Decryption of material technology for medium and high voltage cold shrink tubing

2025-07-25 13:57:43

1.Core Material Duel: Key Performance Analysis of EPDM and Silicone Rubber

The material selection of medium and high voltage cold shrink tubing, as a key protective component at the cable connection of power systems, directly determines the insulation reliability and service life of the equipment. Among numerous materials, EPDM and silicone rubber have become the two mainstream choices, with essential differences in molecular structure and performance between the two.

1.1 Comparison of Mechanical and Electrical Performance

-Ethylene propylene diene monomer (EPDM): As a traditional material, EPDM exhibits excellent ozone resistance and weather resistance due to its saturated main chain structure. Its typical hardness is about 49 Shore A, the tensile strength can reach 11.8 MPa, and the elongation at break is as high as 641%, making it have good resistance to deformation recovery. In terms of electrical performance, the dielectric strength of EPDM is about 19.1 kV/mm, but the dielectric constant is relatively high (5.0-5.6), which may cause larger capacitive currents in high electric field environments.

-Silicone rubber: A low-energy surface is formed by the directional arrangement of side chain methyl groups, and its hydrophobic properties become the biggest highlight. In terms of mechanical properties, the tear strength of silicone rubber (≥ 60 kN/m) is significantly better than EPDM (about 38.6 kN/m), but the elongation at break (≥ 400%) is slightly lower. The electrical performance advantages are reflected in lower dielectric constant (ε ≤ 3.0) and higher volume resistivity (≥ 2 × 10 13 Ω· cm), effectively reducing leakage current.

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Table: Comparison of Key Performance Parameters between EPDM and Silicone Rubber

>Note: Although the breakdown strength of silicone rubber is lower than that of EPDM, surface flashover rarely occurs during actual operation due to its hydrophobicity protection

1.2 In depth analysis of environmental tolerance

-Weather resistance and UV resistance: EPDM maintains good physical properties after 2000 hours of UV irradiation, but is prone to water tree aging in high humidity environments; After the same UV irradiation, the static contact angle of silicone rubber only decreased from 110.9 ° to 102.3 ° (still maintaining HC2 level hydrophobicity), and its small molecule migration mechanism compensated for the surface aging loss.

-Temperature adaptability: EPDM has a narrow working temperature range (-40~105 ℃) and is prone to permanent deformation in high-temperature areas; Silicone rubber can maintain elasticity in a wide temperature range of -60~180 ℃, and its molecular chain flexibility has significant advantages in extremely cold regions.

-Chemical stability: EPDM exhibits excellent performance in acidic and alkaline media, but is prone to swelling when exposed to mineral oil; Silicone rubber has stronger solvent resistance, especially suitable for complex chemical environments such as petrochemical plants.

2. Hydrophobic migration: The "self-healing" defense mechanism of silicone rubber

2.1 Molecular level mechanism of action

The hydrophobic migration ability of silicone rubber is its core technological advantage in becoming the preferred material for high-end cold shrink tubing. This characteristic stems from its unique molecular structure design:

-Main chain structure: It is composed of alternating silicon (Si) - oxygen (O) bonds with a bond energy of up to 447 kJ/mol, which can resist the erosion of ultraviolet radiation (314-419 kJ/mol). The weakly polar surface formed by the side chain methyl group (- CH3) results in a water contact angle of over 110 °.

-Small molecule migration: Oligosiloxane molecules (molecular weight 500-5000) that are not fully crosslinked during the vulcanization process exist as "repair agents" in the matrix. When the surface loses its hydrophobicity due to aging or contamination, these small molecules migrate towards the surface, imparting hydrophobicity to the fouling layer.

2.2 HC level system and performance in polluted environments

The hydrophobicity migration performance is quantitatively evaluated using the water spray grading method (HC method), which is divided into seven levels from HC1 (strongest) to HC7 (hydrophilic):

-Clean surface performance: The initial contact angle of high-quality silicone rubber is greater than 110 °, corresponding to HC1 level. After 2000 hours of UV aging, the material still maintains HC2 grade (contact angle 102.3 °), which reflects its inherent weather resistance.

-Migration ability after pollution: Under the condition of salt density of 0.4 mg/cm 2 (heavily polluted area), hydrophobic migration can reach HC2~HC3 levels within 24 hours, manifested as the formation of separated water droplets on the surface of the pollution layer. The migration speed is inversely proportional to the thickness of the pollution - a 0.3mm pollution layer takes about 4 hours to complete migration, while a 0.9mm thick pollution layer takes more than 12 hours.

-Gradient formula optimization: The fluorosilicone composite gradient layer technology developed by North China University of Science and Technology improves the hydrophobicity migration ability to HC1 level and the contact angle reaches 154 ° through a three-layer process of bottom coating silicone rubber/fluorosilicone blend adhesive, intermediate coating organic composite particles, and surface coating solvent oil.

Three five-year tracking data reveals material differences

Table: Comparison of 5-year operational performance between EPDM and silicone rubber cold shrink tubing

Typical environmental performance:

-Coastal substation (salinity 0.8-1.2g/cm 3):

The EPDM sleeve developed axial cracks after 3 years due to salt crystallization infiltration, resulting in a 35% decrease in dielectric strength; Silicone rubber exhibits hydrophobicity in the salt contaminated layer due to continuous hydrophobic migration, and its alpha coefficient remains stable in the range of 15-25 (normal state).

-Heavy industrial zone (SO ? concentration>50 μ g/m 3):

EPDM surface undergoes sulfurization degradation, resulting in decreased elasticity and seal failure; Although the surface of silicone rubber becomes hydrophilic (HC4 grade), the migration of small molecules restores the fouling layer to HC3 grade within 24 hours, effectively preventing fouling flash.

-Freezing rain area:

EPDM caused micro gaps at the interface with the cable due to low-temperature hardening, resulting in partial discharge (2.5pC); Silicone rubber maintains elasticity at -40 ℃, while its superhydrophobic surface (contact angle>150 °) reduces ice adhesion by 80%.

4. Comprehensive evaluation of cost and lifespan

>Typical case: After replacing EPDM sleeves with silicone rubber in a coastal wind farm, although the initial investment increased by 600000 yuan, the cost of fault repair decreased from an average of 280000 yuan per year to 30000 yuan within 5 years, and the investment payback period was only 2.1 years.

Conclusion: Materials science drives the improvement of power grid reliability

The insulation performance of medium and high voltage cold shrink tubing is essentially a reflection of the design art of polymer materials. EPDM, with its balanced mechanical properties and cost advantages, will still occupy a place in low-pressure conventional environments; Silicone rubber achieves active adaptation to harsh environments through its hydrophobic migration mechanism, making it the preferred choice for medium to high pressure and special scenarios.