What is a Super Absorbent?

The Oxford English Dictionary defines an absorbent as “A substance that soaks up a liquid easily”.  A Super Absorbent is normally an absorbent that will soak up a lot of liquid.  Commonly, this liquid is water or water based.  But what makes an absorbent a Super Absorbent?

When we normally look at absorbents, we think of them absorbing multiple times their own weight in liquid.  For instance, a paper towel may absorb 10 times its own weight in liquid. However, for a Super Absorbent we would normally expect much higher amounts to be absorbed – with some even absorbing up to 1000 times their own weight.

To put this in context, 2 sugar cube sized pieces of Super Absorbent could easily absorb 5l of water.

Another key feature of Super Absorbents is their ability to retain the liquid they have absorbed.  If you think back to the paper towel, it may soak up a lot of water but it tends to drip and if squeezed, will release the water with ease.

If that paper towel was Super Absorbent, apart from holding a lot more water to begin with, it also wouldn’t drip as much and would release very little liquid when squeezed.  This is known as retention.

There is not a defined amount a material needs to absorb or retain to be classed as a Super Absorbent.  However, generally it is accepted that a Super Absorbent will be able to absorb and retain many times its own weight in liquid and many times more than more common absorbent materials.

So, how do Super Absorbents work?

Super Absorbents are normally very long chains of polymer, cross-linked together to keep them from falling apart when they absorb.

Their long chains contain sites which will help bind and retain water that the Super Absorbent takes in.

As these materials take in water, they need to expand and swell to accommodate the liquid.  As a result, the Super Absorbent will swell and expand, getting larger and larger as more water is taken in.

Eventually the cross-linking prevents the Super Absorbent expanding any more.  This is the limit to the absorbent capacity of the material.  If the Super Absorbent was not cross-linked, the material could expand indefinitely, simply becoming dispersed in the liquid.

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