A comprehensive analysis of the water resistance issues of low-smoke halogen-free flame-retardant cable materials.

In actual use environments, cables will inevitably come into contact with moisture. For instance, cables buried underground may be eroded by groundwater, and in some damp industrial environments or outdoor settings, they may also be affected by rainwater, vapor, and other forms of moisture. If the water resistance of low smoke zero halogen flame retardant cable materials is poor, the overall performance and service life of the cables will be greatly compromised, and may even pose potential safety hazards.

Next, let us delve into a detailed discussion on the water resistance of low smoke zero halogen flame retardant cable materials, with the aim of providing a clearer understanding of this issue.

Water Resistance Requirements in Relevant Standards

Water Resistance Standards for Jacket Materials

• In numerous relevant standards, there are clear and detailed requirements for the water resistance of low smoke zero halogen flame retardant jacket materials. For example, in the BS7655-6.1 standard, for LST1/LST3/LST4 types of low smoke zero halogen flame retardant jackets, it is stipulated that the change rate of tensile strength and elongation after immersion in water at 70°C for 168 hours cannot exceed ±30%. Subsequent standards such as EN50525 for TM7 type and IEC62821 for LSHF/ST1 type jackets also adhere to this requirement. Furthermore, the upcoming national standard for low smoke zero halogen flame retardant materials for wires and cables adopts this requirement, reflecting the continuity and consistency of key indicators among standards.
• However, the ST8 halogen-free jacket material in the GB/T12706 standard differs, as it assesses the immersion performance of the material based on weight change after immersion. It is worth noting that jackets that can meet this requirement may not necessarily pass the aforementioned hot water immersion test, indicating that different assessment methods focus on different aspects of material performance.

Water Resistance Standards for Insulation Materials

• For low smoke zero halogen flame retardant insulation materials, different standards also have their respective targeted water resistance requirements. In the EN50525 standard, TI6 and EI8 types of insulation are required to pass a 220V DC resistance test in 10g/L NaCl water at 60°C for 240 hours. The EN50264 standard for railway cables stipulates that insulation for cables with a rated voltage of 0.6kV must pass a 1.5kV DC resistance test in 3% NaCl water at 85°C for 240 hours. The insulation for solar cables in the 2PfG1169/08.2007 standard needs to pass a 900V DC resistance test in 3% NaCl water at 85°C for 240 hours. Additionally, the UL standard for outdoor cables has strict requirements on the change in insulation resistance in water over a long period.
• These requirements aim to assess the stability and reliability of the insulation performance of insulation materials in water under different conditions. Once the insulation performance of insulation materials is compromised by moisture, it may lead to cable shorts and other failures, posing a serious threat to electrical safety. Through the constraints of these standards, relevant manufacturing enterprises can be prompted to pay attention to the water resistance of insulation materials, continuously improve formulas and processes, and ensure that low smoke zero halogen flame retardant insulation materials can maintain good insulation effects even when in contact with moisture in practical applications, thereby safeguarding the normal operation of cable systems.

Factors Influencing Water Resistance

Material Characteristics

• The water resistance of low smoke zero halogen flame retardant cable materials is greatly influenced by the material's own characteristics. Firstly, inorganic hydroxide flame retardants, such as aluminum hydroxide and magnesium hydroxide, are commonly used flame retardants in this type of cable material. While they play a role in flame retardancy, they have the characteristic of being easily absorbent and not moisture-proof. When cables are in damp environments, these flame retardants can absorb a large amount of moisture, increasing the overall moisture content of the cable material and affecting its physical and electrical properties.
• Furthermore, polyolefin resin, as the base material, also has certain deficiencies in terms of water resistance. The structural characteristics of the polyolefin molecular chain make it have a certain affinity for water molecules. Although it is not as hydrophilic as some materials that easily absorb water, water molecules can gradually penetrate into the material under long-term exposure to moisture, disrupting intermolecular forces and the material's microstructure. This leads to swelling, softening, and other issues, resulting in reduced mechanical properties such as mechanical strength and toughness.
• Moreover, the changes in the material's internal structure due to water intrusion may also affect its bonding with other additives and fillers, further weakening the overall water resistance of low smoke zero halogen flame retardant cable materials. Ultimately, this adversely affects the stability and lifespan of cables in practical use.

Environmental Factors

External environmental conditions play a crucial role in the water resistance of low smoke zero halogen flame retardant cable materials.

• In terms of temperature, under high-temperature conditions, the evaporation rate of moisture accelerates, and the moisture inside the cable material attempts to diffuse to the outside. In this process, it may cause thermal stress changes in the material's internal microstructure, leading to defects such as fine cracks, which create conditions for subsequent moisture ingress. When the temperature drops, water vapor in the air is prone to condensing into liquid water on the cable surface, which then penetrates into the cable material.
• Humidity directly affects the moisture content of cable materials. In high-humidity environments, such as the rainy season in the south or some damp basements and mines, the air around the cables contains a large amount of water vapor. This water vapor is constantly adsorbed by the cable material, keeping it in a state of high moisture content for an extended period and accelerating the deterioration of material properties.

Different usage scenarios also exhibit significant differences. For instance, cables used outdoors need to endure complex climatic conditions such as wind, rain, sun, snow, and freezing. Rainwater, snowmelt, and dew caused by diurnal temperature variations all expose cables to frequent contact with moisture. Cables used underground, in addition to facing erosion from groundwater, also encounter a relatively enclosed and damp environment where moisture is difficult to dissipate and accumulates around the cables for extended periods, posing higher requirements for the water resistance of cable materials. Therefore, environmental factors cannot be ignored in the issue of water resistance of low smoke zero halogen flame retardant cable materials, as the manifestation and impact degree of water resistance problems vary significantly under different environments.

Methods to Address Water Resistance Issues

Formula Adjustment and Design

• By reasonably adjusting the composition and ratio of various additives, the water resistance of low smoke zero halogen flame retardant cable materials can be effectively enhanced. For example, in the selection and pairing of flame retardants, nitrogen-based melamine cyanurate and melamine polyphosphate can be used in combination. This compounded flame retardant system not only has the advantages of low toxicity, zero halogen, non-corrosion to cables, and stability to heat and ultraviolet light, but also optimizes the water absorption and other characteristics that affect water resistance compared to some traditional flame retardants.
• At the same time, the use of coupling agents is also crucial. Coupling agents such as vinyltrimethoxysilane, during the mixing and processing, cause the base resin compound with some crosslinked structure to undergo both resin crosslinking and grafting reactions between the resin and silane under the combined action of crosslinking agents and coupling agents. The resulting cable material improves the mechanical properties of the material. The silane grafting reaction of the resin can further undergo crosslinking reactions in the subsequent wet and hot environment of the material, reducing the mechanical property attenuation of the material and thus enhancing the water resistance of the finished product.
• In terms of antioxidants, the combination of primary antioxidants (such as pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] or bis(2-methyl-4-hydroxy-5-tert-butylphenyl) sulfide) and secondary antioxidants (such as dilauryl thiodipropionate or distearyl thiodipropionate) can improve the heat resistance of the product, delay aging, and indirectly ensure the performance stability of the cable material in complex environments, including those with water, thereby enhancing water resistance.
• In principle, these additives interact synergistically, improving the internal structure and chemical properties of the cable material. When faced with moisture erosion, it can better maintain its physical and electrical properties, reducing adverse effects such as decreased insulation resistance and reduced material strength due to water absorption. This leverages the advantages of enhanced water resistance, ensuring reliable operation of cables in damp environments.

Structural Optimization and Improvement

• Adopting appropriate structural improvements has a positive effect on enhancing the water resistance of cable materials. For example, using a structure with a thin layer of PE as the inner insulation and low smoke zero halogen flame retardant insulation as the outer layer. This design provides cable manufacturers with greater reassurance, but it also increases manufacturing costs and places higher flame retardant requirements on the outer layer of low smoke zero halogen flame retardant insulation.
• Additionally, various structural improvement measures such as using waterproof layers and water-blocking tapes can be employed. Waterproof layers can effectively block the intrusion of external moisture, reducing the odds for moisture to come into contact with the main body of the cable material.