A Phosphorus-Containing Polyamide Oligomer as a Synergist for Flame-Retardant PA66/GF Composites: Concurrent Improvements in Flame Retardancy and Electrical Safety

As electronics become smaller and more power-dense, glass-fiber-reinforced polyamide 66 (PA66/GF) must combine halogen-free flame retardancy with electrical safety, including a glow-wire ignition temperature (GWIT) ≥ 775 °C and a comparative tracking index (CTI) ≥ 600 V. Conventional low-molecular-weight flame retardants often require loadings above 15 parts by mass, impairing mechanical and electrical properties. Here, a phosphorus-containing polyamide oligomer (DOPO–MA–oligomerPA6) was synthesized via a DOPO–MAH intermediate from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and maleic anhydride (MAH), followed by polymerization with ε-caprolactam, then melt-blended with aluminum diethylphosphinate (ADP) and melamine polyphosphate (MPP) in PA66/GF. Its oligo-PA6 backbone provides good compatibility with PA66, while its low molecular weight (Mn = 10,984 g mol−1; PDI = 2.569) favors dispersion. The phosphorus groups enhance the efficiency of the ADP/MPP system. With 4 parts oligomer and a total flame-retardant loading of 14 parts by mass per 100 parts PA66/GF, the composite achieved UL-94 V-0 rating, a limiting oxygen index (LOI) of 36.1%, a GWIT of 800 °C, and a CTI of 600 V. Its heat deflection temperature, tensile strength, notched impact strength, and flexural modulus were 178.8 °C, 95.0 MPa, 8.7 kJ m−2, and 8984 MPa, respectively. Compared with the conventional 20-part system, total additive loading decreased by 30%, with simultaneous improvements in flame retardancy, electrical safety, and mechanical properties. This offers a scalable route to high-performance PA66/GF composites.