Laser pyrolysis/time-of-flight mass spectrometry studies pertinent to the behaviour of flame-retarded polymers in real fire situations

The Salford Laser Pyrolysis/Time-of-Flight Mass Spectrometry (LP/TOFMS) technique, which models the behaviour in the so-called dark flame region behind the flame front in a polymer fire, has been applied to investigate flame-retarded polymethylmethacrylate (PMMA), rigid polyurethane foam systems and phosphorus retarded rigid polyurethane foams and a model urethane compound. The laser pyrolysis of aluminium oxide trihydrate (ATH) retarded PMMA produces a large amount of water and carbon dioxide in the volatiles. Also, the amount of the monomer evolved is reduced significantly compared to that obtained from pure PMMA. The implication of these results is that in a real fire situation, ATH influences PMMA pyrolysis in such a manner as to bring about a reduction in the evolved 'fuel' whilst at the same time adding non-combustible gases (e.g. water) to the flame region. Thus is the PMMA flame retarded. The rigid polyurethane foams studied varied in isocyanate index and the molecular weight of the polyols applied. The flame retardance of these materials has been shown to increase with increasing isocyanate index and weight fraction of isocyanate. Laser pyrolysis experiments of these samples showed that the major volatiles evolved were dominated by monomer and oligomers of the polypropylene glycol used to produce the foam, plus lower molecular weight species of which carbon dioxide appeared to be a significant part. An increase in isocyanate index results in a reduction in the extent of monomer/oligomer evolution and an increase in the low molecular weight species. With reference to the behaviour of the foams in a real fire situation, it could be imagined that the monomer/oligomer components and their breakdown products would act as fuel in the flame region while the low molecular weight species dominated by carbon dioxide would be relatively non-flammable. An increase of isocyanate index is equivalent to making less fuel and more of the 'inert gases' available to the burning zone and hence improving the fire resistance of the rigid polyurethane foams. The flame retardant mechanism of phosphorus, introduced as low percentages of dimethyl methylphosphonate, is also attributed to a reduction in fuel evolution via pyrolysis of rigid polyurethane foams