Superabsorbent

Superabsorbent polymers (SAPs) are hydrophilic polymers capable of absorbing and retaining extremely large amounts of water relative to their mass. They are typically crosslinked polymer networks that swell markedly in aqueous environments and hold water after external pressure or deformation.

Most commercial SAPs are based on crosslinked acrylic polymers, especially sodium polyacrylate, or copolymers of acrylamide and acrylic acid. They are produced by free-radical polymerization of acrylic monomers in the presence of a crosslinker, followed by neutralization (to form sodium, potassium, or ammonium salts) and drying to create beads, powders, or fibers. Absorbency and gel strength depend on crosslink density, degree of neutralization, and the presence of electrolytes.

Mechanism: fixed charged groups within the swollen network create an osmotic driving force for water uptake; counterions balance the charge, enabling substantial swelling. Water absorption is much higher in distilled water than in salt solutions, and absorption under load (AUL) decreases with applied pressure.

Applications: disposable diapers and other hygiene products; incontinence products; agricultural soil amendments for moisture retention and slow-release fertilizers; wound dressings and medical devices; industrial uses such as spill control, enhanced oil recovery, and controlled-release systems.

Manufacturing and forms: synthesis via polymerization of acrylic monomers with crosslinkers yields a hydrogel that is subsequently dried into beads, powders, or granules. Neutralization level, particle size, and crosslink density tailor properties such as absorbency, gel strength, and kinetics.

Limitations and environment: SAPs can be sensitive to ionic strength, reducing absorbency in saline environments; high crosslink density improves gel strength but lowers swelling. While some SAPs are designed to be biodegradable, most conventional SAPs are not readily biodegradable; disposal and environmental persistence are considerations.

Safety: generally regarded as safe in consumer products when used as directed; ingestion can pose choking risks.