OFFSET PRESSROOM AIR EVACUATION, WHY?
Offset pressrooms need air evacuation. 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 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. UV curable coatings are near 100% solids, 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.
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 in Aqueous Coatings
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 Curing: Conventional and LED-UV
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.
Conventional UV curing employs mercury arc lamps that produce a broadband output across UV-C, UV-B, UV-A, and into the infrared spectrum. While effective and widely established, mercury arc lamps generate significant heat, produce ozone as a by-product, and require exhaust ducting and cooling systems. A small amount of irritating, pungent ozone is produced especially during lamp start-up, and this must be evacuated from the press area.
LED-UV curing is a newer technology that has rapidly gained acceptance in the offset pressroom. LED-UV systems emit a targeted, monochromatic output, typically centered at 385 nm or 395 nm, focused entirely on initiating the curing reaction. Because LED-UV operates within a narrow wavelength band, it does not produce ozone, does not use mercury, and generates substantially less heat than conventional arc lamps. These characteristics significantly reduce the air evacuation burden associated with UV curing. LED-UV lamps are instant on, instant off, require no warm-up or cool-down period, and offer service life exceeding 20,000 hours. Energy savings of 50% to 70% compared to conventional mercury UV systems have been widely reported. Additionally, the reduced heat output of LED-UV makes it suitable for printing on heat-sensitive substrates such as plastics and films without distortion.
It is important to note that LED-UV inks and coatings require specific photo-initiator systems formulated for the longer wavelength output of LED arrays. Dual-cure formulations, designed to perform under both conventional UV and LED-UV systems, are available to simplify the transition. While LED-UV adoption continues to grow, conventional UV remains preferred for certain applications requiring complex ink sequences, heavy coverage, or robust scuff and scratch resistance across a wider spectrum of substrates.
Explaining Solvent Free
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 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 and polar covalent substances. Water is an inorganic compound, a solvent; it does not possess any carbon and is not organic.
Explaining Low VOC
VOCs are volatile organic (carbon) chemical compounds with a low boiling point. This causes a large number of molecules to evaporate into the air at room temperature. VOCs are varied, numerous, and everywhere. Particular photochemically reactive VOCs have been determined to be air pollutants, and are regulated by governments. For example, EPA publishes a list of VOCs by CAS number. An exception is Propylene Glycol, CAS number 57-55-6, which is not on that list; however, California considers it to be a VOC. Cork reports it as a VOC to meet all 50 states requirements. Most scents, smells, and odors are VOCs. Any offensive odors generated from printing and coating applications should be evacuated.
VOC regulations continue to evolve, and printers should remain aware of both federal EPA guidelines and state-level requirements. States may adopt their own Reasonably Available Control Technology (RACT) rules for offset lithographic printing that can be more stringent than federal standards. Heatset web offset operations with potential emissions of 25 tons per year or more of VOCs are subject to specific emission capture and control requirements. The use of low VOC compliant inks, coatings, fountain solutions, and cleaning materials remains a primary pollution prevention strategy alongside capture and control systems.
Explaining Low Odor
In the conventional offset printing process, inks dry by an oxidative process that can produce odoriferous by-products 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 being 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, as noted above, a small amount of irritating pungent ozone is produced by conventional UV curing mercury arc lamps, especially during start-up. LED-UV systems, by contrast, produce no ozone, thereby eliminating one source of objectionable odor and reducing the overall air evacuation requirement. 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.
Modern Drying Systems and Extended Delivery
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, 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. In LED-UV equipped pressrooms, the elimination of ozone extraction ducting and reduced cooling requirements simplify press design and improve overall air quality within the production environment.
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. Whether a pressroom utilizes conventional drying, aqueous coatings, traditional UV curing, or LED-UV curing, the management of air quality remains essential. Aqueous processes require the removal of water vapor, ammonia, and amines. Conventional UV processes require ozone extraction and heat management. Even LED-UV operations, while significantly reducing ozone and heat concerns, benefit from proper air management to address residual VOCs, fountain solution vapors, and general pressroom air quality.
The trend toward lower VOC materials, LED-UV curing systems, and more efficient air handling equipment continues to improve the pressroom environment. However, no single technology eliminates the need for thoughtful air evacuation planning. A well-designed air management strategy, tailored to the specific processes and materials in use, remains a cornerstone of productive, safe offset printing operations.
Cork understands that every pressroom is different. Whether your operation runs aqueous, conventional UV, LED-UV, or a combination of processes, our team of chemists and coating experts can help you select the right formulations to optimize drying performance, reduce odor, and support your air quality goals.
