Physical Structure

The geometry of ion exchange particles as manufactured today is spherical. The bead diameters can be varied by the method of manufacture, but the standard size of commerce is a Gaussian distribution of beads ranging in diameter from 250 |im (60 mesh US Sieve Series) to about 1000 |im (18 mesh US Sieve Series). The condensation polymers of phenol with formaldehyde may still be supplied as irregularly shaped particles, but even these polymers can be made in spherical bead form, if desired. The spherical geometry arises from the method of manufacture which is by stirring a suspension polymerization of monomer droplets dispersed in an immiscible liquid. The immiscible liquid most used is water properly formulated to maintain droplet integrity throughout the transformation of monomer into polymer.

Recently a number of manufacturers of ion exchangers have developed technology for making monosized particles in which the range of size is very narrow with a uniformity coefficient of less than 1.12. Monosized particles may have an advantage in some catalytic applications if the ion exchanger being used is a gel resin. The monosized gel beads will have an advantage over a Gaussian distribution of beads if the average diffusional path length is shorter for the monosized beads than that for the Gaussian distribution. For macroporous polymers, the bead diameter has a very small impact upon the mass transport because the ingress and egress is through a continuous pore system rather than through a sol-vated polymer network as in a gel polymer.

Ion exchange beads have two internal polymer morphologies: one is a gel in which the network of polymer chains is continuous throughout the bead volume; the other is a macroporous structure in which the bead is constructed from small microgel particles tending towards spherical symmetry and packed together into clusters and arrays of clusters. The macro-porous bead has both a continuous pore phase and a continuous gel phase, whereas the gel bead has only a continuous gel phase. Within the gel bead, there are no pores. Porosity develops only as the polymer chains are solvated by the reaction medium and become solvent separated. Within the macroporous polymers, there are two subgroups: those with a small specific surface area (S) less than about 400 to 500 m2 mL"1 and those with a large specific surface area greater than about 600 m2 mL"1.

The macroporous polymers with a small specific surface area have good accessibility into the core of the bead but the number of catalytic sites on the pore surface is insufficient to provide acceptable rates of catalysis. Consequently, the working phase in these beads is primarily the gel phase of the microgel. The macroporous polymers with a large specific surface area (S) have sufficient catalytic sites on the internal pore surface to give acceptable rates of catalysis and are, therefore, true surface phase catalysts. Table 1 shows these relationships for a family of sulfonated macroporous polymers.

In the macroreticular synthesis of macroporous polymers, large surface areas are achieved only by increasing the level of crosslinking in the polymerizing monomer mixture. The microgel of the resulting polymer is so tightly crosslinked that it is impenetrable even to molecules as small as methylene di-chloride (CH2Cl2). For effective catalysis, the surface phase must be the working arena since the gel phase is impenetrable and also not functionalized. Consequently, mass transport and catalytic effectiveness are influenced quite differently within these three physical structures by the following:

1. Level of crosslinking

2. Bead diameter

3. Solvating nature towards the polymer by the reaction medium

4. Size of the reactants and/or products.

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Solar Panel Basics

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