PFFC - April 2008 - (Page 16)

�� PROCESS MANAGEMENT Drying—Part 3: Paste Inks �� Oil-Based Inks By David Argent Contributing Editor his column will review the two primary �� drying methods of paste inks. 1 | Oil-Based Inks | The main applica�� tion of oil-based inks is in litho printing, �� primarily but not exclusively on absorbent surfaces. These inks have the property of resistance to drying on the fountain system, press rollers, and blankets while �� Energy-Cured Inks drying quickly on the substrate, ensuring a wet-on-wet �� trap. Oil-based inks use a combination of three process- �� es to dry: evaporation of petroleum solvents, absorption into the substrate, and oxidation polymerization of the drying oils in the ink. �� Quick setting occurs by absorption almost immediately, and the press sheets can be stacked at the end of the press without sticking. However, internal �� drying and hardening of the ink takes a much longer time as the oxidative polymerization proceeds. �� Small amounts (<0.5%) of metal dryers are part of the ink formula and serve as catalyst for polymerization of the oil component. Different metals have reaction does not start until the energy is applied during different effects; e.g., cobalt is a surface dryer used drying. The polymerization process is rapid, and all ink in combination with manganese as a through dryer. film is transformed into a solid, glossy finish, so there Oxidation can be retarded by several press facare no �� VOC issues. �� tors, mainly excessive acid pH from the fountain UV curing uses light energy to initiate the po� � �� solution, which is emulsified in the ink. This effect lymerization,� so�the light �must pass through the is amplified on small take-off images, and the rem- entire ink film. Some colors and heavy laydowns edy is to back off the dampening levels. Since oxiare difficult to cure as they do �� not allow UV light �� dation is a chemical reaction, the rate is controlled to penetrate easily. UV curing chemistry utilizes �� by temperature, so cold substrate retards drying. photoinitiators to absorb much of the spectrum. On nonporous substrates used with oil inks, dryThe power output and condition of the lamps and �� ing is achievable only by oxidative polymerization. reflectors significantly affect the amount of UV energy to which the chemistry is exposed. This will have a Therefore, special ink formulations are required that �� do not contain the quick-setting feature. It is critical to significant impact on the finished properties of the UV �� keep water levels as low as possible to keep acidity from ink, coating, or adhesive. Due to the more compact retarding the oxidative drying. To avoid sticking, stacks size, UV lamps lend themselves to inter-station curing. EB electrons have considerably more energy than UV at the end of the press must be low, and coarse spray light and are not significantly affected by the ink film powder often is needed. Winding of the stack also inthickness or color. For the same reason, EB formulations troduces oxygen to keep the drying process active. do not contain photoinitiators. However, an inert nitroSo, lithographic printing with oil inks is an amazgen environment is required to eliminate oxygen ing combination of ink/dampening balance for as a polymerization retarder. Unlike UV optimum print quality and oxidative drying lamps, EB units typically are either on or of the printed product. 2 | Energy-Cured Inks | These inks off and don’t have deterioration issues now are used in offset, flexo, and inkjet (assuming they are set up properly). EB printing, and the technology is advancchemistry more easily meets odor and ing rapidly. The inks are cured—or taste standards for food packaging. SURFACE hardened—by exposure to radiant Energy cure drying continues to be TREATMENT energy. That energy can be in the form an active development area for chemisof UV or EB. Press stability and print try; hardware improvements and market quality are very good, since the chemical expansion can be expected. T �� COMING NEXT MONTH Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and performance. Contact him at 636-391-8180; djvargent@sbcglobal.net. 16 | APRIL 2008 WWW.PFFC-ONLINE.COM http://WWW.PFFC-ONLINE.COM

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PFFC - April 2008 Table of Contents First Glance Calendar of Events From the Editor Web Lines Process Management News Clips Narrow Web & Label Reporter Narrow Web & Label What’s New Products Carton & Box Reporter Carton & Box What’s New Products Green Converting Pouchmaking Show Preview Flexible Packaging Gravure Printing What's New Products Services Directory Classifieds Marketplace Equipment Buyers & Sellers Advertisers Index Experience Speaks

PFFC - April 2008

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