4. PA66 Fabric Surface Treatment
4.1 Surface Modification Method
Blending flame retardants directly into the PA66 polymer matrix, often results in reduced polymer strength, and increased difficulty in melt spinning, especially for ultrafine fibers, Co-polymerizing flame retardants with PA66 is also complex and costly.

Therefore, surface finishing of already formed polymer fibers and fabrics, is undoubtedly the most cost-effective, and convenient modification method.

Surface modification or partial degradation of such fibers, through chemical and physical processes, can generate additional reactive functional groups on the material surface, facilitating more effective grafting or adhesion of coatings, thereby imparting flame retardancy or other functionalities. These processes include chemical reactions, UV irradiation, and plasma treatment.

Sun et al. developed a durable finishing method, using 36% formaldehyde solution to hydroxymethylate the surface of PA66 fabrics, followed by crosslinking with thiourea-formaldehyde flame retardant solution.

Results showed an increase in LOI (Limiting Oxygen Index) from 21.6% to 46.2%. After half an hour of washing, the LOI remained at 45%. The char residue of the treated fabric samples, was significantly higher than that of untreated samples. However, this method involves formaldehyde residues, requiring further improvement for commercialization. To address this, UV-initiated grafting is a more suitable technology for fabric surface modification.

Kundu et al. synthesized a novel phosphorus nitrogen flame retardant intermediate, DOPO based phosphorus nitrogen monomer DOPO-DAAM, and grafted it onto the surface of PA66 fabrics via UV grafting. In the UL94 vertical burning test, PA66 fabrics treated with DOPO-DAAM exhibited no dripping.

The PHRR (Peak Heat Release Rate) of PA66 fabrics grafted with 20% DOPO-DAAM decreased by 22%. Liu et al. studied a combined method of metal ion coordination and surface photografting modification (M/P technology), integrating Fe3+-containing inorganic-organic hybrid structures onto the surface of PA66 fabrics.
Flame retardancy tests showed an LOI of 33.4% for the treated PA fabrics, with no dripping during combustion. The treated fabrics exhibited good wash stability, maintaining a high Fe/C ratio and LOI (27.8%) after 45 washes.
Plasma treatment enables surface modification without altering the bulk properties of the material. Boaretti used microwave plasma technology to surface-treat PA66 fabrics, obtaining flame-retardant fabrics. Optimization using experimental design methods, reduced the flame retardant concentration while achieving optimal fire resistance, with a final LOI reaching 43%.

4.2. Flame Retardant Coating on Nylon Surface
To enhance the flame retardancy of nylon fabrics, post-treatment methods such as dipping and adsorption are more easily implemented.

The research group of Hu Yuan at the University of Science and Technology of China, has done extensive work on the layer-by-layer (LbL) assembly of flame retardant coatings on PA66 fabrics. The basic approach involves sequentially depositing different materials, such as polymer electrolytes, onto PA66 fabrics via LbL assembly to improve flame retardancy.

A typical combination is the four-layer assembly of chitosan (CS), IP6, and oxidized sodium alginate (OSA) (CS-IP6-CS-OSA), which produced PA66 fabrics that did not drip during vertical burning tests. Cone calorimetry results showed a maximum 24% reduction in PHRR for the modified PA66 fabrics.

Different flame retardant materials can be surface-coated in this manner, including borates, (3-aminopropyl)triethoxysilane, polyacrylic sodium (PAS), DOPO derivatives, montmorillonite, and organic-inorganic hybrid flame retardant compounds.

4.3 Modified and Coated PA66 for Flame Retardant
Due to the hydrophobicity and relatively low surface energy of PA66 fibers, surface coating can be challenging. A solution is to first chemically modify (e.g., acrylic acid grafting) and then apply the surface coating.

Kundu synthesized phosphorylated chitosan (PCS) and grafted it onto the surface of PA66 fabrics via UV-initiated graft polymerization. Subsequently, a sol-gel coating containing ammonium polyphosphate APP was applied, resulting in a significant increase in LOI to 34.5% and the elimination of dripping during combustion. This combined modification and coating approach effectively improved the flame retardancy of PA66 fabrics.