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.
Superabsorbent polymers are very high molecular mass, cross‐linked polyelectrolytes that can absorb more than 10 times their mass of water or aqueous solutions. The commercially available superabsorbent polymers are alkali matal salts of poly(acrylic acid) cross‐linked with multifunctional monomers. The polymers are usually made by means of free‐radical‐initiated polymerization of an aqueous solution of the monomers, followed by drying the resulting hydrogel, and grinding the dry polymers to a granular powder. The principal use of superabsorbent polymers is as a liquid absorbent in baby diapers, feminine hygiene products, and adult incontinence products. Smaller volumes are used in food packaging and in water‐blocking tapes for electrical and telecommunication cables. Total global usage of superabsorbent polymers in 2001 amounted to about 1093 kt.
IntroductionSuperabsorbent polymers are very high molecular mass, cross-linked Polyelectrolytes (qv) that can absorb or imbibe more than ten times their mass of water or aqueous solutions. In order to absorb this large quantity of aqueous fluid, the polymers must be only slightly cross-linked so that the polymer chains can adopt widely spaced configurations. And in order to remain largely insoluble, while at the same time being highly expanded, the polymer chains must have very high molecular mass so that the small number of cross-links connect together all the chains. Cross-linked polyelectrolytes absorb more aqueous liquid than do neutral polymers as a result of the added osmotic, swelling pressure of the counterions that balance the high electric charge of the ionized functional groups spaced along the polymer chains (see POLYELECTROLYTES).Although any high molecular mass, cross-linked polyelectrolyte can function as a superabsorbent polymer, the commercially available superabsorbent polymers are alkali metal salts of poly(acrylic acid) cross-linked with multifunctional cross-linkers (see ACRYLIC (AND METHACRYLIC) ACID POLYMERS). Most often the cross-links are formed from comonomers that are incorporated into the polymer during the free-radical-initiated addition polymerization. Common cross-linkers are di-and tri-acrylates or methacrylates. The polymer chains can also be crosslinked after the main polymer chains have been formed. In this case, the crosslinker is multifunctional with groups that can react with the carboxylic acid or carboxylate groups present along the polymer chains. Examples of this type of cross-linker are polyols, polyepoxides, polyamines, and the like. Most commonly, the polymers are made by means of free-radical-initiated polymerization of an aqueous solution of the monomers, followed by drying the hydrogel that is formed and grinding the dry polymer to a granular powder.The principal use of superabsorbent polymers is as a liquid absorbent in disposable hygiene products, which include baby diapers, feminine hygiene products, and adult incontinence products. Smaller volume uses include liquid absorbent pads for packaged meats and water-blocking tapes and coatings for electrical and telecommunication cables.
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