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Pigments

( Originally Published 1963 )



Pigments are the solid coloring matter in inks such as black, white, or any of the common colors. It is generally only the pigment which we see when examining printed matter. Pigments furnish contrast with the background and are responsible for many of the specific properties of the inks, such as specific gravity, opacity or transparency, and permanency to light, heat, and chemicals. The pigments usually determine whether or not a print will bleed in water, oil, alcohol, fat or grease, acid, or alkali. Therefore, the pigment will partially determine whether or not the ink is suitable for offset lithography or for other specific end uses, such as soft food and meat wrappers, soap labels, and other packaging. The pigment to some degree determines whether or not the finished job may be varnished, lacquered, or laminated.

Black pigments. Black pigments are chiefly channel black, furnace black, and lamp black. Channel blacks are made by burning natural gas with an insufficient amount of air so that a very smoky flame is produced, and the resulting soot is collected. They are produced in a large number of different grades by controlling the burning process. Furnace blacks are produced by cracking gas or oil in special furnace-type equipment and are quite different from the channel blacks produced by the open-flame method. Each type of black has specific properties which are used in formulating the various types of black inks for different printing processes. Often it is necessary to use several black pigment types in one formulation. Lamp blacks are made by processes similar to those used for carbon black with oil as a raw material instead of gas.

White pigments. White pigments are of two main types: opaque and transparent. Opaque pigments reflect light at their surface and have the property of covering the background on which they are printed. Opaque white pigments commonly used in inks are titanium dioxide, zinc sulfide, lithopones, and zinc oxides. Mixtures of these may be necessary to achieve a particular result. They may be used to produce opaque white inks. They are also used in combination with any of the color pigments to add opacity or to lighten the color. Transparent pigments do not reflect light at the surface, but permit light to pass through the film of ink and be reflected from the surface on which it is printed. In this manner a light transparent ink can be printed over another darker ink without image loss of the first printing. Transparent white pigments commonly used in inks are aluminum hydrate, magnesium carbonate, calcium carbonate, blanc fixe, barytes, and clays. The transparent whites are also used to reduce color strength of inks, to aid dispersion of some of the color pigments, to help carry some of the heavier pigments, and to make tinting colors of darker pigments.

Inorganic color pigments. This group of pigments consists primarily of mineral components. Several of the more widely used compounds are chrome yellow, chrome orange, molybdate orange, cadmium yellow, cadmium orange, cadmium red, mercuric-sulfide, vermilion, ochre, sienna, umber, iron blue, and ultramarine blue. All of these pigments possess varying qualities of opacity, color fastness, and permanency.

Organic color pigments. These pigments supply the largest variety for manufacture of ink. A few of these pigments are as follows: yellow lakes, hansa yellow, benzidine yellow, orange lakes, para reds, toluidines, fire reds, lithol reds, phloxine or eosine lakes, rhodamine reds, phthalocyanine blues, alkali blues, and green lakes. The origin of most organic colors is a dye material from coal tar. The chemistry of these dyes is very complex. The dyes are used by mixing with transparent materials.

Lithographic inks. Lithographic inks generally are stronger in color value to compensate for the lesser amount applied by this process of printing. Pigments for lithographic use must be free from bleeding tendencies in the presence of fountain solution and the vehicles must have good resistance to emulsification. The tack requirements also are higher. These qualities are required for all lithographic inks. Lithographic inks have many classifications and include those that dry by oxidation, by evaporation at elevated temperatures, by penetration and coagulation, or by combinations of these systems. Each of these types is obtained by the use of drying oils, such as linseed, which are heat treated to develop the special requirements; by combining these oils with synthetic or natural resins or by combining resins with high-boiling petroleum solvents so that the desired characteristics are met.

Regular varnish lithographic inks may contain more than a dozen ingredients to perform successfully on the press and on the paper being used. These include one or more pigments, one or more lithographic varnishes, at least one compound, perhaps an extender, and several driers. The pigments develop the color specified; the vehicles carry the pigment through the process and bond the ink to the paper; the compounds add special properties; the extenders determine the color values; and the driers act to change the fluid ink into a solid. Frequently, synthetic varnishes are used as replacements for the regular drying oil varnishes. These special vehicles speed up drying, form harder and more rub-resistant prints, and frequently are used to produce inks that dry with a gloss. Printing inks for use on metal, which are a special lithographic adaption, call for special pigments that resist elevated temperatures and for synthetic vehicles that dry in varying periods of time from 5 to 10 minutes when processed in an oven varying in temperature from 200 to 400 degrees Fahrenheit. Metal sheets that must be crimped or stretched after baking usually are over-printed or coated with a special varnish that adds to the toughness and flexibility of metal-decorating inks.

Recent developments in quick-set and heat-set lithographic inks were made a success by using special vehicles composed of oils, resins, and high boiling solvents. In this development, great care is taken in the selection of ingredients to avoid any harmful effects on the rollers, blanket, or plate. Faster production, brighter effects, less set-off, better trapping, and sharper printing are features of these special inks. Inasmuch as today's tendency is to produce press-ready inks, the need for mixing and reducing of ink in the pressroom is practically eliminated. However, if adjustment is required, due to some unusual condition, it is advisable that the inkmaker be consulted. This is especially so since many modern inks contain chemicals in their formulation which may not be compatible with common additives normally used.

It is important that the balance between the fountain solution and the ink be correct for best results. If this control cannot be stabilized, it may be necessary to use special dampening solutions, or inks of unusual tolerance, to arrive at good results. Experience is a necessity to establish control under these circumstances. The use of proper fountain solutions is essential to obtain good quality production. The pH values, once established for the plate being used, and the ink and paper being printed, must be carefully maintained within close tolerances. The nature of the fountain solution greatly affects the working property of the ink being used. Too much acid retards drying and causes roller stripping. Too much gum arabic encourages emulsification. Lithographic inks are affected also by the paper being run. Uncoated papers as a rule are less troublesome. Coated papers are subject to picking (hickies) and chalking (excessive vehicle absorption).

Special precautions must be taken in all lithographic inks to avoid these and other difficulties. Vehicles, pigments, and driers must be carefully chosen for each specific use.



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