G.J. Milnes

Use of gas chromatographic analysis of volatile products to investigate the mechanisms underlying the influence of flame retardants on the pyrolysis of cellulose in air

An investigation has been undertaken of the influence of temperature, in the range 300–1000°C, and flame retardant type, on the yields of volatile products and char from flame retardant treated cotton fabrics. The flame retardants studied at commercial levels of application were the three condensed phase active products, ammonium polyphosphate (Amar TR), a phosphonium salt-urea-polycondensate (Proban CC) and a phosphonopropionamide (Pyrovatex CP) and two vapour phase active materials, ammonium polyphosphate-ammonium bromide (Amgard CD) and an antimony (III) oxide-aliphatic bromide (Flacavon H14/587) formulation. H14/587) formulation. Gas chromatographic analysis was restricted to 30 ‘model’ volatile products selected as representative of the pyrolysis process. For clarity, the data are presented in terms of total yields of each of the various generic groups investigated; namely, furans, aldehydes, etc. It was found that the untreated cotton fabric gave no char, with furan yields very much greater than those for the flame retardant treated fabrics. The two vapour phase active flame retardant treatments resulted in low char yields coupled, as was to be expected, with the highest yields of volatiles and CO. The condensed phase retardants, Proban, Pyrovatex and Amgard TR produced large amounts of char, Proban giving most. Significant yields of aromatic species were obtained from the fabrics with the char consolidating Proban and Pyrovatex treatments but Amgard TR, which has a strong dehydration function, gave none. In contrast to the other untreated and flame retardant treated fabrics studied, the antimony/aliphatic bromide treatment (Flacavon) showed two maxima on its yield/temperature profiles. A model for the pyrolytic degradation of cellulose in air has been proposed on the basis of current information concerning char and volatile product formation. This envisages the degradation as two competing series of reaction paths. Volatile flame retardants enhance the decomposition via levoglucosan to form volatile products whilst condense phase active retardants encourage the alternative route to char formation.

Flame retardancy of some ethylene-vinyl acetate copolymer-based formulations

The combustion behaviour and thermal decomposition of ethylene-vinyl acetate copolymer (EVA) (26 wt% vinyl acetate content) formulations containing alumina trihydrate, ammonium polyphosphate, melamine, pentaerythritol and their co-mixtures, were studied using cone calorimetry and thermogravimetric analysis. Formulations containing ammonium polyphosphate burned with the formation of intumescent carbonaceous chars, with EVA acting as a carbonization agent. EVA materials containing ammonium polyphosphate (20 wt%), with a sufficient amount of alumina trihydrate or melamine, were superior to the non-intumescent alumina trihydrate and melamine containing formulations in terms of the heat release rate, mass loss rate and smoke production. Melamine showed some smoke suppressant effect and significant CO reducing properties. However, the melamine-EVA and melamine-alumina trihydrate-EVA showed a very high heat release rate. Thermogravimetric studies showed that oxygen played a favourable role in enhancing the char formation by encouraging active participation of the polymer matrix in the interaction with polyphosphoric acid.

The renaissance of time-of-flight mass spectrometry☆

Abstract Although the time-of-flight mass spectrometer based on the Wiley—McLaren pulsed two-grid ion source has been available since the early 1960s its application has been limited by low resolving power and sensitivity. The renaissance of interest in this instrument is, in part, a consequence of the development of new ionisation techniques such as plasma and laser desorption, which are advantaged by the time-of-flight spectrometers unique ability to provide a complete mass spectrum per event, and its high mass range. This has led to design developments which give major improvements in instrument performance which promise to yield rich rewards particularly in the study of large biomolecules.

Thermogravimetric analysis study of the mechanism of pyrolysis of untreated and flame retardant treated cotton fabrics under a continuous flow of nitrogen

Abstract Samples of unmodified cotton fabrics and cotton fabrics finished with various flame retardant treatments at commercial levels of application were subjected to thermogravimetric analysis in a continuous flow of nitrogen. The relative thermal stabilities of the fabric samples were investigated using various heating rates. Each sample was found to give a characteristic TG curve with three distinct regions. This could be interpreted as an indication of the decomposition occurring in three stages, viz. dehydration, volatile evolution (with char formation) and char removal. All the flame retardant treatments lowered the temperature at which volatiles first appeared. Attempts were made to calculate Arrhenius parameters for the volatile evolution processes using a technique developed at Salford. It was found that fabric samples treated with the commercial condensed phase active-flame retardants, namely ammonium polyphosphate (Amgard TR, Albright and Wilson Ltd) a phosphonopropionamide (Pyrovatex CP, Ciba-Geigy Ltd) and a phosphonium salt-urea-polycondensate (Proban CC, Albright and Wilson Ltd) raise the activation energy compared to that of the untreated cotton fabric. Volatile evolution probably occurs via three-dimensional diffusion. Cotton fabrics treated with the commercial vapour phase-active flame retardants ammonium polyphosphate-ammonium bromide (Amgard CD, Albright and Wilson Ltd) and an antimony(III) oxide-aliphatic bromide (Flacavon, Shill and Seilacher) yielded TG curves which were too complex for simple interpretation.

Laser pyrolysis of polymers and its relation to polymer fire behaviour

Laser-pyrolysis; time-of-flight mass spectrometry has been applied to the study of rapid thermal degradation of polymethymethacrylate (PMMA) and rigid polyurethane foams. The heating rate has little influence on the mechanism of PMMA degradation. In both the laser and low heating rate experiments, monomer was the major pyrolysis product. Laser pyrolysis of rigid polyurethane foams resulted in the evolution of polypropylene glycol as the major species together with low molecular weight species such as CO2 etc. It is considered that polypropylene glycol and its further decomposed species act as fuel for the flame in a real fire situation. The ratio of polypropylene glycol to CO2 in the evolved gases decreased with the increase of isocyanate index. This is associated with the improvement of fire resistance at a high isocyanate index.

Experimental parameters affecting the performance of the Purser furnace: a laboratory-scale experiment for a range of controlled real fire conditions

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 Salfords Purser furnace and reports the findings of a systematic assessment of the experimental parameters that affect the performance of the Purser furnace.

Recent developments in techniques utilising time-of-flight mass spectrometry

Abstract A review is given of a number of techniques which utilise the ability of the time-of-flight mass spectrometer to provide a complete mass spectrum for each ionisation event. These include pulsed laser ionisation, fission fragment mass spectrometry, SIMS and pulsed field desorption. Mention is also made of the improvement in performance of the conventional TOFMS by use of electrostatic ion reflector and time-focussing methods.

GC/MS studies of ABS/CPVC blends

A detailed GC/MS study, of the products of the pyrolysis in air of suitably chosen samples of CPVC/ABS and PVC/ABS blends with and without the smoke suppressant basic iron(III) oxide (FeOOH) present, has been undertaken in order to gain possible insights into the chemical behaviour which results in smoke suppression. Of particular interest is the observation that the presence of the iron(III) compound greatly reduced the concentrations as well as the numbers of aromatic and polycyclic aromatic compounds detected. Such compounds are well known smoke precursors. Thus the mechanism by which FeOOH functions as a smoke suppressant would appear to be via interference with the reactions leading to production of aromatic species.

Laser pyrolysis as a model for fire behaviour behind the flame front

Abstract Two experimental techniques involving the use of laser pyrolysis to model different aspects of polymer behaviour in a fire are briefly reviewed. In the first, a continuous laser provides radiation intensities similar to those occurring in a room fire ( ca. 15 J cm −2 s −1 ). The chemical reactions initiated are monitored using a molecular beam-quadrupole mass spectrometry technique. The second technique utilises pulsed laser heating to generate conditions analogous to those in the very narrow diffusion limited reaction zone at the condensed phase-flame interface of a burning polymer, the so-called ‘dark flame’ region. The rapid chemical reactions occurring are monitored using a fast scanning time-of-flight mass spectrometer. Both experiments provide valuable information pertinent to chemical behaviour during polymer combustion, in particular, that of flame retardant additives.

Applications of time-of-flight mass spectrometry to investigate fire-retardant systems

Abstract An account is given of two mass spectrometric techniques being developed for the investigation of chemical phenomena pertinent to polymer combustion and fire retardancy. Firstly, a systematic investigation of fire retardant materials is being carried out using an experiment in which a ‘SOLID → SOLID + GAS’ reaction is studied under controlled conditions in the ion source region of a small time-of-flight mass spectrometer (TOFMS). The behaviour of polystyrene treated with decabromodiphenyl oxide, with and without the synergists antimony oxide and zinc borate, is reported. In addition, the extremely fast scan capability of the TOFMS is being utilised for monitoring the products evolved when polymer samples are subjected to a very large and rapid temperature rise due to pulsed laser heating. It is hoped that these studies will provide insight into chemical behaviour during plastic ignition under conditions analogous to those occurring in a real fire.