Printing inks

Details Printing inks Printing inks

1998-11-30

2003-10-28

Shosho, Callie (Department: 1714)

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

Synthetic resins

Processes of preparing a desired or intentional composition...

C526S318450, C526S329100, C526S336000

Reexamination Certificate

active

06638995

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to printing ink compositions and methods using printing inks, especially lithographic printing inks. The compositions and methods of the invention employ novel vinyl resins that are prepared using polyfunctional monomers, particularly vinyl resins having low number average molecular weights and broad polydispersities.
BACKGROUND OF THE INVENTION
Printing inks generally include one or more vehicles and one or more colorants as principal components. Printing ink vehicles must meet a number of performance requirements that include both requirements related to the printing process, such as suitable consistency and tack for sharp, clean images, suitable length to avoid fly or mist, or proper drying characteristics, and requirements related to the printed image, such as gloss, chemical resistance, durability, or color. In general, ink vehicles include one or more materials such as vegetable oils or fatty acids, resins, and polymers that contribute to the end product properties, and may include other components such as organic solvents, water, rheology modifiers, and so on that may affect body, tack, or drying characteristics.
Printing inks are employed in a variety of printing processes. Printing processes include letterpress printing, lithographic printing, flexographic printing, gravure and other intaglio processes, screen printing, and ink-jet digital printing. The ink composition is affected by the demands of the process used. For example, inks used in printing operations in which the ink will come into contact with rubber elements, such as the blanket of an offset lithographic press, generally include as the solvent portion of the vehicle only petroleum distillate fractions or other aliphatic solvents that will not adversely interact with rubber.
In lithographic printing, an inked printing plate contacts and transfers an inked image to a rubber blanket, and then the blanket contacts and transfers the image to the surface being printed. Lithographic plates are produced by treating the image areas of the plate with an oleophilic material and ensuring that the non-image areas are hydrophilic. In a typical lithographic printing process, the plate cylinder first comes in contact with dampening rollers that transfer an aqueous fountain solution to the hydrophilic non-image areas of the plate. The dampened plate then contacts an inking roller, accepting the ink only in the oleophilic image areas. The press operator must continually monitor the printing process to insure that the correct balance of the fountain solution and the ink is maintained so that the ink adheres to the printing areas, but only the printing areas, of the plate in order to produce a sharp, well-defined print.
The industry has long sought an offset printing process and associated materials that would not require a separate fountain solution. Waterless plates have been made by applying to the non-image area a silicone rubber, which has a very low surface energy and is not wetted by the ink. The silicone-modified plates are expensive, however, and require expensive, specially-cooled press equipment because the fountain solution of the traditional two-fluid method also serves as a coolant. Other efforts have been directed to producing a single-fluid lithographic ink, i.e., an ink that does not require a separate fountain solution, that can be used with the industry-standard presses and all-metal plates. Parkinson, in U.S. Pat. No. 4,045,232 (the entire disclosure of which is expressly incorporated herein by reference) describes lithographic printing and earlier efforts directed to producing a single-fluid lithographic ink and the tendency of single-fluid inks to be unstable. Parkinson notes that ink emulsions containing a solution of glycerin and salts tend to “break,” with the result that the glycerin wets the inking rollers preventing good inking. Parkinson suggests an improved single-fluid ink obtained by using an additive that includes a resin treated with a concentrated mineral acid, and, optionally, a polyhydric or monohydric alcohol. Preferred polyols are glycerin, ethylene glycol, and propylene glycol. DeSanto, Jr. et al, in U.S. Pat. No. 4,981,517 (the entire disclosure of which is expressly incorporated herein by reference) describe a printing ink that is an emulsion of an oil-based phase and a water-miscible phase. The patentees allege that an emulsion containing a significant portion of water (10% to 21%) and employing phosphoric acid as a critical component has improved stability against phase separation and can be used as a single-fluid lithographic ink. The De Santo, Jr. composition further includes as a diluent and emulsion stabilizer an oil with the properties of No. 1 and No. 2 fuel oils and a polyol emulsifier, of which glycerin and ethylene glycol are the only examples provided.
Nonetheless, due to various drawbacks of the single-fluid lithographic inks that have previously been proposed, including the limited stability and poor definition and toning already mentioned, the industry standard continues to be a dual-fluid lithographic ink that includes an ink component and a separate fountain solution component.
Applicants have now discovered that particular compositions that include a polyol phase dispersed or emulsified in a vinyl resin vehicle phase overcomes these problems in a single-fluid lithographic ink.
Yet another problem observed in printing processes, especially with high speed presses such as web-offset lithographic presses, is what has been termed “ink misting.” Ink misting commonly refers to ink droplets that become airborne during the printing process, for example at the point where the rotating inked rollers separate. When two inked rollers are in contact, as they are for the ink transfer in offset printing, ink splitting can result in the formation of ink filaments that may break to produce ink droplets. When the droplets become larger or more like threads, the problem may be termed “ink slinging.” The high speed of the presses and the modification of ink properties to print on such high speed presses exacerbate the problem of misting or slinging. While the mist can be annoying and cause contamination at low levels, higher amounts of misting can potentially create environmental and/or safety problems. The relative amounts of ink misting for particular inks can be determined by comparing the inks on an inkometer, employing standard testing procedures.
Various methods have been suggested for reducing misting or slinging in inks. For example, different additives have been proposed, including kaolin, anionic or cationic surfactants, or an additive that is an organic acid phosphate, glycerol, or propylene carbonate, as described in U.S. Pat. No. 5,000,787, expressly incorporated herein by reference. It has also been postulated that inks are more likely to produce misting when the ink is less extendible or elastic.
Applicants have discovered that printing inks employing the novel vinyl resins of the invention have surprisingly high resistance to misting and slinging on high speed presses.
A further problem that has been encountered for inks that include vinyl copolymer vehicles has been objectionable odor due to residual monomer. Ripley et al., in U.S. Pat. No. 4,327,011 (expressly incorporated herein by reference) disclose a styrene-acrylic copolymer in a low solvency hydrocarbon for a lithographic printing ink. The copolymer is a linear polymer having a weight average molecular weight of up to about 50,000 in an essentially aliphatic hydrocarbon solvent that boils in the range of 390-595° F. While the authors report “essentially complete conversion of the monomers,” it has been found that copolymers prepared according to the instructions of the Ripley patent typically had objectionable odor due to incomplete conversion of the monomers, as even low levels of residual monomers may cause odor problems. In addition, the polymers of the,Ripley patent exhibit high tacks and have unacceptable misting and slinging. While tack may be lowered w

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