Abstract
Aluminium (AlW), tin (II) (SnW) and zinc (ZnW) tungstates were previously found to increase char formation in polyamide 6.6 (PA66), but have never before been studied for their potential as synergists when present with phosphorus-containing (PFRs) flame retardants. We investigate this gap in the scientific knowledge in this publication. Tungstate-PFR interactions of PA66 composites were investigated by thermal analysis, limiting oxygen index (LOI), UL94, cone calorimetry and evolved gas analysis (TGA-FTIR). Of the three tungstates, AlW in the presence of aluminium diethylphosphinate (AlPi) or a mixture of AlPi and melamine polyphosphate (MPP), promotes the highest level of flame retardancy in terms of a balance of high LOI, V-rating and TGA residual char in air at 500oC and reduction in cone calorimetric peak heat release rate (RPHRR = 80%). This observation has been related to AlW having the highest Lewis acidic properties. While zinc tungstate displays the lowest levels of interaction with either PFR, it shows significant smoke suppressant properties. Elemental analysis of cone calorimetric chars suggests that while some loss of phosphorus occurred from SnW/AlPi/MPP-containing composites, most likely via volatile diethyl phosphinic acid formation, nearly 10% reduction also occurred in the Sn/W molar ratio indicating some volatilisation of tin. The TGA (in air)-FTIR results for carbon dioxide, ammonia and hydrocarbon fuel species evolution enabled discussions of the effect that each tungstate had on the flame retardant mechanisms operating.