Low VOC cationic curable lithographic printing inks

Details Low VOC cationic curable lithographic printing inks Low VOC cationic curable lithographic printing inks

2000-02-07

2002-09-17

Jagannathan, Vasu (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C523S161000, C522S031000, C522S064000, C522S067000

Reexamination Certificate

active

06451873

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to novel lithographic printing ink compositions having low amounts of volatile organic components (VOC) that consist of diluents polymerizable by cationic addition polymerization and cured in the presence of water or fountain solution.
BACKGROUND OF THE RELATED ART
Today, a major goal of national environmental programs is the reduction or elimination of evaporative emissions of organic chemicals into the atmosphere which would eliminate: the health risks associated with continual human exposure to organic vapors in the work-place; the potential contribution of VOC to hydrocarbon and ozone-related pollution of the atmosphere; and the theoretical contribution VOC may make to the atmospheric “greenhouse effect”. Manufacturers across the industrial spectrum have been called upon to look at the formulations of their end products and manufacturing processes and change or alter the product formulations or process steps consistent with the national goal. This is a prodigious challenge, one not easily met in the best of circumstances and not achievable without a severe deleterious impact on end-product properties and/or process cost.
Inks typically include a variety of organic components in their formulation such as one or more organic solvents, monomers, oligomers, crosslinking agents, gelling agents and the like, all selected for their contribution to application-specific ink properties. For lithographic printing inks, conventional web offset heatset inks (WOHS) comprise the largest market segment and are, consequently, a most important segment to convert to VOC-free systems. These inks usually contain a pigment, polymeric binders, additives such as driers or talcs, and volatile solvents or diluents. As their name implies, the inks dry by a setting mechanism. The drying takes place by removal of the solvent when the printed article passes through a high temperature oven where solvent is evaporated into the atmosphere. As such, the drying process for these inks presents a significant opportunity for the printing industry to reduce VOC pollutants emanating from their current printing practices. But that opportunity must be captured without severely affecting the physical printing operation and its cost.
WOHS inks have been formulated that eliminate or substantially reduce VOCs by replacing solvents or diluents in the formulation with acrylate monomers and acrylated oligomers which serve as diluents for pigments, etc. These inks are then cured by free radical initiated polymerization to form polymeric binders. In this instance, as a first choice the formulation is restricted to monomers that polymerize by well recognized free radical initiation mechanisms, where the rate of polymerization is fast and commensurate with the web print speeds of WOHS. When this requirement is is factored into the end properties required of the ink polymeric binder, the choice of monomer is largely restricted to acrylates and the acrylate moiety. But acrylate monomers are relatively expensive for products of this class. Therefore, when coupled with the additional cost pressures inherent in product reformulation, the use of acrylates to reduce VOC produces a severe upward pressure on the cost of WOHS inks.
It is well known in polymer science that there are other means to catalyzing monomers that yield polymers with physical properties potentially useful in low VOC WOHS inks. These monomers polymerize at rates competitive with those achieved by the free radical polymerization of acrylates and, hence, they are compatible with the speed of WOHS printing. These alternative methods include anionic and cationic catalysis, redox initiation, thermal and electron beam. However, each of these approaches have strengths and weaknesses when considered as a basis for reformulating WOHS inks for lower VOC content. Electron Beam initiated polymerization would, for example, require modifications to the physical printing plant to accommodate new equipment at a potentially steep cost. Tonically catalyzed polymerization would present a challenge to the practical feasibility of the overall offset printing process that takes place in the presence of aqueous fountain solutions.
Fountain solutions are typically an aqueous mixture of chemical components used in off-set printing to keep the non-image area of the lithographic printing plate free of ink and often contain basic components to produce high pH, acidic components for low pH or acid/base buffers designed to maintain an intermediate fountain solution pH of about 3, or at least 5-7. Consequently, one skilled in the art would reasonably expect that fountain solutions to seriously retard or stop inks catalyzed by cationic catalysts from curing.
McKie et al, GB2142279A, describes an energy curable printing ink composition consisting of a cationic polymerizable resin, diluent, photoinitiator, and colorant. However, the printing is a screen ink and screen inks typically have a viscosity between 15 and 60 poises. By contrast, UV curable lithographic inks have a viscosity above 100 poises (R. H. Leach, “The Printing Ink Manual”, Fourth Edition, 312 and 498 (1988)). Moreover, screen inks are used in dry transfer printing processes and therefore are not curable in the presence of water or fountain solution.
In general, it is known that water, particularly in the form of high humidity or moisture, poisons anionic and cationic polymerizations. So it would seem unlikely, that any cationically catalyzed ink curing would work in the presence of water or fountain solution; see
UV Curing: Science and Technology,
Vol. II Chapter 6, pgs. 248-282, by S. Peter Pappas, Published by Marketing Technology Corp. Thus, high quality, commercial grade, UV curable lithographic printing ink formulations, polymerizable by cationic addition polymerization and essentially devoid of VOCs and acrylates are desirable.
The object of the present invention is to develop a lithographic printing ink formulation having a low complement of VOC yet fully compatible with commercial printing operations.
It is also an objective of the present invention to formulate low VOC lithographic UV curable printing inks that are cured in the presence of water or fountain solution.
Another object of the invention is to formulate a WOHS ink employing a cationic catalyst and gelling agents compatible with cationic UV cured systems.
SUMMARY OF THE INVENTION
A lithographic printing ink cured by UV radiation in the presence of water or fountain solution comprising: a diluent polymerizable by cationic addition polymerization; a cationic photoinitiator; rheology modifying resin binder and colorant; having a viscosity greater than 100 poises.
DETAILED DESCRIPTION OF THE INVENTION
The images produced by the novel printing ink formulations of the invention are cured following offset printing by a novel process comprising a surprisingly effective cationic catalytic polymerization of the reactive monomer diluents or derivatized oligomers in the formulation. A binder for the ink system is formed that provides a hard coat and a high gloss printed image. Despite the presence of aqueous fountain solution in proximal contact with the formed image subsequent to printing, it has been found that the cationic polymerization of reactive monomers in the uncured ink image is, nevertheless, readily initiated and effectively cures the printed image.
The cationic polymerization catalyst photoinitiators included in the ink formulations of the invention are cationic catalyst precursors that have the capability of releasing cationic catalysts when acted upon by an appropriate agent. Preferably, the initiators release cationic catalyst upon exposure to an energy source such as heat, light or an electron beam. A useful initiator is one which reacts or rearranges upon exposure to ultraviolet (UV) light to release a cationic catalyst which goes unaffected by fountain solution. A particularly preferred cationic precursor is Cyracure 6990, available from Union Carbide. Upon exposure to UV light and subsequent to printing,

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