T. Richard Hull

Flame retardance of poly(methyl methacrylate) modified with phosphorus-containing compounds

Abstract 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.

An investigation into the decomposition and burning behaviour of ethylene-vinyl acetate copolymer nanocomposite materials

Ethylene-vinyl acetate copolymer (EVA) is a widely used material, particularly as a zero-halogen material in the cable industry. It is frequently formulated with large quantities of inorganic filler material, such as aluminium trihydroxide (ATH). Used alone, EVA is known to form a protective layer which can inhibit combustion under well ventilated conditions, though this effect is not observed when used in formulations with ATH. The incorporation of nanoscale clay fillers into EVA appears to reinforce the protective layer. The stages of the decomposition under different conditions is described both for the 10 mg (TGA) and 200 mg (small tube furnace) scales. The latter allows the residues formed to be subjected to further analysis, to elucidate the mechanism of the reduction of decomposition and flammability. Enhancements in the thermo-oxidative stability of the EVA clay material were evident from both tube furnace and TGA experiments. The polymer-organoclay materials, prepared on a two-roll mill, showed poor dispersion when studied by SEM, suggesting that a significant portion was present as a microcomposite. However, when the char was analysed by SEM, layers of protective material were clearly evident on the char surface. From XRD spectra, there was no evidence of order within the polymer-organoclay, but ordering of the outer layer of char was demonstrated. This suggests that for EVA, which melts before burning, organoclay layers become nanodisperse at the surface of the burning polymer. These materials have also been studied in the Purser furnace, designed to replicate the conditions found in fully developed fires. This allows effluent yields, such as O2, CO2 and CO to be determined as a function of fire condition, by controlling the rate of burning and the ventilation rate. The effect of both the nanofillers and the protective layers are reported and discussed, under different ventilation conditions. Specifically, the relationship between equivalence ratio and hydrocarbon and carbon monoxide yield is focussed upon.

Flame retarding poly (methyl methacrylate) with phosphorus containing compounds: comparison of an additive with a reactive approach

Abstract The flame retardance and thermal stability of a methyl methacrylate (MMA) copolymer reactively modified by copolymerisation of the MMA with diethyl (methacryloyloxymethyl) phosphonate (DEMMP) have been compared with those of poly(methyl methacrylate) (PMMA) containing equivalent amounts of the additive diethyl ethyl phosphonate (DEEP). DEEP can be regarded as having a structure similar to that of a DEMMP comonomer unit and therefore the two compounds might be expected to confer about the same levels of flame retardance to PMMA when used at similar concentrations. The incorporation of 3.5 wt.% phosphorus in both cases raises the limiting oxygen index of PMMA from 17.2 to over 22. However, cone calorimetry shows that the MMA/DEMMP copolymer is inherently more flame retardant than PMMA containing DEEP: the former has a significantly lower peak rate of heat release than the latter (449 and 583 kW m −2 , respectively) and gives rise to a greater amount of char. Thermogravimetric analysis (TGA) of the polymers indicates also that the MMA/DEMMP copolymer is more thermally stable than PMMA whilst PMMA containing DEEP is less thermally stable. Dynamic mechanical thermal analysis (DMTA) shows that the MMA/DEMMP copolymer has physical and mechanical properties similar to those of PMMA, whilst the low molecular weight DEEP plasticises PMMA, resulting in a significantly reduced glass transition temperature, T g . A condensed phase mechanism of flame retardance in MMA/DEMMP has been identified.

Combustion toxicity of fire retarded EVA

Abstract A Purser furnace has been used to investigate the combustion toxicity of ethylene-vinyl acetate copolymer (EVA) with and without fire retardants, under different fire conditions. Steady state flaming combustion has been studied at equivalence ratios φ varying from 0.5 to 1.5 by driving the materials through the furnace at 750xa0°C. Yields of CO and CO 2 for EVA containing 27% vinyl acetate, and its fire retarded composites, containing fire retardant fillers are presented. The materials contained 30% EVA and 70% hydrated aluminium oxide (ATH), or 65% ATH and 5% zinc hydroxystannate (ZS), or 5% magnesium borate (MgB) or 5% zinc borate (ZB). In each case the same mass of EVA was used in the determination. The yields of CO per g of polymer from the EVA-fire retardant composite samples showed very similar yields of CO under well ventilated conditions to the pure EVA, but generally higher CO yields than the base polymer under the most toxic fuel rich conditions. The exception to this was the sample containing ATH and zinc borate, which did not take up all the available oxygen under fuel rich conditions, and gave a much lower CO yield, corresponding to an eight-fold reduction in the combustion toxicity. Under fuel rich conditions for EVA, 60% of the carbon was lost as volatile organic species other than CO and CO 2 . For the sample containing zinc borate, this was 50% and for the remaining samples it varied from 20 to 38%. Evidence is presented which indicates that organic material trapped in the solid alumina residue is oxidised to CO, except in the presence of zinc borate, when it appears to be lost as organic carbon.

Thermal degradation and flame retardance in copolymers of methyl methacrylate with diethyl(methacryloyloxymethyl)phosphonate

Methyl methacrylate (MMA) has been free radically copolymerized, both in bulk and in solution, with diethyl(methacryloyloxymethyl)phosphonate (DEMMP), to give polymers which are significantly flame retarded when compared with PMMA, as indicated by the results of limiting oxygen index (LOI) measurements, UL 94 tests, and the results of cone calorimetric experiments. The physical and mechanical properties of the copolymers are similar to those of PMMA, except that the bulk copolymers are slightly crosslinked, and are better than those of PMMA flame retarded to a similar extent by some phosphate and phosphonate additives. Examination of the some of the gaseous products of pyrolysis and combustion, and of chars produced on burning, show that flame retardation occurs in the copolymers by both a condensed-phase and a vapour-phase mechanism. The condensed-phase mechanism is shown to involve generation of phosphorus acid species followed by reaction of these with MMA units giving rise to methacrylic acid units. The methacrylic acid units subsequently form anhydride links, which probably impede depolymerization of the remaining MMA sequences, resulting in evolution of less MMA (the major fuel when MMA-based polymers burn). By undergoing decarboxylation, leading to interchain cyclisation and, eventually, to aromaticisation, the anhydride units are probably also the principal precursors to char.

Synthesis of Mesoporous Silica@Co–Al Layered Double Hydroxide Spheres: Layer-by-Layer Method and Their Effects on the Flame Retardancy of Epoxy Resins

Hierarchical mesoporous silica@Co-Al layered double hydroxide (m-SiO2@Co-Al LDH) spheres were prepared through a layer-by-layer assembly process, in order to integrate their excellent physical and chemical functionalities. TEM results depicted that, due to the electrostatic potential difference between m-SiO2 and Co-Al LDH, the synthetic m-SiO2@Co-Al LDH hybrids exhibited that m-SiO2 spheres were packaged by the Co-Al LDH nanosheets. Subsequently, the m-SiO2@Co-Al LDH spheres were incorporated into epoxy resin (EP) to prepare specimens for investigation of their flame-retardant performance. Cone results indicated that m-SiO2@Co-Al LDH incorporated obviously improved fire retardant of EP. A plausible mechanism of fire retardant was hypothesized based on the analyses of thermal conductivity, char residues, and pyrolysis fragments. Labyrinth effect of m-SiO2 and formation of graphitized carbon char catalyzed by Co-Al LDH play pivotal roles in the flame retardance enhancement.

Ignition temperatures and pyrolysis of a flame‐retardant methyl methacrylate copolymer containing diethyl(methacryloyloxymethyl)‐phosphonate units

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 10 mol% 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 5 mol% 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.

Prediction of CO evolution from small-scale polymer fires

The concentrations of decomposition products of polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(methyl methacrylate) (PMMA) and 19% and 26% vinyl acetate-ethylene copolymers (19% EVA and 26% EVA, respectively) have been studied at equivalence ratios o varying from 0.5 to 1.5 using a Purser furnace. The CO yield of the fire gases increased with increase in fuel/air ratio. For PE, PP, PMMA and 19% EVA and 26% EVA, the CO evolution was independent of the polymer and depended only on o. PS gave higher CO yields at low fuellair ratios, and lower CO yields at high fuellair ratios in comparison with the other polymers studied. The CO yield translates to a fractional effective dose, showing a threefold increase in the fire toxicity in going from fuel lean (o = 0.5) to fuel rich (o = 1.5) conditions.

Burning behaviour of foam/cotton fabric combinations in the cone calorimeter

Abstract The burning behaviours of polyurethane foam/cotton fabric combinations were investigated using cone calorimetry. One non-flame retarded and two flame retarded polyurethane foams containing melamine and melamine plus a chlorinated phosphate respectively, were combined with four cotton fabrics, i.e. two types of commercial cotton, one without flame retardant (CN) and another flame retarded with Proban (CPR); and another two flame retardant cottons which were treated with Pyrovatex (CPY) and (NH4)2HPO4 (CDA) respectively in the laboratory.

Fabrication of Ce-doped MnO2 decorated graphene sheets for fire safety applications of epoxy composites: flame retardancy, smoke suppression and mechanism

Ce-doped MnO2–graphene hybrid sheets were fabricated by utilizing an electrostatic interaction between Ce-doped MnO2 and graphene sheets. The hybrid material was analyzed by a series of characterization methods. Subsequently, the Ce-doped MnO2–graphene hybrid sheet was introduced into an epoxy resin, and the fire hazard behaviors of the epoxy nanocomposite were investigated. The results from thermogravimetric analysis exhibited that the incorporation of 2.0 wt% of Ce-doped MnO2–graphene sheets clearly improved the thermal stability and char residue of the epoxy matrix. In addition, the addition of Ce–MnO2–graphene hybrid sheets imparted excellent flame retardant properties to an epoxy matrix, as shown by the dramatically reduced peak heat release rate and total heat release value obtained from a cone calorimeter. The results of thermogravimetric analysis/infrared spectrometry, cone calorimetry and steady state tube furnace tests showed that the amount of organic volatiles and toxic CO from epoxy decomposition were significantly suppressed after incorporating Ce–MnO2–graphene sheets, implying that this hybrid material has reduced fire hazards. A plausible flame-retardant mechanism was hypothesized on the basis of the characterization of char residues and direct pyrolysis-mass spectrometry analysis: during the combustion, Ce–MnO2, as a solid acid, results in the formation of pyrolysis products with lower carbon numbers. Graphene sheets play the role of a physical barrier that can absorb the degraded products, thereby extend their contact time with the metal oxides catalyst, and then promote their propagate on the graphene sheets; meanwhile pyrolysis fragments with lower carbon numbers can be easily catalyzed in the presence of Ce–MnO2. The notable reduction in the fire hazards was mainly attributed to the synergistic action between the physical barrier effect of graphene and the catalytic effect of Ce–MnO2.