Introduction to Nitrogen-Based Flame Retardants for Nylon

Introduction to Nitrogen-Based Flame Retardants for Nylon

Nitrogen-based flame retardants are characterized by low toxicity, non-corrosiveness, thermal and UV stability, good flame-retardant efficiency, and cost-effectiveness. However, their drawbacks include processing difficulties and poor dispersion in the polymer matrix. Common nitrogen-based flame retardants for nylon include MCA (melamine cyanurate), melamine, and MPP (melamine polyphosphate).

The flame-retardant mechanism involves two aspects:

1. “Sublimation and Endothermic” Physical Mechanism: The flame retardant reduces the surface temperature of the polymer material and isolates it from air through sublimation and heat absorption.
2. Catalytic Carbonization and Intumescent Mechanism in the Condensed Phase: The flame retardant interacts with nylon, promoting direct carbonization and expansion.

MCA exhibits dual functions in the flame-retardant process, promoting both carbonization and foaming. The flame-retardant mechanism and effectiveness vary depending on the type of nylon. Studies on MCA and MPP in PA6 and PA66 reveal that these flame retardants induce cross-linking in PA66 but promote degradation in PA6, resulting in better flame-retardant performance in PA66 than in PA6.

1. Melamine Cyanurate (MCA)

MCA is synthesized from melamine and cyanuric acid in water, forming a hydrogen-bonded adduct. It is an excellent halogen-free, low-toxicity, and low-smoke flame retardant commonly used in nylon polymers. However, traditional MCA has a high melting point (decomposing and sublimating above 400°C) and can only be blended with resins in solid particle form, leading to uneven dispersion and large particle size, which negatively impacts flame-retardant efficiency. Additionally, MCA primarily functions in the gas phase, resulting in low char formation and loose, non-protective carbon layers during combustion.

To address these issues, molecular composite technology has been employed to modify MCA by introducing a complementary flame-retardant additive (WEX), which lowers the melting point of MCA, enabling co-melting and ultra-fine dispersion with PA6. WEX also enhances char formation during combustion, improving the quality of the carbon layer and strengthening the condensed-phase flame-retardant effect of MCA, thereby producing flame-retardant materials with excellent performance.

2. Intumescent Flame Retardant (IFR)

IFR is a significant halogen-free flame-retardant system. Its advantages over halogenated flame retardants include low smoke emission and non-toxic gas release during combustion. Moreover, the char layer formed by IFR can absorb molten, burning polymer, preventing dripping and fire spread.

Key components of IFR include:

• Gas source (melamine-based compounds)
• Acid source (phosphorus-nitrogen flame retardants)
• Carbon source (nylon itself)
• Synergistic additives (e.g., zinc borate, aluminum hydroxide) and anti-dripping agents.

When the mass ratio of phosphorus-nitrogen flame retardants to melamine-based compounds is:

• Below 1%: Insufficient flame-retardant effect.
• Above 30%: Volatilization occurs during processing.
• Between 1%–30% (especially 7%–20%): Optimal flame-retardant performance without affecting processability.

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