Coloring Materials Dyes

Historically, dyes are the oldest class of synthetic specialty chemicals employed in the production of paper Dyes are added to paper at a statistical average rate of only 150 g chromophore per 1 ton of paper and board worldwide. The turnover in dyes (total 500 X 10 ) accounts for 5 % of the specialty chemicals applied to paper but only for 1,5% in view of soHd mass addition (Fig. 3.2 and 3.3). 

Colorants are used in paper manufacture for a variety of reasons, including eye appeal, color coding, and brand identification. The selection of suitable colorants depends for instance upon end-use requirements, physical and chemical properties, and handling characteristics. Coloring is done to achieve the following two general goals  

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Coloring materials, dyes, varnishes, paints, and other coatings and inks... 

Metallic Dyes. MetaUic dyes are among the older hair color materials known. Commercial products are based on a 1% solution of lead acetate in an aqueous, slightly acidic, alcohoHc medium. Precipitated sulfur appears to be essential. The convenience aspect is stressed by the leave-in appHcation method. Actually, the color development is so slow, taking about a week to ten days, that there is no alternative to this technique. Daily appHcation is needed at first. 

Historically the phenazine dyes have played an important part in the dyestuffs industry, although their use has largely been superseded by the more modern, color-fast dyes, in particular those dyes which become chemically bonded to the fibers of the materials being dyed. Amongst the earliest examples of phenazine dyes are those compounds known as the safranines. The discovery of the safranines has been attributed to Greville Williams in 1859 and they were apparently in commercial use shortly after that date, but it was not until 1886 that it was recognized that phenosafranine (138) was indeed a phenazine containing system. 

Anodic oxidation has been employed for water-soluble triphenyl-methane dyes. It has been shown that the formation of dye is an irreversible two-electron oxidation process.21-23 This method has been used for the oxidation of diamino triphenylmethane leuco compounds containing two to four sulfonic acid groups to obtain food-grade colored materials.24... 

The use of the term leuco dye is a common paradox. Leuco color formers are materials that undergo controlled chemical or physical changes resulting in a shift from a colorless state to an intense color. The preparation of leuco color formers takes advantage of the very nature of colored materials themselves. The existence of extended conjugated -system in dyes is responsible for the absorption in the visible region. The chemistry of such rc-system is noted for facile reactivity, particularly to reactions such as reduction, oxidation, and hydrolysis (not hydrolytic cleavage). When n-... 

Most materials exhibit specific colors because they absorb certain wavelengths (colors) from white, ordinary light. A red object exposed to white light, for example, appears red because atoms on its surface absorb all the other colors in the beam and reflect only red. If transparent materials contain coloring materials, such as dyes or pigments, they absorb the characteristic color of the coloring material. 

Because, in the main, they show little contrast with their surroundings, biological materials are very difficult to visualise under a light microscope. Consequently, for many years in histology, visualisation of biological tissues has been assisted by coloration with dyes. These dyes, known as stains, can perform several functions ... 

These methods both of which involve vaporization and condensation of a colored material, have been found suitable only when volatile organic dyes are used as coloring materials. The choice of a suitable dye for colored smoke depends on the following ... 

Azo or diazene compounds possess the —N=N— grouping. Aliphatic azo compounds of the type R—N=N—H appear to be highly unstable and decompose to R—H and nitrogen. Derivatives of the type R—N=N—R are much more stable and can be prepared as mentioned above by oxidation of the corresponding hydrazines. Aromatic azo compounds are available in considerable profusion from diazo coupling reactions (Section 23-IOC) and are of commercial importance as dyes and coloring materials. 

Nile Blue is used as a 0.01 to 0.1 %W/V aqueous solution and is simply added to or mixed with the substrate. The active component of the dye is actually a minor contaminant of the solution, not the blue-colored material [31]. The preparations are viewed with 450-490 nm excitation (an FTTC filter set. Figure 6). Emulsion stability is sometimes an issue in the presence of the cationic blue component of Nile Blue. In this case we use Nile Red, the pure form of this colorant. Nile Red solution is made fresh from a stock solution (0.1%W/V in acetone). This stock is added dropwise to water until a moderate blue color is seen and the solution is used immediately (it deteriorates quickly). For either colorant, the active molecule is fluorescent only when it is in a suitably hydrophobic environment. This usually means neutral lipid droplets [31] but other sites (aggregates of surfactants, the center of casein micelles, cutin plates in some seeds) are possibilities. 

Mordant dyes (and lakes) are dyes combined with metallic salts (mordant means bitter) to form highly insoluble colored materials called lakes. 

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