Historical sketch. A precise time and place origin of printing ink is unknown. The beginning of modern printing inks can be considered to be writing ink, which was used by the Chinese and the Egyptians as early as 2600 B.C. These early inks consisted of lamp black mixed with animal or vegetable oils. Later, earth colors were used in addition to black. The Chinese were printing from hand-cut blocks in the 9th century, 600 years before Gutenberg introduced movable type in Europe. The printer of early times was forced to make his own ink. The ingredients were boiled linseed oil, which was cooked according to his own “secret” formula, and lamp black. The ingredients were mixed together, and then ground by hand with a mortar and pestle.
Inkmaking was established on a commercial scale in Europe in the 16th and 17th centuries. The first ink factory was established in America in 1742. Little color was used in printing inks until the discovery of coal-tar colors. Prior to their discovery, various vegetable, mineral, and earth colors were used. However, the pigments were coarse and of poor quality. They lacked the brilliance of ink manufactured today. The advances made by the synthetic color industry have enabled the inkmaker and the printer today to reproduce virtually any color. However, increased press speeds and new printing processes require the inkmaker to keep pace with all the developments of the graphic arts. Linseed oil has been replaced in part by faster and harder drying synthetic oils. The development of synthetic resins and their use in inks has enabled the inkmaker to produce inks that print on almost any surface from paper to metal, glass, and plastics.
The discovery of the principles of lithography by Senefelder in 1796 required the development of a completely new kind of ink. Ink-making growth and development have been greatest in the past 20 to 30 years. Some of the most notable kinds of ink developed in recent years are flexographic inks, heat-set inks, quicksetting inks, wax-set inks, steam-set or moisture-set inks, and glass inks.
Methods of manufacture. The first step in the manufacture of printing inks, once the formula has been determined, is the mixing of the pigment and the vehicle. This is usually accomplished in a large dough-type mixer. The varnish is first placed in the mixer and the pigment is then added gradually while mixing until all the pigment has been wetted by the vehicle. Fine printing inks require that the pigment be completely dispersed and finely ground. After mixing, the ink is milled. The three-roll mill is most commonly used for grinding the heavier types of ink such as letterpress and offset inks. The rollers of a modern ink mill are made of hardened steel. The surfaces are highly polished and accurately machined. There is a speed differential in the rotation of each of the rollers with respect to each other and a fine adjustment is provided for the clearance between rollers. The dispersion and grinding of the pigment is accomplished by shearing forces generated by this differential speed, as well as the closeness of the roller setting.
A great deal of heat is created in the grinding process. To remove this heat, the centers of the rollers are hollow and water cooled. Ink is passed through the mill several times before it is ground finely enough to meet today’s quality standards. The softer, more fluid types of ink, such as news, comic, flexograph, and rotogravure inks may be ground in colloid mills or ball mills. A colloid mill has two grinding plates; a rotor, which rotates at high speeds, and a stator, which is stationary. By adjusting the clearance between these two plates, the inkmaker controls the degree of grinding and dispersion. A ball mill is a large drum into which all of the ingredients are placed along with a quantity of steel balls. The drum is rotated and grinding is accomplished as the balls rub and fall against each other in the mixture. The fineness of the ink product is determined by the length of time the mill is operating.
While the three-roll mill is most commonly used in the manufacture of printing inks, mills ranging in number of rolls from one to five are used. The problem of grinding and dispersion is a constant challenge to the inkmaker. New methods and machines are constantly being developed to achieve better results.
Ingredients used in printing inks. The ingredients used in the manufacture of printing inks fall into three main classifications: (1) the fluid ingredients or vehicles, (2) the pigment or solid ingredients, and (3) other ingredients, principally driers and compounds.
The type of printing process and the drying system involved deter-mine the type of vehicle used in the manufacture of a particular ink. Printing inks dry in several ways: penetration or absorption, evaporation, oxidation, and precipitation. The function of the vehicle is to act as a carrier for the pigment and as a binder to bond the pigment on the printed surface. The nature of the vehicle determines in a large measure the tack and flow characteristics of the finished ink.
VEHICLES
Nondrying oils. Inks printed on soft absorbent papers, such as news-print inks, dry by absorption of the vehicle into the paper. The vehicle in this instance consists of nondrying, penetrating oils such as petroleum oils and rosin oils, used in combination or modified with various resins to impart suitable tack and flow characteristics.
Drying oils. Most letterpress and offset inks manufactured today dry through oxidation. The chemistry of oxidation is complicated, but consists, essentially, of the absorption of oxygen by drying oils. The driers act as catalysts and speed up this action. Oxidation generally proceeds in two stages. The first stage, during which oxygen from the air is absorbed, does not produce any marked physical change in the vehicle. In the second phase hardening or polymerization of the liquid oil produces a solid film. There are many types of drying oils in use today. The most widely used oil is still linseed oil or litho varnish. Raw linseed oil is not suitable as a printing-ink vehicle. It must be converted to a litho oil by boiling or bodying, during which time the viscosity and other properties are radically altered. The temperature used and the time of cooking determine the “body” or the viscosity of the varnish.
Arbitrary numbers are used to describe the various viscosities of these oils. Number 00000, which is very thin, up to number 00 and number I which are medium viscosity, number 5 and number 6 which are heavy, and finally number 9 and number 10 which are very heavy and tacky are known as “body gum.” Linseed oil varnishes have excellent wetting properties for most pigments. They distribute well, have good transfer qualities, and provide good bonding on paper. Other oils used in printing inks are China wood oil, cottonseed oil, castor oil, perilla oil, soya bean oil, petroleum drying oils, fish oil, rosin oil, synthetic drying oils, and others. Many of these oils are combined with synthetic resins in order to obtain faster and harder drying inks. Chemical modification of these oils often produces marked changes and improvements. The most notable examples of this type of change are the alkyd varnishes. Relatively poor drying oils are converted to hard drying varnishes with desirable characteristics by this means. By combining suitable synthetic resins and other materials into some of the aforementioned oils, varnishes can be made that show a minimum amount of penetration. These are suitable for the manufacture of gloss inks. Gloss inks dry on the surface of the printed sheet and there is little or no penetration of the vehicle.
Solvents. Heat-set inksboth for letterpress and offsetare formulated with high-boiling, slow-evaporating petroleum oils and solvents. These provide maximum press stability, yet dry instantaneously with the application of heat. The selection of the proper synthetic resins for heat-set inks is very important, in order to provide proper solvent release and good drying.
Precipitation drying. Moisture-set inks are made from resins that are insoluble in water but are dissolved in glycol-type solvents. The glycol is readily soluble in water. When moisture, in the form of steam, water spray, or even dampened paper, comes in contact with this type of ink, the glycol is immediately absorbed and the resin is precipitated from the solution, taking the pigment with it.
Quicksetting inks. These inks for both letterpress and offset set very rapidly on contact with paper. The vehicle consists of a carefully balanced combination of resin, oil, and solvent. The solvent is rapidly absorbed by the paper, leaving a relatively dry ink film of resin and oilwhich is subsequently hardened by oxidation.
Caution. Great care should be exercised in mixing and blending different types of inks. The components of inks designed for different printing processes may not be at all compatible with each other.