General Aspects

The first organic ion exchanger that found technical application was a chemically modified natural product, namely a sulfonated coal, described in many patents during the 1930s.

Other exchangers were synthesized by sulfonation or phosphorylation of wood, paper, cotton, lignin and tannins, as well as by the crosslinking of pectins with formaldehyde or epichlorhydrin.

In 1935 the discovery by Adams and Holmes of ion exchange properties in the product of a reaction between phenol, or m-phenylenediamine, with formaldehyde started the development of synthetic organic ion exchangers. These products have a greater importance than those from a natural organic source and have found much wider technical application because of their greater chemical stability and mechanical strength as well as their very different physical and chemical structures.

Synthetic organic ion exchangers are obtained by the two principal reactions used to produce polymeric materials, namely polycondensation or addition polymerization of a mixture of co-monomers. In poly-condensation, incorporation of a trifunctional co-monomer is required while in polymerization the presence of a bifunctional co-monomer is sufficient.

Most commercially available ion exchangers are from polymerization processes which create structures with higher hydrolytic and oxidative stabilities as well as better defined physical features and cross-linkings.

In the case of the polycondensation exchangers, the reaction between a co-monomer that carries base or acid groups and a crosslinking agent (formaldehyde, epichlorohydrine, etc.) is used.

In 1944, D'Alelio found that sulfonated styrene-divinylbenzene copolymers have ion exchange properties. This finding was the beginning of the polymerization ion exchangers.

These structures are made by the polymerization of a mixture of a monovinylic monomer with a basic or acidic group and a divinylic monomer. The achievement of a neutral network, called the precursor or starting material, followed by the introduction of basic or acidic groups by suitable polymer-analogous reactions, is often preferred.

Usually divinylbenzene (DVB) is used as the divinylic monomer and the quantity added, in terms of the percentage in the mixture of co-monomers, defines the degree of crosslinking of the network, although crosslinking side reactions can occur during the polymer-analogous transformations.

The structures created are called 'conventional' or 'gel'-type ion exchangers and generally have about 8 % DVB for crosslinking. This amount is required to achieve a network with both mechanical strength and easy diffusion of exchangeable ions as the exchanger comes into contact with an aqueous phase when swelling of the network occurs.

Meitzner and Oline found that the copolymerization of styrene with DVB in the presence of an appropriate inert compound, called 'diluent' or 'porogene agent', gave a network with significant and measurable physical porosity in the dried state, generally containing internal pores having diameters larger than 3 x 10~9 m. This discovery led to significant progress in the field of synthetic ion exchangers, namely the development of macroporous resins. These exchangers offer the advantage that they can be used with non-aqueous solvents and have much higher sorption rates of ions and non-electrolytes than the conventional gel exchangers.

Polymerization produces exchangers in bead form, with a relatively wide distribution of size, by the suspension polymerization technique. More recently ion exchangers with uniform and controlled bead size have become available.

The polycondensation exchangers often appear as irregular-shaped particles, because they are made by bulk polycondensation followed by grinding of the bulk polymer into smaller particles. However, poly-condensation exchangers can also be made in bead form by reverse-phase suspension polycondensation.

Ion exchangers in fibre form are also known, made by chemical modification of natural and synthetic fibres. Ion exchanger fibres have an improved kinetic performance when compared with the same structures in bead form.

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