A plastic that drinks
A super absorbent polymer is a network of long polymer chains, linked together at points along their length. In the dry state it looks like white sugar or salt. When it meets water, the network swells into a soft gel that can hold hundreds of times its own weight of pure water.
Most commercial SAP is a polyacrylate — sodium polyacrylate for hygiene and industry, potassium polyacrylate for agriculture. Some grades graft these chains onto starch to raise the bio-based content.
Why it absorbs so much
The polymer chains carry many charged groups. In water, the counter-ions (sodium or potassium) try to spread out, and water rushes into the network to dilute them — an osmotic pull. The crosslinks stop the chains from dissolving, so instead of dissolving the network swells.
This is also why SAP absorbs much less salty water: when the surrounding liquid is already full of ions, the osmotic pull is weaker.
Water held as a gel, not a puddle
Water inside SAP is held loosely enough that plant roots can draw it out, but firmly enough that it does not simply drain away under gravity. In a diaper, it is held firmly enough not to come back out under pressure. The degree of crosslinking decides where on that spectrum a grade sits.
Where SAP is used
The same basic chemistry serves very different industries:
- Agriculture and forestry — holding water in the root zone.
- Hygiene — absorbent cores in diapers and sanitary pads.
- Cold chain — gel packs and ice sheets.
- Industry — solidifying liquid waste and blocking water in cables.