First and foremost both Aqueous (water-based) and UV coatings have achieved widespread use in the Graphics Arts Industry as competing top coats. Both offer aesthetic enhancement and protection, adding value to a variety of printed products.
Differences in Curing Mechanisms
Fundamentally, the drying or curing mechanisms of the two are different. Aqueous coatings dry when the volatile coating components (as much as 60% water) are forced to evaporate or are in part absorbed into a porous substrate. This allows the coatings’ solids to coalesce to form a thin, dry to the touch, film.
The difference is UV coatings are formulated using 100% solids liquid components (no volatiles) that cure or photopolymerize in a low-energy photochemical cross-linking reaction when exposed to intense short wavelength ultraviolet (UV) light. The curing process causes a rapid change, turning liquids into solids rather instantly (cross-linking) forming a tough dry film.
Differences in Application Equipment
In terms of application equipment, both low viscosity aqueous & UV coatings can be effectively applied using the last inker in flexo & gravure liquid ink printing processes. In contrast, web and sheet-fed offset litho paste ink printing processes require that a press-end coater be added to apply aqueous or UV low viscosity coatings. Screen processes also are used to apply UV coatings.
Flexo and gravure printing presses have the necessary solvent & aqueous ink drying capacity already installed to effectively dry aqueous coatings. Web offset heat set printing processes also have been shown to have the necessary drying capability to dry aqueous coatings. However, it’s another matter when considering the sheet-fed offset litho printing process. Here the use of aqueous coatings requires the installation of special drying equipment consisting of infrared emitters, hot air knives, and air extraction devices.
Differences in Drying Time
Extended delivery is also recommended to provide extra drying time. When considering the drying (curing) of UV coatings or inks, the difference is in the type of special drying (curing) equipment required. UV curing systems primarily supply UV light supplied by medium pressure mercury arc lamps, or LED sources with sufficient capacity to cure effectively at a required line speed.
Aqueous coatings are fast drying and attention must be paid to clean-up during any press stoppage. The difference is UV coatings stay open on press as long as there is no exposure to UV light. UV inks, coatings, and varnishes do not dry or plug anilox cells. There is no need to clean up between press runs or over a weekend, reducing downtime and waste.
Both aqueous & UV coatings can offer high transparency, and a range of finishes from high gloss, through satin to matte. The difference is UV coatings can offer a significantly higher gloss finish with a discernible depth.
Differences in Coatings
Aqueous coatings generally offer good rub, mar, and block resistance. Specially formulated aqueous coating products can also provide grease, alcohol, alkali, and moisture resistance. The difference is UV coatings typically, go a step further offering far better abrasion, mar, blocking, chemical, and product resistance.
Thermoplastic aqueous coatings for sheet-fed offset litho were developed to in-line wet trap over slow drying paste inks, minimizing or eliminating the need for spray powder used to prevent ink offsetting. Pile temperature needs to be maintained in the range of 85-95®F to avoid softening of the dried coating at higher temperatures, and the potential for setoff & blocking. Advantageously, productivity is improved as coated sheets can be further processed sooner.
The difference is UV coatings applied in-line wet trapping over UV inks are both cured at press-end, and sheets may be further processed immediately. When UV coating over conventional litho inks is considered aqueous primers are recommended to seal and adhere to the inks to provide a base for the UV coating. Hybrid UV/conventional inks can be used to negate the need for a primer.
Influence on People, Food, and the Environment
Aqueous coatings offer clean air, low VOC, zero alcohol, low odor, non-flammability, non-toxicity, and non-polluting properties. Similarly, 100% solids UV coatings produce no solvent emissions, zero VOC’s, and are non-flammable. The difference is wet uncured UV coatings contain reactive components that may have a sharp odor, and can range from slight to severe as irritants, which can cause allergic reactions in some people. Skin and eye contact should be avoided. In a positive note, UV curables are designated as “Best Available Control Technology” (BACT) by EPA, reducing VOC’s, CO2 emissions, and energy requirements.
Aqueous coatings are susceptible to consistency changes throughout a press run due to the evaporation of volatiles, and Ph influence. The difference is 100% solids UV coatings maintain consistency on press as long as there is no exposure to UV light.
Dried aqueous coatings are recyclable, biodegradable and repulpable. The difference is while cured UV coatings are recyclable and repulpable, they are slower to biodegrade. This is because curing cross-links coating components,
producing both high physical and chemical resistant properties.
Aqueous coatings dry with the clarity of water with no aged-related yellowing. The difference is that cured UV coatings can also exhibit high transparency, but care must be taken in formulating because some raw materials can produce yellowing.
Aqueous coatings are able to conform with FDA regulations for both dry and/or wet greasy food contact. The difference is that UV coatings, with the exception of very limited specific formulations, are not able to conform with FDA regulations for either dry or wet/greasy direct food contact.
Aside from differences, aqueous & UV coatings share many benefits to varying degrees. For example, particular formulations can offer heat, grease, alcohol, alkali, and moisture resistance. Additionally, they can offer gluability or glue resistance, a range of COF, imprint ability, hot or cold foil acceptability, the ability to protect metallic inks, increased productivity, in-line processing, work-and-turn capability, energy savings, no set-off, and in sheetfed offset the elimination of spray powder.
Our business at Cork Industries 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/coater a competitive advantage.