New sustainable flame retardant DOPO-NH-functionalized polyamide 6 and filament yarn

This work presents a unique approach for the preparation of a flame retardant (FR) polyamide 6 (PA6) polymer with chemically bonded 9,10-dihydro-9,10-oxa-10-phosphaphenanthrene-10-oxide (DOPO) as a pendant group bridged to the polymer via an –NH- group. A novel phosphonamidate co-monomer (DOPO-A-CLM) was synthesized from DOPO and α-amino-ε-caprolactam (A-CLM). This co-monomer was subsequently used in the hydrolytic polymerization with ε-caprolactam (CLM) in different weight ratios to prepare DOPO-NH-functionalized PA6 (PA6-xDC, x = 7, 10, and 15 wt% DOPO-A-CLM). Chemical incorporation of DOPO-A-CLM into the PA6 backbone decreased the molecular weight of the polymer from 15387 for neat PA6 to 12375, 10516 and 9316 for PA6-7DC, PA6-10DC and PA6-15DC, respectively. The DOPO-NH– pendant group accelerated start of the PA6 thermal decomposition and increased the char residues at 500 ◦C from 1% for PA6 to 4.6, 4.9, and 5.0% for the PA6-7DC, PA6-10DC, and PA6-15DC samples, respectively, indicating crosslinking reactions in the condensed phase. The evolved phosphorus-active species in the gas phase inhibited the PA6 depolymerization, resulting in increased thermo-oxidative stability and about a fourfold higher residue at 500 ◦C in the case of PA6-15DC compared to PA6. The intrinsically flame retardant PA6 filament yarns with chemically bound FR pendant group were successfully melt spun from PA6-10DC, drawn and wound on bobbin. The DOPO-NH– pendant group decreased filament flammability and inhibited flame propagation, resulting in immediate self-extinguishment after flame removal. Incorporation of DOPO-A-CLM decreased the filament tensile properties compared to the neat PA6, which correlates with the decreased polymer molecular weight.