Ink chemistry

Ink chemistry

There's more to ink than meets the eye, says Joy Kunjappu
There are probably as many different definitions of ink as there are types. Perhaps the simplest description is that ink is a liquid or semi-liquid material used for writing, printing or drawing. Chemists view it as a colloidal system of fine pigment particles dispersed in a solvent (Chem. Br., February 2003, p28). The pigment may or may not be coloured, and the solvent may be aqueous or organic.
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.
Other inorganic materials such as clays serve as fillers or extenders, which primarily reduces the cost of pigments, though some also improve ink properties. Metallic pigments like aluminium powder (aluminium bronze) and copper-zinc alloy powder (gold bronze) are used in novel silver and gold inks. Miscellaneous inorganic pigments provide luminescent and pearlescent effects.
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.
A printing ink chemist is primarily interested in preparing a dispersion of pigment particles that does not settle into clumps. Inorganic pigments can be easily dispersed by applying minimal force, but most organic pigments require special milling techniques to produce sub-mm size particles for stable dispersion. In general the colour of the ink arises from organic pigments; the particle size of the pigment governs the colour intensity.
Milling is carried out in two stages: the primary mixing is done with an ordinary mixer and the resultant pre-mix is subjected to secondary grinding in a ball mill or a roller mill. After the primary mixing, the chemist adds chemicals called dispersants or grinding aids to prevent the fine pigment particles from reaggregating during the grinding stage. The correct choice of dispersants, along with the right grinding technique, is the key to obtaining a stable dispersion.
Dispersants stabilise the pigment particles by lowering the mechanical energy needed for grinding. Two classes of compounds are used for this purpose: surfactants and polymers. These compounds adsorb to the pigment particles and form a coating of varying composition and thickness. The resulting modified particle surfaces either attract or repel each other - leading to flocculation or stabilisation, respectively. Flocculation hampers dispersion, and stabilising forces are essential to prevent the fine particles of pigment from settling. The size and shape of the pigment particles dictates the colour intensity, shade and light fastness.
There is a growing tendency these days to exclude organic solvents from commercial products, and inks are no exception. Strict regulations limit the use of volatile organic compounds (VOCs) everywhere from paint to plastic manufacture. As a result, ink chemists have been forced to abandon many efficient and time-tested recipes by replacing organic solvents with water. Water-based inks have in turn introduced new classes of surfactants and polymers into ink chemistry.
An obvious disadvantage of using water as a medium is the increased surface tension of aqueous inks, which makes 'wetting' substrates such as paper or plastics more difficult. A two-pronged approach has helped to alleviate this problem: special surfactants lower the surface tension of inks, while modifying the surfaces of substrates like plastic (eg the corona treatment) enhances the surface energy, and so makes wetting easier. Surfactants have the downside of producing a stabilised foam.
Inks should have a viscosity (loosely called thickness) appropriate to the printing process. Some inks have a butter-like consistency and others have intermediate viscosity. Various polymeric thickening agents are added for this purpose. In this regard, ink chemists are interested in rheology, the study of the relationship between the applied stress and the resulting deformation. Complex fluids like inks show non-Newtonian behaviour, ie their viscosity changes when stirred, although by themselves most of the raw materials in a typical ink composition show the opposite, Newtonian, behaviour. Furthermore, most inks exhibit pseudoplasticity, which essentially means that they become runnier when stirred or spread.
In the past, chemists fine-tuned the properties of solvent-borne inks by including polymers of various molecular weights. These inks contained relatively little solid matter, ie were 'low solids' type, and required large amount of solvent to dissolve high molecular weight polymers. Modern solvent-free inks are high solids types, incorporating monomeric and oligomeric polymer precursors that can be polymerised in situ after applying the ink to the substrate, for example by UV light or a high energy electron beam.
These inks contain easily polymerisable monomeric or oligomeric units mixed with an initiator that produces radicals or ions on irradiation that will initiate the polymerisation process. Electron beam inks do not require an externally added initiator because the electrons can themselves generate radicals. Aside from being solvent-free, these inks cure instantly, giving fast printing speeds. Demand for these inks is currently growing at about 10 per cent per year.
How fast the ink dries governs the speed of the printing process. Drying can involve the absorption or penetration of liquid components into the substrate; evaporating the solvent at a certain temperature; or chemical processes involving oxidation or polymerisation.
A newly developed ink that meets the requirements of a printing process and substrate will be subjected to a number of quality control tests before being marketed. These tests vary with the end application. Some of the tests are termed print quality, block resistance, scrubbing, light fastness, bleeding, 'foamability', shear stability, gloss, water resistance, tape adhesion and drying in air. Print quality tests how good is the print, block resistance tests the transfer of ink from a printed roll to an unprinted surface and 'foamability' indicates the extent of foam generation in an ink formulation, and so on.
In addition to these properties, many speciality inks are designed for other specific end uses. With some new thermochromic and photochromic inks heat and light are needed to produce colour, while electronic ink requires an electric field to induce colour (see Box below and Chem. Br., July 2002, p22). Thermochromic inks help detect temperature changes in a moving part while electronic inks find application in various displays. Magnetic inks incorporate certain magnetic materials in the ink and are used in printing cheque books for efficient screening by cashiers.
As these and many other examples show, ink is a more complex fluid than you might previously have imagined. The paperless society that many people envisage for the future is still a long way off. Meanwhile, ink chemistry should continue to preoccupy scientists for many years to come.