3. Intrinsic Flame Retardancy of PA6, such as melamine phosphate flame retardant
3.1 Copolymerization Modification Method
Intrinsic flame retardancy refers to the method of incorporating flame retardant elements such as nitrogen, silicon, phosphorus, etc., into the main or side chains of polymers through chemical reactions, to achieve flame retardancy.

The preparation of intrinsically flame retardant polymers, generally involves molecular design to produce monomers containing flame retardant elements, followed by polymerization or copolymerization to obtain the intrinsically flame retardant polymers.

Compared to blending, copolymerization can impart inherent flame retardancy, and higher durability to PA66 at the molecular level.

Fu et al. synthesized a reactive flame retardant (TRFR) using pentaerythritol, phosphorus oxychloride, and p-aminobenzoic acid as raw materials. TRFR was then copolymerized with PA66 salt to prepare flame retardant PA66.

Mechanism studies revealed that, the TRFR structure generates incombustible gases during material combustion, forming a dense, porous, and incombustible carbon layer, that prevents continuous polymer combustion. When the TRFR salt content was 3%, the LOI of the flame-retardant PA66 reached 29%, and the UL94 rating was V-0.

Phosphate flame retardants such as ammonium polyphosphate MPP are particularly suitable for PA66 copolymer flame retardancy due to their excellent reactivity, flame retardancy, and environmental friendliness.

Yang et al. first copolymerized bis(4-carboxyphenyl)phenylphosphine oxide (BCPPO) , with adipic acid hexamethylenediammonium salt (AH salt), to obtain intrinsically flame retardant PA66. Experiments showed that BCPPO and AH salt had good copolymerization properties, and their addition did not change the crystalline structure of PA66.

The LOI of flame-retardant PA66 containing 9% BCPPO reached 27.2%, with a corresponding UL94 rating of V-0. The improved thermal stability of the material can be attributed to the formation of a dense carbon protective layer and the synergistic effect of phosphorus and nitrogen.

Lyu et al. first synthesized a halogen-free phosphorus nitrogen flame retardant, poly-N-aniline-phenylphosphoramide (PDPPD), and then copolymerized PDPPD with AH salt. When the PDPPD content was 4.5%, the LOI and UL94 test results of the flame-retardant PA66 reached 28% and V-0, respectively, following the gas-phase flame retardant mechanism.

The key factors in synthesizing intrinsically flame-retardant polymers are the thermal stability, and reactivity of phosphorus-based flame retardants. The thermal stability of phosphonate structures is usually enhanced by attaching them to large sterically hindered groups.

Zhang et al. proposed a strategy to improve the thermal stability of the reactive flame retardant (CPPOA) , by reacting it with hexamethylenediamine to obtain CPPOA salt, which was then copolymerized with PA66 salt to obtain intrinsically flame-retardant PA66.

The LOI and vertical burn rating of PA66 containing 6% CPPOA reached 27.2% and V-0, respectively, with good tensile and impact strength.

Li et al. prepared phosphorus-containing linking group PA66 with inherent flame retardancy, through polycondensation using AH salt and DDP as comonomers. Before polycondensation, DDP reacted with hexamethylenediamine, making the halogen-free and environmentally friendly flame retardant DDP, easier to react with aluminum hydroxide flame retardant AH salt. The tensile strength of PA66 containing 5% DDP was 58.44 MPa, with an LOI of 33.2% and a UL94 rating of V-0, based on vertical burn tests.

Chen et al. prepared flame-retardant copolymerized PA66 using in situ polymerization of AH salt and 2-carboxyethyl(phenyl)phosphonic acid (CEPPA). The flame-retardant modified PA66, exhibited better flame retardancy and charring ability than pure PA66. By adding 6% CEPPA, the LOI increased from 24.0% to 28.0%, and all flame retardant samples were rated UL94 V-0.

Furthermore, thermal stability analysis showed that in situ polymerization, with CEPPA effectively reduced the initial decomposition temperature and increased the char residue.