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INKS

Hangzhou Fancheng Chemical Co.,Ltd | Updated: Jan 03, 2017

The earliest black writing inks, developed before 2500BC, were suspensions of carbon, usually lampblack, in water stabilised with a natural gum or materials like egg albumen. Modern ink formulations are rather more complex. In addition to the pigment, they contain many other ingredients in varying levels. Collectively known as ’vehicle’, these additional ingredients include pH modifiers, humectants to retard premature drying, polymeric resins to impart binding and allied properties, defoamer/antifoaming agents to regulate foam efficiency, wetting agents such as surfactants to control surface properties, biocides to inhibit the fungal and bacterial growth that lead to fouling, and thickeners or rheology modifiers to control ink application. 

Over 90 per cent of inks are printing inks, in which colour is imparted by pigments rather than the dyes used in writing inks. Pigments are insoluble, whereas dyes are soluble, though sometimes these terms are used interchangeably in commercial literature. Ink pigments are both inorganic and organic. Most red writing inks are a dilute solution of the red dye eosin. Blue colour can be obtained with substituted triphenylmethane dyes. Many permanent writing inks contain iron sulfate and gallic and tannic acids as well as dyes. Ballpoint ink is usually a paste containing 40 to 50 per cent dye. 

Most white inks contain titanium dioxide as the pigment, as rutile and anatase in tetragonal crystalline form. However, growing concerns over the known toxicity of heavy metals have led to the replacement of many inorganic pigments such as chrome yellow, molybdenum orange and cadmium red with organic pigments, which offer better light fastness and reduced toxicity. Furthermore, carbon black now replaces spinel black, rutile black and iron black in nearly all black inks. In fact the ink industry is the second largest consumer of carbon black. 

Changes in ink chemistry over the years closely reflect developments in the instruments for ink coating: the pen and the printing machine. The ballpoint pen, the felt-tip marker, and the fibre-tip pen have led to inks containing solutions of dyes in water or organic solvents such as propylene glycol, propyl alcohol, toluene or glyco-ethers. Other ingredients like resins, preservatives and wetting agents are also added. 

Similarly, the composition of printing inks depends on the type of printing process - specifically, how the ink-distribution rollers are arranged in the printing press. The major classes of printing processes are lithography or the offset process, flexography, gravure printing, screen printing, letter press and digital printing. 

The principle of printing is illustrated by the simple stamp pad operation. Here we use a liquid ink that wets the pad. A rubber type dipped in the pad gets wet with the ink, which is pressed against the substrate, say paper, to produce the impression. Clearly, this ink should be a liquid while in the pad and should dry fast on paper. The various printing processes differ in the way the type is impregnated with the ink, although digital printing does not involve movable types. Each process therefore demands an ink that differs in its viscosity and drying efficiency, which is possible by fine-tuning the composition.