Sodium Polyacrylate

25th July 2018

Chemistry

What do disposable nappies and fake snow have in common? A superabsorbent and anionic polyelectrolyte polymer! Or, more simply, sodium polyacrylate. This compound is capable of absorbing 800 times its own weight in water and can be found in disposable nappies, fake snow and even astronaut suits.

What is Sodium Polyacrylate?

Also known as waterlock, sodium polyacrylate is one class of superabsorbent polymers (SAPs) that is dominantly used as a water absorbent in many different industries. With the chemical formula (C3H3NaO2)n, sodium polyacrylate is produced by polymerising a mixture of acrylic acid and sodium acrylate.

Physically, it appears as a white granular solid that is odourless and non-toxic. Chemically, it is a sodium salt of polyacrylic acid and as such is comprised of long chains of repeated acrylic acid subunits. These repeating units are what classify this compound as an anionic polyelectrolyte.

As the name suggests, sodium polyacrylate also contains positively charged sodium ions (Na+). As a dry powder, these sodium ions are bound to polyacrylate chains, facilitated by negatively charged carboxyl groups that keep them joined tightly together.

However, when water is added this carefully organised structure becomes unravelled and the compound becomes superabsorbent.

What Makes Sodium Polyacrylate So Absorbent?

There are several reasons why SAPs like sodium polyacrylate are so absorbent. Firstly, their polymer backbone is generally formed of hydrophilic carboxylic acid groups (-COOH) that love to absorb water.

The presence of water alters the way the sodium ions interact with the polymer chain. This happens because the positively charged hydrogen ions (H+) in water replace the Na+ ions, allowing them to move freely. This is what causes the polymer chain to unravel.

Instead of separating entirely and dissolving, the sodium ions actually form a gel-like solid. This forms the chemistry behind why sodium polyacrylate is able to absorb nearly 800 times its weight in water, a property that makes it an important feature in many industries.

An astronaut floating in space

Astronaut suits contain sodium polyacrylate because of its super-absorbency. This comes in handy if an astronaut ever finds themselves in a position where they can’t reach the loo!

Applications

Like other SAPs, sodium polyacrylate can be used:

  • As a thickening agent
  • To dissolve soap
  • As a wetting and dispersing agent
  • As a chelating agent in detergents
  • To help soil in potted plants retain moisture
  • To improve absorbency in nappies
  • In tampons and other feminine hygiene products
  • In surgical sponges
  • In filtration units to increase the efficiency of vehicles
  • To protect electrical cables from moisture
  • In jelly-filled cold packs in coolers
  • In astronaut suits

Among its many uses, its ability to absorb huge amounts of water as well as its non-toxicity make sodium polyacrylate a common source of fake snow.

Sodium Polyacrylate as Fake Snow

When water is added to sodium polyacrylate, it turns into a transparent slush that looks very similar to icy snow. Its super absorbency means that you can add as much water as is needed to get the amount of fake snow required without the product melting.

The content of water also makes the sodium polyacrylate slush feel cool to the touch, adding an extra element of realism to your fake Christmas set-up. This substance is also easily manipulated to yield the results you want. You can:

  • Refrigerate or freeze it to make it more authentic
  • Dry it out if it is too wet by adding salt, which decreases the amount of water absorbed
  • Rehydrate it if it is too dry by simply adding more water
Snow on a Christmas tree branch

When water is added to sodium polyacrylate, it turns into a gel-like solid that resembles snow. PolySnow is a less-absorbent alternative that produced dryer ‘snow’ flakes.

Why Doesn’t Sodium Polyacrylate Melt?

The reason sodium polyacrylate forms a slush-like consistency without melting is because of its cross-links. In polymer chains, cross-links form a three-dimensional network that has an innate elastic retraction force. This is what prevents the polymer from dissolving.

The number of cross-links present in a polymer chain directly affects the water absorbency, i.e. the ‘swelling’ of the polymer. For example, the more cross-links, the less amount of water a polymer will be able to absorb.

There are relatively few cross-links in sodium polyacrylate, meaning that it is able to absorb huge quantities of water. This is also why it forms a gel-like slush. Other SAPs, on the other hand, behave very differently even though they may appear exactly the same. A good example of this is PolySnow.

What is the Difference Between PolySnow and Sodium Polyacrylate?

PolySnow is another SAP that is widely used for fake snow. It is simply produced by cross-linking sodium polyacrylate. Therefore, the main difference between the two is that PolySnow has more cross-links in its polymer chain.

While this doesn’t sound like a huge difference, this small change in chemical structure dramatically affects how each of these SAPs behaves with water.

Unlike sodium polyacrylate, when water is added to PolySnow it transforms into tighter, dryer flakes that appear much fluffier and much more like snow. This is because of its higher number of cross-links, which correspondingly decreases the amount of water it can absorb.

PolySnow is commonly used on film sets for fake snow. It is particularly useful because it won’t melt in the hotter months, and appears more realistic on camera than the real stuff.

Regrettably, we don’t supply fake snow. We do, however, specialise in bespoke formulations and aren’t strangers to the chemistry of polymers. We can manipulate polymers, like in polyutherane coatings, to suit your needs. Contact us today with any enquiries you may have.  

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