Particle deformability

Particle deformability means that for the same amount of dispersed material, the viscosity at high concentrations is lower than the equivalent dispersion of solid particles, since the droplets are deformable and can thus accommodate each other at rest and squeeze past each other during flow, so that first the maximum packing fraction 0m is higher and the intrinsic viscosity rj is lower [9]. 

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I. Vakarelski, A. Toritani, M. Nakayama, and Higashitani Effects of Particle Deformability on Interaction Between Surfaces in Solutions. Langmuir 19, 110 (2003). 

Formation of agglomerates by powder compaction may involve rearrangement of particles to increase their packing efficiency resulting in the enhancement of interparticle adhesion forces [89]. Furthermore, particle deformation at the point of contact between particles can greatly increase both the contact surface area and interparticle attraction [84]. 

Glassy Particles Deformable Into Ellipsoidal Shapes... 

During compaction, primary particles are packed, re-arranged and can undergo deformation and possibly breakage. These events can occur sequentially or in parallel. The mechanical strength of a tablet may strongly depend on the mechanical properties of the primary particles and the particle-particle interactions within it. It is essential that the particles deform plastically or rupture since the stored elastic strains can weaken the tablet on release (Roberts and Rowe, 1987). 

Density When a powder is poured into a container, the volume that it occupies depends on a number of factors, such as particle size, particle shape, and surface properties. In normal circumstances, it will consist of solid particles and interparti-clulate air spaces (voids or pores). The particles themselves may also contain enclosed or intraparticulate pores. If the powder bed is subjected to vibration or pressure, the particles will move relative to one another to improve their packing arrangement. Ultimately, a condition is reached where further densilication is not possible without particle deformation. The density of a powder is therefore dependent on the handling conditions to which it has been subjected, and there are several definitions that can be applied either to the powder as a whole or to individual particles. 

Guo, H. X., Heinamaki, I, and Yliruusi, J. (1999), Characterization of particle deformation during compression measured by confocal laser scanning microscopy,Int. J. Pharm., 186, 99-108. 

It is suggested that four mechanisms are basically involved in the process of compression of particles deformation, densification, fragmentation, and attrition. The process of compression is briefly described as follows small solid particles are filled in a die cavity and a compression force is applied to it by means of punches and then the formed monolithic dosage form is ejected. The shape of the tablet is dictated by the shape of the die while the distance between the punch tips at the point of maximum compression governs the tablet thickness, and the punch tip configuration determines the profile of the tablet. The compression cycle in a conventional rotary tablet press will be described in detail in this chapter and is illustrated in Figure 1. 

Studies on the rheology of two-phase particulate systems suggest the existence of a deformation threshold that depends on the concentration of particles in the composite. Below this threshold (—68 vol.% for spherical particles), deformation occurs primarily by the flow of the composite matrix. Particles increase the effective viscosity of the matrix by absorbing energy and by forming clusters. 

This eqiiation accoxmts for the effect of the solvent, presence of charged particles (second term) and pectin (third term) on viscosity (r = 0.996). Sxmimarizing, the viscosity of some complex liquids is adequately represented by empirical equations that have as a structural parameter the volume fraction of the dispersed phase. Particle deformation, specific interactions between particles and the presence of a non-Newtonian continuous phase, all which contribute to the structure of a complex liquid, are more difficult to model. 

At low force, the powder particles remain fixed in position and the particles deform elastically. This takes place over a very small range of deformation for diy ceramic powders. This behavior is referred to as a compact body deformation. This deformation can be estimated from the elastic properties of the particles, the void fraction of the powder packing, and the nature of the liquid or binder occupying the voids [71]. 

FIGURE 7.17 Polymer particles deforming under the pressure providing a cushioning effect. Polymer particles coated with colloidal silica particles (from Ref 101). 

Rumscheidt and Mason [14] described particle deformation in a shear field as a function of viscosity ratio (p). There is a minimum and a maximum viscosity ratio where it becomes impossible to reduce the droplet size. The limits described by Karam and Bellinger [15] are 0.005 and 4. Breakup of droplets readily occurs when the viscosity ratio is of the order of 0.2 1. Intuitively, a ratio of 1 would be best because in this case there is a maximum transfer of energy between the... 

The TEM micrographs in Figs. 16a-16c of the undeformed regions of the reconstituted films prepared for mechanical tests revealed that particles were well dispersed and did not coagulate with each other. This proved that HIPS particles of narrow size ranges can be separated from a matrix and put into another matrix without coagulation and without particle deformation or disruption. The tensile stress-strain behavior of these samples is shown in Figs. 17a-17d, while in Table 3 the principal parameters of these curws are summarized. 

During compression, the bulk volume of the material is reduced, resulting in the displacement of the gaseous phase (air).f Further increasing the force leads to particle deformation and rearrangement. At this point, the three principal modes of deformation are as follows ... 

The consequences of such a force distribution on tablet strength can be profound. Particle deformation, whether elastic or plastic, will be proportional to the force applied, and as has been discussed, this deformation is an essential preliminary to the formation of the interparticulate bonds on which tablet integrity depends. Thus, the porosity of the tablet, and hence its strength, will vary within the tablet. The weakest points in the tablet structure will be those that receive the lowest force i.e., on the face of the tablet adjacent to the stationary punch and on the central axis near to the moving punch. Thus, because of its non-uniform density, some parts of a tablet are stronger than others. 

FIGURE 6 A typical Heckel plot showing three phases of particle deformation during in-die method. Source Adapted from Ref. 180. 

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