Multi-layer heat shrink tube and preparation method thereof

Abstract

The invention relates to the field of thermal shrinkage materials, and particularly discloses a multi-layer thermal shrinkage tube and a preparation method thereof.The multi-layer thermal shrinkage tube sequentially comprises an outer shielding layer, a middle insulating layer and an inner semi-conductive layer from outside to inside; the outer shielding layer is prepared from the following raw materials in parts by weight: 10 to 20 parts of EVA (Ethylene-Vinyl Acetate), 20 to 40 parts of LDPE (Low-Density Polyethylene), 0.5 to 2.0 parts of a lubricating agent, 0.5 to 2.0 parts of a dispersing agent, 0.5 to 2.0 parts of an anti-aging agent, 5 to 7 parts of conductive carbon black and 0.5 to 2.0 parts of a cross-linking agent; the middle insulating layer is prepared from the following raw materials in parts by weight: 20 to 30 parts of PE (Polyethylene), 10 to 20 parts of thermal shrinkage elastomer POE (Polyolefin Elastomer), 0.1 to 10 parts of filler, 0.5 to 2.0 parts of lubricant, 0.5 to 2.0 parts of anti-aging agent and 0.5 to 2.0 parts of cross-linking agent; the inner semi-conductive layer is prepared from the following raw materials in parts by weight: 20 to 30 parts of EVA (Ethylene-Vinyl Acetate), 10 to 20 parts of thermal shrinkage elastomer POE (Polyolefin Elastomer), 5 to 15 parts of barium carbonate, 0.5 to 2.0 parts of lubricant, 0.5 to 2.0 parts of dispersant, 0.5 to 2.0 parts of anti-aging agent, 5 to 7 parts of conductive carbon black and 0.5 to 2.0 parts of cross-linking agent.

Description

Technical Field

[0001] This application relates to the field of multilayer heat shrinkable materials, and more specifically, to multilayer heat shrinkable tubes and their preparation methods. Background Technology

[0002] Heat shrink tubing is a tubular material that shrinks when heated, typically made of polymers such as polyolefins, fluororubber, and PVC. Its working principle is based on the memory effect of polymers. During production, the tubing is heated to expand to a certain size and then rapidly cooled to set its shape. When reheated to a specific temperature, the heat shrink tubing shrinks back to its original shape and size, tightly wrapping around the object being protected, thus providing insulation, protection, and labeling.

[0003] With the rapid development of power transmission, rail transportation, aerospace, and new energy, the operating environment of cables is becoming increasingly complex and demanding. Extreme conditions such as high voltage, high temperature, and strong electromagnetic interference place higher requirements on the insulation protection and electromagnetic shielding performance of cables. Multilayer heat shrink tubing, with its layered structure of "shielding layer-insulation layer-functional transition layer," can integrate multiple functions such as electromagnetic shielding, high voltage insulation, and sealing protection, becoming a core component for cable protection in extreme environments.

[0004] However, due to factors such as the softening of the adhesive layer at high temperatures or the cyclic stress caused by thermal expansion and contraction, the aging of electrical trees caused by local electric field concentration under high voltage electric fields, and secondary damage caused by strong electromagnetic interference, the alternating electromagnetic field causes the shielding layer to generate eddy currents and local heating. This, combined with the heat accumulation caused by dielectric loss, accelerates the failure of interlayer bonding and the degradation of the substrate, inducing delamination and cracking.

[0005] To address the aforementioned issues, existing technologies primarily employ methods such as adding compatibilizers to improve interlayer bonding, optimizing crosslinking agent ratios to enhance crosslinking uniformity, and using composite conductive fillers to strengthen shielding performance. However, the improvement effects are limited. Therefore, providing a multilayer heat shrink tubing that can effectively solve the problem of delamination and cracking failure under extreme environments, improve the reliability of cable protection, ensure the safe and stable operation of critical fields such as high-voltage power transmission, rail transportation, and aerospace, and simultaneously promote the technological upgrading of multilayer functional composite heat shrink materials to meet the domestic demand for extreme environment protection components in high-end equipment has significant practical implications and industrial value. Summary of the Invention

[0006] To address the issues of delamination and cracking of multilayer heat-shrinkable materials under conditions such as high pressure, high temperature, and strong electromagnetic interference, this application provides a multilayer heat-shrinkable tube and its preparation method.

[0007] In one aspect, this application provides a multilayer heat shrink tubing, which includes, from the outside to the inside, an outer shielding layer, an intermediate insulating layer and an inner semiconductive layer; The outer shielding layer comprises the following raw materials in parts by weight: 10-20 parts EVA, 20-40 parts LDPE, 0.5-2.0 parts lubricant, 0.5-2.0 parts dispersant, 0.5-2.0 parts anti-aging agent, 5-7 parts conductive carbon black, and 0.5-2.0 parts crosslinking agent; The intermediate insulation layer comprises the following raw materials: 20-30 parts PE, 10-20 parts heat-shrinkable elastomer POE, 0.1-10 parts filler, 0.5-2.0 parts lubricant, 0.5-2.0 parts anti-aging agent, and 0.5-2.0 parts crosslinking agent; The inner semiconductive layer material comprises the following raw materials: 20-30 parts EVA, 10-20 parts heat-shrinkable elastomer POE, 5-15 parts barium carbonate, 0.5-2.0 parts lubricant, 0.5-2.0 parts dispersant, 0.5-2.0 parts anti-aging agent, 5-7 parts conductive carbon black, and 0.5-2.0 parts crosslinking agent.

[0008] By using EVA and LDPE as the matrix resins for the outer shielding layer, the LDPE molecular chain contains short branches and has moderate crystallinity, which gives the shielding layer basic mechanical strength and resistance to environmental stress cracking. It also has good compatibility with EVA, forming a continuous phase. The vinyl acetate side groups disrupt the crystallinity of polyethylene, improve the system's flexibility and processing fluidity, provide active sites, assist LDPE crosslinking, and enhance the crosslinking density and elastic recovery of the shielding layer. The addition of a certain amount of conductive carbon black allows the conjugated structure on the particle surface to form a conductive path, achieving electromagnetic shielding. The carbon black particles can also serve as physical crosslinking points, improving the modulus and heat deformation resistance of the shielding layer.

[0009] Adding a large amount of POE to high-insulation PE results in highly regular PE molecular chains, high crystallinity, and the absence of polar groups, making charge migration difficult. This endows the insulation layer with high volume resistivity and breakdown strength. After cross-linking, a three-dimensional network is formed, restricting molecular chain slippage and improving high-temperature deformation resistance. The long octene branches disrupt the crystallinity of PE, reducing the system's crystallinity and improving the elastic recovery rate of the insulation layer. POE has good compatibility with PE, forming an elastic continuous phase and improving the brittleness of PE. The thermal expansion coefficients of the intermediate insulation layer, outer shielding layer, and inner semiconductive layer are well-matched, resulting in good stability under high temperature, high pressure, and high electromagnetic conditions.

[0010] The inner semiconductive layer uses EVA as the matrix, providing flexibility and processing fluidity. Vinyl acetate side groups enhance compatibility with barium carbonate, prevent inorganic filler agglomeration, assist crosslinking, and improve the structural stability of the semiconductive layer. The addition of a certain mass of POE improves the system's elastic recovery rate, ensuring a tight bond between the semiconductive layer and the cable conductor. Octene side groups increase crosslinking active sites and improve crosslinking density. Barium carbonate, as an inorganic filler, is incorporated into the resin matrix to enhance semiconductivity and wear resistance. Its high dielectric constant allows for adjustment of the semiconducting dielectric properties, preventing dielectric mismatch with the insulation layer. The addition of a certain mass of conductive carbon black forms a semiconductor pathway, reduces the volume resistivity of the semiconductive layer, eliminates interfacial charge accumulation between the conductor and insulation layer, and prevents localized electric field distortion.

[0011] Preferably, the PE in the intermediate insulating layer includes 5-7 parts of PE-g-(MAH-co-IA) and 1-3 parts of PE-g-GMA.

[0012] The PE backbone of PE-g-(MAH-co-IA) exhibits good compatibility with the intermediate HDPE / POE layer, the outer LDPE / EVA layer, and the inner EVA / POE layer. The polyolefin segments can generate a certain degree of entanglement. The carboxyl and ester groups in the grafted chains are strongly polar groups that can undergo transesterification with the vinyl acetate of the outer shielding layer EVA and form hydrogen bonds and ionic bonds with the polar side groups of the inner semiconductive layer EVA and the hydroxyl groups on the surface of barium carbonate, thereby improving the interfacial shear strength and making the product less prone to interlayer delamination. The epoxy groups of PE-g-GMA have dynamic covalent reactivity, which can undergo ring-opening reactions with the carboxyl groups of PE-g-(MAH-co-IA) and the hydroxyl groups on the filler surface to form chemical bonds. This assists the DCP / TMPTA system, extending the crosslinking reaction from the coupling between HDPE / POE molecular chains to the crosslinking between molecular chains, grafts and fillers. Under high temperature and pressure, this bond can alleviate interlayer stress concentration through breakage and recombination, avoiding interface cracking due to stress accumulation, making the crosslinking network more uniform, eliminating electric field distortion and stress concentration caused by network defects. At the same time, the PE backbone ensures its compatibility with the matrix resin and avoids phase separation.

[0013] Preferably, the mass ratio of PE-g-(MAH-co-IA) to PE-g-GMA is (2-4):1.

[0014] The former focuses on static interface strengthening through filler dispersion and interlayer chemical bonding, while the latter focuses on dynamic stress relief through dynamic covalent bonds and system integration. The two form a static and dynamic dual-compatibility system, which not only ensures the stability of interlayer bonding but also copes with stress fluctuations under high temperature and high pressure, thus avoiding interface cracking.

[0015] Preferably, the filler in the intermediate insulating layer includes one or more of silane-modified MgO-TiO2 heterojunction nanofillers and CTAB-modified nano-montmorillonite.

[0016] Preferably, the mass ratio of silane-modified MgO-TiO2 heterojunction nanofiller to CTAB-modified nano-montmorillonite is 4:(1-3).

[0017] The heterojunction structure with MgO as the core and TiO2 as the shell forms a dielectric gradient transition zone. From the HDPE matrix to the TiO2 shell and then to the MgO core, the dielectric constant gradually increases, avoiding electric field distortion caused by local dielectric abrupt changes. This significantly increases the voltage at which electrical trees are induced. Montmorillonite, a layered silicate, is modified with CTAB (hexadecyltrimethylammonium bromide), which inserts alkyl chains between the montmorillonite layers, widening the interlayer spacing. The layers form a two-dimensional barrier network within the matrix, forcing electrical trees to bypass the layers, thus greatly extending the path length. Furthermore, the hydroxyl groups on the surface of the montmorillonite layers can capture active free radicals generated during electrical tree growth, preventing free radical-induced HDPE molecular chain breakage and delaying electrical tree propagation. When both fillers are used together, the silane-modified MgO-TiO2 point structure inhibits electrical tree initiation at its source, while the CTAB-modified montmorillonite layer structure blocks electrical tree growth during the process. Together, they form a dual inhibition system for initiation and growth. The point- and surface-combined inorganic phases form a three-dimensional expanded barrier network, resulting in significantly better electrical tree inhibition.

[0018] Meanwhile, the polar groups of PE-g-(MAH-co-IA) form chemical bonds and hydrogen bonds with the hydroxyl groups and modified groups on the surface of silane-modified MgO-TiO2 and CTAB-modified montmorillonite, breaking the van der Waals forces between filler particles, preventing agglomeration, and ensuring that the filler is uniformly dispersed in the matrix. This avoids the formation of local high electric field regions due to filler agglomeration, eliminates the core inducing factors caused by electrical treeing, and forms a physical support network with uniform filler distribution. This inhibits the thermal movement of polymer chain segments and further reduces the CTE of the intermediate layer, narrowing the difference in CTE between the intermediate and outer and inner layers, resulting in better thermal expansion adaptability.

[0019] Preferably, the crosslinking agent in the intermediate insulating layer comprises dicumyl peroxide and trimethylolpropane triacrylate in a weight ratio of (1-3):1.

[0020] At the crosslinking temperature, DCP decomposes into free radicals to initiate crosslinking, providing a basic framework for the crosslinking network. TMPTA trifunctional acrylate monomers undergo addition reactions with macromolecular free radicals, connecting multiple macromolecular chains and improving the density of the crosslinking network. At this mass ratio, it can ensure that DCP provides enough free radicals to initiate crosslinking, while avoiding excessive crosslinking and increased brittleness caused by excessive TMPTA. It also eliminates the gaps between HDPE molecular chains, preventing the formation of conductive channels by charge breakdown of gaps under high voltage, resulting in more stable product performance.

[0021] Preferably, the intermediate insulating layer further includes 0.1-1.0 parts of a benzotriazole anticorona agent.

[0022] Benzotriazole derivatives contain nitrogen-containing heterocyclic structures and have strong electron-trapping capabilities. Under high voltage and high electromagnetic environments, they can capture free electrons and ions, prevent charge accumulation at interface defects or filler agglomerations, reduce local electric field strength, suppress corona discharge, and delay the transformation of electrical trees from initiation points to dendritic cracks. The MgO-TiO2 heterojunction regulates dielectric uniformity, and the anti-corona agent eliminates residual free electrons and inhibits the initiation of electrical trees, resulting in more stable product performance.

[0023] Secondly, this application provides a method for preparing multilayer heat shrink tubing, characterized by the following steps: mixing raw materials of each layer uniformly according to their respective mass proportions, then co-extruding, curing, expanding, and cooling to produce multilayer heat shrink tubing; the co-extrusion temperature of the outer layer is 160-170℃, the co-extrusion temperature of the middle layer is 155-165℃, and the co-extrusion temperature of the inner layer is 150-160℃. The cooling rate is 3-5℃ / min.

[0024] The three layers exhibit a temperature gradient, with a higher outer layer and a lower inner layer. This design prevents degradation and the formation of weak interfaces when the high-temperature resin melt contacts the low-temperature layer, ensuring full cross-linking of components and uniform melt flow across each layer. This allows the melts to adhere tightly during cross-linking, preventing bubble formation, reducing electric field distortion points and stress concentration points, while simultaneously promoting interfacial contact between fillers and resin, improving the chemical bonding efficiency of fillers and resins. The cooling process eliminates internal stress, and gradient cooling refines and homogenizes the grains, resulting in uniform interlayer stress. The cooling shrinkage rates of the three resin layers are relatively consistent, eliminating shear stress caused by localized shrinkage differences and protecting the interlayer chemical bonding interfaces from damage.

[0025] In summary, this application has the following beneficial effects: This application provides a specific formulation for a multilayer heat shrink tubing, consisting of an outer shielding layer, a middle insulating layer, and an inner semiconductive layer. This formulation exhibits good electromagnetic shielding, insulation, and dielectric properties, while also ensuring well-matched thermal expansion of each layer and effective interlayer bonding. The addition of a specific mass ratio of PE-g-(MAH-co-IA) and PE-g-GMA synergistically ensures uniform dispersion of all components, preventing localized agglomeration or phase separation, thus avoiding electric field distortion and uneven expansion caused by component inhomogeneity. The addition of MgO-TiO2 heterojunctions eliminates electric field distortion, suppressing electrical treeing at its source, while montmorillonite sheets block electrical tree growth, inhibiting its propagation. The chemical cross-linking network formed by the composite cross-linking agent interweaves with the physical cross-linking points of the HDPE crystalline regions, limiting molecular chain thermal slippage, reducing CTE, and avoiding the brittleness of a purely cross-linked system. The addition of an anti-corona agent to capture free electrons inhibits electrical treeing initiation, while an anti-aging agent captures oxidative free radicals, delaying electrical tree propagation. The components form an organic whole through "three-dimensional synergy, bridge synergy, and multiple synergy," which completely solves the problems of uneven expansion and electrical tree cracking under high temperature, high pressure, and high electromagnetic environment from the molecular chain, chemical bond, and microstructure levels, and greatly improves the stability of the product. Attached Figure Description

[0026] Figure 1 : Diagram of heat shrink tubing structure. Detailed Implementation

[0027] To further aid in understanding the technical solution of the present invention, several specific embodiments are provided to describe the technical solution of the present invention in more detail. All described embodiments are only some embodiments of the present invention, not all of them; embodiments can be combined with each other, and the same or similar concepts or processes may not be repeated in some embodiments. The following embodiments are further illustrations of the present invention, but the present invention is not limited thereto.

[0028] The components used in the preparation examples in this application refer to those obtained through the preparation methods described in the examples. Unless otherwise specified, the experimental reagents are all from commercially available brands or obtained through conventional preparation methods.

[0029] PE-g-GMA was purchased from Licocene PE MA 4351; EVA was purchased from BASF Yangzi V5110J; LDPE was purchased from Cartel Petrochemical FD0274; PE was purchased from Yanshan Petrochemical X-9300GK; POE was purchased from ExxonMobil 8203; montmorillonite was purchased from Corward 127-18-4; barium carbonate was purchased from Neosin BC-16; and conductive carbon black was purchased from TIMCAL ENSACO. Preparation Example

[0030] Preparation Example 1: PE-g-(MAH-co-IA) 100g of PE was dried to remove water. 5g of maleic anhydride, 2g of itaconic acid, 0.5g of dicumyl peroxide, and 2g of white oil were weighed and mixed evenly. The mixture was then extruded using a twin-screw extruder at a screw speed of 200 rpm. The temperatures in the following zones were set as follows: Zone 1: 120℃; Zone 2: 140℃; Zone 3: 160℃; Zone 4: 170-175℃; Zone 5: 170℃; Die head: 165℃; Residence time: 2 min. The extrudate was then cooled with water and pelletized to obtain a crude product. The crude product was extracted with acetone using a Soxhlet extractor for 8 h and finally dried under vacuum at 80℃ for 12 h to obtain PE-g-(MAH-co-IA).

[0031] Preparation Example 2: Silane-modified MgO-TiO2 heterojunction nanofiller 35g tetrabutyl titanate and 8.0g glacial acetic acid were slowly added dropwise to 100mL anhydrous ethanol. Separately, 12.8g magnesium nitrate was dispersed in 50mL anhydrous ethanol, and 12mL deionized water was added dropwise. The mixture was slowly mixed, stirred at 30℃ for 2h, aged in an oven at 60℃ for 12h, dried in an oven at 105℃ for 8h, heated to 200℃ and held for 1h, then heated to 500℃ and held for 3h. After natural cooling to room temperature, the powder was ground and sieved to obtain MgO-TiO2 heterojunction powder. 100mL of 80vt% ethanol aqueous solution was added, along with 3g KH-550. The powder was ultrasonically dispersed for 25min, heated to 65℃ and stirred at a constant temperature for 3.5h, centrifuged, washed with water, and dried to obtain silane-modified MgO-TiO2 heterojunction nanofiller.

[0032] Preparation Example 3: CTAB-modified montmorillonite 10g of montmorillonite was added to 200mL of deionized water, along with 0.5g of NaCl. The mixture was ultrasonically dispersed at 400W for 30min, and then stirred at 80℃ for 1h. 3.5g of CTAB was added to 50mL of deionized water, heated and stirred until homogeneous, and then slowly dripped into the montmorillonite dispersion. The mixture was stirred at a constant temperature for 4h, allowed to stand and cool, filtered and separated, washed with water, vacuum dried, and ground through a 200-mesh sieve to obtain CTAB-modified nano-montmorillonite. Example Example 1

[0033] Raw material weighing: Outer shielding layer: 15 parts EVA, 30 parts LDPE, 0.5 parts zinc stearate, 0.5 parts polyethylene wax, 1 part PE-g-MAH, 0.5 parts antioxidant 1010, 0.3 parts antioxidant 168, 0.2 parts UV-531, 6 parts conductive carbon black, 1.0 part DCP, 0.5 parts TMPTA; Intermediate insulation layer: PE 25 parts, POE 15 parts, montmorillonite 7 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 part, TMPTA 0.5 parts; Inner semiconductive layer: 25 parts EVA, 15 parts POE, 10 parts barium carbonate, 0.5 parts zinc stearate, 0.5 parts polyethylene wax, 1 part PE-g-MAH, 0.5 parts antioxidant 1010, 0.3 parts antioxidant 168, 0.2 parts UV-531, 6 parts conductive carbon black, 1.0 part DCP, and 0.5 parts TMPTA.

[0034] The preparation steps include: mixing each layer of raw materials evenly, then co-extruding multiple layers, cooling to obtain a multi-layer heat shrink tubing, with the outer layer co-extrusion temperature at 165℃, the middle layer co-extrusion temperature at 160℃, the inner layer co-extrusion temperature at 155℃, holding at 160℃ for 10 min, pre-cooling to 70℃ for 2 min, reheating to 130℃, introducing dry compressed nitrogen, increasing the pressure at a uniform rate until the tube diameter expands to twice its original size, with a pressure increase rate of 0.05 MPa / min, holding the pressure for 3 min, and then cooling to room temperature at 5℃ / min to obtain a multi-layer heat shrink tubing. Example 2

[0035] The only difference between this embodiment and Embodiment 1 is that: Intermediate insulation layer raw materials: PE 17 parts, PE-g-(MAH-co-IA) 6 parts, PE-g-GMA 2 parts, POE 15 parts, montmorillonite 7 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 parts, TMPTA 0.5 parts. Example 3

[0036] The only difference between this embodiment and Embodiment 1 is that: Intermediate insulation layer raw materials: PE 17 parts, PE-g-(MAH-co-IA) 2 parts, PE-g-GMA 6 parts, POE 15 parts, montmorillonite 7 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 parts, TMPTA 0.5 parts. Example 4

[0037] The only difference between this embodiment and Embodiment 1 is that: Intermediate insulation layer raw materials: PE 17 parts, PE-g-(MAH-co-IA) 8 parts, POE 15 parts, montmorillonite 7 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 parts, TMPTA 0.5 parts. Example 5

[0038] The only difference between this embodiment and Embodiment 1 is that: Intermediate insulation layer raw materials: PE 17 parts, PE-g-GMA 8 parts, POE 15 parts, montmorillonite 7 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 parts, TMPTA 0.5 parts. Example 6

[0039] The only difference between this embodiment and Embodiment 1 is that: Intermediate insulation layer raw materials: PE 17 parts, PE-g-(MAH-co-IA) 6 parts, PE-g-GMA 2 parts, POE 15 parts, CTAB modified nano-montmorillonite 7 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 parts, TMPTA 0.5 parts. Example 7

[0040] The only difference between this embodiment and Embodiment 1 is that: Intermediate insulation layer raw materials: PE 17 parts, PE-g-(MAH-co-IA) 6 parts, PE-g-GMA 2 parts, POE 15 parts, silane-modified MgO-TiO2 heterojunction nanofiller 4 parts, CTAB-modified nano-montmorillonite 3 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 parts, TMPTA 0.5 parts. Example 8

[0041] The only difference between this embodiment and Embodiment 1 is that: Intermediate insulation layer raw materials: PE 17 parts, PE-g-(MAH-co-IA) 6 parts, PE-g-GMA 2 parts, POE 15 parts, silane-modified MgO-TiO2 heterojunction nanofiller 3 parts, CTAB-modified nano-montmorillonite 4 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 parts, TMPTA 0.5 parts. Example 9

[0042] The only difference between this embodiment and Embodiment 1 is that: Intermediate insulation layer raw materials: PE 17 parts, PE-g-(MAH-co-IA) 6 parts, PE-g-GMA 2 parts, POE 15 parts, silane-modified MgO-TiO2 heterojunction nanofiller 7 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 parts, TMPTA 0.5 parts. Example 10

[0043] The only difference between this embodiment and Embodiment 1 is that: Intermediate insulation layer raw materials: PE 17 parts, PE-g-(MAH-co-IA) 6 parts, PE-g-GMA 2 parts, POE 15 parts, silane-modified MgO-TiO2 heterojunction nanofiller 4 parts, CTAB-modified nano-montmorillonite 3 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 part, TMPTA 0.5 parts, 1.2, 3- benzene 1 part of triazole . Comparative Example

[0044] Comparative Example 1 Raw material weighing: Outer shielding layer: 15 parts EVA, 30 parts LDPE, 0.5 parts zinc stearate, 0.5 parts polyethylene wax, 1 part PE-g-MAH, 0.5 parts antioxidant 1010, 0.3 parts antioxidant 168, 0.2 parts UV-531, 6 parts conductive carbon black, 1.0 part DCP, 0.5 parts TMPTA; Intermediate insulation layer: PE 25 parts, POE 15 parts, zinc stearate 0.5 parts, polyethylene wax 0.5 parts, antioxidant 1010 0.5 parts, antioxidant 168 0.3 parts, UV-531 0.2 parts, DCP 1.0 part, TMPTA 0.5 parts; Inner semiconductive layer: 25 parts EVA, 15 parts POE, 10 parts barium carbonate, 0.5 parts zinc stearate, 0.5 parts polyethylene wax, 1 part PE-g-MAH, 0.5 parts antioxidant 1010, 0.3 parts antioxidant 168, 0.2 parts UV-531, 6 parts conductive carbon black, 1.0 part DCP, and 0.5 parts TMPTA.

[0045] The preparation steps include: mixing each layer of raw materials evenly, then co-extruding multiple layers, cooling to obtain a multi-layer heat shrink tubing, with the outer layer co-extrusion temperature at 165℃, the middle layer co-extrusion temperature at 160℃, the inner layer co-extrusion temperature at 155℃, holding at 160℃ for 10 min, pre-cooling to 70℃ for 2 min, reheating to 130℃, introducing dry compressed nitrogen, increasing the pressure at a uniform rate until the tube diameter expands to twice its original size, with a pressure increase rate of 0.05 MPa / min, holding the pressure for 3 min, and then cooling to room temperature at 5℃ / min to obtain a multi-layer heat shrink tubing.

[0046] Performance testing

[0047] Test 1: Interlaminar shear strength was tested according to standard JC / T773.

[0048] Test 2: Using the needle-plate electrode method, with needle-plate electrodes (needle diameter 50μm, spacing 2mm), a 50Hz power frequency voltage was applied at a rate of 50V / s, and the voltage at which the first electrical tree of 100µm appeared was recorded.

[0049] Test 3: 100℃, polarization field strength of 30kV / mm, for 1000h, observe whether cracking / delamination occurs.

[0050] The test results are shown in Table 1.

[0051] Table 1

[0052] In conjunction with Examples 1-5, Comparative Example 1, and Table 1, the carboxyl / ester groups of PE-g-(MAH-co-IA) form chemical bonds with the polar side groups of EVA and the hydroxyl groups of montmorillonite in the interlayer, while the epoxy groups of PE-g-GMA form dynamic covalent bonds to relieve stress. The synergistic effect of both upgrades the interlayer bonding from entanglement to chemical bonding and dynamic buffering, significantly improving strength. The compatibilizer promotes uniform dispersion of montmorillonite, eliminates local electric field distortion, and maximizes the physical barrier effect of the montmorillonite sheets. The compatibilizer and crosslinking network synergistically restrict chain segment movement, inhibiting the growth of electrical trees.

[0053] Although the epoxy groups of PE-g-GMA in Example 3 can form dynamic bonds, it lacks the strong polar groups of PE-g-(MAH-co-IA) to form static bonds with the filler / resin. The interfacial anchoring force is insufficient, and the strength is slightly lower than that in Example 2. The promoting effect of PE-g-GMA on filler dispersion is weaker than that of PE-g-(MAH-co-IA). The uniformity of montmorillonite dispersion is slightly worse, the electric field distortion points increase, and the voltage drops.

[0054] Example 4 relies solely on static chemical bonding, lacking the dynamic stress buffer of PE-g-GMA. At high temperatures, interlayer stress cannot be released, resulting in a slight decrease in interfacial strength. While the filler exhibits good dispersibility, the absence of dynamic bonding to optimize system homogeneity leads to a lower initiation voltage compared to the composite system.

[0055] Example 5: PE-g-GMA cannot form strong static bonds with the filler / resin; the interfacial bonding force mainly relies on dynamic bonds, resulting in the lowest strength. The filler dispersibility is generally poor, with numerous electric field distortion points, and the initiation voltage is close to that of Example 1.

[0056] Comparative Example 1: There is no filler support or physical barrier. The system uniformity depends solely on cross-linking. There are many electric field distortion points, such as cross-linking defects and interfacial voids, which easily lead to electrical trees. At high temperatures, the movement of molecular chains intensifies, and the accumulation of interlayer stress leads to delamination. Under high electric fields, electrical trees grow rapidly and eventually penetrate the insulation layer, causing cracking.

[0057] Based on Examples 2, 6-9, and 10 and Table 1, the CTAB montmorillonite in Example 6 was uniformly dispersed and had a better physical barrier effect than the unmodified montmorillonite. However, it lacked the dielectric homogenization effect of MgO-TiO2, and the local electric field distortion was not completely eliminated.

[0058] Example 7 shows that the interlayer spacing of CTAB-modified montmorillonite is increased, resulting in a tighter bond with the polar groups of the compatibilizer. The MgO-TiO2 heterojunction synergistically enhances the overall system integrity. The MgO-TiO2 heterojunction forms a metastatic gradient, eliminating electric field distortion. The CTAB montmorillonite sheets form a two-dimensional barrier network, extending the electrical tree path and causing a significant voltage increase. Without electrical trees, the composite filler and cross-linked network synergistically resist high-temperature and high-electric-field erosion.

[0059] In Example 8, the increased proportion of CTAB montmorillonite enhanced physical barrier properties, but insufficient MgO-TiO2 weakened the dielectric regulation effect, resulting in a slightly lower induction voltage than in Example 7.

[0060] Example 9: MgO-TiO2 adjusts dielectric uniformity and suppresses the initiation of electrical treeing, but lacks the lamellar barrier of montmorillonite. Once electrical treeing is initiated, it grows rapidly. Therefore, the initiation voltage is lower than that of the composite system, resulting in insufficient stability and the appearance of slight electrical treeing.

[0061] Example 10: The nitrogen heterocyclic structure of the anti-corona agent captures free electrons, eliminating charge accumulation under high voltage. Combined with the dielectric regulation of MgO-TiO2 and the physical barrier effect of CTAB montmorillonite, it induces the highest voltage. The anti-corona agent molecules adsorb at the filler resin interface, enhancing interfacial bonding strength, which is slightly higher than in Example 7. The anti-corona agent captures active free radicals required for the growth of electrical trees, completely inhibiting the initiation and propagation of electrical trees, exhibiting high stability.

[0062] The specific embodiments described herein are merely illustrative of this application and are not intended to limit it. Those skilled in the art can make modifications to these embodiments without contributing any inventive step after reading this specification, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A multilayer heat shrink tubing, characterized in that: From the outside to the inside, it includes an outer shielding layer, a middle insulating layer, and an inner semiconductive layer; The outer shielding layer comprises the following raw materials in parts by weight: 10-20 parts EVA, 20-40 parts LDPE, 0.5-2.0 parts lubricant, 0.5-2.0 parts dispersant, 0.5-2.0 parts anti-aging agent, 5-7 parts conductive carbon black, and 0.5-2.0 parts crosslinking agent; The intermediate insulation layer comprises the following raw materials: 20-30 parts PE, 10-20 parts heat-shrinkable elastomer POE, 0.1-10 parts filler, 0.5-2.0 parts lubricant, 0.5-2.0 parts anti-aging agent, and 0.5-2.0 parts crosslinking agent; The inner semiconductive layer material comprises the following raw materials: 20-30 parts EVA, 10-20 parts heat-shrinkable elastomer POE, 5-15 parts barium carbonate, 0.5-2.0 parts lubricant, 0.5-2.0 parts dispersant, 0.5-2.0 parts anti-aging agent, 5-7 parts conductive carbon black, and 0.5-2.0 parts crosslinking agent.

2. The multilayer heat shrink tubing according to claim 1, characterized in that: The intermediate insulating layer contains 5-7 parts of PE-g-(MAH-co-IA) and 1-3 parts of PE-g-GMA.

3. The multilayer heat shrink tubing according to claim 2, characterized in that: The mass ratio of PE-g-(MAH-co-IA) to PE-g-GMA is (2-4):

1.

4. The multilayer heat shrink tubing according to claim 1, characterized in that: The filler in the intermediate insulating layer includes one or more of silane-modified MgO-TiO2 heterojunction nanofillers and CTAB-modified nano-montmorillonite.

5. The multilayer heat shrink tubing according to claim 4, characterized in that: The filler in the intermediate insulating layer includes silane-modified MgO-TiO2 heterojunction nanofiller and CTAB-modified nano-montmorillonite, with a mass ratio of 4:(1-3).

6. The multilayer heat shrink tubing according to claim 1, characterized in that: The crosslinking agent in the intermediate insulating layer comprises dicumyl peroxide and trimethylolpropane triacrylate in a weight ratio of (1-3):

1.

7. The multilayer heat shrink tubing according to claim 1, characterized in that: The intermediate insulating layer also includes 0.1-1.0 parts of a benzotriazole anticorona agent.

8. The multilayer heat shrink tubing according to claim 1, characterized in that: It also includes a flexible rubber insulation layer, the raw materials of which include EPDM rubber and high melt index EVA hot melt adhesive.

9. A method for preparing a multilayer heat shrink tubing according to any one of claims 1-8, characterized in that: The process includes the following steps: mixing each layer of raw material evenly according to the mass fraction, then co-extruding, curing, expanding, and cooling to produce a multi-layer heat shrink tube. The co-extrusion temperature of the outer layer is 160-170℃, the co-extrusion temperature of the middle layer is 155-165℃, and the co-extrusion temperature of the inner layer is 150-160℃.

10. The method for preparing a multilayer heat shrink tubing according to claim 9, characterized in that: The cooling rate is 3-5℃ / min.

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