ABSTRACT:The desired performance characteristics of absorbent products such as diapers include a low number of leaks, adequate liquid absorption capacity, and high skin dryness. By measuring the absorption behavior of mixtures of cellulose fluff pulp and superabsorbent polymers in nonimmersed conditions, we identify four major parameters that affect the swelling of the polymer in a composite: the Donnan ion-exclusion effects of limiting the amount of liquid in contact with the superabsorbent polymer, the restriction of swelling by the physical constraints of the fibrous network that surrounds the polymer particles, the restriction of swelling by the capillary tension provided by the pores between the cellulose fibers, and the osmotic pressure of extracted polyelectrolytes. Swelling of the superabsorbent polymer in composites is always smaller than the value measured by immersion of the polymer followed by centrifugation. Donnan exclusion of ions from the gel phase results in about 16% less swelling than when the polymers are swollen in excess liquid. The swelling restriction imposed by the fiber network, which surrounds the granules, reduces swelling a further 10%. In addition, the presence of small pores between the fibers exerts capillary tension on the polymer and lowers the swelling by another 10%. The external compression of the composite pads does not reduce the extent of swelling of the superabsorbent polymer because the stress is supported by the fibers.