Insights into the effect of different macromolecular architectures on the charring ability of polyethylene

The induction period preceding the ignition is a fundamental stage in controlling the protection of polymeric materials from fire. In fact, at this stage, the formation of a non-pyrolyzable carbonaceous layer (char) onto the material surface could prevent the development of a sustained flame, hence delaying or preventing polymer burning. In this work, the charring ability of four samples of high- and low-density polyethylene was evaluated in different scenarios, namely isothermal oxidative conditions and cone calorimetry tests at different heat fluxes, aiming at assessing the possible influence of the polymer macromolecular architecture on their charring ability. First, the selected polymers were characterized from a thermal and rheological point of view; the obtained results allowed for disclosing the different microstructure of the materials, in terms of molecular weight and possible presence of short- or long-chain branching. Then, isothermal thermogravimetric analyses and cone calorimetry tests were performed, demonstrating a different charring ability of the investigated polyethylenes. Finally, the observed behaviours were correlated to the different chemical structure and to the specific macromolecular architecture of the polymers, allowing a systematic evaluation of the relationship between the molecular weight or the presence of branched structures and the mechanism and rate of char formation.