The additives as flame retardants and smoke suppressants for polyester thermosets

The use of tin oxide, zinc stannate and zinc hydroxystannate as flame retardants and smoke suppressants in both halogenated and nonhalogenated polyester resin thermosets has been investigated. Oxygen Index studies showed that zinc stannate in conjunction with the resin based on dibromoneopentyl glycol (DBNPG) was the most efficient combination. This was also the case when the same resin containing less bromine was used (10% instead of 28.3%). It was however the tin oxide which caused the greatest decrease in smoke production. Nitrous Oxide Index tests showed the tetrabromophthalic anhydride (TBPA) and chlorendic anhydride (HET) resins to burn via a vapour phase mechanism and the DBNPG resin to burn via a condensed phase mechanism. Thermogravimetry indicated that the tin oxide did not interact with any of the resins, the additive being recovered almost quantitatively after degradation in air. The Zn/Sn additives interacted with the DBNPG resin with additive being almost completely lost every time. Some interaction was observed with the HET and TBPA resins but this was to a much lesser extent. XRD results indicated the possible formation of tin bromide from a DBNPG sample containing zinc hydroxystannate and this finding would support the theory of a condensed phase mechanism for the DBNPG resin. The SEM photographs indicated the formation of allne powdery layer on the surface of the burnt sample. Elemental analysis by. SEM showed that this contained both zinc and tin, as did the smoke. Residue analysis gave some indication of the temperature at which metal and halogen were volatilised from each sample. The results suggested that for the DBNPG resin with the Zn/Sn additives, it is very likely that tin bromide could be formed as both the tin and bromine are lost in the same temperature ranges. The same could be said for the TBPA resin but to a much lesser extent but for the HET resin it is unlikely that tin chloride could be formed as the volatilisation temperatures do not coincide. In conclusion, a condensed phase mechanism is proposed for the DBNPG resin with the Zn/Sn additives incorporated. It is suggested that this involves the formation of tin bromide which is then hydrolysed in the flame to yield tin oxide and hydrogen halide which can then act as a radical trap and suppress the flame reactions.