Flashpoint and burning of thin molten plastic pool above hot boundary

Abstract

The melting and dripping of burning thermoplastics can cause a new ignition and form a plastic pool fire, resulting in a significant fire risk. This work investigates the burning dynamics of polyethylene (PE) vs polypropylene (PP) pools fully melted at 380–410 °C on a hot plate with a controlled area and initial temperature. For PE, three burning patterns are observed and defined under different bottom boundary temperatures. When the boundary temperature is lower than the melting point of thermoplastic, burning Pattern I (near-limit flame) appears shortly and extinguishes quickly. Above the melting point of PE, the flame becomes stronger and can last for the longest period before quenching (Pattern II: transitional flame). PP does not have this transitional-flame stage due to a higher melting point and lower pyrolysis point. When the plastic pool temperature exceeds its flashpoint of about 300 °C (∼60 °C below its pyrolysis point), the flame becomes intense and quickly burns out the molten pool (Pattern III: intensive flame). The burning processes of molten thermoplastics show a clear difference from the burning of ethanol and paraffin wax. This study promotes the understanding of the melting and burning of plastics in real fire scenarios and helps quantify the hazards of dripping and flooring fires.