Powder caking is considered as the undesired aggregation of particles resulting in the transformation of a free-flowing powder into a coherent solid mass. These might simply be large lumps that break down readily into their constituent primary particles. Alternatively caking may result in the complete and irreversible fusion of the entire particulate contents of a silo or other container. Whether mild or extreme, caking is a significant problem for a wide range of process industries in terms of loss of product quality or process performance, and can therefore have substantial impact on the financial health of a business. The aim of this review is to identify the current state of knowledge regarding the phenomenon of powder caking. Of particular concern are the underlying physical and chemical mechanisms, the role of process variables such as temperature, moisture content and consolidation and their dynamics, the range of experimental methods to assess caking and methods to prevent it. The main challenge of the subject is to be able to predict reliably the caking propensity of a powder product at a protracted time scale in the future, and this requires a detailed understanding of the factors listed above.
Throughout the review, areas for further research have been identified that will take us towards meeting this challenge. The role of interparticle forces in caking has been examined. Further work is required to characterise irreversible, non-equilibrium adhesive contact due to molecular rearrangement. The full role of piezoelectric, and pyroelectric charging in caking also requires further investigation.
A review of the formation of solid bridges between particles has identified two main processes; sintering and solvent evaporation. Research into sintering stems from the technologies of powder metallurgy and ceramic manufacture which involve elevated temperature and pressure. The applicability of this research to powder caking has hardly been addressed and its suitability not been examined. It is therefore recommended that further work is directed to develop the established concepts of sintering into the area of powder caking.
A small body of work has provided evidence of metastability in solid bridges causing the morphology of the bridge to evolve over protracted timescales. It is suspected that this condition is endemic in powder caking and therefore more research is recommended in this area. The formation of solid bridges from mixtures of solutes by solvent evaporation is a common phenomenon in caking. There are apparent contradictions regarding the nature of the solid bridge produced from mixed components which would benefit from further work.
The caking of amorphous powders has received a large amount of attention in the literature and models to predict caking kinetics are showing promise. The remaining uncertainty in this area relates to the behaviour of multi-component mixtures of particles. It is recommended that this area is targeted for further work. The published work relating to the dynamics of caking has been reviewed. Attempts at transient heat and moisture transfer modelling have been directed at materials that cake through dissolution and recrystallisation. More theoretical and experimental work is required in the area to develop a universal modelling tool to describe caking by this mechanism. The role of transient heat and mass transfer in caking by viscous flow, creep or sintering has not been addressed. These processes have been shown to be dependent on temperature and moisture content, and therefore it would be worthwhile to focus further work in this area. A review has been conducted of the wide range of tests to measure the strength and extent of powder caking. Of the conventional mechanical test methods, shear cell testing appears to be the most suitable, particularly if a cell was developed that had full humidity and temperature control by air percolation, and was instrumented to give changes in sample volume during time consolidation. For materials that cake by creeping, it is possible that creep testing could be reliably extrapolated to predict future caking propensity as long as the various creep mechanisms are adequately understood and accounted for. Recent developments in the application of indentation to measure powder flow could be applied to diagnose the early stages of caking. The method is sensitive, and requires very small amounts of material. It is recommended that the suitability of this technique for caking is considered in future work. The application of NMR measurements to caking looks a strong candidate for further investigation. It is recommended that the technique is coupled with more rigorous cake strength easurements. So far only amorphous materials have been studied. It would be interesting to apply NMR to the caking of crystalline or multi-component systems.
Finally the published work relating to anti-caking agents has been reviewed. The mechanisms by which these reportedly operate are various including i) competing with the host powder for available moisture, ii) acting as a surface barrier between the host particles, (preventing the formation of liquid bridges, decreasing inter-particle friction, dissipating electrostatic forces, or inhibiting crystal growth of solid bridges), iii) increasing the Tg of an amorphous phase, or iv) forming a moisture-protective barrier on the surface of hygroscopic powders using e.g. lipids. All of these mechanisms could potentially be deployed to reduce caking in multi-component formulations, and therefore further research in this area is strongly recommended.