The chemistry of staying dry: how a powder holds 300 times its weight

Countries & Cities · Published 21 October 2026 · Updated 21 October 2026 Swipe more like this →

In short: Imagine a material that can turn into a gel, absorbing hundreds of times its own mass in liquid. From baby diapers to self-healing concrete, superabsorbent polymers (SAPs) are the invisible workhorses of modern hygiene and engineering, capable of locking away water with molecular precision.

Superabsorbent polymers, often called "slush powder," function as hydrogels that trap aqueous solutions through hydrogen bonding. Their performance is highly sensitive to chemistry: in deionized water, an SAP can absorb 300 times its weight—becoming up to 99.9% liquid. However, the presence of valence cations (like salt) significantly hinders this process; in a 0.9% saline solution, that same capacity drops to approximately 50 times its weight.

The physical properties of these gels are engineered through "cross-linking." Low-density cross-linked SAPs offer higher absorbent capacity and create softer, stickier gels. In contrast, high-density cross-linked polymers provide lower capacity but maintain firmer shapes under pressure. This balance is critical for diverse applications, ranging from personal hygiene products like sanitary napkins to industrial uses such as blocking water penetration in underground power cables.

The history of SAPs dates back to the 1960s, when the USDA developed a starch-grafting resin to improve soil water conservation. This "Super Slurper" technology eventually led to commercial use in 1978 for feminine napkins in Japan and bed liners in the United States. Today, while synthetic polyacrylates remain dominant, research is expanding into natural materials like soy proteins and polysaccharides to achieve similar performance.

Manufacturing typically involves three methods: gel, suspension, or solution polymerization. Gel polymerization is currently the most popular for producing sodium polyacrylate, the world's most common SAP. In this process, UV radiation drives the reaction to create "logs" of sticky gel, which are then shredded and dried into granules. These particles can even be treated with surface cross-linking to improve their ability to swell under physical load.

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