Which is more absorbent?
Nylon or cotton, and why?

Cotton and nylon are both water absorbent because of their molecular structures and how those structures interact with water molecules.

Cotton is made primarily from cellulose, which is a carbohydrate produced by plants. Cellulose comes from a family of carbohydrates known as polysaccharides. Polysaccharides are long chains of simple sugars that contain several hydroxyl groups on each sugar. These hydroxyl groups interact very strongly with water because of the polarity of the oxygen-hydrogen bond. A single molecule of cellulose can contain hundreds of hydroxl groups.

The affinity that nylons have for water also comes from polar bonds. Nylons are a class of materials that vary in structure, but all contain a characteristic "amide" bond. This bond is typically formed between a carboxylic acid and an amine, and contains carbon, oxygen, nitrogen, and hydrogen atoms. The polar nature of the carbon-oxygen double bond, as well as the nitrogen-hydrogen bond, provides binding sites for water molecules. Nylons are also long chains of smaller molecules, each linked by one of the amide bonds. Each of these linkages provides a site where water can bind.

Although both cotton and nylon have numerous sites where water can be bound (a driving force for absorbency), cotton has far more of these sites than does nylon. Cotton can absorb up to 27 times its weight in water, whereas Nylon 66 will absorb only around 10 percent of its weight water.

Where did the name nylon come from? This new polyamide was first introduced as a synthetic replacement for silk stockings in both New York (Ny) and London (lon) in 1940.

This month's Whizard is Dr. Mark Van Dyke, a senior research scientist in the Materials Development section of the Mechanical and Materials Engineering Division.

The Lighter Side SwRI Home

March 25, 2013