The term dyes entered into our vocabulary well before the term pigments.1 Lexical sources define dye as a soluble or insoluble coloring matter, and pigment as a substance that imparts black, white or a color to other materials. Pigments are specifically described as insoluble powdered substances that are mixed with a liquid in coating materials such as paint and ink. A fair amount of confusion existed in the early days by using the terms dyes and pigments interchangeably. According to the American Dry Color Manufacturers’ Association,2 “…[pigments are] any coloured, black, white or fluorescent particulate solid, which is insoluble in, and essentially unaffected by, the vehicle or substrate in which it is incorporated. It will alter the appearance of an object by the selective absorption and/or scattering of light. A pigment will retain a crystal or particulate structure throughout the colouring process.”
Ink makes use of pigments and dyes, even though the use of dyes is limited to some kinds of writing, security, and ink jet ink. Most printing inks use pigments for their color efficacy. Pigments can be natural (plant or animal origin) or synthetic, and belong to both organic and inorganic classes. Synthetic analogues of natural pigments are also known.
Natural pigments occurring in plants and animals are the sources of a variety of specific functions. For example, the pigment chlorophyll that makes plants green participates in the central process of photosynthesis to produce the plant’s food by absorbing light. In addition, plant leaves contain the pigments belonging to the class of carotenes, which are orange-yellow in color. The variation in these pigments gives rise to the colorful foliage of aspens, poplars, maples and similar deciduous trees, widely seen in New England and other places at the onset of the fall season. The familiar pH indicator made from red cabbage juice functions with the help of another class of pigments known as anthocyanins, which belong to the generic class of flavonoids.
In animals, pigments occur in visual organs, skin, hair and blood. The pigment rhodopsin is found in the rods of the retina of the vertebrate eye, and is associated with the sensitivity of vision. Deposition of the pigment melanin in the human skin as a result of excessive exposure to sunlight or ultraviolet (UV) light has a protective function, and its amount is dependent on genetic and environmental factors. The well-known respiratory protein, hemoglobin, present in the red blood cells is a red color bearing pigment.
All of these cases are examples of natural organic pigments. Naturally occurring inorganic mineral pigments such as red ochre were used in ancient times for aesthetic and religious purposes. In prehistoric times, different inorganic pigments were also used in cave drawings, as can be seen from the still-preserved sites in some parts of France, Spain and Africa.
The advent of synthetic organic pigments was marked by the synthesis of Mauve by Perkin in 1856.3 Since then, different classes of chromophores (color-bearing molecular features) were added, the most important recent addition being diaryl pyrrolopyrroles in 1983. Synthetic pigments provided the much-needed color strength and durability to coatings, as well as other properties such as relative nontoxicity and fastness. The area of dyes has also undergone a metamorphosis based on modern technological applications, as in the use of dyes called laser dyes that function as the active medium in dye lasers.4,5
Pigments are one of the most important ingredients in ink. Familiar definitions consider inks as dispersions of pigments in vehicles. The oldest inks were black in color, and used some form of carbon as the pigment.6 Even the selection process of pigments for ink has become a highly specialized subject, such that books dedicated to this topic appear as multi-authored volumes.7 A high level of research activity, both fundamental and applied, on pigments in their own right and in the context of applications is indicated by the publication of journals on pigments.8
As mentioned earlier, pigments in ink are both inorganic and organic types. Many inorganic pigments are either obtained from natural sources or processed from mineral precursors. Inorganic pigments are classified according to their color and functional properties as white, colored, black, extenders, metallic, and miscellaneous pigments. Each of these classes finds use in ink.
Titanium dioxide (TiO2) is the most powerful and popular white pigment.9 It takes the lion’s share of more than 65% of the total inorganic pigments produced globally every year (about 3.7 million tons). Other white pigments such as zinc oxide, lithopone and zinc sulfide are less frequently used in ink. TiO2 exists in three different crystalline forms: rutile (tetragonal), anatase (tetragonal) and brookite (rhombic). Only rutile and anatase have useful pigment properties. Rutile is the thermally stable form and anatase transforms into rutile at over 700ºC. They have the highest refractive index of all white pigments, and provide the highest coverage for ink coatings. Rutile has the greatest opacity, and anatase has the greatest whiteness.
The colored inorganic pigments such as Chrome yellow, Molybdenum orange, Iron red and Cadmium red once used lavishly in ink have given way to more efficient organic synthetic pigments, due to toxicity concerns and fastness. These pigments were widely used in artistic ink, with the result that the restoration of fading art works is a major endeavor today. For example, environmental factors such as temperature, relative humidity, light intensity and air quality induce chemical reactions at microscopic levels, causing the aging of valuable collections in museums.10 One instance of blackening arises from the conversion of white lead carbonate into black lead sulfide on exposure to hydrogen sulfide gas in air. In this respect, TiO2 is extremely inert. Moreover, the use of heavy metal containing formulations are discouraged and regulated due to their toxic nature and difficulties in their disposal. The Occupational Safety and Health Administration (OSHA) has stipulated standards regulating the use of metals like cadmium, molybdenum and lead. However, it is heartening to note that research continues to develop newer inorganic pigment systems without toxic metals. A case in point is a very recent effort to produce inorganic yellow-red pigment with the formulas CaTaO2N and LaTaON2.11
Even though black-colored compounds such as spinel black, rutile black and iron black can impart black color to coatings, the ink industry mainly uses carbon black to obtain black color. The ink industry is the second-largest consumer of carbon black, the major consumer being the rubber tire industry, where a consumption rate of more than 90% is reported. Carbon blacks are available in a wide spectrum of particle size, ranging from 5 to 500 µm (1 (Greek Mu) = one millionth of a meter). They are usually made by the partial oxidation or thermal cracking of hydrocarbons.
Another class of inorganic pigment generally used in ink is extenders or fillers. This class includes natural clays and synthetically produced materials. The materials’ chief function is to reduce the cost of pigments, although they can also improve the technical and application properties. For example, alumina hydrate promotes gloss and flow properties, calcium carbonate helps to adjust the tint strength, and clays act as viscosity modifiers. Extenders are largely made from natural sources, sometimes by the benefication of minerals. Fine particulate, precipitated or fumed silica finds use as flatting agents.
Metallic pigments consist of small flakes of metals. The pigments’ inclusion in ink formulations yields a metallic finish and appearance to the coating. For example, aluminum powder (aluminum bronze) and copper-zinc alloy powder (gold bronze), respectively, are used for attaining silver and gold appearances. They are available both in the leafing grade, where the particles are in a platelet form, and in the non-leafing powder form which are used in making metal sheen ink. They are made in ball mills.
Miscellaneous inorganic pigments used in ink are mainly the luminescent and pearlescent (nacreous) types. Luminescent pigments may either be the fluorescent type or the phosphorescent type. The processes of fluorescence and phosphorescence differ in the time delays involved between the absorption and emission of light. Semiconducting sulfides of zinc and cadmium are examples of these pigments. However, organic luminescent pigments are more popular in ink.
Pearlescent pigments are used in ink to obtain a pearl-like appearance. They mimic the reflection mechanism in natural pearl. Pearl has multiple layers of calcium carbonate and protein. Incident light undergoes multiple reflection between these layers, resulting in the special appearance. Similarly, in pearlescent pigments, flakes of the mineral mica (lower refractive index) coated with layers of titanium dioxide (higher refractive index) create the impression of luster and sheen, on reflection of light. By manipulating the thickness of coating on mica, a range of colors can be achieved. Other pigments such as (gamma)Fe2O3 and CrO2 (magnetic pigments) that are used as magnetic recording media in the tape and disk forms could find imaginative application in ink also.
Some of the physical properties of inorganic pigments include particle size, shape, size distribution, specific surface area, sieve residue, hardness, density, refractive index and oil absorption. The important crystal structures generally found are cubic (sphalerite lattice as in CdS), tetragonal (rutile lattice as in TiO2), rhombic (geothite lattice as in (alpha)-FeOOH), hexagonal (corundum lattice as in (alpha)-Fe2O3), and monoclinic (monazite lattice as in PbCrO4). Chemical parameters such as acid value, alkali value and pH of the aqueous suspensions are useful tools in characterizing the pigments.12
The optical properties emanating from the interaction between light and the particles produce various colors due to selective absorption of light, whiteness due to nonselective scattering, and blackness due to nonselective absorption. The absorption process is generally through electronic transitions of the ions, involving molecular orbitals in metals and valence band/conduction band in semiconductors.13 A spectral reflection curve is a useful benchmark for the physico-optical properties of pigments and pigment coatings. The important theoretical treatments correlating the reflection spectra and optical properties of a pigment are based on colorimetry, the Kubelka-Munk theory, the theory of multiple scattering, and the Mie theory. The refractive indices of pigments, especially white pigments, are important as they decide the hiding power of the pigment, based on the Lorentz-Lorenz equation where the difference in the refractive indices between the pigment and medium is of consequence.
Synthetic organic pigments are the most generally used pigments in modern-day ink. Even high school students, who often perform paper chromatography demonstrative experiments to separate colored pigments from ink samples, appreciate the role of these pigments in ink. These pigments are comparatively costly. But, their superior beneficial properties such as lightfastness, tinctorial strength and low toxicity outweigh this weakness. The ink industry is the main consumer of synthetic organic pigments, its share being more than 40% of the worldwide consumption.
Organic pigments are mainly available in the following five forms.
1. Press cake quality that contains about 70% water, and directly comes from the filter press in which the aqueous pigment slurry is processed. This type is useful in waterborne ink. One advantage is that the pigment dispersion problem is minimal.14
2. Toner quality powder, which is almost 100% dry pigment. This is produced from the press cake quality and is very dusty. Much energy is expended to disperse this quality, as the adsorbed air and other gases must be completely displaced before the wetting needed for the dispersion process is achieved.
3. Flush quality that is made from press cake quality by displacing the water content with resin containing vehicles. This quality is compatible with many coating formulations.
4. Dispersion quality that contains about 50% of the pigment loading. The ink maker can bypass the dispersion stage by selecting appropriate compositions of these dispersions.
5. Microencapsulated quality that directly interacts with the polymer matrix in which they are usually embedded. They are generally prepared by precipitating preformed polymers onto the particle surface or by adsorption of gaseous monomers followed by polymerization.
The problem of nomenclature of pigments was a thorny issue. Manufacturers named their products using their own attributes such as shade, color strength and chemical source. Currently, the accepted nomenclature is the Colour Index (CI) system, the one standardized under the combined auspices of the United Kingdom’s Society of Dyers and Colourists, and the United States’ Association of Textile Chemists. These organizations have suggested a Colour Index name to reflect the hue as in the case of PB 15 (Pigment Blue) for the copper phthalocyanine blue, and a Color Constitution number to associate with the structural features as 74160 for PB 15. Chemical Abstract Service (CAS) registry number is also a unique index of pigments, such as [147-14-8] for PB 15. CI index numbers point to the chronological and structural details of the pigment.14
Fluorescent organic pigments, especially those that fluoresce in daylight, have gained prominence these days, owing to a variety of applications in security ink to prevent forgery, traffic light signals, safety applications, poster boards, and advertising. Many of these pigments are available as polymer composites. Fluorescent pigments are currently used in flexography, screen, lithography and letter-press ink, as well as in paint and plastics.
Organic pigments are available with superior fastness, a property that is becoming important in ink applications. There are many situations in which ink coatings are being exposed to factors like light, weather, moisture, heat and solvents. Chemically and photochemically inert pigment grades have been developed for outdoor applications. One approach is to control the particle size of the pigment to reduce the detrimental effect of solar radiation. Thus newer technology has helped ameliorate the color-fading problem even in relatively high dilutions.
The ultimate color producing effect of pigments is related to the constitutional structure. Molecular structure is the prime factor that is further influenced by the solid state properties. The structural feature — called a chromophore — is responsible for causing color that results from the absorption of near ultraviolet and visible region of light. For this, the electronic transitions should occur at wavelengths corresponding to the UV and visible regions. A common structural moiety that effects such a transition is provided by the conjugate double-bonded system. By extending the conjugate system to a certain level, color can be induced to a molecule. Thus, 1,6-diphenyl hexatriene (H5C6-CH=CH-CH=CH-C6H5) is colored, whereas 1,4-diphenyl butadiene (H5C6-CH=CH-CH=CH-C6H5) is not.15 Conjugation pushes the (Greek pie-pie) and n-(Greek pie) transition energies to higher wavelengths.
Control of physical characteristics of the organic pigments can have beneficial effects on the performance of ink. For example, phthalocyanine blue that can exist in (Greek alpha) and (Greek beta) crystalline forms exhibits blue and cyan (greener) shades, respectively. Thus the nature of stacking of the lattices has a role in the color generation.16 A consequence of stacking crystals is the anisotropic character exhibited by them. For example, beta copper phthalocyanine blue shows dichroism, exhibiting different colors when viewed in different angles. Anisotropy can also produce differential surface polarity within the crystal lattice. Thus brick-shaped crystals of beta form perform well in polar ink (as in alcohol-based packaging ink), and rod-shaped crystals of beta form perform well in low polarity ink (as in lithographic ink).
Since, in ink technology, pigments are mainly used in making dispersions, their nature of aggregation is of significance. Although the crystal size may be controlled by manipulating the process parameters and additives, a minimum size is needed in aiding the proper handling of pigment powders. But an optimum size range is desired so that the solid pigment particles can be easily dispersed. The cohesive forces operating between crystals in the aggregates have a role in the dispersion process. For strong aggregation, the area of contact between crystals and the strength of cohesion per unit area of contact are important that influence the dispersion process. Pigment dispersion procedures take note of these factors and use appropriate additives and grinding techniques that result in a stable dispersion devoid of flocculation.
Organic pigments based on novel structural moieties do not emerge routinely. Any new pigment candidate has to pass rigorous tests in terms of chemical, toxicological, ecological and performance parameters to be viable. There is an urgent need to develop newer members to meet the stringent requirements of emerging printing technologies, focusing on improved characteristics like color, rheology, gloss, flow, transfer and color fastness.