Ink can be a complex medium, composed of solvents, pigments, dyes, resins, lubricants, solubilizers, surfactants, particulate matter, fluorescers, and other materials. The components of inks serve many purposes; the ink’s carrier, colorants, and other additives control flow and thickness of the ink and its appearance when dry.
Pigment inks are used more frequently than dyes because they are more color-fast, but they are also more expensive, less consistent in color, and have less of a color range than dyes.
Pigments are solid, opaque particles suspended in ink to provide color. Pigment molecules typically link together in crystalline structures that are 0.1–2 µm in size and comprise 5–30 percent of the ink volume. Qualities such as hue, saturation, and lightness vary depending on the source and type of pigment.
Dye-based inks are generally much stronger than pigment-based inks and can produce much more color of a given density per unit of mass. However, because dyes are dissolved in the liquid phase, they have a tendency to soak into paper, making the ink less efficient and potentially allowing the ink to bleed at the edges of an image.
To circumvent this problem, dye-based inks are made with solvents that dry rapidly or are used with quick-drying methods of printing, such as blowing hot air on the fresh print. Other methods include harder paper sizing and more specialized paper coatings. The latter is particularly suited to inks used in non-industrial settings (which must conform to tighter toxicity and emission controls), such as inkjet printer inks. Another technique involves coating the paper with a charged coating. If the dye has the opposite charge, it is attracted to and retained by this coating, while the solvent soaks into the paper. Cellulose, the wood-derived material most paper is made of, is naturally charged, and so a compound that complexes with both the dye and the paper's surface aids retention at the surface. Such a compound in common use in ink-jet printing inks is polyvinyl pyrrolidone.
An additional advantage of dye-based ink systems is that the dye molecules interact chemically with other ink ingredients. This means that they can benefit more than pigmented ink from optical brighteners and color-enhancing agents designed to increase the intensity and appearance of dyes. Because dyes get their color from the interaction of electrons in their molecules, the way the electrons can move is determined by the charge and extent of electron delocalization in other ink ingredients. The color emerges as a function of the light energy that falls on the dye. Thus, if an optical brightener or color enhancer absorbs light energy and emits it through or with the dye, the appearance changes, as the spectrum of light re-emitted to the observer changes.
A more recent development in dye-based inks are dyes that react with cellulose to permanently color the paper. Such inks are not affected by water, alcohol, and other solvents.[citation needed] As such, their use is recommended to prevent frauds that involve removing signatures, such as check washing. This kind of ink is most commonly found in gel inks and in certain fountain pen inks.
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