Innovative systems and applications - Physical Gels from Biological and Synthetic Polymers - page 347

Figure 11.13 Scanning electron micrograph of a superporous hydrogel (SPH). Reproduced with permission from
Omidian et al.( 2005 ) © 2005 Elsevier.

4. Foaming aid, foam stabilizer

3. Cross-linker

5. Oxidant

2. Neutralization
1. Monomer dilution

6. Reductant

7. Bicarbonate

SPH

Figure 11.14 Steps in the production of a superporous hydrogel (SPH). Reproduced with permission from
Omidian et al.( 2005 ) © 2005 Elsevier.

added to generate gas bubbles. Dispersion and dissolution of HCO 3 − increases the pH of
the reaction medium to a level at which the initiator decomposes faster. When the
formation of initiator radicals reaches a certain level, polymerization proceeds rapidly
and the reacting mixture becomes viscous over time. At the same time, HCO 3 − reacts
with the acid component to produce the CO 2 required for the blowing process.
The two processes, i.e. the cross-linking reaction (gelation) and foaming processes, need
to be controlled so that 80% of the foaming develops before gelation. Then the rest of the
foaming occurs, the cross-linking reaction takes place and temperature increases by 5 10°C.

As cross-linking proceeds further and the temperature reaches its maximum, the foam turns
into a

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flexible, rubber-like material. Then the synthesized foam is added to a non-solvent,
usually ethanol, in several batches until it is completely dehydrated, to stabilize the product
and prevent it from shrinking. This results in a solid, brittle, porous product which can be
ground into particles (like SAP particles), sliced into absorbent sheets or machined to a
desired shape and size. Dried SPHs swell fast in water and to a size a few hundred times their