It’s well known and understood that washed clothing dries when the wash water is evaporated. Similarly, paint dries when the diluent solvent is evaporated, leaving a film of dry paint solids.
Aqueous coatings dry as volatile components (mainly water), about 60% of the typical formula composition, evaporate or are absorbed by a porous substrate. Coalescence is the process that causes the remaining solids, resin molecules, to join or link together forming a thin, dry, flexible film. The coating is 90% dry during this rapid initial drying phase, resulting in a coated printed substrate, dry to the touch for handling. Additional drying, post-cure, taking place over time is responsible for the development of final coating properties.
Cork aqueous coatings are comprised of mixtures of aqueous emulsions, combined with a variety of other ingredients to improve properties and performance. Commonly included are an amine, a plasticizer, waxes, a surfactant, coalescent aids, and an anti-foam. These coatings have produced great productivity gains for the printer/converter. Fast-drying aqueous coatings have given the sheetfed offset printer the ability to seal oxidizing sheetfed inks in-line and move on to further processing. This, without waiting for the underlying, still wet, inks off-line time to dry thoroughly. Productivity gains result from the ability to move faster to die-cutting, gluing, and packing operations.
In the case of aqueous coatings wet trapped in-line over litho inks in the sheetfed offset printing process, the film formed is micro-porous allowing oxygen to reach the underlying setting, but not yet dry inks. Drying may be said to have taken place when the coated sheet can be handled safely, permitting additional processing without creating scrap.
How are aqueous coatings dried when applied in-line in hi-speed printing processes?
Well simply, drying systems are built that are able to remove a large amount of water by evaporation in a very brief time. Yes, a very brief time! The time to move a sheet through the delivery of a litho sheet-fed press at speeds that can reach 20,000 impressions per hour.
So how is this accomplished? For one, extended delivery is offered by press manufacturers, which allows more time to dry. Second, the dryer manufacturers have improved drying systems over time, increasing the capability to evaporate great quantities of water, very quickly. Incorporated are devices to provide continuous drying airflow, and exchange throughout the drying space.
These may include hot, warm, and ambient air knives that direct a blast of water evaporating air toward the aqueous coated surface. Also included may be multiple panels of short (preferred), and/or medium wave IR emitters. Medium wave IR, slow to heat up and cool down, radiates energy that heats the air layer above the substrate. Short wave IR radiates heat energy that will not heat the air layer above the substrate, but will be absorbed very efficiently by any darker image areas, and even lighter areas penetrating the substrate. IR also supplies a hot dry air source in some configurations. Too much IR heat can result in set-off, blocking, and even re-softening of an aqueous coating in a pile that gets too hot. This is critical, especially when coating both sides of a substrate.
Equally important in any functionally efficient drying system is the exhaust air. The air that has impacted the wet aqueous coated substrate is now moisture-laden. This air must be effectively removed, exhausted from the drying and delivery space, including the space beneath the press.
It is critical to remove and exhaust this moist air or drying will be impeded!
Never forget that a voluminous supply of warm dry air must be continually available in order to dry aqueous coatings by the required evaporation of a large amount of water. Keeping it simple, it behooves us to remember that wet laundered clothing will not dry very rapidly hanging on a line on a windless, damp, humid day. However, try it on a sunny, windy day and marvel at how fast drying takes place.
Ideally, a properly designed printing process drying system consists of enough capacity in terms of time, heat, airflow, and exhaust to dry effectively at press speed. Using the minimum amount of energy necessary, the result should be a printed/aqueous coated substrate that can be safely processed further.
Once again, let’s talk about the goal of producing a print that can be handled at the end of the press. In conventional litho printing, emulsified inks set and dry (eventually) by oxidation polymerization. Setting is understood to occur when volatile solvents evaporate and/or are absorbed with other low viscosity materials into the printed substrate. When inks set fast enough on a given substrate the sheet can be handled without setoff, marking, or smearing occurring. Ink film thickness also is a factor in ink setting and drying. We are often asked to provide an aqueous coating that will dry effectively over very heavy ink coverage. However, the more ink there is, the more time it takes for them to set and dry. Even a dry aqueous coating over an unset, heavy ink film will not resist marking, or smearing when rubbed lightly.
Control of the litho printing process is most important. It is said that nothing affects ink setting and drying more than ink and water balance. The adoption of alcohol-free fountain solutions and electronically charged grained plates, makes (OVER DAMPENING) all the easier. An ink high in water pick-up is an ink that has a compromised drying rate. Duke tested ink water pick-up (emulsification) should not exceed a range of between 40-55%. Low pH (high acid) fountain solution will also retard ink drying. Target pH of between 4.0-5.0. Soya based inks are known to be slower drying.
Drying is no mystery, but many elements are involved. All must be controlled in order to optimize the result expected from aqueous coating, especially some of the slower drying very high gloss formulations.
Aqueous coatings, low in VOC, have also played a major role in the conversion from solvent-based liquid ink & coating systems by the Flexo and Gravure printing market segments. Aqueous coatings mesh naturally with liquid printing systems, as additional equipment to apply and dry coatings is not required, as is the case in the offset printing market segment.
Corks’ business is the development and formulation of Aqueous, energy-curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings and adhesives. Contact us for your next project!