Scouring The Matrix
How the bio-preparation of cotton is becoming a commercial reality.
Nature does an excellent job of protecting living plants from invasion by harmful external agents that could interfere with a plants healthy growth.In the case of cotton fiber, the shield that nature uses for protection consists of the cuticle and primary cell wall, which remain with the fiber after growth has stopped and the fiber has been removed from the seed.The cuticle/primary cell wall membrane is located in the outermost zone of the cotton fiber and consists of a thin matrix of waxes, pectins, proteins, and other substances.This matrix renders the cotton fiber quite resistant to water absorption in the absence of wetting agents, and can cause subsequent wet-processing problems for the textile manufacturer.No matter if the textile material is in the form of a cotton fiber, yarn, knitted or woven fabric, it is essential that the substrate be subjected to one or more chemical processes that can remove or modify the cuticle to make the cotton fiber water absorbent.It is imperative that the substrate be adequately prepared if the textile material is to be given a wet-processing treatment such as bleaching, dyeing, printing, or finishing.Lack of adequate and consistent preparation continues to be a source of quality problems for the textile manufacturer, and improvements in this area continue to be eagerly sought. Conventional PreparationOne of the earliest techniques for preparation of cotton fabric involved the use of potash, the by-product of wood combustion.A major component of wood ash is potassium oxide, and when wood ash is added to water, the potassium oxide reacts to produce potassium hydroxide, a strong alkali.Early textile chemists prepared their fabric for subsequent processing by treating the fabric in hot slurries of potash, followed by neutralization of the treated fabric in solutions of buttermilk.Although it can be argued that the chemicals used by ancient textile chemists were of natural origin, the release of these natural substances did not have a positive impact on the environment.Today, alkaline scouring of cotton is still the most widespread commercial technique for removing or interrupting the fiber cuticle to make the fiber absorbent for processing. However, the modern textile chemist uses sodium hydroxide in place of potash, and acetic acid in place of buttermilk, but the release of these modern chemicals into the effluent still does not have a positive environmental impact.In addition to the alkali used in preparation and the acid used in neutralization, other chemicals also are included in typical preparation baths.These chemicals include: wetting agents that reduce the surface tension of water so that the preparation chemicals more easily can penetrate the cotton fiber; emulsifying agents that assist in removing waxy materials; chelating agents that remove polyvalent metal ions such as calcium, magnesium, iron or other salts that can have a harmful effect on subsequent wet-processing operations; and polymeric materials that also can act as chelating agents or as pickup enhancing agents for use in continuous preparation processes.These various chemicals tend to be used in excessively high amounts, and this fact is another source of needless environmental contamination. EnzymesIt has been known for a very long time that enzymes can be used as positive environment alternatives to harsh chemicals in textile wet-processing. Enzymes are nothing more than biological catalysts, consisting of complex, three-dimensional proteins that are composed of polypeptide chains, and usually have either a fungal or a bacterial source. These proteins only accelerate reactions: enzymes cannot cause a reaction to occur that ordinarily would not occur; however, in the absence of enzymes an extremely long time may be required for some reactions to occur. A primary textile use of enzymes is in the promotion of hydrolysis of specific substrates, a process for converting water-insoluble material to products that dissolve in water and can be washed away.Until recently, enzymatic hydrolysis has found widespread textile use in only three areas: desizing, stone washing, and bio-polishing. DesizingThe enzymatic removal of the warp size, starch, from fabric by use of the enzyme, amylase, is a very old process that first was begun about 1850. Although starch can be removed from fabric by use of acid hydrolysis, such a process can have a detrimental influence on the strength of the cotton fabric if the process is not carefully controlled. Also, acids have a much more negative environmental impact than does amylase.Enzymatic hydrolysis of starch has long been accepted as being by far the preferred process from a standpoint of ease of control and environmental stewardship. Stone WashingThe popular washed-down or worn look that is so prevalent in denim jeans initially was obtained by the use of pumice stones that were soaked in sodium hypochlorite or potassium permanganate oxidizing agents. When jeans were tumbled with these bleach-soaked stones, dye was destroyed where the stones rubbed against the fabric.Since the decomposition of dye occurred in a random manner, a distinctive look was obtained. Needless to say, the chemicals used in the process were by no means environmentally benign.During the 1980s it was discovered that the same look could be obtained by the use of cellulase enzyme. When denim jeans were tumbled with solutions of cellulase enzyme under the proper conditions of temperature and pH, the surface of the dyed fiber was hydrolyzed, releasing dye in a random manner to produce the desired effect. Enzymatic washing of jeans gradually has replaced the use of environmentally harsh chemicals in the stone washing process on an international level. Bio-PolishingCellulase is highly effective in removing loose fibers from fabric surfaces. The term, bio-polishing, has been coined to describe the process. Bio-polishing has benefit both in producing a smooth fabric surface and in helping a dyed fabric retain its color depth during laundering. In the later process, cellulase enzyme is included in the detergent formulation to hydrolyze any fibers brought to the fabric surface by abrasion during laundering. By eliminating surface fiber, the light scattering phenomenon that reduces color depth is avoided. Bio-PreparationAs a result of the international research efforts directed to treatment of cotton fabric with cellulase enzyme, it was discovered that cellulase ostensibly had the ability to make the cotton absorbent.In fact, fabric treated with cellulase was more absorbent, but the cellulase enzymes used in various investigations were not pure. Now it is known that it is the pectinase impurity present in cellulase that is responsible for the greatly improved water absorbency of cotton that is found to occur when cotton is subjected to a mild cellulase treatment.Of course when cotton is subjected to a vigorous treatment with cellulase, the cotton becomes absorbent because so much of the cotton structure is destroyed by hydrolysis.The pectin which is present in the cotton fiber cuticle is a powerful biological glue. This glue consists of polygalacturonic acids that to a great extent have been converted to calcium, magnesium, iron, or other salts during fiber growth. These pectin salts are not very soluble in water and serve to bind the waxes and proteins together in the cuticle to form the fibers protective barrier, the primary cell wall.Not all pectinase enzymes are equally effective in promoting hydrolysis of the pectin. The best ones seem to be those that can function under slightly alkaline conditions even in the presence of chelating agents. Most conventional pectinases are not active under those very commercial conditions that promote the best interruption of the cuticle matrix to achieve superior fabric absorbency. Alkaline PectinaseA very powerful, novel alkaline pectinase has been isolated by scientists at Novo Nordisk. This robust pectinase functions well at moderate temperatures under buffered, mildly alkaline conditions - even when chelating agents and selected wetting agents are included in the preparation bath.The possible mechanism by which this new alkaline pectinase functions is illustrated schematically in the figure.As shown in Figure 1, pectinase has a quite specific three-dimensional structure and is adsorbed onto the three-dimensional pectin substrate in lock-and-key fashion. At the interface between the pectinase/pectin associated complex, hydrolysis of the pectin is very strongly accelerated.