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.
Source: Chemistry in Britain
Acknowledgements
Joy T. Kunjappu
Further Reading
- Joy T. Kunjappu, Essays in ink chemistry. New York: Nova Science Publishers, 2001.
- The printing ink manual (5th edn), R. H. Leach and R. J. Pierce (eds). London: Blueprint, 1993.
- Chemical technology in printing and imaging systems, J. A. G. Drake (ed). Cambridge: RSC, 1993.
- Surface phenomena and additives in water-based coatings and printing technology, M. K. Sharma (ed). Plenum Press, New York, 1991.
- Chemistry and technology of UV and EB formulations for coatings, inks, and paints, G. Webster (ed). New York: Wiley/SITA, 1997.
- Chemistry and technology of water-based inks, Pat Laden (ed). New York: Chapman and Hall, 1997.
For some recipes of writing and drawing inks. - The Society of Dyers and Colourists.
Technical and trade journals
- Ink World
- Ink Maker
- Paint and Coatings Industry
- Coatings World
- Flexo
- Modern Paint and Coatings
- Paper Film Foil Converter
- European Coatings Journal
- Journal of Coatings Technology
A colourful palette
Pigments are considered to be the chief constituent of an ink and contribute about 50 per cent of its cost. A pigment is essentially any particulate solid - coloured, black, white or fluorescent - that alters the appearance of an object by the selective absorption and/or scattering of light. It occurs as a colloidal suspension in ink and retains a crystal or particulate structure throughout the colouring or printing process.
Organic pigments in modern inks are identified by a Colour Index System number that reflects the colour shade or hue, and structural and chronological details (order of synthesis) of the pigment. For example the well-known blue pigment copper phthalocyanine blue is PB 15. The colour intensity (strength) of a pigment increases as the particle size reduces, and the opacity peaks around a particle size of 0.3?m. The molecular structures of four important pigments used in ink are shown below.
Other speciality pigments are also in demand. Fluorescent pigments have a variety of applications, such as in security inks to prevent forgery, in traffic light signals, poster boards and advertising. Pearlescent pigments used in other inks reflect light in the same way as natural pearls. However, instead of comprising multiple layers of calcium carbonate and protein, pearlescent pigments contain flakes of mineral mica (lower refractive index) coated with layers of titanium dioxide (higher refractive index).
Stabilising influences
Surfactants are surface active agents that lower the surface tension of the solvent in which they dissolve. Surfactants have multiple functions in an ink formulation. Primarily they act as stabilising agents for pigment dispersions. With the advent of water-based inks, they have an additional function as wetting agents - keeping the surface tension of the aqueous medium low so that the ink interacts favourably with the substrate. Careful choice of surfactants is often necessary to avoid problems with ink foaming - the break in ink flow that sometimes occurs when bubbles form at a pen tip, for example.
Foam is almost unavoidable during ink manufacture, and results from the release of various gases, such as the adsorbed gas in pigment released at the dispersion stage, as well as from the air introduced during mixing. Surfactants adsorb on the liquid-air interface in the foam and stabilise it, thereby accelerating its formation.
Foaming may be overcome by two approaches: it can be prevented by antifoaming agents and ’cured’ by defoaming agents. These agents include various hydrophobic solids, fatty oils and some special surfactants, which work by penetrating the liquid-air interface in the foam and slowing foam formation.
In general surfactants are structures that contain a hydrophobic hydrocarbon chain and a polar group. If the polar group is ionic, two classes of surfactants result: cationic and anionic. Ionic surfactants are particularly good at stabilising foams, and ink chemists try to avoid them in an ink formulation. Another class of zwitterionic surfactants contains both positive and negative groups in the same molecule in addition to the hydrophobic group. In non-ionic surfactants, a block of ethylene oxide groups usually imparts polarity.
Sodium dodecyl sulfate, SDS, is a well known anionic surfactant
CH3(CH2)11OSO3-Na+
Cetyl trimethyl ammonium bromide (CTAB) is a cationic surfactant
C16H33(CH3)3N+Br -
Dodecyl octaethyleneglycol monoether is a non-ionic surfactant
CH3(CH2)11(OCH2CH2)8OH
N-n-Dodecyl-N,N-dimethyl betaine is a zwitterionic surfactant
CH3(CH2)11N+(CH3)2CH2COO-
As the concentration of surfactants increases in a solution, some of the physical properties of the solution will change sharply at a concentration called the critical micelle concentration (CMC), (see Fig). Above CMC, the surfactant molecules come together to form spherical aggregates (micelles) in which the core is populated with hydrophobic chains and the corona by polar groups. The average number of surfactant molecules in each micelle structure is known as the aggregation number, which is about 60 in the case of SDS micelles.
Surfactants aggregate on the surface layers at the liquid-air and the solid-liquid interfaces. In the former case, the surface tension of the liquid reduces and in the latter case the solid (pigment)-liquid interface is modified. Either way, the net result is to make the application of the ink to its substrate (eg paper) easier.
Multi-task polymers
Polymers have multiple functions in fine-tuning the properties of an ink. In the past, naturally occurring polymer resins found use in inks and coatings, but modern inks contain many synthetic polymers. One of the main functions of polymers in ink is to serve as dispersants, either alone or coupled with surfactants. They also help to adjust viscosity and to modify rheological properties. Other important functions include aiding film formation and improving the mechanical and specific properties of inks, such as ’washability’ and abrasion resistance.
Nitrocellulose based polymers were the main player in solvent-borne inks, but polyacrylates are most familiar in modern water-borne inks. Various polyacrylate homopolymers and copolymers are widely used, although other classes such as polyurethanes and polyesters are useful in imparting specific properties. Basic properties like the glass transition temperature - at which the polymer transforms from a glassy or hard state to a flexible state - must be controlled to achieve the appropriate blocking resistance (causing ink to adhere only to its substrate) and minimum film forming temperature (MFFT).
The reactivity of polymers with other components in the ink decides the final properties of the ink coating. For example, polymer-surfactant interactions detract from fine properties like viscosity and dispersion stability, affecting the applicability and colour strength of the ink.
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Ink ingredients
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Sales of various inks in the US* ($m)
| Offset | 2200 |
| Flexo | 1100 |
| Gravure | 600 |
| UV/EB | 300 |
| Screen | 250 |
| Letterpress | 100 |
| Inkjet | ~300 |
| Total sales in the US: | ca 4700 |
* Total sales worldwide are estimated to be $13,500m (ca ?9000m). The breakdown is expected to follow the same trend as above.
Source: David Savastano, Ink World, 2001
Digital and electronic inks
Non-impact printing (NIP) technology (’plateless printing’) is becoming popular these days, with the proliferation of computers, office copiers, fax machines and laser printers, and home and office ink jet printers. By definition, NIP accepts electronic input (for example, digital) and uses ’impactless’ electrostatic, dielectric inkjet, thermal transfer or magnetic printing technologies to put an image onto substrates. Digital printing is the merging of the graphic design system (scanner or computer) with the printing unit.
Inkjet technology is the fast growing segment in the NIP sector (Chem. Br., August 2000, p39). Here an electrical pulse forces the printer to eject an ink drop. The ink for this purpose has the same general composition as other inks, but has some special characteristics: for example, very small particle sizes are required to pass through the fine nozzle and very low viscosity is needed for free ink flow. Drop on Demand (DOD) and Continuous Ink Jet (CIJ) are the two main inkjet printing technologies. Piezo Ink Jet DOD technology, in which a piezo crystal pushes a drop of the ink when prompted by a frequency regulated energy impulse, dominates the market.
Electronic inks represent the latest development in inks that is expected to change the concept of printing itself. These inks are now used in sign boards, and the display can be changed electronically, without resorting to liquid crystal displays (LCDs) or light emitting diodes (LEDs). Electronic inks change colour when an electric field is applied to them. The ink is made up of tiny bubbles of a dark coloured dye in which light coloured particles are suspended. These plastic-encapsulated particles are printed on a conductive material and some look light and some look dark when electricity is applied, so creating images. Normally the ink is not visible and reveals images only on applying electricity. Microcapsules of proprietary particulate materials mixed with the appropriate binders form the main constituents of these inks.
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Six top international ink companies and their sales
Source: David Savastano, Ink World, November 2001 |
Contact and Further Information
Joy T. Kunjappu
Chemical consultant and Visiting professor
Chemistry at Barnard College of Columbia University and at Yeshiva University, both in New York, US
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