Polymeric phosphorus – nitrogen PIN FR achieved UL 94 V-0 (3.2 mm) at 0.3 % P. The experimental polymeric PIN FR was synthesised by reacting DDP (9,10-Dihydro-10-(2,3-dicarboxypropyl)-9-oxa-10-phosphaphenanthrene 10-oxide) with decamethylene diamine and caprolactam (both nitrogen containing) resulting in a nitrogen-containing DOPO-derivative polymer*. This polymer was then blended into polyamide (PA6) to achieve phosphorus contents of 0.3 – 0.5 %. At 0.3% P, UL 94 V-0 was achieved (V-2 for neat PA6) and LOI (limiting oxygen increase) was increased to nearly 26% (below 22% for neat PA6). However, smoke and carbon monoxide release were considerably increased and mechanical properties were significantly deteriorated compared to neat PA6. The flame retardant action was identified as principally gas phase, with low char formation. The authors note that the PIN FR obtained in this study had a relatively low molecular weight (5 9000 – 7 5000 g/mol) and that polymerisation method improvement could enable higher molecular weights to be achieved, possibly leading to less deterioration in PA6 mechanical properties. The authors cite previous studies showing that other PIN FRs can achieve UL 94 V-0 in PA6, e.g. AlPi (aluminium diethylphosphinate) at 15% loading.
“Polymer-type flame retardants based on a DOPO derivative for improving the flame retardancy of polyamide 6: Preparation, properties and flame retardancy mode of action”, B. Liang et al., Polymer Degradation and Stability 225 (2024) 110807, https://doi.org/10.1016/j.polymdegradstab.2024.110807
DOPO = 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide