Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1998-11-23
2001-09-04
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...
C522S143000, C522S170000, C522S909000
Reexamination Certificate
active
06284816
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to low viscosity energy curable flexographic printing inks and methods for using same.
2. Description of Related Art
Low viscosity and good flow are the most important factors affecting liquid ink behavior and improved printability. These factors are particularly important for formulating inks for flexographic printing.
Schwartz et al. in U.S. Pat. No. 4,468,255, disclose rheology modifiers for arylide yellow pigment dispersions. The rheology modifiers which are derived from diarylide pigments improve the fluidity of non-aqueous arylide pigment dispersions prepared from either monoarylide or diarylide yellow pigments. Schwartz et al. inla series of patents (i.e., U.S. Pat. Nos. 4,946,508; 4,946,509; 5,024698; and 5,062,894) have disclosed modified azo pigments for use in conventional, solvent and water based inks and coatings to function as rheology control agents. In each of these patents, Schwartz et al. modify an azo pigment (e.g., a diarylide pigment, a monoazo pigment, a disazo pyrazolone pigment and the like) by grafting a polyalkylene oxide to the pigment so that water based inks made from these pigment compositions exhibit high coloring strength, cleaner shades, lower rheology, and enhanced gloss compared with conventional water based inks.
To increase printing throughput, ultraviolet (UV) or electron beam (EB) curable inks have been developed that allow printers to reduce the solvent content of the ink. It is always a challenging task for a UV or EE curable liquid ink formulator to develop ink formulations with a viscosity low enough for improved flow, while at the same time maintaining other essential characteristics, such as to cure, adhesion, low odor, etc. Traditional organic and inorganic pigments used in formulating energy curable liquid inks, are poorly dispersed in the vehicles used in the systems. As a result, poor pigment wetting leads to a thixotropic structure. If the inks are not subjected to high shear, the apparent viscosity will remain high and the ink will exhibit poor transfer resulting in poor printability. Flow additives have been tried, however, with limited success for different pigments. The situation becomes more critical in the case of cationic curable inks, where not all the pigments and additives can be used due to the presence of basic functionalities. In addition, some energy curable inks of the prior art contain fugitive solvent (such as water, alcohols and the like) as part of the vehicle to reduce viscosity, aid in pigment dispersion, and modify flow during printing. However, after printing but before curing, the fugitive solvent is typically removed to prevent interference with the curing process and to prevent it from having an adverse affect on the printed image. The curing delay required by solvent removal, as well as attendant unpleasant odors, are further undesirable limitations to printing. Furthermore, solvent removal is desirable for environmental reasons.
A UV cured cationically polymerized printing ink is disclosed by Seng in Patent Application DE 195 00 968.1 for use in indirect letterpress printing or dry offset printing. Seng lists the differences between letterpress and flexographic printing and the list includes critical differences in ink viscosities. In particular, Seng discloses that printing inks used in flexographic printing have viscosities in the range of 0.01 cps to 2 cps at 20° C. and in the range of 3 cps to 100 cps, and more preferably in the range of 4 cps to 10 cps. Seng's inks are cationically polymerized and differ from the very high viscosity printing inks used in indirect letterpress printing in solvent content. While inks of this viscosity range are useful in high shear, indirect letterpress printing, such inks have high tack which causes them to be unsuitable for low shear applications such as flexography, which requires low tack inks.
Although improvements have been made in energy curable cationic ink formulations, there continues to be a need for energy curable ink formulations (including free radical initiated curable formulations) which have a high color strength, cleaner shade and enhanced gloss but retain a suitably low viscosity as required for flexographic printing.
SUMMARY OF THE INVENTION
In one aspect, the invention is an energy curable flexographic ink comprising a pigment, a rheological additive having the structure:
P-(U-Y)
s
wherein P is the residue of an organic colorant, Y is a polyalkylene oxide moiety, U is a linking moiety covalently bonding Y to P and s is an integer from 1 to 3; and an energy curable liquid vehicle; wherein the ink is substantially free of fugitive solvent and has a viscosity ranging from about 50 cps to about 3,000 cps,and more preferably from about 50 cps to about 1,500 cps.
In another embodiment of this invention, this energy curable flexographic ink further comprises a polymerization initiating system activatable by actinic radiation.
In yet another embodiment of this invention, there is a method of flexographic printing and curing a flexographic ink comprising the steps of: preparing the energy curable flexographic ink described supra; printing the ink onto a substrate surface to form the ink image; and subjecting the ink image to actinic radiation or thermal energy to form a cured ink image.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a novel solvent-free, energy curable flexographic ink having a low viscosity and its application. The term “energy curable or cured” as used herein, in reference to ink means an ink which can be cured, hardened, polymerized, or crosslinked by the action of actinic radiation such as UV or EB radiation and the like, from a radiant energy source or from a thermal energy source by heating with a conductive or radiant heat source such as a platen, an oven, infrared (IR), microwave, and the like.
The energy curable ink of this invention comprises a pigment, a rheological additive and an energy curable liquid vehicle is substantially free of fugitive solvent. The terms “substantially free of fugitive solvent” and “solvent-free”, as used herein in reference to inks, means free of a liquid component (e.g., water, lower alcohols, alkanes, aromatics, aliphatics, ketones, acetates and the like) which, after printing, is evaporated, imbibed into a substrate surface, or both, and does not remain as an essential component of the cured ink. Further, these terms are not intended to exclude trace or residual solvents resulting from the manufacture of ink components prior to ink formulation.
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Chatterjee Subhankar
Laksin Mikhail
Merchak Paul A.
Schwartz Russell J.
Jagannathan Vasu
Persley Sidney
Shosho Callie E.
Sun Chemical Corporation
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