Ink Drying
Mechanisms of ink drying - Absorption, evaporation and oxidative polymerization.
News inks and rubber-base offset inks dry primarily by absorption into uncoated stocks. Drying oils are sometimes added to news inks to reduce ruboff. Ink jet inks dry by absorption to some extent. Gravure, Flexo and web offset inks dry primarily by evaporation.
Inks for sheetfed offset (not rubber based) generally dry by oxidation polymerization. The greatest cause of drying problems in sheetfed offset is high pH dampening solution. A pH below 4 inactivates drier by hydrolysis. Heatset inks dry by evaporation in hot air dryers. Heat softens the resin, but resin is rehardened by passing over chill rolls. Process occurs very rapidly, completing in a second or less.
Drying oils dry by oxidative polymerization. Oxygen in the air adds to double bonds of the drying oil molecules to form hydroperoxides. The resulting free radicals are very reactive. These attack other molecules and attach forming new (larger) free radicals. This causes polymerization. Since many of the molecules have multiple reactive sites (i.e. double bonds) some crosslinks are formed creating a network. Drying of Quickset Inks - Quickset inks contain resin, drying oil and a high boiling hydrocarbon solvent. Total solvent is about 15%.
When printed on paper, some solvent is absorbed into the paper. This caused the ink film to increases in viscosity, leading to rapid setting. Oil is absorbed faster into coating on coated paper than into uncoated paper. (Coating is more oil-wet than paper fibers which are water-wet.) Thus, quickset inks dry quicker on coated paper. Some oxidation polymerization occurs after the initial setting.
Chemical and physical processes are speeded up by increasing the temperature. Thus, IR radiation can be used to speed the setting of ink. The drying time for sheetfed inks can be reduced by 75% by use of IR drying. The IR radiation drives the ink solvent into the paper rapidly, causing the resins in the to precipitate. This results in fast setting of the ink. Final drying continues by oxidative polymerization.
Energy cured inks are ones which cure, or harden, under exposure to radiant energy. The radiant energy may be in the form of UV light or high energy electrons. The advantages of these materials are
- Nearly instantaneous curing and “drying”.
- Increased productivity.
- Good stability on press.
- No VOC's when curing is complete.
- Reduction of waste.
- Improved quality.
- Good product resistance.
- Printable on multiple substrates.
- Good ink “mileage”.
The disadvantages are
- High up-front equipment cost.
- Expensive inks.
- Pigmented and opaque systems may be difficult to cure.
- Possible toxicology issues.
- Limited shelf life.
These inks “dry” or set by an acrylic polymerization. The reaction is initiated by a catalyst called a photoinitiator. The photoinitiator requires less energy to activate than the molecules of the vehicle. The photoinitiator molecule absorbs a photon and fragments into free radicals or ions. The most common photoinitiators involve free radicals, which attack double bonds in the monomer, or oligomer, contained in the formulation and add to the molecule.
As larger free radicals are formed and add to double bonds in the chain, a polymer is formed. The photoinitiators may be aromatic ketones or esters, acetophenones, benzoin derivatives or benzil ketals. Multifunctional acrylates (two or more acrylic double bonds in the molecule) may be used to crosslink linear polymer molecules. One inhibitor to the curing is oxygen from the air that adsorbs on the surface of the wet inks. This is counteracted by passing the web through a chamber filled with nitrogen.
This oxygen problem is more severe for electron beam cured inks, which require a nitrogen blanket to cure. UV cured ink for flexography is used for label printing, carton packaging and flexible packaging. UV cured rotary letterpress ink is also used for label printing. UV cured offset inks are used for paper carton board, business forms, leaflets, tickets, labels, film and foil, plastic and metal. UV cured screen printing ink is used for printed circuits and glass bottle decoration.
Rubber based Inks - Used for sheetfed duplicators or small presses. These dry by absorption and coalescence. These consist of cyclized rubber particles, which are dispersed in the varnish. These particles coalesce (come together) when printed and form a rub resistant thin film. Plastisol inks used in screen printing behave similarly. These consist of a PVC homopolymer dispersed in a phthalate plasticizer. A tough vinyl film is formed when the ink is heated.
Deinking - After spending great effort to get a liquid ink to dry, solidify or cure, sometimes we want to go the other way. With the increased use of recycled fibers it becomes important to remove ink from the fiber before recycling. Between a quarter and a third of paper in the US is recycled, with larger percentages in Europe and Japan. The value of wastepaper has be increased greatly due to deinking.
There are three processes of deinking: washing, flotation and dispersion. Waste newspaper can be deinked by simple washing with a surfactant or washing soda. The surfactant or soda softens the ink and mobilizes it. The news ink is removed similarly to soil from laundry. For flotation, the paper is macerated (ground-up) and the dried or cured ink is separated from the fiber and floated away with foam.
Dispersion is a combination of the other two processes. After cleaning by flotation, the fiber is collected and pressed. It is then dispersed in a disk refiner and washed to remove remaining dirt or ink. It is difficult and costly to deink UV cured ink. A mixed waste stream is less valuable than a product printed with a single process.