Nylon 66 (PA66) has good mechanical properties, abrasion resistance and chemical resistance, and is widely used in engineering plastics and chemical fibers. When PA66 is made into fibers or products, it is easy to be ignited due to its lightness and large specific surface area, and it is mostly in a vertical state during use. After being ignited, the flame spreads upwards quickly, so it is more difficult to modify the flame retardant of PA66 fiber.

According to the different constituent elements, flame retardants suitable for PA66 materials can be divided into halogen flame retardants, inorganic flame retardants, nitrogen flame retardants and phosphorus flame retardants. 

Halogen-based flame retardants have high flame retardant efficiency, but most of them will produce toxic gases during use.

Inorganic flame retardants generally require a large amount of addition to achieve better flame retardant effects, and the compatibility with the PA66 matrix is not good, so nitrogen-based flame retardants and phosphorus-based flame retardants are mainly used.

Nitrogen-based flame retardants mainly include melamine, melamine cyanurate (MCA), etc. MCA has a good flame-retardant effect, has a similar chemical composition to nylon, and has good compatibility, so it is widely used in the field of flame-retardant PA66.

Phosphorus flame retardants have the advantages of good thermal stability, low toxicity and long-lasting flame retardant effect. Phosphorus flame retardants suitable for nylon can be divided into additive phosphorus flame retardants and reactive phosphorus flame retardants.

1. Preparation of flame-retardant PA66 resin

(1) DDP pretreatment

In order to make it easier for DDP to participate in the polycondensation reaction of PA66, DDP is firstly aminated.

(2) Preparation of nitrogen-phosphorus synergistic flame-retardant PA66 resin

The weighed hexamethylene diamine adipate (AH salt), MCA and DDP-HMDA are prepared into an aqueous solution with a mass fraction of about 65% and 2% caprolactam is added. After mixing evenly, add the autoclave and turn on the stirring to 100 r/min. In order to prevent the product from being oxidized, the gas in the kettle was replaced with high-purity N2 three times. The temperature in the pressure holding stage is controlled to be 210-220°C, the temperature in the depressurization stage is 265°C, and the temperature in the finishing stage is 265-275°C.

• Infrared analysis

The analysis shows that DDP is introduced into the PA66 molecular chain through copolymerization.

• Relative viscosity and molecular weight

From PA66 to FRPA66-4, the relative viscosity and molecular weight decrease is mainly due to the large steric hindrance effect of the rigid structure of DDP, which hinders the polymerization of PA66, which makes the polymerization difficult, the viscosity decreases, and the degree of polymerization decreases.

• Melting and crystallization

The melting point (Tm) of PA66 is 260.43℃, and the crystallization temperature (Tc) is 215.87℃. With the increase of DDP content, both Tm and Tc of flame-retardant PA66 show a decreasing trend. The Tm of FRPA66-4 decreases to 250.78℃, and the Tc decreases to 203.74℃. Similarly, the crystallinity (Xc) of PA66 is 34.58%, and the Xc of FRPA66-4 is reduced to 29.21%. This is because DDP participates in the polymerization reaction of PA66 and polymerizes to the PA66 molecular chain, which destroys the regularity and symmetry of the PA66 molecular chain. The introduction of DDP will also reduce the density of hydrogen bonds between PA66 molecules and weaken the interaction between PA66 molecules. Force, the melting point, crystallization temperature and crystallinity of flame-retardant PA66 decrease.

• Mechanical performance analysis

The introduction of DDP will inevitably cause the decrease of the mechanical properties of the flame-retardant PA66. This is due to the decrease of the molecular weight and crystallinity of the flame-retardant PA66, which leads to the decrease of the mechanical properties of the material. In addition, the introduction of DDP reduces the density of the hydrogen bond between the PA66 molecules, resulting in a decrease in the mechanical properties of the material. .

• Flame retardant performance analysis

After the introduction of DDP, the flame retardant effect of PA66 is significantly improved. With the increase of DDP addition, the number of droplets gradually decreases and the burning time is shortened. When the amount of DDP added is 4%, the droplets fall to the non-ignited absorbent cotton, and the vertical burning test is UL94 V-0. The limiting oxygen index of FRPA66-4 can be increased to more than 30%, reaching the flame-retardant level. It shows that DDP and MCA have a good synergistic effect and can improve the flame retardant properties of PA66.

• Conclusion

Combining in-situ polymerization and copolymerization to prepare a nitrogen-phosphorus synergistic flame-retardant PA66 resin through melt polycondensation. When the MCA content is 2% and the DDP addition is 4%, the limiting oxygen index of FRPA66-4 is increased to 30.6% , The vertical combustion test reaches UL94 V-0.