Chain Extension and Synergistic Flame-Retardant Effect of Aromatic Schiff Base Diepoxide on Polyamide 6/Aluminum Diethylphosphinate Composites

A novel phosphorus-nitrogen-based hyperbranched polysiloxane for improving the fire safety of PA6 with suppressed melt droplets and good mechanical properties

The combustible defects of polyamide 6 (PA6), especially the flammable melt-dripping behavior, have greatly limited its application in some particular fields. In this work, a halogen-free hyperbranched polysiloxane (PBDSi) containing DOPO and Schiff base was designed via Michael's addition reaction and dehydration-condensation reaction. Results showed that the char yield (Yc) of PBDSi attained 37.9 wt%, confirming the satisfactory charring behavior of PBDSi for preparing flame-retardant PA6. Just by adding 3 wt% of PBDSi, the serious melt droplets of PA6 were suppressed effectively. The prepared PA6/PBDSi-3 with 5 wt% of PBDSi could achieve the highest value of limited oxygen index (LOI) of 27.2 %, while that of PA6 is 21.0 %. Meanwhile, PA6/PBDSi-3 obtained an apparent reduction in the peak heat release rate (PHRR) value of 31.1 % compared with pure PA6. The cooperated effect of DOPO, Schiff base, and polysiloxane that contributed to generating a silicon-phosphorous-rich char layer and releasing incombustible volatiles that were determined to be the essential factor for the improved fire safety of PA6/PBDSi were explored intensively. Inspiringly, PA6/PBDSi composites exhibited a slight mechanical loss concerning PA6, overcoming the great challenge of developing additive flame-retardant materials to balance mechanical properties and fire safety.

Terminal group effects of DOPO-conjugated flame retardant on polyamide 6: Thermal stability, flame retardancy and mechanical performances

Two DOPO-conjugated flame retardants with or without amino terminal groups (DOPO-NH₂ and DIDOPO, respectively) were synthesized and incorporated into polyamide 6 (PA6). Results demonstrated the DOPO-NH₂ endowed superior thermal, flame retardant and mechanical performances to PA6 composites. With the same loading of 15 wt%, DOPO-NH₂ can catalyze the PA6 matrix more effectively and result in more residues at high temperature. The PA6 composites containing DOPO-NH₂ exhibited higher LOI (28.0%) compared to 25.0% for the sample containing DIDOPO, and the lower heat release capacity and peak heat release rate. Furthermore, the overall mechanical properties of PA6 composites containing DOPO-NH₂ outperformed the samples containing DIDOPO, even superior to that for PA6. Such a significant difference can be mainly attributed to the existence of amino-terminal group, which can interact with carboxyl group in PA6 as confirmed by dynamic mechanical analysis, improving the compatibility between the flame retardant and PA6 matrix.

A cross-linked charring strategy for mitigating the hazards of smoke and heat of aluminum diethylphosphonate/polyamide 6 by caged octaphenyl polyhedral oligomeric silsesquioxanes

Aluminum diethylphosphonate (ADP) is a highly efficient phosphorus-based flame retardant, widely used in polyamide 6 (PA6). However, ADP/PA6 releases large amounts of heat and smoke under high heat flux, which commonly means serious hazards to life and property. Octaphenyl polyhedral oligomeric silsesquioxanes (OPS) is an organic-inorganic hybrid silicon compound, playing flame retardant role in condensed phase. In this work, combustion behaviors of OPS/ADP/PA6 were investigated by limited oxygen index (LOI), UL94 and cone calorimeter (CONE) tests. The LOI and UL94 rating results did not change obviously, while the CONE data and smoke density data showed the synergistic effect of OPS and ADP in PA6. For 2.5%OPS/7.5%ADP/PA6, the peak values of heat, smoke and CO release rate (pk-HRR, pk-RSR, D_s, max with/without pilot flame and pk-COP) decreased by 60.2%, 82.1%, 45.9%/38.3% and 80.4% respectively, compared with 10%ADP/PA6. Moreover, 2.5%OPS/7.5%ADP/PA6 produced 337.5% more residue than 10%ADP/PA6. TGA, TG-IR, SEM-EDS, XPS and py-GC/MS were used to further explore the synergistic mechanism of OPS and ADP. It was verified that the cross-linked charring strategy apparently has weakened the hazards of smoke and heat of PA6. This work proposed a possible technical approach to solve both fire risk and heat/smoke hazards of PA6.