On the flammable behavior of non-traditional dusts: Dimensionless numbers evaluation for nylon 6,6 short fibers

Safety parameters assessment is not sufficient to fully understand the flammable and explosive behaviour of a combustible dust and correctly manage potential risk. A correct evaluation requires the identification of flame propagation path as well as the limiting step controlling fire propagation, through evaluation of dimensionless numbers (Biot, Damköhler, Thiele, Sherwood, Thiele modulus numbers). Herein, these aspects were investigated for non-traditional dusts, made of nylon 6,6 short fibers. To this purpose, flammability parameters including minimum ignition energy (MIE), the maximum pressure of explosion and the deflagration index were assessed and combined with results of extensive physical-chemical characterization, by means of several techniques (TGA/DSC, FTIR, XRD). In particular, thermogravimetric analysis highlighted the presence of homogeneous and heterogeneous phase phenomena activated at different temperatures and heating rates. The homogeneous phase processes are controlled by the pyrolysis process strictly dependent on the dust size and its decomposition kinetics. The most flammable sample is characterized by smaller dimensions and a fast decomposition kinetics at low temperature. Heterogeneous flame propagation is controlled by the intrinsic heterogeneous reaction. The most reactive sample is characterized by the highest value of specific surface area and by intense exothermic phenomena at low temperature, as evidenced by the analysis of the solid residue. As a main conclusion, the processes involving nylon fibres that may modify the key parameters influencing the flammable/explosive behaviour are also discussed.