Sunday, May 25, 2003
How Absorption Works:
Superabsorbent polymers are crosslinked networks of flexible polymer chains. The most efficient water absorbers are polymer networks that carry dissociated ionic functional groups. Except for the molecular-sized chains that make up the network, this picture of a network is remarkably similar looking to the mass of cotton fibers. The difference is that cotton takes up water by convection – water is "sucked" up, wetting the dry fibers; SAPs work by diffusion on the molecular level, since their "fibers" are actually long chained molecules. Diapers include higher or lower amounts of each which determines the absorption properties of the particular diapers.
Water diffuses into a particle of superabsorbent polymer when the concentration of water is initially lower in the interior of the particle. As water travels into the particle, it swells to accommodate the additional molecules. Because the polymer molecules are crosslinked, they do not dissolve in the absorbing liquid.
Absorbency under load and stability of the gel against shear are important properties of superabsorbent polymers and relate strongly to diaper performance. Diaper leakage was closely correlated to the stability of gel to shearing. More rigid superabsorbent particles, created by increasing the crosslinking, allows for a higher gel modulus and helps the particle withstand the shearing from the baby’s weight.
The most commonly available superabsorbent polymers are hard, dry, granular powders that look much like clean white sand or granular table sugar. When these polymer particles are placed in water, a slurry of water and the particles is formed. Gradually the superabsorbent polymer absorbs the water, turning into a soft, rubbery gel. On average, fluffed cellulose pulp fibers will absorb about 12 g of water per gram of dry fiber, whereas superabsorbent polymers will absorb up to 1,000 g of water per gram of polymer. 1
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Tuesday, May 20, 2003