Paper Direct Dyes

This is an important class of dyes for the dyeing of paper. Direct dyes are also called substantive dyes [4,5] because they tend to have a high affinity to cellulose fibers due to their linear molecular structure and a system of conjugated double bonds and usually also exhibit good wetfastness properties with the addition of a fixative. 

Although it has been reported (138) that decolorization of wastewater containing reactive azo dyes with sodium hydrosulfite is possible only to a limited extent, others have demonstrated good reduction (decolorization). For example, using zinc hydrosulfite for the decolorization of dyed paper stock (139) resulted in color reduction of 98% for azo direct dyes (139). A Japanese patent (140) describes reducing an azo reactive dye such as Reactive Yellow 3 with sodium hydrosulfite into its respective aromatic amines which ate more readily adsorbable on carbon than the dye itself. This report has been confirmed with azo acid, direct, and reactive dyes (22). 

The 1 1 copper complex azo dyes are used both as reactive dyes for cotton and as direct dyes for paper. (For definitions of reactive dyes, which form covalent linkages with the substrate, and direct dyes, which bind more weakly, see refs. 23 and 24, respectively.) Typical monoazo dyes are Cl Reactive Violet 1 (23) and Cl Reactive Blue 13 (25), and the bis-copper-ed dye, Cl Direct Blue 80 (26). The navy blue dye, Cl Reactive Blue 82 (27), is a typical disazo dye. 

Water-soluble phthalocyanine dyes may contain only sulfonic acid groups but normally contain both sulfonic acid and sulfonamido groups. They are used as direct dyes for paper and as reactive dyes for cotton. Representative examples are Cl Direct Blue 199 (38) and Cl Reactive Blue 71 (39). These dyes are also used for the ink jet printing of textiles (see Section 9.12.6.2). 

Direct dyes are attracted to the textile, according to their substantivity, by intermolecular forces without the need of mordant. They are used to color cotton and paper leather, silk, and nylon, and are also used as pH indicators or as biological... 

These are defined as anionic dyes with substantivity for cellulosic fibres applied from an aqueous dyebath containing an electrolyte. The forces that operate between a direct dye and cellulose include hydrogen bonding, dipolar forces and non-specific hydrophobic interaction, depending on the chemical structure and polarity of the dye. Apparently multiple attachments are important, since linearity and coplanarity of molecular structure seem to be desirable features (section 3.2.1). The sorption process is reversible and numerous attempts have been made to minimise desorption by suitable aftertreatments (section 10.9.5). The two most significant non-textile outlets for direct dyes are the batchwise dyeing of leather and the continuous coloration of paper. 

Direct dyes have only modest fastness to washing, which may be improved by after-treatments such as metal-complex formation (section 5.5.3) or by diazotisation of the dye on the fibre and further coupling of the diazonium salt with an insoluble coupling component (section 1.6.14). In addition to their use on cotton and viscose, direct dyes are important in the dyeing of leather. The cheapest members of this class are also used in the coloration of paper, since for this purpose fastness properties are largely irrelevant and price is all-important. 

The nomenclature given for these componnds is a nniversally recognised system for the naming of dyestuffs devised by the Society of Dyers and Colourists as part of their Colour Index (Cl). The Cl Generic Name is made up of the application class, the hne and a nnmber. Acid dyes are nsed on wool and polyamide, direct dyes on cel-lulosic fibres, paper and leather, disperse dyes on polyester fibres, reactive dyes on cellnlosic fibres and basic dyes on polyacrylonitrile and paper. 

Dyeing of paper has been traditionally carried out with acid, direct and basic dyes, whilst newer systems are based on cationic direct dyes. These cationic direct dyes are related structurally to reactive dyes with the labile chlorines replaced by pendant cationic groups, e.g. (2.50). 

Direct dyes are applied directly to cellulosic fibers in the presence of electrolytes. Major applications are to cotton, regenerated cellulose, leather, and paper. 

Direct Dyes. These water-soluble anionic dyes, when dyed from aqueous solution in the presence of electrolytes, are substantive to, i.e., have high affinity for, cellu-losic fibers. Their principal use is the dyeing of cotton and regenerated cellulose, paper, leather, and, to a lesser extent, nylon. Most of the dyes in this class are polyazo compounds, along with some stilbenes, phthalocyanines, and oxazines. Aftertreatments, frequently applied to the dyed material to improve washfastness properties, include chelation with salts of metals (usually copper or chromium), and treatment with formaldehyde or a cationic dye-complexing resin. 

In spite of the reactive dyes with their outstanding wet-fastness, the direct dyes continue to hold a large share of the market for inexpensive cellulose and paper dyes. Apart from the new developments to replace the old established benzidine dyes, con-... 

In contrast to some naturally occurring dyes like indigo or kermes, which must be vatted or mordanted to be applied in textile dyeing, direct or substantive dyes can be used on cellulosic fibers directly . Their use is widespread because of their easy handling. Today, direct dyes still account for ca. 10% of the world textile dye consumption. For use on paper, see Section 5.3. Recent research on direct dyes concentrated on the replacement of possibly carcinogenic benzidine dyes [44],... 

With the increasing popularity of the continuous method of dyeing in the paper industry, new direct dyes were developed with particularly good cold-water solubility without any impairment to the substantivity. Improvements of this kind with regard to substantivity and solubility can be seen in the further development of the widely used, but only moderately soluble, cotton dye C.I. Direct Red 81, 28160 [2610-11-9] (4). 

Substantive anionic direct dyes can be repolarized by incorporating an excess of cationic groups [11] without impairing the substantive properties. Several dyes of this kind have been developed for the paper industry, e.g., C.I. Basic Red 111, [118658-98-3] (25). 

It can be prepared either by using appropriate direct dyes or reactive dyes. In the latter case, pure linters (the raw material for high-quality paper) are suspended in water, and the solution of the reactive dye is added. For example, the dye 15 reacts via the sulfonic acid group in the side chain. The reaction mixture is then made alkaline and the dye reacts with the linters. After completion of the reaction, the fiber pulp is centrifuged, washed electrolyte-free, and processed on a paper machine to form the pH paper. Paper produced in this way is mostly bonded onto a plastic material and used as pH test strip. 

In this context we note that the curriculum for the planned M.S. program in Forensic Chemistry at Northeastern University which is discussed in detail in Reference 2 will contain a new course entitled "Forensic Materials" as a step in the direction indicated above. The abstract of this course is as follows Forensic Materials (2 Quarter Hours) Fundamental types of solids, such as metals, ceramics, minerals, organic solids, including drugs, polymers, plastics, fibers their properties and determination by modern methods. Forensically important materials such as alloys, glass, soils, fibers, wood, paper, rubber, dyes, paints, ink, and their determination. Illustration of various materials as associative or dissociative items of evidence. 

The rather extensive literature on the absorption of dyes, especially direct dyes, by cellulose has been reviewed by Vickerstaff. Vickerstaff has also reviewed the literature on salt absorption however, three papers might be mentioned as pertinent to the work presented here, namely, those of Usher and Wahbi, Neale and Standring, and Farrar and Neale, and particularly the last. 

Taking into account high capacity and selectivity of ion exchange resins for different dyes, they seem to be proper materials for dyes sorption from textile effluents. Applicability of the anion exchange resins in the removal of acid, reactive, direct dyes widely used in the textile industry, from aqueous solutions and wastewaters, was confirmed in some papers [2,15, 20, 23, 25-28]. 

---