Research on the Thermal Stability of Low Smoke Zero Halogen Flame Retardants
Low smoke zero halogen flame retardants are widely used in fields such as wires and cables, building materials, and electronic appliances due to their environmental friendliness, low toxicity and low smoke characteristics. Its thermal stability is the key factor determining the flame retardant performance and service life of the material in high-temperature environments. The following is a review from the two aspects of inorganic flame retardants and organic flame retardants, combined with the current research status of their thermal stability.
First, the thermal stability of inorganic low smoke zero halogen flame retardants
Aluminum hydroxide (ATH) and magnesium hydroxide (MH)
ATH: The decomposition temperature is approximately 200-220℃, and the material temperature is reduced through endothermic decomposition (2Al(OH)₃ → Al₂O₃ + 3H₂O↑). Studies show that the addition of ATH can significantly increase the initial temperature of thermal decomposition and the amount of residual carbon of the material, but a high filling amount (usually >50%) is required to achieve effective flame retardancy.
MH: The decomposition temperature is approximately 340-490℃, which is higher than ATH. It is suitable for engineering plastics with higher processing temperatures. The flame retardant efficiency of MH is superior to that of ATH, but it still requires a high filling amount and has a significant impact on the mechanical properties of the material.
Synergy effect: The combined use of ATH and MH can optimize thermal stability. For example, when the addition ratio of ATH/MH is 1:1, both the thermal decomposition temperature and the amount of residual carbon of the material reach the optimum.
Boron-based flame retardants
Boron-based flame retardants such as zinc borate isolate heat and oxygen by forming a glassy coating. They have excellent thermal stability and their decomposition temperature is usually above 300℃. Studies have shown that when boron-based flame retardants and phosphorus-based flame retardants work in synergy, they can significantly improve the thermal oxidation stability of materials and the quality of residual carbon.
Expandable graphite (EG
EG begins to expand at 220℃ and rapidly expands to 100-280 times its original volume at 230-280℃, forming a porous carbon layer. Its thermal stability depends on the expansion temperature and the particle size of graphite, and it is suitable for flame retardancy in high-temperature environments.
Second, the thermal stability of organic low smoke zero halogen flame retardants
Phosphorus-based flame retardants
Red phosphorus: An efficient flame retardant, but it is prone to moisture absorption and oxidation, and requires microencapsulation treatment. The flame retardant mechanism of red phosphorus is condensed phase flame retardancy, which is achieved through the dehydration and carbonization of catalytic materials. Studies show that the combination of red phosphorus and magnesium hydroxide can significantly improve the thermal stability and flame retardancy grade of materials.
Ammonium polyphosphate (APP) : Decomposition temperature >256℃, promoting the formation of carbon layers by releasing ammonia gas and polyphosphate. The thermal stability of APP is superior to that of organophosphorus flame retardants and it can be used alone or in combination with other flame retardants.
Nitrogen-based flame retardants
Melamine and its salts dilute the oxygen concentration through gas-phase flame retardancy, but their flame retardant efficiency is relatively low when used alone. They are usually compounded with phosphorus-based flame retardants to form intumescent flame retardants (IFRs). The thermal stability of IFR depends on the synergistic effect of phosphorus and nitrogen elements, and a dense carbon layer can be formed at a relatively low temperature.
Silicon-based flame retardants
Silicate flame retardants achieve flame retardancy by absorbing heat to decompose and dilute flammable gases. They have excellent thermal stability, and their decomposition temperature is usually above 300℃. Studies have shown that the combination of silicon-based flame retardants and phosphorus-based flame retardants can significantly improve the thermal oxidation stability and mechanical properties of materials.
Third, research methods and improvement directions for thermal stability
Thermogravimetric analysis (TGA
TGA is the main method for evaluating the thermal stability of flame retardants. By analyzing the thermal decomposition temperature, residual carbon content and thermal decomposition kinetic parameters of materials, the formula of flame retardants can be optimized.
Synergistic flame retardant system
Through the synergistic effect of inorganic-inorganic and inorganic-organic flame retardants, the thermal stability of materials can be significantly improved. For example, systems such as ATH/MH and APP/PER/MEL (Ammonium polyphosphate/pentaerythritol/melamine) exhibit synergistic effects in terms of thermal stability.
Surface modification technology
Ultrafine, nanoscale and surface coating technologies can improve the compatibility of flame retardants with polymers and enhance thermal stability. For instance, the flame retardant efficiency of nano-scale magnesium hydroxide is significantly better than that of micron-scale products.
Fourth, application cases and performance comparison
Wire and cable
Low smoke zero halogen flame retardant cable materials usually adopt the ATH/MH compound system, combined with cross-linking technology to improve thermal stability. For example, irradiated cross-linked PE-based composites have the highest thermal stability at an irradiation dose of 15.8 kGy.
Epoxy resin
The addition of DOPO (9, 10-dihydro-9-oxa-10-phosphoafine-10-oxide) and its derivatives to epoxy resin can significantly improve the thermal stability and flame retardancy grade. For example, the epoxy resin composite material containing DOPO achieved the V-0 grade in the UL-94 test, and the LOI value could reach 35.2%.
Polyolefin
Adding intumescent flame retardants (IFR) to polyolefin materials can achieve low smoke and halogen-free flame retardancy. Studies have shown that the addition of IFR can reduce the heat release rate (HRR) and smoke release rate (SPR) of the material and improve its thermal stability.
Fifth, Future development trends
“效 率” 可 以 翻 译 为 “high efficiency”
Develop a flame retardant system with high flame retardant efficiency and low addition amount to reduce the impact on the mechanical properties of materials.
Multifunctionalization
Flame retardants are developing towards multiple functions such as smoke suppression, anti-dripping and toughening, to meet the demands of complex application scenarios.
“Environmentally friendly”
Halogen-free, low-toxicity and recyclable flame retardants will become mainstream, in line with the development trend of green chemistry.
“Nanoscale”
Nano flame retardants have excellent dispersibility and interfacial interaction, which can significantly improve the thermal stability and flame retardant performance of materials.
Vi. Conclusion
The thermal stability of low smoke zero halogen flame retardants is one of the core indicators for their performance evaluation. Through the synergistic effect of inorganic flame retardants and organic flame retardants, surface modification technology and nanometization and other means, the thermal stability of flame retardants can be significantly improved to meet the demands of different application fields. In the future, with increasingly strict environmental protection regulations and the continuous improvement of material performance requirements, low smoke zero halogen flame retardants will develop in the direction of high efficiency, multi-functionality and environmental protection.
