Drying Part 3: Paste Inks
- Published: April 01, 2008, By By David Argent Contributing Editor
This column will review the two primary drying methods of paste inks.
- Oil-Based Inks
The main application 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 drying quickly on the substrate, ensuring a wet-on-wet trap. Oil-based inks use a combination of three processes 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 (E<0.5%) of metal dryers are part of the ink formula and serve as catalyst for polymerization of the oil component. Different metals have different effects; e.g., cobalt is a surface dryer used in combination with manganese as a through dryer.
Oxidation can be retarded by several press factors, mainly excessive acid pH from the fountain solution, which is emulsified in the ink. This effect is amplified on small take-off images, and the remedy is to back off the dampening levels. Since oxidation is a chemical reaction, the rate is controlled by temperature, so cold substrate retards drying.
On nonporous substrates used with oil inks, drying is achievable only by oxidative polymerization. Therefore, special ink formulations are required that do not contain the quick-setting feature. It is critical to keep water levels as low as possible to keep acidity from retarding the oxidative drying. To avoid sticking, stacks at the end of the press must be low, and coarse spray powder often is needed. Winding of the stack also introduces oxygen to keep the drying process active.
So, lithographic printing with oil inks is an amazing combination of ink/dampening balance for optimum print quality and oxidative drying of the printed product.
- Energy-Cured Inks
These inks now are used in offset, flexo, and inkjet printing, and the technology is advancing rapidly. The inks are cured — or hardened — by exposure to radiant energy. That energy can be in the form of UV or EB. Press stability and print quality are very good, since the chemical reaction does not start until the energy is applied during drying. The polymerization process is rapid, and all ink film is transformed into a solid, glossy finish, so there are no VOC issues.
UV curing uses light energy to initiate the polymerization, so the light must pass through the entire ink film. Some colors and heavy laydowns are difficult to cure as they do not allow UV light to penetrate easily. UV curing chemistry utilizes photoinitiators to absorb much of the spectrum.
The power output and condition of the lamps and reflectors significantly affect the amount of UV energy to which the chemistry is exposed. This will have a significant impact on the finished properties of the UV ink, coating, or adhesive. Due to the more compact size, UV lamps lend themselves to inter-station curing.
EB electrons have considerably more energy than UV light and are not significantly affected by the ink film thickness or color. For the same reason, EB formulations do not contain photoinitiators. However, an inert nitrogen environment is required to eliminate oxygen as a polymerization retarder. Unlike UV lamps, EB units typically are either on or off and don't have deterioration issues (assuming they are set up properly). EB chemistry more easily meets odor and taste standards for food packaging.
Energy cure drying continues to be an active development area for chemistry; hardware improvements and market expansion can be expected.
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; This email address is being protected from spambots. You need JavaScript enabled to view it..
