Growth mechanisms in melt agglomeration in high shear mixers

This paper presents a review of factors affecting the agglomerate formation and growth mechanisms in melt agglomeration in high shear mixers. The agglomerate formation occurs either by distribution or immersion or by a combination of both mechanisms. Distribution is promoted by a low binder viscosity, by a high impeller speed, and by a small binder particle size. Effects of the liquid saturation of the agglomerates, impeller speed, particle properties, binder viscosity, and electrostatic charging on the subsequent agglomerate growth are discussed, and experimental results are presented. The agglomerate growth becomes controlled by the balance between the agglomerate strength and the shearing forces. If the agglomerate strength is sufficiently high to resist the shearing forces of the rotating impeller, the dominant agglomerate growth mechanism will be coalescence. The shearing forces will give rise to breakage if the agglomerate strength is too low, and then agglomerate growth will occur by a simultaneous buildup and breakdown of agglomerates, possibly combined with growth by layering of fragments upon larger agglomerates. Provided that the liquid saturation is sufficiently high, a higher agglomerate strength is primarily caused by a higher binder viscosity, a smaller particle size, an irregular particle shape, and by densification of the agglomerates.