Quantification of Zinc Hydroxystannate** and Stannate** Synergies in Halogen-containing Flame-retardant Polymeric Formulations

Abstract

Zinc stannate and hydroxystannate are used as synergists in fire-retardant systems in conjunction with halogenated species where their behavior is generally considered to be similar to antimony-containing synergists while offering the additional properties of smoke suppression and relative nontoxicity. The literature most often compares relative synergistic behaviors qualitatively but this article determines such behavior quantitatively using Lewin’s synergistic effectiveness parameter, ES, calculated from sample limiting oxygen index (LOI) data. Flame-retardant formulations comprising zinc stannate (ZS), zinc hydroxystannate (ZHS), or antimony III oxide (ATO) in combination with selected and polymer-compatible bromine-containing flame retardants were formulated in commercial grades of poly(vinyl chloride) (PVC)), thermoset polyester resin, and polyamide 6. PVC formulations simulated both commercial cable and plastisol applications and comprised either a phthalate or aryl phosphate ester as plasticizer in combination with selected synergists. All formulations were subjected to flammability testing using LOI, UL94 (vertical sample mode), and cone calorimetric (including smoke analysis) methods. For all PVC/synergist combinations containing the phthalate plasticizer, synergy was evident with 2.0 > ES > 1.0 and a relative effectiveness order ATO > ZHS > ZS. Zinc borate present as a cosynergist also has a quantifiable, positive effect. In the presence of the phosphate ester plasticizer, the reverse order is observed with marginal levels of synergy being evident (1.2 > ES > 1.0). In polyester resin formulations, ATO and ZHS exhibit similar levels of synergy when present with dibromoneopentyl diglycol with the former being superior when decabromodiphenyl ether is the flame retardant. However, in all polyamide 6 formulations, the highest levels of synergistic effectiveness are observed (ES ≥ 5.0) with ZS. Maximum values of ES correspond to a molar ratio of Sn/Br<0.3 suggesting the formation of SnBr2 and SnBr4 as the effective flame-retarding species.