Filtration and sieving

Filtration can be used to distinguish between dissolved and dispersed components in a colloidal dispersion. Membrane filtration involves passing a suspension through a thin, porous membrane, which is usually polymer or ceramic in nature, but could also be woven fabric or metal fibres. Filtration is also a common method 

Ultrafiltration, nanofiltration and hyperfiltration (also termed reverse osmosis) can provide better separations of colloidal-sized material. The pore size ranges for these overlap somewhat ultrafiltration (10 - 200nm pores), nanofiltration (0.5-10 nm pores) and hyperfiltration ( 2 nm pores). Although there is no exact conversion between pore size and filtration cut-off size, the 1-2 nm pores in the nominal 100 - 200 grnol nanofiltration and hyperfiltration membranes have size cut-offs of about 1 nm in diameter, so that these membranes can be used to separate dissolved from dispersed species [42,43). Care should be taken in using such small pore size ultrafilters since the results are easily influenced by 

Both of these effects act to restrict the passage of species to those of smaller size than the nominal size of the ultrafilter pores themselves. These effects can be minimized, if not eliminated, by a number of techniques. In some cases, membrane exclusion can be dealt with by reducing the solution pH. Another way to avoid 

For small-scale work, such as the characterization of colloidal dispersions, field-flow fractionation (FFF) can be used to separate out small quantities of various size fractions within the full, practical, colloidal size range (from 1 nm to tens of micrometres) [46]. 

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Many parts of polymer dispersion manufacturing plants (Figure 1) tend to be open to the atmosphere. In many locations, due to the nature of the chemicals used and the normally open filtration and sieving processes, are critical points at which microbial contamination may occur. 

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