Abstract:

Objective Polyamide 66 (PA66) fiber is widely used in textiles, automotive, and electronics due to its excellent mechanical properties and heat resistance, but with insufficient flame retardancy. Although traditional flame retardants can improve the flame retardancy of PA66, a high loading amount often deteriorates the spinnability of PA66 and significantly reduces the mechanical properties of the fiber. Carbon dots (CDs), as a typical organic-inorganic hybrid zero-dimensional carbon nanomaterial, are expected to enhance the flame retardancy of PA66 while improving its spinnability and strengthening the mechanical properties of PA66 fiber. Specifically, heteroatoms (e.g., N, O, P) in CDs scavenge combustion free radicals to terminate chain reactions, while their carbon-rich core promotes dense char layer formation, blocking heat and oxygen transfer for efficient flame retardancy at low addition levels. Moreover, CDs’ nanoscale size and surface functional groups form hydrogen bonds with PA66 molecular chains, ensuring homogeneous dispersion to maintain melt fluidity and spinnability, and constructing strong interfacial interactions to transfer stress and restrict molecular chain slippage, thus reinforcing mechanical properties.

Method Polyethylene terephthalate (PET) waste was used as a precursor to achieve large-scale preparation of PET waste-based carbon dots (rPET-CDs) via a solvent-free one-step pyrolysis method. Specifically, 1 500 g of PET bottle flakes and 975 mL of ethanolamine were added to a 5 L high-pressure quick-release reaction kettle. The reaction was carried out at 260 ℃ for 56 h, followed by cooling to approximately 80 ℃ and dispersion with ethanol. Subsequently, the mixture was filtered to remove larger particles (>220 nm), and ethanol was recovered. Finally, the product was vacuum-dried and pulverized to obtain rPET-CDs powder, with a yield of 1 796 g.

Results The prepared rPET-CDs exhibited a spherical structure with an average particle size of 2.07 nm and surface functional groups such as —OH and —NH₂, demonstrating good thermal stability. Using rPET-CDs as a flame retardant, flame-retardant PA66 composites (PA66/rPET-CDs) were prepared via melt blending. The addition of rPET-CDs improved the flame retardancy of PA66. When the loading amount of rPET-CDs was 3%, the limiting oxygen index (LOI) of the PA66/rPET-CDs composite reached 29%, and the peak heat release rate (pHRR) decreased by 10.64%. Flame retardancy mechanism studies revealed that the incorporation of rPET-CDs into PA66 reduced the pore size of the char residue after combustion and made the char layer more continuous, demonstrating a certain solid-phase flame retardant effect. SEM images showed that the char layer of pure PA66 contained numerous interconnected pores, whereas when the loading amount of rPET-CDs increased to 3%, the pores in the char layer significantly decreased, and the structure became denser, exhibiting superior thermal-oxygen shielding properties. This effectively inhibited the diffusion of heat and flammable gases, thereby enhancing the flame retardancy of the material. The addition of rPET-CDs reduced the spinning temperature of PA66 from 285 ℃ to 270 ℃. This is likely because the abundant oxygen-containing functional groups on the surface of rPET-CDs formed hydrogen bonds with PA66 molecular chains, reducing the hydrogen bonding interactions between PA66 molecular chains. This decreased the apparent viscosity of the melt, lowered the spinning temperature, and moved it away from the thermal degradation-sensitive region, thereby mitigating thermal degradation and gelation during spinning and improving the stability of PA66 spinning. In terms of mechanical properties, the addition of rPET-CDs synergistically enhanced the tensile strength and elongation at break of PA66 fibers, which may be attributed to the abundant —OH/—NH₂ groups on the surface of rPET-CDs. The functional groups on the surface of rPET-CDs formed hydrogen bonds with the amide bonds of PA66 molecular chains, creating cross-linking points between the molecular chains and enhancing the mechanical properties of the fiber. Additionally, rPET-CDs acted as a plasticizer in PA66, increasing the elongation at break of PA66 fibers. The reduction in spinning temperature further corroborates the plasticizing effect of rPET-CDs in PA66.

Conclusion The addition of rPET-CDs improved the flame retardancy of PA66. Flame retardancy mechanism studies indicated that the incorporation of rPET-CDs into PA66 reduced the pore size of the char residue and made the char layer more continuous, contributing to a solid-phase flame retardant effect. However, the decreased densification degree of the char residue limited the improvement in flame retardancy. Furthermore, the addition of rPET-CDs significantly enhanced the spinnability and mechanical properties of PA66. The spinning temperature was reduced from 285 ℃ to 270 ℃, which helped mitigate thermal degradation and gelation, thereby improving the stability of PA66 spinning. Mechanically, the addition of rPET-CDs synergistically increased the tensile strength and elongation at break of PA66 fibers, likely due to the abundant —OH/—NH₂ groups on the surface of rPET-CDs. These functional groups formed hydrogen bonds with the amide bonds of PA66 molecular chains, creating cross-linking points and enhancing the mechanical properties of the fiber. Additionally, rPET-CDs acted as a plasticizer, increasing the elongation at break of PA66 fibers.

Key words: flame-retardant fiber, PET waste, carbon dot, polyamide 66, flame retardancy, spinnability, functional fiber