Cotton reactive dyeing

In addition to giving high washfastness on cotton, reactive dyes usually give bright shades. The latter property arises from the fact that reactive dyes are often acid dye structures... 

Cyanuric chloride is also used for the introduction of a link group into dyes. The S Ar reactivity of the chlorotriazine system leads to covalent bonding with the OH groups of cellulosic fibres, e.g. cotton (reactive dyes, e.g. cibacron and procion dyes, e.g. 12 [175]) ... 

Phthalocyanine Dyes. In addition to their use as pigments, the phthalocyanines have found widespread appHcation as dyestuffs, eg, direct and reactive dyes, water-soluble dyes with physical or chemical binding, solvent-soluble dyes with physical or chemical binding, a2o reactive dyes, a2o nonreactive dyes, sulfur dyes, and wet dyes. The first phthalocyanine dyes were used in the early 1930s to dye textiles like cotton (qv). The water-soluble forms Hke sodium salts of copper phthalocyanine disulfonic acid. Direct Blue 86 [1330-38-7] (Cl 74180), Direct Blue 87 [1330-39-8] (Cl 74200), Acid Blue 249 [36485-85-5] (Cl 74220), and their derivatives are used to dye natural and synthetic textiles (qv), paper, and leather (qv). The sodium salt of cobalt phthalocyanine, ie. Vat Blue 29 [1328-50-3] (Cl 74140) is mostly appHed to ceUulose fibers (qv). 

The material, made by a two-step diazotization of each naphthalenic sulfonic acid derivative, is typically used in the form of the neutralized sodium salt. A similar sulfonic acid-based azo dye (4) which falls into the class of reactive dyes is also shown (76). This compound, made similarly to (3), is used as a blue dyestuff for cotton and wool. 

Dyes may be classified according to chemical stmcture or by thek usage or appHcation method. The former approach is adopted by practicing dye chemists who use terms such as a2o dyes, anthraquinone dyes, and phthalocyanine dyes. The latter approach is used predominantiy by the dye user, the dye technologist, who speaks of reactive dyes for cotton and disperse dyes for polyester. Very often, both terminologies are used, for example, an a2o disperse dye for polyester and a phthalocyanine reactive dye for cotton. 

In the name Procion Red H-E 7B, Procion is the Zeneca trade name for its range of reactive dyes for cotton. Red denotes the main color of the dye. H-E denotes the dye to be hot dyeing and an exhaust dye (high fixation), and 7B denotes it to be a very bluish red dye, ie, a magenta. 

Forma n dyes bear a formal resemblance to a2o dyes, since they contain an a2o group but have sufficient stmctural dissimilarities to be considered as a separate class of dyes. The most important forma2an dyes are the metal complexes, particularly copper complexes, of tetradentate forma2ans. They are used as reactive dyes for cotton (81) is a representative example. 

Dyes for cellulose fiber include the direct, sulfur, vat, a2oic, and reactive dyes. R D activities of world dye manufacturers have been focused on the area of reactive dyes, because reactive dyes offer brighter shades and excellent wet-fastness and have been increasingly used for dyeing cotton. 

Wax Printing. This is a special case of resist printing and widely used in African designs. Wax is first printed onto cotton fabric to give a patterned effect, and the fabric is then dyed. The wax resists penetration of the dye and reserves the fabric. The wax is then removed by either alkaU washing or solvent when it is also recovered for future use. Other colors can then be printed, if so desired, on the nondyed areas. A2oic dyes or reactive dyes can be used for this. 

Copper and chromium are used for complexing a number of dyes such as the coppered direct and reactive dyes for cotton and metaUi2ed and neutral metal complex acid dyes for nylon, wool, etc. Examples are Direct Blue 218 [28407-37-6] (Cl 24401) (317), Reactive Violet 2 [8063-57-8] (Cl 18157) (318), and Acid Black 52 [5610-64-0] (Cl 15711) (319). 

In 1956, ICI commercia1i2ed the first dyes promoted as reactive dyes for cotton. These dyes were marketed under the trade name Procion and contained the dichlorotria2ine reactive group bridged to dye through an amino group. 

The phosphonic acid reactive dyes were appHed to cellulose under slightly acid pH rather than alkaline pH required for other ceUulosic reactive dyes. This feature made them especially attractive for one bath appHcation with disperse dyes to cotton/polyester blends. A review of these dyes appears ia Refereace 5. 

Substrates. The principal use for reactive dyes is dyeing textiles ceUulosics constitute the largest use. Cotton is very nearly pure cellulose, and as such has three free hydroxyl groups per glucoside unit that act as dye sites for reactive dyes. 

In view of the immense commercial importance of phthalocyanines as pigments, it is perhaps surprising that only a few are of importance as textile dyes. This is primarily due to the size of the molecules they are too large to allow penetration into many fibres, especially the synthetic fibres polyester and polyacrylonitrile. An example of a phthalocyanine dye which may be used to dye cellulosic substrates such as cotton and paper is C. I. Direct Blue 86 (96), a disulfonated copper phthalocyanine. In addition, a few blue reactive dyes for cotton incorporate the copper phthalocyanine system as the chromophoric unit (Chapter 8). 

In one study [90], enzyme pretreatment increased colour yield without affecting fastness properties. However, pretreatment of cellulosic fibres with cellulase lowered the subsequent fixation of homobifunctional triazine reactive dyes but did not impair the fixation of other types of reactive dyes [91]. Another study suggested that the enhanced brightness of reactive dyeings was greater with triazine dyes than with vinylsulphone types when cotton was pretreated or aftertreated with cellulase [92]. 

Combined dyeing and easy-care finishing of cotton using bis-nicotinotriazine reactive dyes and DMDHEU in a pad-dry-HT steam process [325]... 

Carboxymethylated Carpet printing/dyeing acid, metal-complex dyes Cotton vat, limited reactive dyes... 

One of the earliest fibre pretreatments for improving the dyeability of cotton is of course mercerisation (section 10.5.4). However, more recent research interest in this area has been generated by environmental concerns about reactive dyeing, aiming to enhance substantivity for the modified fibre so that higher absorption and fixation are obtained. This results in less dye (hydrolysed or still active) in the effluent. A further objective is to minimise the usage of electrolyte in the application process. This area has been thoroughly reviewed [392,393]. 

Polyepichlorohydrin and dimethylamine Polymerisation of epichlorohydrin in carbon tetrachloride with boron trifluoride/ether catalyst, then reaction with dimethylamine. Applied to cotton by exhaust method or pad-dry. Scheme 10.65 Good yields with direct dyes using only 2 g/l salt. Excellent build-up with most reactive dyes only 10% of normal salt usage needed for low-reactivity dyes and none for highly reactive types. Washing fastness very good but light fastness impaired. 

A more radical approach to pretreatment reverses the conventional reactive dyeing concept by preparing a reactive cotton cellulose capable of reacting covalently with suitable dyes containing, for example, aliphatic amino groups. In an initial attempt, cotton was... 

Imaginative use has been made of triazine and sulphatoethylsulphone reactive dye chemistry in the application of pretreatments to nylon [405]. The concept resembles that used to make cotton cellulose reactive before dyeing with aminoalkylated dyes, as discussed earlier (Schemes 10.67 and 10.68). In this case, nylon becomes the reactive partner by pretreatment with a reactive multifunctional crosslinking agent ... 

A polyacrylamide with a molecular mass of 1.87 x 105 was prepared by polymerising a 5% w/v aqueous solution of acrylamide monomer in the presence of 0.15% w/w benzyl alcohol and 0.025% w/w potassium persulphate for 45 minutes at 80 °C. This product was effective in preventing the bleeding of direct dyes and hydrolysed reactive dyes from dyed cotton, which was simply dipped in a 1% solution of the polyacrylamide and dried in air [450]. 

As already mentioned, some lubricants can be difficult to remove by washing and surfactants are often added to overcome this problem [463]. Lubricants can impair fastness properties, particularly those of disperse dyes. They may influence the uptake of dyes either positively or negatively, although seldom seriously except where it results in unlevelness. For example, knitting oils can increase the yield of relatively oleophilic reactive dyes on cotton and yet with highly hydrophilic types they may cause dye-resist effects [467]. 

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