OFFSET PRESSROOM AIR EVACUATION, WHY?
The control of humidity is critical in offset printing in order to optimize productivity. It is well known that dry air causes paper to lose its moisture so that wrinkling, paper jams, and register issues are likely due to a pick-up in static electricity. A humidity level of 45 to 50% is recommended for paper storage, pre-press, and pressroom environments.
The process of offset printing involves interactions between many materials and as such can be called a combination of differing chemical processes. In these, there is the potential for action between inks, coatings, fountain solutions, water, washes, metals, rubbers, and plate coatings. Additionally, depending on the exact process, there are the effects of humidity, air movement, air knives, temperature, IR & UV energy, spray powder, and air evacuation.
Coatings, both aqueous and UV, are routinely used in the graphic arts industry. While aqueous coating formulations vary somewhat depending on projected end use, they are approximately 60% water and 40% solid materials. The latter consists of thermoplastic resin solids, in combination with other additives to improve properties. Included would be an amine/ ammonia to control pH, plasticizer, waxes, surfactants, coalescent aids, optical brighteners, and an anti-foam.
Coalescence in Aqueous Coatings
Aqueous coatings dry when the large water component and amine/ammonia are removed from coating solids. Heat and air are used to dry aqueous coatings. Water is evaporated more effectively in the presence of low humidity air therefore maximum air flow is more conducive to coating drying speed than heat. When the volatile non-solid materials are evaporated or in part absorbed into absorbent substrates, the resin solid molecules join or link together in a process called coalescence.
Coalescence is the process by which two or more separate masses of miscible substances pull each other together if they make the slightest contact. In coatings, it is the bonding of polymer particles to create a film. As a result of this initial fast drying phase, the coating is approximately 90% dry with a thin film formed allowing safe sheet handling.
Post cure continues over time to complete full coating property development. UV curable coatings are near 100% solids, have low VOC, and contain no polluting solvents. A typical formula consists of film forming reactive acrylate resins, (oligomers and monomers), photo-initiators, and additives such as flow and slip agents, and optical brighteners.
UV curing or “photopolymerization” is a photochemical cross-linking reaction process that uses intense UV light absorption to turn liquids into solids almost instantly. Photo-initiators change UV light energy into chemical energy to effect cure.
Solvent Free Coatings
Aqueous coatings are typically described as being solvent free, low VOC, low odor, and alcohol-free. When aqueous coatings are described as being solvent free, it means that they are free of the volatile hydrocarbon solvents and thinners that have been historically used in the formulation of fast drying gravure and flexo solvent inks and coatings.
Since a solvent is defined as a substance that dissolves a solute, water in this context is a solvent. Solvents are used to dissolve binders (resins and film formers). Water is known as the universal solvent because it can dissolve both ionic & polar covalent substances. Water is an inorganic compound, a solvent, and it does not possess any carbon. Therefore, it is not organic.
Low VOC Coatings
VOC’s are volatile organic (carbon) chemical compounds with a low boiling point which causes a large number of molecules to evaporate into the air at room temperature. VOC’s are varied, numerous and everywhere. Particular photochemically reactive VOC’s have been determined to be air pollutants and are regulated by governments. Most scents, smells, and odors are VOC’s. Any offensive odors generated from printing and coating applications should be evacuated.
In the conventional offset printing process, inks dry by an oxidative process that can produce odoriferous byproducts such as volatile aldehydes. In the case of aqueous coatings, odor can result from odoriferous ammonia/amines, or in some cases residual acrylic monomers. Fountain solution alcohols and glycols can also be objectionable with isopropanol especially odoriferous.
UV curing inks and coatings can be the source of objectionable acrylic odors, and odors from photo-initiators such as benzophenone, or decomposition products. Additionally, a small amount of irritating (pungent odor) ozone is produced by UV curing low-pressure mercury arc lamps, especially during start-up. Regardless of the source of objectionable odors, it is prudent to take steps to remove them from the vicinity of the press by effective air evacuation techniques.
Effective Air Evacuation Techniques
In consideration of contemporary high-speed offset press operations, drying systems have been built that can evacuate a large volume of water by continuous evaporation. Extended delivery is a plus in extending drying time. Various devices designed to provide the capacity to quickly evaporate great quantities of water may be incorporated. Accelerating this continuous drying air flow process are hot, warm, and/or ambient air knives, as well as short and/or medium IR emitters. Exhaust air is equally important in any high performing, efficient drying process.
When applying aqueous coatings, the water component that is evaporated will produce moisture-laden air in and around the press. This air must be evacuated from the press drying and delivery space including the area beneath the press. It is critical that this moisture-laden air be removed or drying at speed will be retarded. In conclusion, WHY is air evacuated? Air evacuation is needed not only to create an effective printing and coating operation, but also to create a desirable pressroom environment.
Our business at Cork is the development and formulation of Aqueous, energy curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings and adhesives. Cork thrives on its ability to formulate novel, useful specialty products that offer the graphic arts industry printer and coater a competitive advantage. Give us a call on your next project.