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

Abstract

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 (Amgard 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. 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.