Large, dry granules are formed from fine powder mixtures by wet-granulation. This process is quite complex but an accepted view holds that the liquid solution containing a binder, e.g., polymer, wets and spreads in the interstices between primary particles, forming liquid bridges that subsequently dry. As liquid evaporates from the bridges it leaves behind solid bridges or “necks” that impart mechanical strength to the dry granule. The process of solid bridge formation is not unique to granulation and plays a significant albeit unwanted role in powder caking. There is very little in the literature that describes the properties of solid bridges during granule formation and powder caking. To simplify the problem, the solid bridge is assumed, in most cases, to be non-porous and of similar chemical composition and physical properties as either the primary powder particles or the solidified binder material [1].

The above picture is an overly simplistic view of a complex problem especially if the original fine powder(s) are themselves at least partially soluble in the binder solution. In this case, the liquid dissolves some solid powder and forms liquid bridges of a very complex composition [2]. Upon drying, these bridges exhibit intricate patterns of crystallization that are both time and composition dependent. This behavior imparts complex morphology to the drying bridge as well as time dependent strength to the forming dry granule [3]. The questions answered during this presentation are (i) what kind of solid bridge will actually form inside the granule as liquid evaporates, (ii) what will characterize its strength and (iii) where will it break when subjected to a mechanical load?

References

1. Tardos, G.I. and R. Gupta, “Forces generated in solidifying liquid bridges between two small particles”, Powder Technology, 86(1), 29-35, (1996).

2. Farber, L, G.I. Tardos and J.M. Michaels, “Evolution and structure of drying material bridges of pharmaceutical excipients: Studies on a microscope slide”, Chem. Eng. Sci., 58, 4515-4525, (2003).

3. Bika, D., Tardos, G.I. , Panmai, S., Farber, L., Michaels, J.N., “Strength and morphology of solid bridges in dry granules of pharmaceutical powders”, Powder Technology, Vol. 150, pp. 104-116, (2005).