Cotton Fiber and its Chemical Structure
The chemical composition of cotton, when picked, is about 94 percent cellulose; in finished fabrics is it 99 percent cellulose. Cotton contains carbon, hydrogen, and oxygen with reactive hydroxyl groups. Glucose is the basic unit of the cellulose molecule. Cotton may have as many as 10,000 glucose monomers per molecule. The molecular chains are arranged in long spiral linear chains within the fiber. The strength of a fiber is directly related to chain length. Hydrogen bonding occurs between cellulose chains in a cotton fiber. There are three hydroxyl groups that protrude from the ring formed by one oxygen and five carbon atoms. These groups are polar meaning the electrons surrounding the atoms are not evenly
distributed. The hydrogen atoms of the hydroxyl group are attracted to many of the oxygen atoms of the cellulose. This attraction is called hydrogen bonding. The bonding of
hydrogen's within the ordered regions of the fibrils causes the molecules to draw closer to each other which increases the strength of the fiber. Hydrogen bonding also aids in moisture absorption. Cotton ranks among the most absorbent fibers because of Hydrogen bonding which contributes to cotton's comfort. The chemical reactivity of cellulose is related to the hydroxyl groups of the glucose unit. Moisture, dyes, and many finishes cause these groups to readily react. Chemicals like chlorine bleaches attack the oxygen atom between or within the two ring units breaking the molecular chain of the cellulose.
Chemical structure of Cotton Fiber
Bleaching
Bleach is a chemical that removes colors or whitens, often via oxidation. Common chemical bleaches include household "chlorine bleach", a solution of approximately 3–6% sodium hypochlorite (NaClO), and "oxygen bleach", which contains hydrogen peroxide or a peroxide-releasing compound such as sodium per-borate, sodium per-carbonate, sodium persulfate, sodium per-phosphate, or urea peroxide together with catalysts and activators, e.g. tetraacetylethylenediamine and/or sodium nonanoyloxybenzenesulfonate. To bleach something is to apply bleach, sometimes
48
as a preliminary step in the process of dyeing. The bleaching of textiles appears to have been known as early as 300 B.C. when soda ash was prepared from burned seaweed and used to clean cloth. Then the cloth was treated with soured milk to reduce its alkalinity. The bleaching process was completed when the cloth was exposed to the Sun. This type of sun bleaching typically took several weeks. A Swedish chemist discovered chlorine gas in 1784 and succeeded in demonstrating its use for decolorizing vegetable dyes. Fifteen years later a patent was awarded for a bleaching powder formed by the absorption of chlorine gas into dry hydrate of lime. Following World War I the technology for shipping liquid chlorine was developed. This allowed for on-site production of sodium hypochlorite in textile mills and led to the development of other chlorine-based bleaches. In 1928, the first dry calcium hypochlorite bleach containing 70% available chlorine was produced in the United States. This material largely replaced bleaching powder in commercial bleaching.
Hydrogen peroxide was prepared as early as 1818 but did not find use in the bleaching of textiles until much later. By 1930, the prices of peroxides had dropped sufficiently to allow the use of hydrogen peroxide in the bleaching of cotton, wool, and silk. By 1940, 65% of all cotton bleaching was done with hydrogen peroxide.
Textile bleaching is one of the stages in the manufacture of textiles. All raw textile materials, when they are in natural form, are known as 'greige' material. This greige material will be with its natural color, odor and impurities that are not suitable for clothing materials. Not only the natural impurities will remain on the greige material but also the add-ons that were made during its cultivation, growth and manufacture in the form of pesticides, fungicides, worm killers, sizes, lubricants, etc.
The removal of these natural coloring matters and add-ons during the previous state of manufacturing is called scouring and bleaching.
Bleaching Chemistry
The process of bleaching can be summarized in the following set of chemical reactions: Cl2 (aq) + H2O (l) H+ (aq) + Cl- (aq) + HClO (aq)
The H+ ion of the hypochlorous acid then dissolves into solution, and so the final result is
effectively:
Cl2 (aq) + H2O (l) 2H+(aq) + Cl- (aq) + ClO- (aq)
Hypochlorite tends to decompose into chloride and a highly reactive form of oxygen: ClO-Cl-+ O
This oxygen then reacts with organic substances to produce bleaching or antiseptic effects.
Reactive Dyes
The best dyes, by far, to use for cotton and other cellulose fibers are the fiber reactive dyes. They are much brighter, longer-lasting, and easier-to-use than all-purpose dyes. Reactive dyes first appeared commercially in 1956, after their invention in 1954 by Rattee and Stephens at the Imperial Chemical Industries Dyestuffs Division site in Blackley, Manchester, United Kingdom. Reactive dyes are used to dye cellulosic fibres. The dyes contain a reactive group, either a haloheterocycle or an activated double bond, that, when applied to a fibre in an alkaline dye bath, forms a chemical bond with a hydroxyl group on the cellulosic fiber. Reactive dyeing is now the most important method for the coloration of cellulosic fibres. Reactive dyes can also be applied on wool and nylon; in the latter case they are applied under weakly acidic conditions. Reactive dyes have a low utilization degree compared to other types of dyestuff, since the functional group balso bonds to water, creating hydrolysis.
50
Reactive dyes are categorized by functional group:
Dyestuffs with only one functional group sometimes have a low degree of fixation. To overcome these dyestuffs containing two different reactive groups (i.e. one
monochlorotriazin and one vinyl sulfone) were created.
Bifunctional Dyestuffs containing two groups are also known as bifunctional dyestuffs,
though some still refers to the original combination. Other type of bifunctional dyes has been introduced. The first bifunctional dye made where more tolerant to temperature deviations (better process). Other bifunctionals are created, some with fastness (better quality) or only fixation degree (better environment/economy) in mind.
Reactive dyes have good fastness properties owing to the bonding that occurs during dyeing. Cotton is made of cellulose molecules which react with the dye .During reactive dyeing the H atom in the cellulose molecule combines with the cl atom in the dyeing process and results in a bond. Trifunctional dyestuffs also exist. Procion MX. Many people feel quite strongly that the best dye choice for the beginner, as well as for many experienced dyers, is Procion MX, because this dye is very easy to work with.because this dye is very easy to work with.
The dyes are relatively non-toxic. Other advantages include the remarkable wash fastness found in all fiber-reactive dyes.
Optimum reaction temperatures for Procion MX dyes are between 95° and 105°F = 35° to 41°C (except for turquoise, which prefers up to 130°F = 55°C)
Cibacron F. Another very good fiber reactive dye for artists and crafters to use is the Cibacron F line. (Don't confuse Cibacron F with just plain Cibacron! They can be completely different types of dyes; be sure you get the type with the "F" suffix.) Like Procion MX dyes, Cibacron F dyes can be used in warm water, instead of extremely hot water like some dyes. Its advantages over Procion MX dyes are that it 'keeps' better in solution, so you can store and possibly even buy it already in solution (liquid form),
avoiding the safety hazard of breathing dye powder; it is also much easier to wash the excess dye out of the fabric when using Cibacron F dyes than when using Procion MX. However, the Cibacron F line has one major drawback when compared to the Procion MX line: there is not as wide a choice of colors.
Cibacron F dyes can be used according to the exact same recipe as Procion MX dyes, and even mixed freely with them. However, Cibacron F dyes are slightly less quick to react than Procion MX type, which is why they can be stored so much longer after dissolving them in water than the Procion MX type dyes; this also means that slightly higher temperatures are needed.
Optimum temperatures for Cibacron F dyes are slightly higher than for Procion MX dyes. Ciba says 55° to 65°C (130° to 150°F); ProChem (Sabracron F) says 45 to 55°C. (113° to 130°F) Drimarene K. This is the remaining popular "cool water" fiber reactive dye. It requires higher temperatures still than Cibacron F, but does not require steaming.
Instructions for Drimarene K dyes can be found at Batik Oetoro; they are very similar in action to MX type dyes, except for requiring a minimum temperature of 35°C or 95°F. The greatest drawback, besides the need to find a warm place for the dye reaction to occur, is the lack of a truly rich red.
Drimarene K has optimum temperatures around 60°C (140°F) for most colors, 80°C (176°F) for turquoise and a couple of others.
Procion H dyes are usually used for silk painting; they require steaming or simmering to fix to cotton or silk. They are chemically similar to Procion MX dyes, being
monochlorotriazines, but they are far less reactive, and will not work at all well at room temperature.
52
Procion H dyes, may be fixed to cotton using a high-pH solution such as sodium silicate at room temperature, or by moderate heating. Vinyl sulfone dyes are particularly useful for chemical resist dyeing, in which two different types of fiber reactive dyes are used to print foreground and background in different colors. Remazol dyes are more suitable for dyeing for later discharge (bleaching) than are other fiber reactive dyes.
Levafix. made by Dystar, for small-scale users. Their temperature optimum is around 50°C (122°F). This is lower than the optimum temperature of 60° to 80° C. for the Drimarene K dyes, which can be used in "cold" dyeing, but higher than the optimum temperature of about 35° to 40°C (95° to 105°F) for Procion MX. It is not necessary to get quite as high as the optimal temperature in order to have acceptable results.