Source: http://www.agclassroom.org


Cotton may soon be afforded the same upscale recognition as wool, thanks to new technology. ARS chemists Jeanette M. Cardamone and William N. Marmer have found a way to make cotton more chemically similar to its high-end counterpart-wool-so that a fabric blend of the two can be easily dyed.



Samples of wool/cotton crosswoven blends dyed in a single dye bath. The solid-color swatches were pretreated so that both the wool and cotton yarns would pick up the dye evenly. In the untreated fabrics, the cotton stayed largely undyed.


Peggy Greb (K9139-1)


Dyeing a cotton/wool blend is difficult because the two fibers have different chemical makeups. Wool, which is sheep hair, is made of animal proteins, while cotton is made of plant cellulose-the main part of a plant's cell wall. Normally, when wool and cotton are blended together, two separate dye baths are required because the wool takes up most of the dye.


"The process we developed is a new single-bath dyeing procedure called union dyeing," says Cardamone. She and Marmer are with the Hides, Lipids, and Wool Research Unit at ARS' Eastern Regional Research Center (ERRC), in Wyndmoor, Pennsylvania. "This process helps textile manufacturers overcome a major technological barrier: dyeing all-natural blends to the same depth of shade in one step."


In the conventional procedure, Cardamone explains, cotton is dyed first and wool second. Wool is dyed in an acidic environment at high temperatures, and cotton is dyed in a nonacidic environment at lower temperatures. This difference requires that the wool and cotton be dyed either separately or sequentially in one bath in which the pH and temperature levels are changed. "Our improved method uses pretreatments to make the cotton as receptive to dye uptake as the wool," she says.


Opposites Attract


Cardamone and Marmer's simple approach is to reverse the chemical charge of cotton from negative to positive before dyeing; wool is already positive. To do this, they use cationic fixatives- positively charged ions-which are typically used after cotton is dyed to help it keep its color. Applying the fixatives before dyeing gives both fiber components of the fabric a positive charge. Since the dye is negatively charged-and opposites attract-the cotton and wool dye to a uniform shade because the dye is attracted equally to both fibers. This union-dyeing process uses one dye in one bath, under one set of conditions. Cotton industry officials are excited about the new process. "This technology is easy to adopt," says John Turner, a senior chemist with Cotton Incorporated in Cary, North Carolina. It doesn't require elaborate equipment or expense.


In the past, there was no satisfactory method for cotton mills to dye blends. This technology could potentially increase the use of cotton. "Cotton Incorporated wants to expand the use of cotton and make it more profitable for cotton farmers and the textile industry. "This process gives cotton an upscale market. A cotton/wool blend would have greater value than a 100-percent cotton item,"