm. After hot pressing, the surface of the conductor shielding layer was cleaned with deionized water.

[0139] Step Six: Online Testing

[0140] The conductor with the conductor shielding layer was passed through an eddy current flaw detector and an online resistivity testing device. The eddy current flaw detector detected no leaks or uneven thickness areas in the shielding layer. The volume resistivity was measured online using a coaxial three-electrode system, and the measurement method complied with GB / T3048.3-2007 "Electrical Performance Test Methods for Wires and Cables - Part 3: DC Resistance Test of Conductors". The volume resistivity was 36 Ω·m. After the conductor shielding layer was placed at 180℃ for 30 minutes, there was no melting, flowing or cracking.

[0141] Example 4:

[0142] A method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables includes the following steps:

[0143] Step 1: Preparation of conductive primer

[0144] A conductive primer is prepared, consisting of epoxy resin, latent curing agent, conductive carbon black, silane coupling agent, and organic solvent, wherein the latent curing agent is dicyandiamide; the conductive primer is stable below 80°C and cured above 120°C, with a volume resistivity of no more than 100 Ω·m after curing.

[0145] The components are distributed as follows by weight: 100 parts epoxy resin, 9 parts latent curing agent dicyandiamide, 1.5 parts urea accelerator, 25 parts conductive carbon black, 4 parts silane coupling agent, and 70 parts organic solvent; the urea accelerator sets the curing temperature of dicyandiamide to 130°C; the organic solvent is a mixture of acetone and xylene in a 1:1 volume ratio, in which both epoxy resin and dicyandiamide are dissolved; the silane coupling agent is γ-glycidoxypropyltrimethoxysilane, whose epoxy groups participate in the curing reaction of epoxy resin, and whose siloxane groups condense with the hydroxyl groups on the surface of the metal conductor.

[0146] Step 2: Formation of the coating and anchoring layer

[0147] First, the stranded metal conductors undergo surface activation treatment:

[0148] Aluminum stranded conductor: Immerse in a 10% (w / w) mixed acid of phosphoric acid and nitric acid for 25 seconds. The volume ratio of phosphoric acid to nitric acid in the mixed acid is 3:1. Rinse with deionized water and then dry with cold air.

[0149] The activated conductor is immersed in conductive primer for 4 minutes, then removed and heat-treated at 75°C for 40 minutes to allow the epoxy resin to reach the B-stage semi-cured state, forming a conductive anchoring layer on the conductor surface and in the stranded gaps. The conductive anchoring layer does not swell or fall off in the electrophoretic deposition solution. The criterion for judging the B-stage semi-cured state is: when the semi-cured conductive anchoring layer is immersed in acetone for 30 minutes, the weight change rate is not greater than 2%.

[0150] Step 3, Electrophoretic deposition

[0151] Preparation of electrophoretic deposition solution: Composed of deionized water, nano-conductive carbon black, cationic waterborne polyurethane, dispersant and pH adjuster, with pH value adjusted to 5.5; the cationic waterborne polyurethane has a softening point of 110-130℃ and a glass transition temperature of -30-0℃; by weight, the ratio is 1000 parts deionized water, 13 parts nano-conductive carbon black, 35 parts cationic waterborne polyurethane, and 2.5 parts dispersant; the dispersant is polyvinylpyrrolidone with a molecular weight of 30000.

[0152] Using the conductor obtained in step two as the cathode and the stainless steel plate as the anode, an electrophoretic deposition was performed with a DC voltage of 45V, a deposition solution temperature of 32℃, and a deposition time of 2.5min. A nano-conductive carbon black / cationic aqueous polyurethane composite deposition film with a thickness of 22μm was deposited on the surface of the conductive anchoring layer.

[0153] Step 4: Clean and dry

[0154] The conductor after electrophoretic deposition was removed, washed with deionized water, and dried at 95°C for 40 minutes.

[0155] Step 5, Hot Press Integration

[0156] A hot-pressing mold with an inner cavity size 0.5 mm larger than the outer diameter of the conductor was used. Before hot pressing, a water-soluble release agent, polyvinyl alcohol solution, was coated on the inner surface of the mold. The dried conductor was placed in the mold and hot-pressed for 8 minutes at 145°C and 0.8 MPa. The cationic waterborne polyurethane in the composite deposition film softened and flowed to fill the surface pores. The latent curing agent in the conductive anchoring layer was activated above 130°C, and the epoxy resin was completely cured. The anchoring layer and the deposition film were integrated into an integrated conductor shielding layer with a total thickness of 0.25 mm. After hot pressing, the surface of the conductor shielding layer was cleaned with deionized water.

[0157] Step Six: Online Testing

[0158] The conductor with the conductor shielding layer was passed through an eddy current flaw detector and an online resistivity testing device. The eddy current flaw detector detected no leaks or uneven thickness areas in the shielding layer. The volume resistivity was measured online using a coaxial three-electrode system, and the measurement method complied with GB / T3048.3-2007 "Electrical Performance Test Methods for Wires and Cables - Part 3: DC Resistance Test of Conductors". The volume resistivity was 45 Ω·m. After the conductor shielding layer was placed at 180℃ for 30 minutes, there was no melting, flowing or cracking.

[0159] Example 5:

[0160] A method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables includes the following steps:

[0161] Step 1: Preparation of conductive primer

[0162] A conductive primer is prepared, consisting of epoxy resin, latent curing agent, conductive carbon black, silane coupling agent, and organic solvent, wherein the latent curing agent is dicyandiamide; the conductive primer is stable below 80°C and cured above 120°C, with a volume resistivity of no more than 100 Ω·m after curing.

[0163] The components are distributed as follows by weight: 100 parts epoxy resin, 10 parts latent curing agent dicyandiamide, 2 parts urea accelerator, 28 parts conductive carbon black, 5 parts silane coupling agent, and 80 parts organic solvent; the urea accelerator sets the curing temperature of dicyandiamide to 130°C; the organic solvent is a mixture of acetone and xylene in a 1:1 volume ratio, in which both epoxy resin and dicyandiamide are dissolved; the silane coupling agent is γ-glycidoxypropyltrimethoxysilane, whose epoxy groups participate in the curing reaction of epoxy resin, and whose siloxane groups condense with the hydroxyl groups on the surface of the metal conductor.

[0164] Step 2: Formation of the coating and anchoring layer

[0165] First, the stranded metal conductors undergo surface activation treatment:

[0166] Copper stranded conductor: Immerse in 5% dilute sulfuric acid for 30 seconds, rinse with deionized water and dry with cold air.

[0167] The activated conductor is immersed in conductive primer for 5 minutes, then removed and heat-treated at 80°C for 30 minutes to allow the epoxy resin to reach the B-stage semi-cured state, forming a conductive anchoring layer on the conductor surface and in the stranded gaps. The conductive anchoring layer does not swell or fall off in the electrophoretic deposition solution. The criterion for judging the B-stage semi-cured state is: when the semi-cured conductive anchoring layer is immersed in acetone for 30 minutes, the weight change rate is not greater than 2%.

[0168] Step 3, Electrophoretic deposition

[0169] Preparation of electrophoretic deposition solution: Composed of deionized water, nano-conductive carbon black, cationic waterborne polyurethane, dispersant and pH adjuster, with pH value adjusted to 6.0; the cationic waterborne polyurethane has a softening point of 110-130℃ and a glass transition temperature of -30-0℃; by weight, the ratio is 1000 parts deionized water, 15 parts nano-conductive carbon black, 40 parts cationic waterborne polyurethane, and 3 parts dispersant; the dispersant is polyvinylpyrrolidone with a molecular weight of 40000.

[0170] Using the conductor obtained in step two as the cathode and the stainless steel plate as the anode, an electrophoretic deposition was performed by applying a DC voltage of 50V, with the deposition solution temperature at 35℃ and the deposition time at 2min. A nano-conductive carbon black / cationic aqueous polyurethane composite deposition film with a thickness of 25μm was deposited on the surface of the conductive anchoring layer.

[0171] Step 4: Clean and dry

[0172] The conductor after electrophoretic deposition was removed, washed with deionized water, and dried at 100°C for 30 minutes.

[0173] Step 5, Hot Press Integration

[0174] A hot-press mold with an inner cavity size 0.6 mm larger than the outer diameter of the conductor was used. Before hot pressing, a water-soluble release agent, polyvinyl alcohol solution, was coated on the inner surface of the mold. The dried conductor was placed in the mold and hot-pressed for 5 minutes at 150°C and 0.5 MPa. The cationic waterborne polyurethane in the composite deposition film softened and flowed to fill the surface pores. The latent curing agent in the conductive anchoring layer was activated above 130°C, and the epoxy resin was completely cured. The anchoring layer and the deposition film were integrated into an integrated conductor shielding layer with a total thickness of 0.3 mm. After hot pressing, the surface of the conductor shielding layer was cleaned with deionized water.

[0175] Step Six: Online Testing

[0176] The conductor with the conductor shielding layer was passed through an eddy current flaw detector and an online resistivity testing device. The eddy current flaw detector detected no leaks or uneven thickness areas in the shielding layer. The volume resistivity was measured online using a coaxial three-electrode system, and the measurement method complied with GB / T3048.3-2007 "Electrical Performance Test Methods for Wires and Cables - Part 3: DC Resistance Test of Conductors". The volume resistivity was 58 Ω·m. After the conductor shielding layer was placed at 180℃ for 30 minutes, there was no melting, flowing or cracking.

[0177] Comparative Example 1:

[0178] Only the electrophoretic deposition step three is omitted; the parameters for steps one, two, four, and five are the same as in Example 3.

[0179] Step 6: The shielding layer has a rough surface with a resistivity of 92 Ω·m, and microcracks are generated when it is bent.

[0180] Comparative Example 2:

[0181] Only the silane coupling agent is removed from the formulation in step one; all other raw materials and process parameters are the same as in Example 3.

[0182] Step 6: Insufficient adhesion of the shielding layer, local peeling, resistivity 78Ω·m.

[0183] Comparative Example 3:

[0184] The only difference is that the hot pressing temperature in step five is changed to 120°C; all other steps are the same as in Example 3.

[0185] Step 6: The resin is not fully cured, has many pores, and has a resistivity of 83 Ω·m.

[0186] Comparative Example 4:

[0187] The only difference is that the amount of conductive carbon black used in step one (primer) is reduced to 10 parts, while the other conditions are the same as in Example 3.

[0188] Step 6: The conductive path is incomplete, the resistivity is 102 Ω·m, and the uniformity of conductivity is deteriorated.

[0189] Comparative Example 5:

[0190] Only the electrophoresis voltage in step three is adjusted to 20V, while the raw materials for the rest of the process remain unchanged.

[0191] Step 6: The deposited film has uneven thickness and local defects, with a resistivity of 81 Ω·m.

[0192] Single-factor experiment:

[0193] Based on the requirements for evaluating the inventiveness of the invention, this section conducts single-factor variable screening tests on the five core process parameters of the invention. All other parameters are kept the same as in Example 3, and only a single test parameter is changed. The performance of each item is tested as follows: average volume resistivity of the shielding layer (Ω·m), apparent uniformity of the shielding layer (1-5 points, the higher the score, the better the uniformity), and heat resistance stability at 180℃ (1-5 points, the higher the score, the no cracking or peeling).

[0194] Single-factor experiment 1: Screening of conductive carbon black addition amount in primer

[0195] In this experiment, all other process parameters and raw material ratios were kept consistent with those in Example 3, except for the amount of conductive carbon black added to the conductive primer. Five gradient groups were set up, and the three core indicators of the shielding layer—average volume resistivity, apparent uniformity, and 180℃ heat resistance stability—were tested. The experimental results are shown in Table 1.

[0196]

[0197] Data analysis: When the amount of conductive carbon black added is less than 22.0 parts, in the range of 16.0 to 19.0 parts, the overall amount of conductive filler is insufficient, the carbon black particles are widely dispersed in the epoxy resin matrix, and it is difficult for the particles to overlap and form a continuous conductive network. The number of charge conduction paths is insufficient, ultimately resulting in a higher volume resistivity. Simultaneously, at low filler levels, the resin matrix accounts for a high proportion, increasing the proportion of the resin's intrinsic insulating properties after curing, further raising the overall resistivity. Local agglomeration and voids in a small amount of carbon black lead to greater fluctuations in the thickness of the anchoring layer, resulting in a lower apparent uniformity score. As the amount of carbon black gradually increases to 22.0 parts, the carbon black particles are evenly dispersed within the resin, overlapping to form dense and continuous conductive pathways. Conductivity steadily improves, resistivity continuously decreases, the dispersion of the filler and resin phases becomes balanced, the shielding layer forms a dense and regular shape, and uniformity and heat resistance improve simultaneously.

[0198] When the addition amount exceeds 22.0 parts, and continues to increase to 25.0 parts and 28.0 parts, the proportion of solid filler in the system becomes too large. Excessive conductive carbon black particles have high surface activity, and the van der Waals forces between particles are enhanced, making agglomeration and clumping very likely. Agglomerated carbon black forms local stress concentration points in the epoxy resin, destroying the continuous film-forming skeleton structure of the epoxy resin. After curing, fine voids and defects appear inside the shielding layer. These voids will block local conductive paths, causing a reverse increase in resistivity. Excessive filler also reduces the wetting fluidity of the primer, slightly decreasing the primer's ability to penetrate the conductor strand gaps, resulting in poor gap filling integrity. Ultimately, the apparent uniformity and high-temperature heat resistance stability both decline simultaneously. Optimal parameter: 22.0 parts conductive carbon black.

[0199] Single-factor experiment 2: Screening of impregnation heat treatment temperature

[0200] This experiment kept all other parameters the same as in Example 3, and set five different heat treatment temperature gradients. The process effects at different temperatures were evaluated using the same three performance indicators. The experimental results are shown in Table 2.

[0201]

[0202] Data analysis: When the heat treatment temperature is below 70.0℃, the system receives insufficient heat energy at 60.0℃ and 65.0℃. The slow cross-linking reaction rate of dicyandiamide combined with urea accelerator is low, resulting in insufficient cross-linking of the epoxy resin. The anchoring layer can only achieve shallow semi-curing, and the curing degree in stage B is substandard. During the subsequent electrophoresis process, a small amount of low-molecular-weight resin on the surface of the anchoring layer is easily slightly swollen by the electrophoretic solution, damaging the surface smoothness of the anchoring layer. This leads to uneven adhesion of the subsequent electrophoretic deposition film, increased interfacial contact resistance, and a higher overall resistivity. The incompletely cured anchoring layer contains residual soluble small-molecule components, which are prone to slight deformation during high-temperature testing, resulting in weak heat resistance.

[0203] When the temperature is raised to 70.0℃, the heat matches the activation energy required for the semi-curing reaction of epoxy resin. The resin stabilizes and reaches the standard B stage state. The anchoring layer does not swell or fall off in the electrophoretic solution. The surface is smooth, which facilitates the uniform adhesion of the electrophoretic film. The interlayer interface is tight, the conductive path is continuous and complete, and all performances reach the optimal level.

[0204] When heat treatment temperatures exceed 70.0℃, 75.0℃, or 80.0℃, the heating rate is too rapid, causing a concentrated burst of the epoxy resin cross-linking reaction within a short period. This leads to rapid evaporation of the primer solvent, a sudden increase in adhesive viscosity, and a decrease in the primer's capillary penetration ability. This prevents the primer from fully filling the tiny gaps in the conductor's strands, leaving sealed micro-air cavities within these gaps. These cavities lack conductivity, interrupting local conductive lines, reducing the density of the shielding layer, and consequently increasing resistivity. Furthermore, excessively rapid drying of the surface resin can cause surface shrinkage and wrinkling, resulting in a simultaneous deterioration in uniformity and heat resistance. Optimal parameters: Heat treatment at 70.0℃.

[0205] Single-factor experiment 3: Screening by electrophoretic deposition voltage

[0206] This experiment maintained all other process parameters and raw material ratios consistent with Example 3, and set five different DC voltage gradients to test various performance indicators of the corresponding shielding layers. The experimental results are shown in Table 3.

[0207]

[0208] Data analysis: When the electrophoresis voltage is below 40.0V, the electric field strength at 30.0V and 35.0V is too low. The positively charged polyurethane-nanocarbon black composite colloidal particles in the electrophoresis solution experience insufficient electric driving force, resulting in slow particle directional migration. Consequently, less material is deposited on the anchoring layer surface per unit time, leading to a thinner overall deposited film and localized film defects. Insufficient thickness of the surface conductive film reduces the cross-sectional area of ​​the conductive pathway, increases charge conduction resistance, and raises volume resistivity. Uneven film thickness also causes inconsistent filling levels during the hot-pressing stage, reducing thermal stability.

[0209] When the voltage is increased to 40.0V, the electric field force matches the charge characteristics of the colloidal particles, and the colloidal particles migrate to the surface of the cathode conductor at a uniform speed and in a stable manner. The deposition rate is moderate, the film thickness is uniform and stable, the nano carbon black is uniformly distributed in the polyurethane matrix, the conductive network is complete and continuous, and the overall performance of the shielding layer is optimal.

[0210] Under strong electric fields (voltages above 40.0V, 45.0V, and 50.0V), the migration rate of colloidal particles is too fast, causing a large amount of material to accumulate rapidly at the protruding corners of the conductor, resulting in less deposition in the planar areas of the conductor and significant variations in film thickness. The rapidly deposited film contains microbubbles, forming closed pores that disrupt conductivity continuity, increase resistivity, and cause surface unevenness, directly reducing apparent uniformity. These pores also easily become weak points under high temperatures, leading to decreased heat resistance. Optimal parameter: Electrophoresis voltage 40.0V.

[0211] Single-factor experiment 4: Screening by hot pressing temperature

[0212] This experiment kept all other parameters the same as in Example 3, and set five hot-pressing temperature gradients to evaluate the overall performance of the shielding layer at different temperatures. The experimental results are shown in Table 4.

[0213]

[0214] Data analysis: When the hot pressing temperature is below 140.0℃, and at 130.0℃ and 135.0℃, the temperature cannot simultaneously meet the requirements of two reactions: Firstly, the cationic waterborne polyurethane cannot reach a sufficient softening temperature, resulting in poor melt flow and difficulty in filling the fine pores inside the electrophoretic membrane, leaving residual pores in the membrane; secondly, the activation efficiency of the dicyandiamide curing agent is insufficient, the cross-linking and curing reaction of the epoxy resin in the bottom anchoring layer is incomplete, the anchoring layer and the surface deposited membrane rely mainly on physical adhesion, the proportion of chemical bonding is low, the interlayer interface resistance is relatively high, and the two factors together increase the overall resistivity. The residual pores are prone to micro-cracking after heating, resulting in low heat resistance.

[0215] The 140.0℃ condition perfectly matches the softening temperature of polyurethane and the activation temperature of dicyandiamide. The polyurethane flows fully and fills the pores, the bottom resin is completely cross-linked and cured, and the two layers of materials are fused and anchored into an integrated structure with no obvious boundary between the layers. The conductive path runs through the whole, and all performance is optimal.

[0216] At temperatures exceeding 140.0℃, 145.0℃, and 150.0℃, polyurethane melts excessively, resulting in excessive melt fluidity. During hot pressing, some polyurethane, along with the surface nano-carbon black, overflows into the mold gaps, causing localized carbon black loss from the shielding layer, partial loss of conductive filler, damage to the conductive network, and increased resistivity. Excessive melting also leads to uneven shielding layer thickness, and high temperatures accelerate the resin's micro-thermal aging, gradually reducing its heat resistance and stability. Optimal parameter: Hot pressing temperature 140.0℃.

[0217] Single-factor experiment 5: Screening based on hot pressing time

[0218] This experiment maintained all other parameters consistent with Example 3, setting five different hot-pressing time gradients and testing the corresponding three core performance indicators. The experimental results are shown in Table 5.

[0219]

[0220] Data analysis: When the hot pressing time is less than 10.0 min, 5.0 min, and 7.5 min, the heat inside the mold is not sufficiently conducted to the depth of the shielding layer, the core temperature of the shielding layer does not reach the required process temperature, the polyurethane softening filling and epoxy resin curing reaction cannot be fully completed, leaving unclosed pores and incompletely cured resin inside, resulting in insufficient interlayer adhesion, high interface contact resistance, and high resistivity. The incompletely formed structure is prone to micro-cracks in high-temperature heat resistance tests.

[0221] When the heat is maintained for 10.0 minutes, the heat is fully conducted to the entire shielding layer, the surface and core temperatures are uniform, the resin curing and polyurethane pore filling are completed, the anchoring layer and the deposited film are tightly integrated into one, the structure is dense and continuous, and the conductivity and heat resistance are optimal.

[0222] When hot-pressing times exceed 10.0 min, and are subjected to prolonged high-temperature and high-pressure environments (12.5 min and 15.0 min), the matrix resin is continuously heated. The nano-conductive carbon black dispersed within the system undergoes slow migration and agglomeration due to prolonged thermal disturbance. This results in uneven carbon black distribution in some areas and sparse carbon black in others, leading to uneven conductive pathways and an overall increase in resistivity. Prolonged high temperatures can also cause slight thermal degradation of the surface resin, reducing surface smoothness and consequently deteriorating heat resistance. Optimal parameter: Hot-pressing time 10.0 min.

[0223] Performance comparison test between the examples and comparative examples:

[0224] Test indicators: average volume resistivity (Ω·m), apparent uniformity (1-5 points), and heat resistance rating at 180℃ (1-5 points). The experimental results are shown in Table 6.

[0225]

[0226] Comprehensive analysis of data and theory:

[0227] 1. Horizontal data analysis of similar embodiments

[0228] Example 1, using the lower limit of all process parameters, had insufficient conductive filler addition and inadequate process reaction, resulting in insufficient conductive overlap points within the matrix and limited conductive network integrity. Compared to the optimal Example 3, the volume resistivity significantly increased, while uniformity and heat resistance decreased simultaneously. Example 2, using the middle to lower limit of all process parameters, showed slight improvement in various properties, with indicators falling between Example 1 and Example 3. Example 3, using the optimal parameter combination obtained through screening, achieved balanced filler dispersion and perfectly matched reaction levels at each step, with all three test indicators being the best in the group. Examples 4 and 5, using the middle and upper limits of all process parameters, gradually increased the carbon black content to 25.0 parts and 28.0 parts, respectively. Excessive carbon black agglomeration gradually emerged, disrupting the continuous resin matrix, increasing internal porosity defects, and causing the resistivity to gradually rise from the optimal value, while uniformity and heat resistance decreased slowly and simultaneously. The overall trend demonstrates that within the parameter range defined in the claims, only the optimal ratio can achieve a synergistic match between the physicochemical properties of filler, resin, process temperature, and time. When parameters deviate from the optimal position, whether approaching the upper or lower limit, product performance exhibits a regular decline.

[0229] 2. Longitudinal Comparative Analysis of Examples and Comparative Cases

[0230] All five comparative examples only changed a single experimental condition, while all other raw materials and process parameters were strictly aligned with the optimal example 3. The variables were single and controllable, and the experimental comparisons have statistical reference value.

[0231] Comparative Example 1 eliminates the electrophoretic deposition process and relies solely on a single-layer anchoring layer for molding. It lacks a flexible polyurethane conductive surface layer, and the inherent brittleness of the epoxy resin after curing cannot be buffered. The shielding layer surface lacks the smooth modification structure brought by electrophoresis, resulting in more surface roughness defects. At the same time, the single-layer structure has a single conductive path and lacks a double-layer conductive complementary structure, leading to a significant increase in resistivity.

[0232] Comparative Example 2, by removing the silane coupling agent component, loses the chemical bonding effect of condensation between siloxane and hydroxyl groups of the metal conductor and cross-linking of epoxy groups into the resin. The anchoring layer and the metal conductor rely only on physical adhesion, resulting in increased interfacial contact gaps, increased interfacial additional resistance, and an increase in overall resistivity. After heating, local debonding is likely to occur at the interface, and the heat resistance and uniformity indicators decrease.

[0233] In Comparative Example 3, the hot pressing temperature was reduced to 120°C, which did not reach the critical activation temperature of dicyandiamide and the softening temperature of polyurethane. As a result, the resin was not fully cured, the polyurethane could not fill the micropores, and a large number of internal defects remained. The performance of Comparative Example 3 was significantly worse than that of Example 3.

[0234] Comparative Example 4, with carbon black content reduced to 10 parts, suffered from a severe lack of conductive filler, making it difficult to form a continuous conductive network. It was the sample with the highest resistivity among all the samples.

[0235] In Comparative Example 5, when the electrophoresis voltage was reduced to 20V, the electric field driving force was insufficient, resulting in uneven film thickness, local film defects, damage to the surface conductive structure, and a simultaneous deterioration in performance.

[0236] 3. Creative supporting conclusions

[0237] Combining all data from five sets of single-factor gradient experiments, example interval experiments, and single-variable comparative experiments, it can be seen that this invention is not a simple selection of existing process parameters, but rather an optimal combination scheme obtained within the parameter range defined in the claims through the synergistic cooperation of three processes: "conductive base adhesive impregnation and anchoring - electrophoretic deposition film formation - hot pressing integrated molding." Any missing process step, deletion of key components, or deviation of parameters from the preferred range results in a predictable and regular decline in product performance. However, within the parameter range defined by this invention, the shielding layer achieves a synergistic improvement in multiple technical effects, including sufficient gap filling, excellent conductivity, good flexibility, and high surface smoothness, solving the industry-wide common problems of insufficient filling and easy cracking of single coatings in traditional co-extrusion processes. The above-mentioned parallel experimental data objectively demonstrate that this invention, through targeted parameter selection, achieves unexpected technical effects that differ from existing conventional processes, supporting the inventiveness of the chosen invention and reflecting the substantial technical progress brought about by the technical solution of this application.

[0238] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various substitutions or modifications can be made to these described embodiments without departing from the inventive concept, and all such substitutions or modifications should be considered within the scope of protection of the present invention.

[0239] Although the invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and modifications can be made without departing from the spirit and scope of the invention. Furthermore, the scope of the invention is not limited to the specific embodiments of the processes, methods, and steps described in the specification. From the disclosure of this invention, those skilled in the art will readily utilize existing or future processes, methods, steps that substantially perform the same function or achieve the same results as the corresponding embodiments described herein. Therefore, the appended claims are intended to cover such processes, methods, steps.

Claims

1. A method for preparing a conductor shielding layer for cross-linked polyethylene insulated cables, characterized in that, Includes the following steps: Step 1, Preparation of conductive primer: Prepare conductive primer; the conductive primer is composed of epoxy resin, latent curing agent, conductive carbon black, silane coupling agent and organic solvent; the latent curing agent is dicyandiamide or its derivative; the conductive primer is stable below 80℃ and cured above 120℃, and the volume resistivity after curing is not greater than 100Ω·m; Step 2, Dipping and Anchoring Layer Formation: The stranded metal conductor is immersed in the conductive primer for 1-5 minutes, then removed and heat-treated at 60-80℃ for 30-60 minutes until the epoxy resin reaches the B-stage semi-cured state, forming a conductive anchoring layer on the conductor surface and in the stranding gaps; the conductive anchoring layer does not swell or fall off in the electrophoretic deposition solution of Step 3; Step 3, Electrophoretic Deposition: The conductor obtained in Step 2 is used as the cathode and immersed in the electrophoretic deposition solution. A stainless steel plate is used as the anode, and a DC voltage is applied to perform electrophoretic deposition, depositing a nano-conductive carbon black / cationic aqueous polyurethane composite deposition film on the surface of the conductive anchoring layer. The DC voltage for electrophoretic deposition is 30-50V, the deposition time is 2-4min, the deposition solution temperature is 25-35℃, and the thickness of the resulting composite deposition film is 15-25μm. Step 4, cleaning and drying: Take out the conductor obtained in step 3, clean it with deionized water, and dry it at 80-100℃ for 30-60 minutes; Step 5, Hot-pressing integration: The conductor obtained in Step 4 is placed in a hot-pressing mold and hot-pressed at 130-150℃ and 0.5-2MPa for 5-15 minutes; the cationic waterborne polyurethane in the composite deposited film softens and flows at 130-150℃, filling the surface pores; the latent curing agent in the conductive anchoring layer is activated above 130℃, the epoxy resin completes curing, and the anchoring layer and the deposited film layer are integrated into an integrated conductor shielding layer with a total thickness of 0.1-0.3mm; Step 6, Online Testing: The conductor with the conductor shielding layer obtained in Step 5 is passed through an eddy current flaw detector and an online resistivity testing device to test the uniformity, continuity and volume resistivity of the shielding layer. The volume resistivity is not greater than 100 Ω·m.

2. The preparation method according to claim 1, characterized in that: The components of the conductive primer in step one, by weight, are: 100 parts epoxy resin, 5-10 parts latent curing agent dicyandiamide, 0.5-2 parts accelerator, 16-28 parts conductive carbon black, 2-5 parts silane coupling agent, and 50-80 parts organic solvent; the accelerator is a urea accelerator, which sets the curing temperature of dicyandiamide to 130°C; the organic solvent is a mixture of acetone and xylene in a volume ratio of 1:1, in which both epoxy resin and dicyandiamide are dissolved.

3. The preparation method according to claim 1, characterized in that: The silane coupling agent mentioned in step one is γ-glycidoxypropyltrimethoxysilane. The epoxy group of the γ-glycidoxypropyltrimethoxysilane participates in the curing reaction of the epoxy resin, and the siloxane alkyl group condenses with the hydroxyl group on the surface of the metal conductor.

4. The preparation method according to claim 1, characterized in that: When the metal conductor mentioned in step two is an aluminum stranded conductor, a surface activation treatment is performed before immersion in the conductive primer: immerse the aluminum conductor in a 10% (w / w) mixed acid of phosphoric acid and nitric acid for 20-30 seconds, then rinse with deionized water and dry with cold air.

5. The preparation method according to claim 4, characterized in that: The volume ratio of phosphoric acid to nitric acid in the phosphoric acid-nitric acid mixture is 3:1; when the metal conductor is a copper stranded conductor, a surface activation treatment is performed before immersion in the conductive primer: the copper conductor is immersed in 5% dilute sulfuric acid for 15-30 seconds, then rinsed with deionized water and dried with cold air.

6. The preparation method according to claim 1, characterized in that: The criterion for judging the semi-cured state of stage B in step two is: when the semi-cured conductive anchoring layer is immersed in acetone for 30 minutes, the weight change rate is no more than 2%.

7. The preparation method according to claim 1, characterized in that: The electrophoretic deposition solution in step three is composed of deionized water, nano-conductive carbon black, cationic waterborne polyurethane, dispersant and pH adjuster, with a pH value of 4.0-6.0; the softening point of the cationic waterborne polyurethane is 110-130℃ and the glass transition temperature is -30-0℃.

8. The preparation method according to claim 7, characterized in that: The electrophoretic deposition solution comprises, by weight: 1000 parts deionized water, 8-15 parts nano-conductive carbon black, 20-40 parts cationic waterborne polyurethane, and 1-3 parts dispersant; the dispersant is polyvinylpyrrolidone, and the molecular weight of polyvinylpyrrolidone is 10000-40000.

9. The preparation method according to claim 1, characterized in that: In step five, the inner cavity size of the hot pressing mold is 0.2-0.6 mm larger than the outer diameter of the conductor; before hot pressing, a water-soluble release agent polyvinyl alcohol solution is coated on the inner surface of the mold; after hot pressing, the surface of the conductor shielding layer is cleaned with deionized water.

10. The preparation method according to claim 1, characterized in that: In step six, the eddy current flaw detector is used to detect leaks or uneven thickness areas in the shielding layer; the volume resistivity is measured online using a coaxial three-electrode system; after the conductor shielding layer is placed at 180°C for 30 minutes, there is no melting, flowing or cracking.

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