Properties particle dispersions

Figure 4c also describes the spontaneous polymerisation ofpara- s.yX en.e diradicals on the surface of soHd particles dispersed in a gas phase that contains this reactive monomer (16) (see XylylenePOLYMERS). The poly -xylylene) polymer produced forms a continuous capsule sheU that is highly impermeable to transport of many penetrants including water. This is an expensive encapsulation process, but it has produced capsules with impressive barrier properties. This process is a Type B encapsulation process, but is included here for the sake of completeness. 

A convenient way to understand particle dispersion is to consider the process in four successive parts the nature of particles and surfaces, adsorption onto particles, interface properties, and forces of attraction and repulsion. 

In summary, dispersants are effective for particle dispersion and crystal growth inhibition, but do not normally have surface-active properties such as oil emulsification. Chelants and antiprecipitants frequently inhibit crystal growth better than dispersants, but are ineffective for particle dispersion. Flocculants are effective for aggregating particles, the opposite function of a dispersant. 

Energetic composites can be considered as particle-dispersed polymeric matrices. The mechanical properties of these systems are crucial to their dimensional and ballistic stability. Their ability to deform without rupture and to recover is important for successful performance. 

Investigations of the rheological properties of disperse systems are very important both from the fundamental and applied points of view (1-5). For example, the non-Newtonian and viscoelastic behaviour of concentrated dispersions may be related to the interaction forces between the dispersed particles (6-9). On the other hand, such studies are of vital practical importance, as, for example, in the assessment and prediction of the longterm physical stability of suspensions (5). 

In practical application, it was reported that the platinum particles dispersed in highly porous carbonized polyacrylonitrile (PAN) microcellular foam used as fuel-cell electrocatalyst160 have the partially active property. The fractal dimension of the platinum particles was determined to be smaller than 2.0 by using the potentiostatic current transient technique in oxygen-saturated solutions, and it was considered to be a reaction dimension, indicating that not all of the platinum particle surface sites are accessible to the incoming oxygen molecules. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

Sodium silicate wetting, emulsifying, penetrating, and other surface-active properties do not appear to impact ink removal efficiency in flotation (36). Flotation can also remove some of the paper filler and coating particles dispersed in the pulp. Addition of certain cationic oiganic polymers such as poly(dia11yldimethylammonium chloride) to pulp improves the removal efficiency of these particles during flotation (37,38). 

It has also been suggested that steady state low shear dynamic measurements in the melt could be a convenient method for the study of particle dispersion in relation to flller properties, which might also correlate with mechanical properties of the composite [48,49]. 

The state of the art in friction and wear of PTFE-filled rubbers include the effects of many important system parameters, such as the composition of the rubber formulation, particle dispersion, bulk mechanical properties, ability of transfer film formation, and the chemistry between PTFE powder and the rubber matrix. Although the present study has explicitly highlighted the potential of PTFE powder in rubber matrixes with significant property improvements in the friction, wear, and physical properties, it has simultaneously opened a new field regarding the use of PTFE powder in rubber compounds, with some challenging tasks for chemists, engineers, and material scientists. 

Since needlelike particles of controlled shape and size can be prepared (see Fig. 1.14, for example), the effects of asymmetry of the particles on flow and optical properties of dispersions can be studied in a systematic manner. 

The preceding section indicated some of the complications that arise when the optical properties of dispersions are calculated without placing narrow limitations on the size and refractive index of the particles. The difficulties are still large but somewhat more manageable if only the refractive index is given full range, the particle dimensions being somewhat restricted. This is the situation that was treated by Mie in 1908. 

The type of chosen polymer and additives most strongly influences the rheological and processing properties of plastisols. Plastisols are normally prepared from emulsion and suspension PVC which differ by their molecular masses (by the Fickentcher constant), dimensions and porosity of particles. Dimensions and shape of particles are important not only due to the well-known properties of dispersed systems (given by the formulas of Einstein, Mooney, Kronecker, etc.), but also due to the fact that these factors (in view of the small viscosity of plasticizer as a composite matrix ) influence strongly the sedimental stability of the system. The joint solution of the equations of sedimentation (precipitation) of particles by the action of gravity and of thermal motion according to Einstein and Smoluchowski leads 37,39) to the expression for the radius of the particles, r, which can not be precipitated in the dispersed system of an ideal plastisol. This expression has the form ... 

A characteristic feature of colloidal dispersions is the large area-to-volume ratio for the particles involved. At the interfaces between the dispersed phase and the dispersion medium characteristic surface properties, such as adsorption and electric double layer effects, are evident and play a very important part in determining the physical properties of the system as a whole. It is the material within a molecular layer or so of the interface which exerts by far the greatest influence on particle-particle and particle-dispersion medium interactions. 

A most important physical property of colloidal dispersions is the tendency of the particles to aggregate. Encounters between particles dispersed in liquid media occur frequently and the stability of a dispersion is determined by the interaction between the particles during these encounters. 

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