Sintering kinetics of UHMWPE nascent powders by high velocity compaction: Influence of molecular weight

The sintering mechanism by high velocity compaction of UHMWPE nascent powders has been investigated with regard to the number of impacts, Nhits (otherwise the total impact energy), and the molecular weight, Mw. The mechanical energy provided to the powder produces a partial melting and subsequent welding of the powder particles. A Nhits threshold value is necessary to generate mechanical cohesion in the powder-compacted billets owing to the recrystallized interfacial material which provides wetting and welding of the powder particles. Mechanical strength increases with increasing Nhits owing to the increasing fraction of recrystallized material. However, stiffness decreases in parallel due to the lower crystallinity of the recrystallized material compared to the original nascent powder. The Mw dependence of the mechanical properties of the various compacted powders is attributed to the interface strength of the welded particles in relation to the kinetics of interfacial chain diffusion. The mechanism of particle welding is then discussed in terms of chain diffusion with regard to the time scale of the process. Wool’s welding criterion based on the reptation of chains over their own radius of gyration through the interface only applies for Mw < 106 g/mol. It turned out that above this Mw value efficient welding is readily obtained after chain diffusion over a distance close to the stacking long period thanks to the occurrence of co-crystallization.