Developing flame retardant solutions for partially aromatic polyamide with phosphine oxides

Partially aromatic polyamides owing to their excellent thermal stability are widely used in high temperature applications, however, like their aliphatic counterparts, they are readily flammable and more challenging to process. In this work, several organophosphorus flame retardants were synthesized and compounded with partially aromatic polyamide and evaluated for their processability, thermal, and fire behaviour. The compounds containing a commercial flame retardant, Exolit® OP 1230 (EX), and two new flame retardants, namely 1,4-phenylenebis(diphenylphosphine oxide) (MP) and (1,1′-biphenyl]-4,4′-diylbis(diphenylphosphine oxide) (BP), showed self-extinguishing capability (i.e., UL94 V0 class) with 4 wt% phosphorus (P) loading, together with a substantial reduction in the pHRR (up to 47 %), with respect to the pristine PAP. Rheological measurements on extended timescales were used to assess the melt stability of partially aromatic polyamide compounds. The presence of MP and BP in the polymer matrix did not trigger any excessive degradation phenomena such as chain scission, branching, or crosslinking reactions, thus, allowing a stable processability similar to a pristine partially aromatic polyamide sample. Finally, analysis of evolved gases during thermal decomposition revealed that MP and BP mainly exert a flame inhibition effect quite early in the decomposition process.