Particle morphology, effects

Hughes, P., Jackson, G.V., and Rothon, R.N., Particle morphology effects on the performance of PMMA filled with aluminum hydroxide in a variety of fire tests, Makromol. Chem. Makromol, Symp., 74, 179, 1993. 

The dynamics of particle morphology can be used to an advantage, to counteract the effect of sintering ofthe copper particles. As Fig. 8.13 shows, a Cu/ZnO catalyst slowly loses activity, which is attributed to sintering. Exposing the catalyst for a short time to a highly reducing mixture of C02-free synthesis gas restores the activity. 

There are a number of concepts concerning the structure of small particles which have a bearing upon geometrical catalytic effects (e.g. 41-43). These follow both from the surface imaging results, and a detailed experimental (13-15) and theoretical (44-47) study of particle morphologies. 

Structure sensitivity is not a single phenomenon but can often be regarded as a set of independent, but interlinked mechanisms. At the very beginning it is important to realize that the term particle size effect (PSE) not only refers to the size of active component particles but moreover comprises effects deriving from peculiarities in their morphology, that is, their shape and structure. 

To be effective, these fillers have to be used at high loadings and it is essential to minimise any associated loss in important properties such as toughness. It is this aspect that largely determines the optimum particle morphology, rather than the flame retardancy. 

An issue that has been receiving increasing attention is the deleterious effect of fillers on the scratch resistance of polymers, as measured by the loss in surface appearance. The understanding of this problem is still at a rudimentary stage, but it appears that the problem can be minimised by control of particle morphology [28] and correct choice of surface treatments [29]. 

In general, the filler industry recognises these limitations, and tries to use a few relatively simple parameters that, taken in combination, give an approximate, working definition of morphology appropriate to the application in mind. The parameters that are most likely to be encountered are specific surface area, average particle size, effective top size and oil adsorption. The measurement and application of these are discussed in more detail below. 

Crowder TM, Rosati JA, Schroeter JD, Hickey AJ, Martonen TB. Fundamental effects of particle morphology on lung delivery predictions of Stokes law and the particular relevance to dry powder inhaler formulation and development. Phanna Res 2002 19 239-245. 

Bulk Density. The bulk densities determined by the tapping method for three powders were 0.424, 0.426 and 0.425 g/ml. From a theoretical viewpoint, particles with a smaller diameter should contribute to a tighter packing and thus lead to an increase in bulk density. This difference was not noted. Again, this could result from the difference in particle morphology. The increased tendency to expand or form internal voids in small size fractions (8) may have offset the expected effect of particle size itself. Therefore, no significant difference was observed. 

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