2.Flame Retardant Blending of PA66.
Flame retardant blending involves the addition of materials, such as aluminum hypophosphite , phosphorus flame retardant etc.

2.1 Phosphorus Based Flame Retardants.
Phosphorus based flame retardants function in the condensed phase, by reacting with polymers, to facilitate char formation, making them particularly effective for oxygen or nitrogen containing polymers.

Red phosphorus is a crucial phosphorus based flame retardant, highly efficient in nylon. Through coaxial electrospinning, a nanofiber with a core-shell structure, is prepareing using PA66, and red phosphorus hybridized nano-graphene (NG-RP). NG-RP, enhances the flame retardancy and mechanical properties of the nanofibers,while maintaining the color and gloss of PA66.

Glass fiber reinforced PA66 composites,with red phosphorus flame retardant exhibit exceptional strength and flame retardancy, widely used in electrical engineering. However, the addition of glass fibers reduces the flame retardancy of PA and creates a "wicking effect," making PA66 more susceptible to combustion, resulting in poor flame retardancy, and limiting its application.

Red phosphorus, on the other hand, improves the flame retardancy of PA66 composites, while preserving the mechanical properties of nylon. Research shows that appropriate post treatment can enhance the crystallinity, and crystallization rate of red phosphorus flame-retarded glass fiber-reinforced PA66 composites, significantly improving their mechanical and flame retardant properties.

Aluminum diethylphosphinate AlPi is a novel environmentally friendly phosphorus based flame retardant, with high thermal and chemical stability. with a phosphorus content of up to 23.85%, AlPi promotes carbon formation in the condensed phase, and eliminates high-energy radicals in the combustion zone.

Phosphorus compounds are often used in combination with other flame retardants, including nitrogen compounds, nano-fillers, and metal hydroxides. Aluminum dicyclohexyl phosphinate (ADCP) has been synthesized and applied to flame retard PA66. ADCP/PA66 composites exhibit good flame retardancy, while maintaining the mechanical properties of the base material.

The optimal dosage of ADCP is 15%, achieving a V-0 rating in the UL94 vertical burning test.9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives are typical phosphorus based flame retardants, effective in both the gas and condensed phases. The novel flame retardant intermediate DOPO, possesses active P-H groups, enabling easy introduction of the phosphaphenanthrene group, into other molecules to create new flame retardant molecules.

A melt flame retardant has been synthesized, through the addition reaction of DOPO with 2,4,6-triallyloxy-1,3,5-triazine, and applied to flame retard PA66. The modified PA66 has an LOI of up to 29.4%, and a dense carbon layer, is visibly formed on the surface after combustion. Compared to pure PA66, the PHRR and average effective heat of combustion, are reduced by 26.5% and 19.3%, respectively.

Synergistic flame retardation of glass fiber reinforced PA66, is achieved using a composite of two novel flame retardant intermediates, DOPO-based bisphenol A (DPOH), and aluminum diethylphosphinate (ADP). Thanks to simultaneous action in both the gas and condensed phases, it inhibits heat release while increasing char yield and LOI.

continous......