Abstract
Zinc stannates (ZSs) and related metal metallates are attracting increasing attention as alternatives to antimony trioxide (ATO) for use as synergists with halogen-containing flame retardant systems (HFRs). Like ATO, ZSs show synergistic effects with both chlorine and bromine-containing FRs (BFRs) and are reported to be effective in a wide range of polymers, including PVC, polyolefins, polyester resins, polystyrene and polyamides. However, unlike ATO, ZSs are non-toxic, appear to promote char formation in char-forming polymers and can function additionally as smoke suppressants. Previous work in our laboratories has shown that ZS is more effective as a synergist than ATO when used with BFRs in polyamide 66 (PA66), producing reductions in peak heat release rates of more than twice than those achieved with ATO in cone calorimetric experiments, coupled with as much as one tenth the smoke evolution. Use of ZS in place of ATO also produced better results in UL94 tests with some BFRs in PA66. However, the exact mechanism of synergism of ZSs with HFRs is not known yet. The aim of this project was to elucidate the mechanism of synergism of zinc stannate (ZS) with HFRs as non-toxic and environmentally benign alternative to ATO. Besides, possible similarities with a number of related metal metallates including zinc hydroxy stannate (ZHS), calcium stannate (CS) and copper stannate (CuS) were investigated. PA66 as a versatile engineering polymer has been used as a model polymer. Poly(pentabromobenzyl acrylate) (PPBBA), commercially used as an effective flame retardant (FR) in combination with ZS in PA66, was selected as a model FR.
In the present work, a comparative study of the mechanisms of action of the selected metal metallates and metal oxides (MM/MOs) including ZS, ZHS, CS, CuS and ATO with the polymeric BFR, PPBBA, both on its own and when incorporated in PA66 has been carried out, using various evolved gas analysis (EGA) techniques for investigation of nature and quantity of evolved species, solid residue analysis techniques for investigation of chemical structure of remaining residues as well as quantification of elemental volatilization at different temperatures and heating regimes and thermogravimetric analysis. EGA techniques utilized in this thesis include pyrolysis coupled with gas chromatography mass spectroscopy (Py-GCMS) Pyrolysis coupled with Fourier transfer infrared spectroscopy (Py-FTIR), thermogravimetric analysis coupled with Fourier transfer infrared spectroscopy (TGA-FTIR) and tube furnace, and analysis of solid residues were conducted using X-ray photoelectron spectroscopy (XPS), X-ray florescence spectroscopy (XRF) and Fourier transfer infrared spectroscopy (FTIR).
Simultaneous thermogravimetric analysis/differential thermal analysis (TGA/DTA) was also used to compare the thermal profiles of thermal degradation, under both air and nitrogen atmospheres. Experiments on mixtures of PPBBA with ZS and ATO in the absence of PA66, showed that both ZS and ATO delayed the release of bromine-containing compounds from PPBBA and that, in the case of ATO, as expected, significant quantities of antimony bromides were released. However, with ZS, the formation of zinc bromides seems to be the principal fate of bromine, with little evidence for the formation of tin bromides, in marked contrast to earlier suggestions that it is the formation of volatile tin halides that are responsible for the vapourphase synergistic effects of ZS with HFRs. CS showed similar, however more pronounced effect on delaying liberation of bromine to vapour phase and very little or no calcium and tin were volatilized even at temperatures above 650 °C. The pyrolysis of PA66 is known to follow a complex series of parallel and consecutive reactions, but neither ATO nor CuS was found to have much effect on the nature and distribution of the volatile pyrolysis products under nitrogen. However, ZS and CS appears to catalyse reaction of decomposition of PA66 and cause minor formation of charred residues (ca. 25% more char at 450°C under air atmosphere), although there is little evidence of either zinc, tin or calcium being chemically incorporated in these residues. Very little to no metal volatilization was observed when ZS and CS were incorporated in PA66. Addition of PPBBA to PA66 without synergist lead to a significant reduction in the thermal stability of PA66 accompanied by the release of brominated compounds into the vapour phase with no significant char formation, consistent with PPBBA acting entirely as a vapour phase FR. Pyrolysis and combustion studies of the ternary systems, PA66+PPBBA+MM/MOs, confirmed that ATO acts almost entirely as a vapour phase FR synergist but that ZS and CS act principally in the condensed phase by char promotion, probably due to in situ formation of zinc and tin and calcium bromides which as Lewis acids act as dehydration catalysts and promote char formation. Besides some level of intumescence was observed for these systems. It is noteworthy that ZS and CS considerably reduced the vapour phase activity of PA66+PPBBA by trapping a considerable portion of bromine that would otherwise be released into the vapour phase. Moreover, it is confirmed that the interaction of bromine is primarily with zinc in ZS and not with tin.