The potential of metal oxalates as novel flame retardants and synergists for engineering polymers

Based on their known decomposition to carbon dioxide, carbon monoxide and the respective oxide, six metal (calcium, manganese (II), iron (II), copper (II), tin (II) and zinc) were synthesised and assessed for their potential flame retardant activity in the absence and presence of selected flame retardants. Initially they were assessed when impregnated on cotton as a screening process and then selectively compounded with polyamide 6.6 (PA66), as a typical engineering polymer. Only manganese (II) and iron (II) oxalates alone reduced the burning rate of cotton, whereas together with ammonium bromide, calcium and iron (II) oxalates showed an apparent additional burning rate reducing effect. Derived synergistic effectivity (Es) values fall within the limits 0<Es<1 indicating a less than additive interaction. TGA/DTA analysis of oxalate/PA66 blends suggested that only zinc oxalate (ZnOx) offers both possible flame retardant activity in terms of enhanced residue formation ≥500oC, coupled with acceptable stability in molten PA66. When compounded with PA66, in the presence and absence of either aluminium diethyl phosphinate (AlPi)-based or selected polymeric bromine-containing flame retardants, LOI values increased in most PA66/ZnOx/flame retardant blends but UL94 test ratings were disappointingly low and more likely than not, “fails”. PA66/ZnOx blends with AlPi and AlPi/MPP gave poor plaques suggesting that thermal interactions were occurring during compounding. The bromine-containing blends had better processibility and both TGA and cone calorimetric studies showed that the PA66/poly(bromopentabromobenzyl acrylate)/ZnOx sample not only yielded the highest residues in air and nitrogen at 500 and 580oC, but also the lowest peak heat release rate value of 398 compared with 1276 kW/m2 for pure PA66. The derived Es value for this blend is 1.17 suggesting a small level of synergy between the zinc oxalate and poly(pentabromobenzyl acrylate) flame retardant. The possible role of zinc bromide is discussed