Ignition of low-density polyethylene slabs by a small flame

Slabs of low-density polyethylene (LDPE) were exposed to the wake of a lean hydrogen-oxygen flat flame. The ignition delay and initial flame velocity after the ignition were measured at several gas-air equivalence ratios and distances from the igniting flame. When ignition occurred, the surface temperature was far lower than that required for pyrolysis in the absence of oxygen. Small amounts of char formed on the polymer surface during the delay, consistent with the involvement of oxygen in solid-phase preignition processes. Plots of ln(delay) versus l/(absolute temperature) were linear and the activation energy was derived from the Arrhenius equation, 64 + or - 10 kJ/mol. Initial rates of flame development decreased with increased separation between the polymer and the igniting flame, but unlike those reported for poly(methyl methacrylate), they were independent of the duration of the preceding delay except when the polymer was very close to the flame. The results are explained by a model in which both the ignition delay and the subsequent rate of flame development depend on the concentration of species associated with the chain- propagation steps of the combustion process.Des plaques de poly\ue9thyl\ue8ne de faible densit\ue9 ont \ue9t\ue9 plac\ue9es dans le sillage de la flamme plane d'un m\ue9lange pauvre en hydrog\ue8ne et en oxyg\ue8ne. Le d\ue9lai d'allumage et la vitesse initiale de propagation de la flamme apr\ue8s l'allumage ont \ue9t\ue9 mesur\ue9s en fonction de plusieurs rapports d'\ue9quivalence gaz-air et \ue0 diverses distances de la flamme d'allumage. Apr\ue8s l'allumage, la temp\ue9rature superficielle \ue9tait de beaucoup inf\ue9rieure \ue0 celle requise pour la pyrolyse en l'absence d'oxyg\ue8ne. De petites quantit\ue9s de produits de carbonisation se sont form\ue9es \ue0 la surface du polym\ue8re pendant le d\ue9lai, ce qui concorde avec les r\ue9sultats relatifs au r\uf4le de l'oxyg\ue8ne dans le processus de pr\ue9allumage en phase solide. La variation de ln(d\ue9lai) en fonction de l/(temp\ue9rature absolue) \ue9tait lin\ue9aire et l'energie d'activation, 64 + ou - 10 kJ mole[-1], a \ue9t\ue9 calcul\ue9 e \ue0 partir de l'\ue9quation d'Arrh\ue9nius. Les vitesses initiales de propagation de la flamme diminuaient lorsque la distance entre le polym\ue8re et la flamme augmentait mais, contrairement aux vitesses obtenues avec le poly(m\ue9thylm\ue9 thacrylate), elles \ue9taient ind\ue9pendantes de la dur\ue9e du d\ue9 lai d'allumage, sauf dans le cas o\uf9 le polym\ue8re \ue9tait tr\ue8s pr\ue8s de la flamme. Les r\ue9sultats sont expliqu\ue9s \ue0 l'aide d'un mod\ue8le dans lequel le d\ue9lai d'allumage et la vitesse de propagation de la flamme d\ue9pendent de la concentration des substances associ\ue9es aux \ue9tapes de la propagation en cha\ueene de la combustion.Peer reviewed: YesNRC publication: Ye