The water resistance assessment of low smoke zero halogen flame retardants needs to be combined with their material properties, testing standards and actual application scenarios. The following is a specific analysis:
First, the core indicators for water resistance assessment
Changes in mechanical properties after immersion in water
Standard requirements: As stipulated in the BS7655-6.1 standard, for the LST1/LST3/LST4 type sheaths, after being immersed in water at 70℃ for 168 hours, the change rate of tensile strength and elongation at break should be ≤±30%.
Actual impact: If the strength of the material drops significantly after absorbing water (e.g., >30%), it may lead to cracking and peeling of the sheath during long-term use.
Rate of change of weight
According to the GB/T12706 standard, the weight change of ST8 halogen-free sheath material after immersion in water is evaluated. However, it should be noted that this indicator has no direct correlation with mechanical properties.
Actual impact: Some materials may meet the standards through weight changes, but their mechanical properties still fail to satisfy the requirements.
Electrical performance stability
Insulation water resistance test: As stipulated in the EN50525 standard, TI6 type and EI8 type insulations need to pass a 220V DC resistance test in 10g/L NaCl water at 60℃ for 240 hours.
Actual impact: Long-term immersion in water may lead to a decrease in insulation resistance and increase the risk of short circuits.
Second, the water resistance performance of different types of low smoke zero halogen flame retardants
Epoxy resin flame retardant
Features: Excellent fire resistance, but sensitive to water. Long-term exposure will lead to performance degradation.
Application suggestion: It should be combined with a waterproof coating or structural design to avoid direct exposure to damp environments.
Ammonium aluminum phosphate flame retardant
Features: It has good water resistance and can be used for a long time in a humid environment without affecting the flame retardant effect.
Application suggestion: Suitable for cable sheaths in damp places such as subways and tunnels.
Aluminum hydroxide flame retardant
Characteristics: It has good water resistance, but its stability decreases at high temperatures, which may lead to a decline in flame retardant performance.
Application suggestion: The processing temperature needs to be controlled to avoid exceeding its decomposition temperature (approximately 200℃).
Third, technical paths for enhancing water resistance
Formula optimization
Hydrophobic modification: By adding silane coupling agents (such as LC-100) to enhance the hydrophobicity of the filler surface and reduce the water absorption rate.
Crosslinking technology: Irradiation crosslinking or silane crosslinking technology is adopted to enhance the material’s density and reduce water penetration.
Structural design
Composite structure: It adopts a composite structure of inner PE (polyethylene) and outer halogen-free flame-retardant insulation to enhance the overall water resistance.
Thickness optimization: Increasing the thickness of the sheath layer can reduce the water penetration rate, but it is necessary to balance cost and performance.
Additive application
Hydrophobic agent: Add fluorine or silicon hydrophobic agents to form a waterproof barrier.
Nano-fillers: Nano-scale magnesium hydroxide or silicon dioxide can improve interfacial compatibility and reduce moisture adsorption.
Fourth, the challenge of water resistance in practical applications
Cable manufacturing process
Extrusion temperature: Excessively high temperatures may cause inorganic flame retardants to decompose, generating pores and affecting water resistance.
Screw design: Improper screw compression ratio can lead to the decomposition of hydrated oxides, generating bubbles and reducing the material’s strength.
Environmental factors
Long-term immersion in water: For instance, photovoltaic cables need to pass a 900V DC resistance test in 3%NaCl water at 85℃ for 240 hours. The requirement for water resistance is extremely high.
Temperature fluctuations: Base materials such as polyethylene are sensitive to temperature. The internal stress caused by thermal expansion and contraction often leads to interface cracks.
Cost and performance are balanced
High filling amount: Inorganic flame retardants require a high filling amount (>50%) to achieve flame retardant effects, but this will reduce the material’s flexibility and tear resistance.
Composite materials: The adoption of composite flame retardant systems (such as phosphorus-nitrogen synergy) can reduce the filling amount while enhancing water resistance and flame retardancy.
Fifth, Conclusions and Suggestions
Selection of evaluation methods
Mechanical properties (tensile strength, elongation at break) and electrical properties (insulation resistance) are given priority as core evaluation indicators.
The rate of change in water immersion weight is combined for auxiliary judgment, but its limitations should be noted.
Suggestions for material selection
In humid environments: Ammonium aluminum phosphate or hydrophobic modified aluminum hydroxide flame retardants should be given priority.
High-temperature environment: Irradiation cross-linking technology or composite structures are adopted to enhance the heat resistance and water resistance of materials.
Process optimization direction
Control the extrusion temperature and screw design to prevent the decomposition of flame retardants and the formation of pores.
Add hydrophobic agents and nano-fillers to enhance the interfacial compatibility and waterproof performance of the material.
Through the above assessment and optimization, the water resistance of low smoke zero halogen flame retardants can be significantly enhanced to meet the demands of various application scenarios.
