How superabsorbent polymer is made

A pinch of this powder can hold hundreds of times its own weight in water, which is why diapers work.

stations: 6
things upstream: 10
raw sources: 5
steps from the ground: 5

The short version

Superabsorbent polymer starts as acrylic acid partly neutralized with sodium hydroxide in water, plus a cross-linker that stops the chains from dissolving. Once polymerized it sets into a wet gel, which gets chopped, dried, ground and sieved. A last round of cross-linking at each grain's surface finishes the powder, which can absorb 300 times its weight in pure water.

Neutralize the monomers

Acrylic acid is blended with sodium hydroxide in water, so that part of its acid groups become sodium salt. A cross-linker joins the mix. One patent warns that this warm solution can start reacting early in the pipe on its way to the reactor.

Goes in
Acrylic acid Caustic soda
Set into a wet gel

An initiator starts the chains, and the cross-linker ties them into a net. More cross-linking gives a firmer gel that swells less, and less gives a softer, stickier one that holds more. One patent puts the finished hydrogel at about 40 to 80% water.
Chop the rubbery mass

A cutter mill or chopper breaks the gel into coarse lumps, 2 to 10 mm in one patent. Pieces that size dry faster, and going much finer is hard because the wet bits stick together.
Dry with hot air

Hot air or infrared drives the water out of the lumps. If the dryer runs too hot, only the outside dries, and the grinder makes unwanted fines later. The polymer leaves as a solid with little moisture left in it.
Grind and sieve the grains

Pin, hammer or roll mills reduce the dry solid to powder. Sieves then keep the wanted size range and send only those grains forward.

150 to 850 µm in one patent
Cross-link each grain's surface

The powder is sprayed with or stirred into a cross-linking solution, then heat treated, which forms a second set of links in the outer layer of each grain. Too much agent over-links the surface and hurts the finished properties.

End of the lineSuperabsorbent polymer Next it goes into sanitary pad, disposable diaper.

Worth repeating

In the early 1960s, US Department of Agriculture researchers trying to conserve water in soil made a starch-based resin that absorbed over 400 times its weight.
Until the 1920s, the best absorbers were fiber products such as tissue, cotton and sponge, which hold up to about eleven times their weight.
In a 0.9% saline solution, absorbency falls from about 300 times the polymer's weight to about 50.