Waterlock (Sodium Polyacrylate)
Sodium polyacrylate, which can hold up to 800 times its weight in water, has found applications from home to Hollywood.
An intriguing chemical demonstration begins with a beaker containing approximately half a liter of water. A teaspoonful of a white powder is added to the water, and the combination is stirred until all of the water has become a gel. The beaker may even be held upside down without any of the material falling out. Could this powder with such an intriguing property possibly have a practical application?
The white powder is sodium polyacrylate, which is also known as waterlock. Parents of infants might have been the first to realize that a compound that can absorb 800 times its weight in water would be an outstanding innovation to revolutionize the disposable diaper. Even better, the fully loaded waterlock feels only moist rather than wet, so a baby with a wet waterlock diaper is not particularly uncomfortable.
The structure of the molecule is the key to a teaspoon of waterlock absorbing 500 mL or more of water. When sodium polyacrylate is dry, the long polymer chains are coiled. The outside of the coil has numerous groups that are attracted to water, and when the molecule gets wet, it uncoils, which exposes even more water-loving groups. Water molecules can also link two uncoiled polymer molecules, which creates the gel.
In addition to disposable diapers, waterlock has found several other applications. It is often used on movie sets to create fake snow. By controlling how much water is added to the sodium polyacrylate, set decorators can create fluffy realistic snow. If the snow gets too firm, the addition of salt to the moist waterlock will pull the water out of the polymer and the gel becomes more slushy.
Waterlock’s ability to hold water has also been used in protecting buildings in the path of large-scale wildfires and brush fires. In the event that a fire-fighting team must withdraw from an area, the powdered waterlock can be mixed with water to create a fire retardant polymer gel, which is sprayed on the house and sticks to everything. The gel holds in enough water to protect the dwelling as the fire passes. Because the gel holds the water rather than allowing it to evaporate, it uses far less water than trying to soak the house with water, and has been extremely successful at reducing property damage in widespread fire.
More information is available at http://www.m2polymer.com/html/history_of_superabsorbents.html