Printing processes require that printing inks be both fluid and capable of subsequent ready conversion to a dry, smudge-resistant film once in place on paper or another substrate. One common type of printing ink consists primarily of a pigment and a binder which are suspended or dissolved in a volatile diluent. The binder serves the purpose of adhering the pigment to the printed substrate. the diluent must subsequently be removed by allowing it to evaporate either at room temperature, or for higher printing speeds, by heating. Large amounts of heat are needed to vaporize the ink diluent, which requires that substantial quantities of fuel be consumed. Since large volumes of air must be drawn over the drying ink film to remove the diluent vapor, frequently much heat is wasted. Further, the evaporation of the ink diluent into the open atmosphere can be a source of pollution. In many instances, it is necessary or desirable to burn off or otherwise remove the solvent vapor from the drying air before discharging it to the atmosphere. Additional fuel is consumed and special equipment must be installed for this purpose.
A second common type of printing ink consists of mixtures containing oleoresinous varnishes and/or drying oils which set by air oxidation. These inks set slowly so it is necessary to take precautions to avoid set-off (transfer) or ink between printed sheets.
Inks may also be formulated which contain drying oils or the like together with some binder and volatile diluent. In the usual printing operation, heat is applied to such an ink immediately after it is printed onto the substrate. At this stage, the diluent is driven off, reducing or minimizing problems of ink set-off. However, the ink is not fully set and rub resistance is inadequate. A subsequent drying stage is necessary in which the drying oil or the like hardens, as by oxidation and/or polymerization.
While it is necessary that a printing ink set rapidly and conveniently after printing, it is equally necessary that it not set or dry on the press. Inks containing volatile components may thicken on the press as the solvent evaporates, making it difficult or impossible to control the printing process. Air oxidizing varnished, drying-oils, and the like, may thicken or gum on the press on exposure to the atmosphere. Such inks are especially inclined to "skin over" when the press is shut down during a run. An ink which is not subject to such changes while on the printing press is said to "stay open".
Ink technologists have sought to achieve inks which set rapidly with a low level of energy input to initiate setting, which do not release polluting materials to the atmosphere, and which stay open to on press, while at the same time meeting the physical and mechanical requirements of the printing process. Much attention has been directed to highly chemically reactive formulations. After printing, such inks are set by polymerization and/or cross-linking which is initiated by heat or radiation. A typical chemically reactive system contains polyfunctional acrylate esters (often in combination with other unsaturated materials), one or more ultraviolet photoinitiators, pigment and a variety of secondary components to control ink physical properties. See U.S. Pat. No. 3,804,640 and U.S. Pat. No. 3,881,942, both to Buckwalter. In normal use, such inks are printed on presses equipped with ultraviolet lamps which expose the film on the paper or other substrate immediately after printing. The inks set rapidly and with essentially no emission of polluting materials. Inks based on such highly reactive materials, however, have limited storage stability. The acrylates and other reactive materials in common use have been found to be chemically incompatible with some pigments which are desirable in printing inks. They present toxicity hazards, or are dangerous eye irritants, and are frequently skin sensitizers. When the inks are designed to be set by an ultraviolet light initiated reaction, the ultraviolet radiation itself and the high voltages necessary to power the ultraviolet lamps are additional health and safety hazards.
U.S. Pat. No. 3,024,213 to Ludlow discloses a heat drying ink vehicle based on polyvinyl chloride plastisols, more particularly finely divided polyvinyl chloride polymer dispersed in liquid plasticizers, all of which have low viscosity and solubility parameters closely matched to polyvinyl chloride, and containing a compatible thermoplastic resinous binder to increase ink cohesion and tack. Further improvements in inks based on polyvinyl chloride and vinyl chloride copolymer plastisols are disclosed in U.S. Pat. No. 3,760,724 to Budzinski. These inks are set by applying heat which causes the liquid plasticizers and polyvinyl chloride or vinyl chloride copolymer to co-dissolve. Thus, such polyvinyl chloride plastisol-based inks are readily heat-settable without significant release of volatile materials to the atmosphere. These inks, however, do not have good press running characteristics. Undesirable build-up of material on the blanket of lithographic offset presses occurs when polyvinyl chloride plastisol inks are used. Further, both Ludlow and Budzinski require the use of at least one plasticizer in such polyvinyl chloride plastisol inks. Such plasticizers may, as Ludlow discloses, damage ordinary natural rubber rolls on printing presses. It is thus necessary that ink rolls of polyurethane rubber or other plasticizer resistant compositions be used.