Flame retardant phosphonate-functionalised polyethylenes

Abstract

The two-step synthesis of phosphonate-functionalised polyethylenes with enhanced thermal and flame-retardant properties has been described. Solution phase copolymerisation of ethylene and 11-bromo-1-undecene using the commercially available early transition metal catalyst, rac-ethylenebis(indenyl)zirconium dichloride, produced poly(ethylene)-co-(11-bromo-1-undecene) with comonomer incorporation levels up to 6.10 mol%. Solvent-free post-polymerisation modification of the poly(ethylene)-co-(11-bromo-1-undecene) with the phosphite esters, P(OⁱPr)₃ and P(OPh)₃ successfully converted the bromide group into the corresponding phosphonate group on polyethylene (PE–PO(OR)₂). The phosphonate-functionalised polyethylene was characterised by NMR and IR spectroscopy, GPC and DSC. Higher thermal stability (higher T_10%, T_50%, T_max and % residue at 700 °C) was determined by TGA with respect to pure LDPE. Microscale combustion calorimetry (MCC) tests indicate significant lower values of heat release capacity (HRC), peak heat release rate (pHRR) and total heat release (THR) compared to those of LDPE, indicating an enhanced flame retardancy. Importantly, blends of LDPE and PE–PO(OR)₂ with weight ratios of 90 : 10, 95 : 5 and 99 : 1 demonstrate higher thermal stability compared to LDPE despite the low concentration of the phosphonate group. Additional incorporation of traditional inorganic flame retardant additives (aluminium hydroxide, ATH), LDPE : ATH : PE–PO(OR)₂ with 80 : 10 : 10 wt%, presents higher thermal stability than pure LDPE or blended LDPE with PE–PO(OR)₂ due to a synergistic effect from the combination of the phosphonate group and ATH.