Overprint coatings are used: to extend the life of a printed image; to provide aesthetic enhancements to the piece. To determine whether a print job on YUPO should be coated, you must fully understand your customer's expectations, end use requirements, life expectancy of the piece and budget. Choosing the most appropriate coating should strike a balance between its cost and the benefits.

Thoroughly discuss the details of the print job with your ink and coating suppliers. You must be certain that their products are compatible with and properly formulated for the unique properties of YUPO, and that they meet the end use requirements. Clear communication is essential among all parties involved.

Suitable tests for end use requirements should be conducted to analyze the effectiveness of a coating. This will require a thorough understanding of how the print job will be used. If imprinting or applying an adhesive to a coated print job, be sure the coating material is compatible. For best results, develop testing procedures that duplicate end use conditions.

Aqueous Coatings
As the name implies, aqueous coatings use water as the primary solvent system to produce a glossy, non-yellowing finish. As an overcoating, they provide rub- and abrasion resistance qualities ideal for a variety of end use applications. Aqueous coatings may also be used to prime YUPO's surface or ink films prior to printing or overcoating.

Overprint Varnishes
Overprint varnishes are essentially high viscosity, non-pigmented inks. They're used primarily for aesthetic purposes. For instance, they are used to prevent finger smudges or to provide a glossy or dull cast to all or portions of the printed piece. They also offer limited protection to a printed surface but are not usually selected for that purpose.

Primers
On YUPO, primers are used to improve the receptivity of UV inks and coatings, and to reduce the impact of gas ghosted images which may develop when printing two sides with heavy ink coverage. Common primers include aqueous coatings and varnishes.

Ultraviolet Curable Coatings
UV coatings are used to impart unique characteristics for a specific end use, such as gloss or resistance to the damaging effects of a particular chemical. Once cured, they become highly cross-linked, three-dimensional, non-yellowing films. They're extremely tough and scuff resistant, yet they are also very decorative too, adding smoothness and high gloss to any piece. For more information about UV inks and coatings, please refer to the section titled "UV Curable Inks and Coatings".

A. Aqueous Coatings

Introduction
The use of aqueous coatings is complex, so consult a qualified coating supplier before using them on YUPO Papers. It's also important to run a trial on YUPO before an actual production run to enable you to determine the ideal press conditions, coating weight and drier conditions.

1. Primer and Coating Qualities
Although not as glossy or as durable as UV coatings, aqueous coatings are less "temperamental" and can be printed over wet or dry ink films. For this reason, they're often used as primers for UV coatings. And since they are gas permeable, inks can dry under them when applied in-line.

2. Drier Conditions & Requirements
The coater's drying equipment must have a large enough capacity to evaporate the water and amine in the coating. Since YUPO is non-absorptive, it will not absorb excess water from the coating. This can soften the ink film, lengthen the drying time considerably and cause blocking in the loads.

Drier requirements depend on the coating method, the desired coating weight, the coating solids content and the production speed. In some cases, slower production speeds or running a lower wet coating weight are required for the coating to dry. Lower coating weights typically reduce the gloss level as well as the protective properties of the aqueous dry coating film.

For jobs using YUPO, a drier system that includes infrared driers, high velocity hot air impingement (air knives) and a well-vented exhaust system is recommended. Since adequate drying is the key to successful aqueous coating of YUPO, the drier's exhaust system must remove enough moist air so that a constant supply of fresh dry air is available to dry the coating as the sheets pass through. IR driers supply heat that accelerates the ink drying process beneath the aqueous coating.

3. Monitor Load Temperatures
A load temperature that's too high may cause the ink to pass through the coating and setoff. Load temperatures must be monitored closely when printing dark colored inks since they absorb more heat. If the sheets in the load begin to distort, be sure the inks are formulated for YUPO and contain a low solvent content (less than 3%). Then, adjust drier heat to reduce the load temperature. Production speed, coating weight and coating solids content may have to be adjusted to ensure the coating dries adequately. Lighter basis weight sheets are more susceptible to temperature distortion.

4. The ideal conditions for drying aqueous coatings on YUPO are:
* Maximize air flow of dryer
* Dryer air temperature 120~15Oº F (49~66º C)
* Dryer air relative humidity below 50%
* Reduce press speed by 10% or more
* Load temperatures 90~95º F (32~35ºC)
* Spray powder used in the delivery
* Reduce lift sizes to 3 inches
* Fan the loads if the pile temperature exceeds 95º F (35ºC)
Under ideal conditions, aqueous coatings should set almost immediately. Complete curing of the coating film will take longer.

5. Two-Sided Aqueous Coating
Aqueous coating two sides is difficult for any paper and is particularly challenging with non-absorptive materials like YUPO. If a narrow range of coating conditions is not met when applying an aqueous coating on the second side, the applied coating on the first side may soften and cause blocking.
Two-sided aqueous coating is not a routine job, and extreme caution must be exercised. It's essential to work closely with your coating/ink supplier to determine optimum conditions.

6. The following recommendations should be consistently followed:
* The topside must completely dry before coating the second side. The time required will depend on the degree of ink coverage.
* If the load temperature of either side exceeds 90ºF (32ºC), fan the load to reduce the load temperature.
* Apply less coating, with a lower viscosity, on the second pass.
* The coating film of the second pass should dry completely before sheets enter the delivery. The residual amine and moisture will soften the previously applied coating, causing setoff and blocking.
* Maximize the airflow on the sheets in the drier.
* Reduce the load size of the second pass. This will depend on the caliper or basis weight of the paper plus the level of ink coverage.
* Use a more evenly distributed spray powder—the same type used on YUPO.
* Anti-setoff powder is applied on both sides of aqueous coating to keep space and to avoid setoff.
To check for full drying, take a coated sheet of YUPO from the center of the pile. Apply moisture with a cotton swab and fold the sheet over so that the coating is face-to-face in the moist area. If the coating sticks to itself, additional drying time is needed before printing the backside.

7. How to Prevent Blocking
Follow these key points to prevent blocking:
* Avoid extreme conditions such as excess temperature, high humidity and high load pressure.
* Do not run heavy or excessive levels of coating or multiple coating applications.
* The highest gloss coatings may offer less block resistance due to their polymer composition and their minimum of wax and slip agent additives.
* Wait at least 72 hours, and preferably longer, for inks and coating to dry before running printed materials through binding and finishing operations.
* Conduct tests to determine the performance and production limits of the printing coating equipment.
* Consult qualified coating suppliers on the formulations and coating conditions that will resist blocking.

8. Effects of High Temperatures
Most aqueous coatings will perform adequately at temperatures up to 140ºF (60ºC) before softening and becoming tacky. When this occurs, coated materials that are facing may stick together, or other materials in contact may stick to them. Pressure may aggravate the tendency to stick. For example, if aquoues coated materials are stacked or boxed in cartons and shipped in a hot truck, blocking may result.

9. Effects of Various Liquids
The types of liquids that come in contact with an aqueous coating can have an impact on its performance. A liquid that raises the pH of the coating to an alkaline level (above pH 7) will soften the coating by partially solubilizing the coating resin. This will weaken the coating, reduce its rub resistance and shorten its effective life. For example, restaurant menus that are aqueous coated should not be washed with cleansers containing ammonia or other high pH (alkaline) cleaning liquids; they will soften the coating and expose the inks to potential rub-off. Test liquids that will come in contact with the coating during its end use to determine their compatibility.

10. Consult Your Suppliers
Pre-test inks for their compatibility with YUPO and aqueous coatings. Some additives used in offset inks can act as solvents with aqueous coatings and accentuate slow drying.

Off-line aqueous coating requires the use of wax-free inks. Waxes will bloom to the ink film surface over time, making it difficult to coat. Inks used for in-line coating may contain a low polyethylene wax concentration that increases rub resistance. Do not use microcrystalline, carnauba, paraffin, Fischer-Tropsch or Teflon® waxes. If in doubt, use ink certified wax-free by your ink supplier.

Several alkali-sensitive ink pigments will change color in contact with the amines and solvents present in aqueous coatings. The effects are compounded with YUPO Paper due to its alkalinity and non-absorptivity. The pH of the residual moisture in the coating may increase while in contact with the alkaline surface of YUPO. If the pH is raised high enough, it causes alkali-sensitive inks to change colors. In addition, the non-absorptive surface of YUPO does not allow residual amines from the coating to escape, causing the alkaline pH of these materials to change the ink's color. Prevent color changes by discussing the performance expectations of your finished piece with your suppliers.

Avoid using these pigments, in full color or tint strengths:
* Reflex-alkali blue
* Rhodamine reds (yellow and blue shades)
* Red lake C
* Lithol rubine
* Methyl violet

A. Overprint Varnish
Introduction
Compared to UV and aqueous coatings, varnishes are not considered as rub resistant, durable or glossy; when protective qualities impact the selection of an overprint coating process, their lower durability should be taken into account. Depending on their formulation, noticeable yellowing of the coating film may occur over time and affect the longevity of the print appearance.

1. Applying Varnishes
Varnishes are applied either as a solid covering the entire printed image or in localized areas ("spot varnish") for visual enhancement. Usually, varnishes are applied in-line, using the last printing unit of a multi-color press. They can also be applied off-line as a second pass through the press. Infrared driers may be used to accelerate the drying process. Spray powder must be used in the delivery to prevent setoff. However, this gives the final printed piece a rougher feel compared to properly applied UV and aqueous coatings.

2. In-Line varnishing
In-line varnishing is recommended for YUPO for a number of reasons. However, applying varnish in a second pass is not recommended since a second pass increases the risk of static and setoff problems and may give the printed piece an undesirable, sandpaper-like finish by varnishing over offset powder from the initial press run.

3. Primers: To reduce gas ghosting
Wax-free varnishes are often used as primers to reduce the effects of gas ghosted images that may form with two-sided printing of YUPO. When used as a primer, varnishes are laid down on the second side prior to overprinting the inks. The tack of the primer varnish should allow inks to wet trap.

4. Press Conditions
The same conditions for offset printing YUPO apply when using an overprint varnish. Varnish requires only half the amount of ink that's required to cover the same area. Greater care must be taken during handling to avoid setoff and blocking, since too much varnish can cause blocking. Finally, varnishing will increase a print job's drying time.

5. Formulation
Varnishes must be specially formulated for use on YUPO Papers, and they must be compatible with the inks used on the print job. Because they're similar to lithographic inks, varnishes must consist of high solids, contain less than 3% mineral oil and dry by oxidation.