John Milnes

Very recently, deoxyribonucleic acid (DNA) has proven to be an efficient renewable, natural flame suppressant and retardant, due to its intrinsic intumescent features. In our previous work we have explored its flame retardant activity by applying as a surface coating on cotton fabrics and observed that on exposure to heat, the DNA was able to form a foamed char on the surface of the fabric.
These remarkable results have stimulated this study on in-depth understanding of mechanism of thermal degradation of DNA. A number of characterization techniques have been exploited to investigate DNA decomposition, namely: thermogravimetry coupled to infrared spectroscopy or combined with differential thermal analysis, pyrolysis-combustion flow calorimetry and analysis of degradation residues. A scheme of DNA degradation mechanism is proposed to account for the results of the study.

Our recent work has demonstrated that deoxyribose nucleic acid (DNA) can act as an effective flame retardant when applied to cotton fabrics as a thin coating. DNA acts as a Lewis acid and promotes the dehydration of cotton cellulose to form char, limiting the production of volatile species. Here, the effect of heating rates on thermal degradation behaviour has been studied in order to understand the thermo-oxidation behaviour under slow heating and fast (flash) pyrolysis. The low heating rate effect has been studied using thermogravimetry coupled with infrared spectroscopy and pyrolysis-combustion flow calorimetry, whereas high heating rates effect was obtained by performing thermogravimetry at 200 – 500oC/min and flash-pyrolysis tests coupled with infrared spectroscopy. The information obtained by the latter has been successfully employed in order to i) better explain the results collected from combustion tests and ii) demonstrate that the type of the gaseous species and their evolution, as well as the char formation, as a consequence of the fabric thermo-oxidation, are independent of the adopted heating rate.

This paper presents a methodology developed to quantitatively record the real-time melt and burn dripping behaviour of thermoplastic polymers. Six different commodity polymers were tested for their melt dripping behaviour exposed to convective heat in a purpose built electric furnace. The number, diameters and shapes of individual drops were measured and found to be influenced by the mechanism of decomposition of each polymer type. By conducting thermogravimetric analysis and measuring the viscosity of both the polymers and their molten drops, it could be established that the melt dripping is a combined effect of physical melting and polymer decomposition, which results in decrease in the viscosity of the molten drops. The effect of fire and heat on melt dripping was also observed in a UL-94 equivalent test where it was observed that the behaviour is quite different from pure melt dripping. Relationships between the glass transition temperature and melt viscosity with melt/flame dripping and burning intensity of these polymers have been observed. These will be studied in detail in a subsequent publication. (C) 2012 Elsevier Ltd. All rights reserved.

Pyrolyses of the reactively flame retarded polystyrene copolymers styrene/diethyl(acryloyloxyethyl)phosphate(S/DEAEP), styrene/diethyl(methacryloyloxyethyl)phosphate(S/DEMEP), styrene/diethyl(methacryloyloxymethyl)phosphonate(S/DEMMP) and styrene/diethyl(acryloyloxymethyl)phosphonate(S/DEAMP) have been investigated with a view to obtaining information pertinent to the mechanism of their flame retardant behaviour. Studies were also carried out on the additive polystyrene systems containing triethylphosphate (TEP) and diethylethylphosphonate (DEEP) for comparison. All the systems contained 3.5 wt% of phosphorus. A range of techniques were used, namely TG with EGA, DSC, SEM, laser and microfurnace pyrolysis mass spectrometry and isothermal pyrolysis/GC-MS, to study the decompositions under a range of conditions. In the case of the additive systems, the additive was shown to be evolved before polymer decomposition occurred. Very little, if any, char residues were observed. Thus the main mechanism of fire retardant action of the phosphorus incorporated into the polystyrene as an additive would occur in the vapour phase. This mechanism prevailed regardless of whether the additive was a phosphate (TEP) or a phosphonate (DEEP). The effectiveness of the fire retardant action would be limited as the fire retardant and fuel did not volatilise together. There was evidence that some interaction occurred in the condensed phase. In all the copolymers the phosphorus content of the char was substantial. This is characteristic of the condensed phase fire retardant action of phosphorus. SEM studies showed the interior of the char to be a network of channels which would give the char a sponge-like interior which would enhance thermal insulation. The surfaces were relatively dense thus providing a barrier to escape for any gaseous products formed in the interior. Char formation and cross-linking are assumed to be the result of the presence of the strong phosphoric and phosphonic acids resulting from initial pyrolysis. Since phosphonic is the weaker acid, the polymer degradation and release of volatile products may be less inhibited in the case of the phosphonate-containing copolymers compared to the phosphate-containing copolymers. This is consistent with their shorter times to ignition. There was also evidence for some potential phosphorus vapour phase fire retardant action as phosphorus-containing species were identified among the pyrolysis products for all samples. The rate of volatile evolution from the copolymers was reduced compared to that of the corresponding additive system.

The combustion behaviour of polystyrene flame retarded by the incorporation of phosphorus-containing compounds has been studied by LOI and cone calorimetry. Both 'reactive' and 'additive' approaches to the incorporation of the phosphorus have been applied and assessed. The data obtained show that the reactive approach results in enhanced char formation during combustion due to a condensed phase mechanism. Flame retardation by the additive systems occurred exclusively in the vapour phase via both chemical and physical interactions. The main advantage of the reactive approach was the maintenance of the physical and chemical properties similar to those of the homopolymer. The phosphorus environment was also a significant factor in terms of the level of flame retardance achieved. Phosphonate species were more effective than were phosphate species.

The combustion behaviour of polystyrene flame retarded by the incorporation of phosphorus-containing compounds has been studied by LOI and cone calorimetry. Both ?reactive? and ?additive? approaches to the incorporation of the phosphorus have been applied and assessed. The data obtained show that the reactive approach results in enhanced char formation during combustion due to a condensed phase mechanism. Flame retardation by the additive systems occurred exclusively in the vapour phase via both chemical and physical interactions. The main advantage of the reactive approach was the maintenance of the physical and chemical properties similar to those of the homopolymer. The phosphorus environment was also a significant factor in terms of the level of flame retardance achieved. Phosphonate species were more effective than were phosphate species.

MMA has been copolymerised with pentavalent phosphorus-containing monomers and the flame retardance of the resulting copolymers has been assessed by limiting oxygen indicies (LOI) and cone calorimetry experiments. The thermal stability of the copolymers has also been assessed by conventional thermogravimetric analysis (TGA). Poly(methyl methacrylate) (PMMA) modified with phosphorus-containing additives have also been synthesised and the flame retardance assessed. All of the modified PMMA samples contain 3.5 wt.\%, allowing a comparison of the relative merits of an additive and a reactive approach to flame retardance. The chemical environment of the phosphorus in terms of flame retardance achieved is also considered in this paper. The incorporation of 3.5 wt.\% phosphorus in both reactive and additive approaches increases the limiting oxygen index of PMMA from 17.8 to over 21. However, cone calorimetry shows that the phosphorus-containing copolymers are inherently more flame retardant than PMMA and the PMMA modified with phosphorus-containing additives. The methyl methacrylate (MMA) copolymers have significantly reduced peak rates of heat release and leave substantial char residue during combustion, as compared to PMMA. Cone calorimetry has also shown that the phosphates are more effective flame-retardants for PMMA than are the phosphonates in both additive and reactive approaches. TGA of the polymers indicates that the copolymers are more thermally stable than PMMA whilst PMMA containing the additives are less thermally stable. A condensed phase mechanism in which diethyl(methacryloyloxymethyl)phosphonate reduces the flammability of PMMA has been identified.

The mechanism of flame retardancy and smoke suppression by melamine in flexible polyurethane foam has been investigated using cone calorimetry, TG, DSC and pyrolysis/GC/MS. The cone calorimetric results indicate that the addition of melamine into polyurethane foam was very effective at reducing heat release rates and suppressing smoke and CO production during the initial combustion stage. GC/MS analysis showed that the volatiles evolved from polyurethane foam, pyrolysed at 350°C in air, consisted mainly of the 2,6- and 2,4- isomers of toluene diisocyanate. Such species together with other aromatic compounds are accepted as being the main contributors to smoke released from polyurethane foam in a fire. Cone calorimetry, DSC and pyrolysis/GC/MS experiments indicated that interaction occurs between melamine and the evolved toluene isocyanate fraction arising from the decomposition of polyurethane foam. The resulting polymeric structures so formed will reduce the amount of aromatic smoke precursors evolved thus suppressing smoke in the event of a fire. This polymeric structure will also degrade to a char, reducing the amount of combustibles volatilized and hence the rate of heat release. The char would form a protective layer on the surface of the polyurethane foam. A mechanism for this important melamine-isocyanate interaction is proposed.

The ignition and pyrolysis of some copolymers of methyl methacrylate (MMA) with diethyl(methacryloyloxymethyl)phosphonate (DEMMP) have been studied by a simple tube furnace and by isothermal and non-isothermal TGA. The results indicate that copolymers containing DEMMP thermally degrade, under both air and nitrogen, by a mechanism which is more complex than that (simple depolymerization) for poly(methyl methacrylate) (PMMA). The copolymers containing 10mol% or more of DEMMP are inherently flame retardant in that they fail to autoignite at 480°C and take longer to autoignite at 490°C than PMMA or MMA-DEMMP copolymers containing only 5mol% DEMMP. The formation of a carbonaceous residue or char on combustion of the MMA?DEMMP copolymers suggests that flame retardance is due mainly to reactions in the condensed phase.

Yields of toxic combustion products from burning materials vary considerably between different stages and types of fire, eg fully ventilated, vitiated. It is important that any laboratory studies of such phenomena be carried out as close as possible to real fire conditions. The Purser furnace has been developed to widen the range of fire types amenable to laboratory-scale investigation. This paper describes Salford?s Purser furnace and reports the findings of a systematic assessment of the experimental parameters that affect the performance of the Purser furnace.