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Coating RejUVinates LAQkluster Digital Prints With a little testing, UV and aqueous coatings can add "pop" to digital output. By Rob Andrews T he opportunity of adding pizazz to your on demand digital printing is out there for the taking, if you're willing to grab it. UV and AQ coatings have been used in the litho, fexo, and gravure printing industry for decades. Over the past couple of years, these coatings have been used in-line and off-line on digital printing as well. Digital prints, while versatile with what and how they can offer it, just don't have the "pop" needed to get noticed. That's where UV and AQ coatings come in. These coatings can offer various gloss levels, scuff resistance, and slip that the print alone can't. Depending on the coating you choose, you can get a super high-gloss UV coating, a scuff-resistant AQ coating, a matte UV coating, or anything in between. It all comes down to application, testing, and approval. left side of the card had an aqueous primer applied to it. Then, a UV coating was applied over the whole card. Where there was no aqueous primer (or toner) on the card, the UV coating absorbed into the substrate. So Many Choices Pop Goes the Graphic Relatively inexpensive offline application equipment is now offered by many different vendors. Generally, this equipment requires a low viscosity coating in order to be applied successfully. Lower viscosity coatings are typically better for flow, leveling, and— for AQ coating—dry speed. Lower viscosity UV coatings also tend to show improved adhesion characteristics over digital prints. This is due to the monomer (diluent) selection in the coating. Most UV monomers that are lower in viscosity tend to be mono- and di-functional. Having a lower functionality, UV raw material helps the UV coating to adhere to the print underneath for many reasons, including the UV coating cures slower and is less susceptible to physical shrinkage, which pulls the coating away from the print. Also, these materials tend to wet out and create stronger cohesive bonds to the digital toners. AQ coatings have one major advantage over UV coatings: physical hold-out. Some digital toners require a substrate that is fairly porous. This allows the toner to print and adhere better than it would to a substrate that has a good, solid surface. AQ coating polymers and resins are, by comparison, extremely large materials (>3,000 molecular weight). UV coating oligomers and monomers are, by comparison, very small materials (<500 molecular weight). When the coatings are applied over a porous substrate, the lowest molecular weight materials tend to absorb into the stock. When this happens, the net effect is a blotchy look. The illustration shows an example of this. The Getting a visual "pop" of gloss is easy for a UV coating, but not so for an AQ coating. On the flipside, getting a very low gloss matte coating is easy for an AQ coating, but not so for a UV coating. "Satin" coatings are easy for each to achieve—if you can define what "satin" is. Because a UV coating generally starts with a gloss in the 90s, a mid-point, or satin, would be about 50. A gloss of 50 is nearly the gloss level of a regular AQ coating. As such, a mid-point/satin gloss would be about 25 to 35. So identifying the desired gloss number or type of gloss level is extremely helpful in determining what coating you need. In the end, to create that successful visual pizazz, it all comes down to testing. Matching the right coating (or coating combination) to the application and digital toner, then testing that combo, is the key. Ask the questions and get the answers: • What coating viscosity can this piece of equipment handle? • Does the job require a high-gloss coating, a satin coating, or a matte coating? • Does the job just require physical protection? If so, from what? • Are there any other special circumstances that this piece will be subject to? Once you have the answers to those questions, the testing can begin. ◗◗ 20 QUICK PRINTING / July 2013 The left side of the card had an aqueous primer applied to it. Then, a UV coating was applied over the whole card. Where there was no aqueous primer (or toner) on the card, the UV coating absorbed into the substrate. Rob Andrews is lab director at ACTEGA Kelstar. For more information, visit MyPRINTResource.com/10314886. w w w. M y P R I N T R e s o u r c e . c o m