Nitrogen-based halogen-free flame retardants are a type of highly efficient flame retardant additives with nitrogen as the core functional group and no halogen. Their working principle combines both physical barrier effect and chemical chain reaction intervention mechanism. The following is a systematic analysis from the core action path, synergistic enhancement mode and typical application scenarios:
First, the core operating principle
Condensed phase flame retardant mechanism
Expanded carbon layer construction
Nitrogen-based flame retardants (such as melamine cyanurate, MCA) release a large amount of non-flammable gases (such as NH₃, N₂) when heated and decomposed, and these gases form a porous and expanded carbon layer on the material surface. This carbon layer has the following characteristics:
Thermal insulation barrier: The thermal conductivity is reduced to 0.05-0.1 W/(m·K), which is only 1/5 of that of ordinary carbon layers, effectively slowing down heat transfer.
Oxygen barrier: The porosity can reach 80%-90%, and the gas diffusion coefficient is reduced to 10⁻¹² m²/s, significantly inhibiting oxygen permeation.
Structural support: The triazine ring structure endows the carbon layer with high strength (compressive strength up to 5-10 MPa), preventing the carbon layer from collapsing at high temperatures.
Enhanced thermal stability
The decomposition products of nitrogen-based compounds (such as melamine dimer) can form hydrogen bond networks with polymer chains, increasing the initial decomposition temperature (T₅%) of the material by 50-80 ° C and delaying the thermal degradation process.
2. Gas-phase flame retardant mechanism
Free radical quenching
The nitrogen-containing free radicals such as NH₃ and HCN produced by decomposition react with the active free radicals (H·, OH·) from the combustion chain reaction in the gas phase:
The rate constants of reactions such as NH3 +H/cdotp→NH2 ⋅+H2 and HCN+OH/cdotp→NCO+ H2O are as high as 10⁹-10¹⁰ L/(mol·s), which can interrupt over 90% of the chain reactions.
Dilution effect
Each gram of nitrogen-based flame retardant decomposes to release 300-500 mL of gas, diluting the concentration of flammable volatile substances to less than one-third of the lower explosive limit (LEL), directly suppressing the spread of flames.
Second, the synergy and efficiency enhancement model
1. Synergy with phosphorus-based flame retardants
P-N synergy effect
Phosphoric acid generated from the decomposition of phosphorus-based flame retardants (such as APP) catalyzes the carbonization of materials, while the gases released by nitrogen-based compounds cause the carbon layer to expand, forming a composite structure of “internal catalysis – external expansion”. For example:
The limiting oxygen index (LOI) of the APP/MCA compound system in PP can reach 32%, which is 15% higher than that of the single component.
The peak heat release rate (PHRR) of the carbon layer decreased by 65%, and the total heat release (THR) decreased by 40%.
2. Synergy with inorganic fillers
Porous structure strengthening
When nitrogen-based flame retardants are compounded with layered silicates (such as montmorillonite, MMT), the layered structure of MMT can be embedded in the expanded carbon layer, forming a “nano-sheet layer – micrometer pore” composite system:
The thermal conductivity of the carbon layer was further reduced to 0.03 W/(m·K).
The oxygen permeability decreased to 10⁻¹⁴ m²/s, approaching the barrier level of metal oxides.
Third, typical application scenarios and performance optimization
1. Field of electronics and electrical appliances
PC/ABS alloy is flame-retardant
When MCA is compounded with silicone modifiers at an addition amount of 15% :
Passed UL-94 V-0 grade (0.8mm thickness), smoke density grade (SDR) ≤75.
The retention rate of impact strength reaches 85%, which is superior to the 60% of halogen-based flame retardants.
Wire and cable sheaths
The application of APP/MCA/ZnB (Zinc borate) ternary system in PVC:
Vertical burning complies with the IEC 60332-3-24 standard, and the probability of burning droplets igniting the cotton pad is 0.
The tensile strength is increased by 20%, and the retention rate of elongation at break is over 90%.
2. Transportation field
Battery packs for new energy vehicles
DOPO derivatives (nitrogen-phosphorus structure) and MCA synergistic flame-retardant epoxy resin:
The thermal runaway trigger temperature (T youdaoplaceholder4) has been raised to 280 ° C, which is 100 ° C higher than that of pure resin.
Passed the needle-puncture test of GB/T 31467.3, with the maximum temperature rise < 50℃.
Interior materials for high-speed railways
Microencapsulated MCA flame-retardant polypropylene
It meets EN 45545-2 HL3 grade (smoke toxicity FED < 0.5, peak heat release rate < 60 kW/m²).
The odor grade is ≤ grade 3 (VDA 270 standard), which meets the requirements of in-vehicle air quality.
Fourth, technical challenges and optimization directions
Compatibility improvement
Through grafting modification (such as MCA grafted maleic anhydride) or in-situ polymerization, the dispersed particle size was reduced to 50-100 nm, and the retention rate of impact strength was increased to over 90%.
Enhanced hydrolysis resistance
Nitrogen-based flame retardants are treated with hydrophobic coating layers (such as fluorosilicone resin), reducing the water absorption rate to below 0.2%, and the LOI attenuation rate after wet heat aging is less than 5%.
Low-cost path
The development of bio-based nitrogen-based flame retardants (such as chitosan derivatives) reduces the raw material cost by 30% compared with traditional MCA and is completely biodegradable.
Nitrogen-based halogen-free flame retardants have become the mainstream alternative to halogen-based flame retardants through a dual mechanism of “physical barrier by expanded carbon layer + chemical inhibition by free radical quenching”, combined with the synergistic effect of multiple components. In the future, it is necessary to further resolve the compatibility contradiction between it and polar matrices and develop modification technologies that adapt to high-temperature and high-humidity environments.
