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Manufacture of superabsorbents
Superabsorbent acrylic acid polymers are produced by copolymerizing acrylic acid with a "crosslinker", a compound with two or more double bonds. This is mostly carried out as a solution polymerization. Redox systems like K2S.2O8/N / A2S.2O5 or hydrogen peroxide / ascorbate are common and inexpensive initiators for this polymerization. The crosslinkers create cross-links between the individual polymer chains, which leads to the insolubility of the polymer. Diacrylate esters, allyl methacrylates, triallylamines or tetraallyloxyethanes are used.
When enough chains are linked together and a macromolecular network is formed, the system changes from a viscous mass to an elastic solid ("gel point"). The amount of crosslinking substances determines the water absorption capacity of the gel. A high crosslinking density limits the possibility of expansion of the polymer backbone.
After synthesis, the insoluble superabsorbent particles are in the form of granules. Despite their high affinity for water, these particles are not hygroscopic, so that their storage and transport are unproblematic.