Sodium Polyacrylate

OVERVIEW

Sodium polyacrylate (SO-dee-um pol-ee-AK-ruh-late) is an odorless, grainy white powder. Its most impressive property is its ability to absorb large amounts of fluid, up to 800 times its volume of distilled water and lesser amounts of other liquid mixtures. This property accounts for one of its primary applications, in the manufacture of disposable diapers. Diapers made from sodium polyacrylate are able to absorb up to 30 grams of urine for each gram of diaper.

KEY FACTS

OTHER NAMES:

Acrylic acid polymer sodium salt; polyacrylic acid sodium salt; PAAS

FORMULA:

-[-CH₂-CH(COONa)-]_n-

ELEMENTS:

Carbon, hydrogen, oxygen, sodium

COMPOUND TYPE:

Polymer of an organic salt

STATE:

Solid

MOLECULAR WEIGHT:

Varies

MELTING POINT:

Not applicable

BOILING POINT:

Not applicable

SOLUBILITY:

Insoluble in water; swells in water

HOW IT IS MADE

Sodium polyacrylate is produced by the reaction between acrylic acid (H₂C=CHCOOH) and its sodium salt (H₂C=CHCOONa). The product of this reaction is a long-chain copolymer consisting of alternate units of acrylic acid and sodium acrylate. A copolymer is a polymer made of two different monomers, in this case, acrylic acid and sodium acrylate. What makes this polymer different from most other kinds of polymers is that adjacent polymer chains are able to cross link with each other. The hydrogen on a carboxyl group (-COOH) on one chain reacts with a double bond (-C=C-) on an adjacent chain, forming a link that holds the two chains together. Cross-linking occurs at many points in the polymer, resulting in the formation of a mesh-like web consisting of polymer chains.

When water is added to the polymer, it forces carboxyl groups away from each other, forcing the mesh to open up and make space for water molecules to fill the gaps in the polymer. As more water is added, the carboxyl groups stretch even farther apart, making room in turn for yet more water molecules to be absorbed by the polymer. If the polymer is allowed to dry out, water molecules leave gaps in the compound, the empty spaces between carboxyl groups collapse, and the polymer returns to its original size. It can then be stored and re-used any number of times.

COMMON USES AND POTENTIAL HAZARDS

The primary use of sodium polyacrylate is in the manufacture of baby diapers. The need for some sort of disposable diaper first arose during World War II because of a shortage of cotton, from which cloth diapers are made. The compound now has a number of other applications. For example, it is used to pack poultry, red meat, fish, fresh-cut fruits and vegetables, and fresh whole berries to keep them moist and fresh. Fluids from washing these foods during processing may accumulate inside a package and provide an environment for the growth of bacteria that cause foods to spoil. A packaging material made of cellulose and sodium polyacrylate absorbs these fluids and prevents them from being squeezed out of the package.

Interesting Facts

• Sodium polyacrylate was first developed by researchers for the National Aeronautics and Space Administration. The material was used for diapers worn by astronauts while they were on long space trips.

Sodium polyacrylate is also used as a thickening agent in medical gels used to treat bed sores, which are open wounds that develop when a person is bed-ridden for too long. The compound is also added to detergents and to potting soils to help retain water. The compound is now being used in some parts of the world where there is insufficient rain to allow crops or lawns to grow. It absorbs moisture when rain does fall and holds it in place until plants can absorb the water.

In industry, sodium polyacrylate is used in filtration units that remove water from airplane and automotive fuel. It is also used as a thickening agent in coatings and adhesives used in the upholstery, drapery, carpet, paper, paint, wallpaper, printing, and textile industries. The compound is also used to thicken certain liquid products applied by spraying, such as cleaning products. Finally, it is sometimes used to prevent fluid loss in oil wells.

Sodium polyacrylate is a mild irritant to the skin, eyes, and respiratory tract. It may cause redness, itching, and pain on the skin or in the eyes; and coughing, shortness of breath, and inflammation of the respiratory tract. Questions have been raised about possible health effects on babies who wear disposable diapers containing sodium polyacrylate. Some people suggest that a baby's tender skin may be more sensitive to the irritation caused by sodium polyacrylate than the skin of an adult. The compound was removed from tampons in 1985 because some women who left their tampons in place too long experienced unacceptable levels of irritation caused by sodium polyacrylate in the product.

Words to Know

COPOLYMER

A polymer that consists of two or more different monomers.

POLYMER

A compound consisting of very large molecules made of one or two small repeated units called monomers.

FOR FURTHER INFORMATION

Allison, Cathy. "Disposable Diapers: Potential Health Hazards?" BCParent Online. http://www.bcparent.com/articles/baby_talk/disposable_diapers.html (accessed on November 10, 2005).

"Environmental Assessment." U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition. http://www.cfsan.fda.gov/∼acrobat2/fnea0427.pdf (accessed on November 10, 2005).

Mebane, Robert C., and Thomas R. Rybolt. Plastics and Polymers. New York: Twenty-First Century, 1995.

"Sodium Polyacrylate." Center for Advanced Microstructures and Devices, Louisiana State University. http://www.camd.lsu.edu/msds/s/sodium_polyacrylate.htm (accessed on November 10, 2005).

"Sodium Polyacrylate." Flinn Scientific Company/Department of Chemistry, Iowa State University. http://avogadro.chem.iastate.edu/MSDS/Na_polyacrylate.pdf (accessed on November 10, 2005).