Pigments
Pigments - Most pigments are used for all of the printing processes. Pigments are classified as carbon black, organic, white inorganic and colored inorganic.
Carbon Black - Is major ingredient in black inks.
Chemically is mostly carbon with small amounts of mineral matter and some volatile materials of the form CxOy. (The exact composition is variable).
The typical particle size is 20-30 nm (10-9 m), the smallest of the pigments. Note that these may be truly nanoparticles. The smallest particles might be individual "Bucky Balls" (single polymer molecules of carbon, possibly with other atoms encapsulated or bonded, e.g. C60). The CxOy improves the flow properties and serves as a natural wetting and dispersing agent. The amount of carbon varies between 90 and 99% depending on the method of manufacture.
Organic Pigments - Used for process-color printing because they form transparent films. Most are derivatives of aromatic hydrocarbons benzene, naphthalene or anthracene.The pigment molecules contain chromophoric groups such as =C=NH, -CH=N-, and -N=N-. The electrons in the double bonds selectively absorb some visible wavelengths and change energy levels. The color results from the wavelengths not absorbed. Recall the subtractive color theory.
The electrons in the chromophores are generally coupled to those in the conjugated aromatic rings. Dyes are soluble in the vehicle, while pigments are generally suspended in the vehicle. Pigments are preferred to prevent bleeding and fading. Dyes can be converted to pigments by reacting with phosphomolybdic acid and/or phosphotungstic acid. When reacted with both are known as PMTA pigments.
These include the rhodamines, Victoria blue and red, green, blue and violet PMTA pigments. Azo pigments, which have the -N=N- group, include diarylide yellow, lithol rubine, red lake C, toluidine red, Hansa yellow, DNA orange and napthol red. Phthalocyanine pigments are restricted to the blue and green regions of the spectrum. The phthalocyanine structure consists of a multiple rings including 4 benzene rings, 8 nitrogens and copper coordinated with 4 of the nitrogens.
The phthalocyanines include Copper Phthalocyanine Blue and Copper Polychlorophthalocyanine Green. The green is obtained by chlorinating the blue form. Each of these has two crystal structures. The green shade beta form of the Copper Phthalocyanine Blue is a good process cyan. Organic pigments may be prepared as flushed pigments. They are prepared as dyes in aqueous solutions, converted to pigments, precipitated, filtered and washed.
The filter cake is mixed with a viscous varnish in a large dough mixer, a process known as flushing. The varnish eventually replaces the water adsorbed on the particles. Some water separates and is poured off. The remainder is removed by heating under vacuum. The flushed pigment is sold to the ink manufacturer. Flushed pigments do not contain grit or unground, dry particles.
They are easily mixed with other ink ingredients. They have high color strength and low moisture content. 90% of paste inks use flushed pigments. If, instead of flushing, the filter cake is dried in an oven and then ground, the powdered product is called dry color. Pigments may also be sold in the form of chips, pigment dispersed in a solid resin. Chips are mixed into solvent or varnish to dissolve the resin binder, dispersing the pigment and making the ink.
Chips are usually used for liquid inks for flexo and gravure. Chips containing an organic pigment (60% by weight) in acrylic resin (40%) are used to produce water based inks. Treating the surface of the pigment with a suitable resin, before drying or flushing, makes it more easily dispersible. This process creates resinated pigments, prevents agglomeration and makes the pigment more easily wettable by varnish. Often the natural resin rosin or rosin derivatives are used to make resinated pigments.
White inorganic pigments - Major white pigments are titanium dioxide (TiO2), calcium carbonate (CaCO3) and clay. (There are no white organic pigments). The clay used is a hexagonal platelike form of aluminosilicate called kaolin. It has low opacity and is used as an extender for letterpress and screen inks. Barium sulfate and fumed silica are also sometimes used as extenders in inks. These white pigments are all used to whiten paper.
Colored Inorganic Pigments - Mostly the iron blues which are based on ferric ferrocyanide. These are formed by the reaction
K4Fe(CN)6 + 2FeSO4 -> Fe2[Fe(CN)6] + 2 K2SO4
The Fe2[Fe(CN)6] (ferrous ferrocyanide) is oxidized to Fe4[Fe(CN)6]3 (ferric ferrocyanide) with a strong oxidizing agent such as sodium dichromate or sodium chlorate. Different shades of blue are obtained by varying the pH, temperature and time of heating during oxidation. These were used by printers for all printing inks through the mid 19th century, before the discovery of synthetic organic dyes.