(379d) Caking of Binary Powder Mixtures

In the food and pharmaceutical industries, water soluble amorphous components - in particular various carbohydrates - are ubiquitous. Water acts as a plasticizer on water soluble amorphous material. Plasticizers are compounds that increase the free volume (i.e. the volume that is not occupied by molecules) and therefore decrease the glass transition temperature. The molecular mobility increases and the material soften. As the water content of an amorphous matrix increases, the glass transition temperature decreases. A dramatic decrease of viscosity at temperature above the glass transition temperature can be observed. Indeed the amorphous matrix softens and becomes more and more plastically deformable at temperatures above the glass transition temperature, influencing the stability of a powder bulk. As the viscosity declines viscous flow can develop. If two amorphous particles are in contact, they can stick together and build viscous bridges at their contact point. This is the sintering phenomenon which provokes undesired agglomeration and caking, i.e. the change from a bulk of flowable particles to a sticky mass. Sintering and caking are not only depending on temperature and water content. Indeed mechanical stress applied on a bulk can lead to viscoelastic deformation and amplifies the caking if the particles are sticky. And as sintering is a kinetic phenomenon, storage time has also a great influence on the degradation of flowability. Crystalline particles may also form interparticle bridges but the mechanism of bridge formation differs from sintering. Practically crystalline particles have to be exposed to relative humidity higher than their critical humidity (i.e. the relative humidity at which the crystalline substance starts to take up moisture from the surrounding atmosphere and to dissolve without any viscosity development) to cake. This phenomenon leads to liquid bridges formation between particles that solidify after drying. The critical humidity of crystalline substance being often higher compared to the relative humidity responsible of softening of amorphous substance, they are consequently less prone to caking than the latter. Caking mechanisms of pure amorphous and pure crystalline products are known. However the caking behaviour of powder mixes remains difficult to predict. In case of a powder mix, the properties of each single component have to be taken into account: particle size, particle shape and concentration of each component in the mix. In the food industry, many recipes and products contain mixtures of crystalline and amorphous substances in their powdered form in various particle size distributions. Therefore, knowledge of the caking behaviour of mixtures is important. Actually the most sensitive component of a mix will lead the mixture to cake. Hence it is useful to know which amount of sensitive components, like amorphous ingredients, can be added to a mix without impairing its properties during storage or processing. In the current study, binary mixes of crystalline and amorphous substances in their powdered form and at different concentrations were prepared. The caking behaviour of the mixtures was investigated using time-consolidation measurements with a ring shear tester. The mixes were stored at controlled temperature, relative humidity and consolidation stress. The flowability after storage was analysed as a function of the components' particle size distribution and concentration. This study presents the relationship between the internal factors (mix components characteristics), the external factors (relative humidity, temperature and consolidation stress) and the resulting caking behaviour of binary mixtures.