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Title: Novel metal complex fire retardants for engineering polymers
Author: Holdsworth, Alistair F.
ISNI:       0000 0004 5367 8747
Awarding Body: University of Bolton
Current Institution: University of Bolton
Date of Award: 2015
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The aims of this project were to develop non-halogenated, non-toxic and environmentally benign synthetic inorganic compounds capable of imparting flame retardant and/or synergistic activity with selected conventional flame retardants to selected engineering polymer systems. Following a comprehensive literature survey with specific focus centered upon the degradation and fire retardancy of polyamide 6,6 (PA66), an initial study of the effect of several metal oxalates on the burn rate of impregnated cotton in combination with sources of bromine and phosphorus was performed, with some promising synergistic activity detected with the former. Following this, a matrix of potential non-toxic inorganic flame retardant candidates (FRCs) was devised from a range of non-toxic, water-soluble precursors. And 180 FRCs were successfully synthesised and a cursory characterisation was conducted using X-Ray Fluorescence (XRF). These were screened for potential flame retardant properties with polyamide 6,6 (PA66) as a suitable engineering polymer. PA66 pellets were powdered and mixed with each FRC in 3:1 mass ratio, and subjected to simultaneous differential thermal analysis/thermogravimetric analysis (TGA/DTA), to which a range of selection parameters were applied, assessing their stability and char-forming potential. All FRCs which did not conform to these conditions were eliminated, reducing the number to 60. Differential mass analysis was performed on TGAs of the 60 FRCs to better define their char promotion potential in PA66, thereby narrowing these down to 16 samples: aluminium, zinc and tin (II) tungstates (AlW, ZnW and SnW respectively); zinc molybdate (ZnMo); iron aluminate (FeAl); iron (III) hypophosphite (FeHP); zinc oxalate (ZnOx); three stannic metal nitrite complexes (SnMNO where M = Mn, Cu or Zn), tin (II) oxide (SnO), hydrogenphosphite (SnH2PO3), triphosphate (SnTP) and phenylphosphonate (SnPhPO3); and tin (IV) silicate (SnSi). The 16 selected were synthesised on a larger scale and fully characterised, of which 6 were discounted due to difficulties in synthesis scaling, or characterisation. The final 10 FRCs were selected for small-scale fire testing in PA66 (at 5 wt%), using UL94, LOI and cone calorimetry with additional analysis provided by TGA/DTA. All samples were prepared by melt compounding, during which two of the samples (ZnMo and FeHP) displayed erroneous activity during processing and so were eliminated. The remaining 8 FRCs had little effect on PA66 LOI performance with only two samples (AlW and SnW) marginally improving UL94 performance. All samples reduced cone peak heat release rate (PHRR) to levels ranging from 70 % (ZnW) to 53 % (FeAl) compared to the PA66 control. Two FRCs (SnPhPO3 and SnH2PO3) incorporated at a higher level (10 wt%) showed improved performance iv although insufficient for achieving acceptable levels of flame retardancy. Further studies were undertaken to test potential synergism of the FRCs with several phosphorus- and bromine containing flame retardants. Combinations of 3 phosphorus-containing flame retardants (PFRs) with each of the 4 FRCs, FeAl, AlW, SnW and ZnW were studied. A degree of synergy with AlW and SnW combined with aluminium phosphinate (AlPi) was observed and for SnW with a combination of AlPi with melamine polyphosphate (MPP) with respect to UL94 and LOI performance. A moderate reduction in cone PHRR was observed for all FRCs combined with AlPi, and a significant reduction for all with AlPi/MPP. AlW, SnW and ZnW combined with MPP showed moderate improvements in UL94 and LOI and slight reductions in PHRR relative to the MPP control. These results indicate a degree of synergism of several FRCs with PFRs, the greatest effect being observed with the combined vapour and condensed-phase active AlPi/MPP. Two polymeric brominated flame retardants (BrFRs) were selected: brominated polystyrene, (BrPS) and poly(pentabromopolybenzl acrylate (BrPBz), and were compounded with AlW, ZnW and SnW into PA66. AlW displayed significant antagonism with both BrFRs while SnW and ZnW both showed promising improvements in terms of UL94 and LOI performance and significant reductions in cone-derived PHRR values. These results were replicated at various differing Sn:Br and Zn: Br molar ratios. Mechanisms of action of AlW, SnW and ZnW were studied with both PFRs and BrFRs using TGAFTIR and analysis of cone-derived chars. Based on the detailed analysis, the inherent action of these tungstates alone was proposed to be condensed phase, Lewis-acid catalysed cross-linking of PA66), thereby promoting the formation of char, whereas any vapour phase activity for the FRCs such as SnPhPO3 and SnH2PO3 could originate from volatilisation of either phosphorus or metal (Sn, Fe, Zn) acting as vapour-phase radical quenching agents. With regards to the specific mechanism of action of the tungstates with other flame retardant elements, in combination with BrFRs, formation of metal halides and oxyhalides are proposed to contribute to condensed-phase char promotion and vapour-phase radical quenching. In combination with PFRs, increased chain cross-linking promotes the formation of greater char, stabilised by the presence of phosphorus groups and metal oxide residues, coupled with vapour phase activity inherent to AlPi. In conclusion, of 180 FRCs synthesised and screened for potential FR behaviour in PA66, the tungstates in particular were observed to possess both some level of flame retardant behaviour as well as function as synergists with selected phosphorus- and bromine-containing flame retardants. A method for screening future FRCs was also developed.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available