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Overprint Coatings/Varnishes 

Overprint varnishes (OPVs) are coatings applied to an already printed sheet or web. The primary reasons for applying an OPV are gloss enhancement; stain resistance; edge fusion resistance; burnish or scuff resistance; and resistance to discoloration from absorption of impurities in the environment. There are three basic types of OPVs: oil based, water based, and radiation curable.

1. Oleoresinous (oil-based resin) coatings are similar to lithographic paste inks without any pigmentation. The resin systems are typically alkyds. Typical variables will be wax addition for scuff resistance, solvent content, and dryers for ease of setting or drying. Oleoresinous OPVs typically have high viscosity (4000 cps or more) and are applied through the ink train of an offset lithographic press.

Oleoresinous OPVs, like paste inks, set and dry quickly on Teslin substrate. Therefore, offsetting or blocking is not a problem when applying them to Teslin substrate. However, the amount of press varnish required is higher than typically required for paper.

To completely seal Teslin substrate's absorptive surface for stain resistance, the suggested amount of oleoresinous OPV to apply to is 3.0 lbs. per side, per ream (500 sheets of 25 x 38) [4.45 g/m2 per side]. This quantity has been attained in a single pass on press. A double bump of varnish may be required to achieve the recommended coverage. The amount of press varnish applied can be measured using a TMI pocket paper scale. Call TMI at (800) 678-3221, and request item number TMI 70-02-00.

Flint / VVW #76191 and Braden Sutphin #711V1800 Tuff Scuff are oleoresinous OPVs that have been successful in demanding applications. Other oil-based OPVs will work when applied in the proper amount.

Three simple tests can help determine if an adequate amount of press varnish has been applied:

I. 60-second coffee stain: Pour a few drops of coffee on varnished Teslin substrate. Leave for 60 seconds, then see if it wipes away cleanly. It should all come off.

II. 60-second linseed oil test: Pour a few drops of linseed oil on varnished Teslin substrate. Leave for 60 seconds, then see if it wipes away cleanly. It should all come off.

III. 60-second RIT dye test: Pour a few drops of RIT #15 black dye solution on varnished Teslin substrate. Leave for 60 seconds, then see if it wipes away cleanly. It should all come off.

2. Water-based or aqueous coatings typically have styrenic or acrylic binding systems. Viscosities of these OPVs range from 250 - 400 cps. Such coatings exhibit good holdout on Teslin substrate. A higher level of coating may be required compared to paper. Water-based OPVs can demonstrate brittleness on Teslin substrate, as well as on paper. Aqueous OPVs are generally clearer than oil-based OPVs, which enhance the look of a finished, printed Teslin substrate. Water-based coatings typically are dried with infrared heaters or hot air ovens. Maximum airflow and minimum temperature to achieve drying of the OPV will yield the best results.

Common water-based OPV application methods are:

I. Dedicated coating tower: Typically found on more expensive presses at large print shops.

II. Through-the-fountain system: This only works on Dahlgren-type, continuous dampening systems.

III. Conversions: Several companies produce add-on aqueous coaters that apply the aqueous coating on the last printing unit's blanket.

IV. Off-line: For example, a Bilhofer coater/laminator.

3. Ultraviolet (UV) radiation curable coatings offer excellent gloss and scuff resistance. UV curable OPVs can be applied either through the ink train (high viscosity coating) or through a dedicated coater (low viscosity coating). Radiation curable coatings are solvent-free and ideal for hard-to-dry substrates. UV curable OPVs, as the name suggests, are cured when exposed to UV light. Typically, lights that supply the energy required for curing range from 200 - 300 watts/in.

High viscosity UV curable varnishes Mira-Glos ® 2305 from Morton International and RCA03025R from Sun Chemical/GPI have been used successfully when used on dry offset/letterpress. These UV curable, press applied varnishes do not require a separate primer when placed over wax-free ink print coverage.

Due to Teslin substrate’s absorptive nature, a primer is required prior to applying a low viscosity UV curable OPV. The primer seals the absorptive surface and provides holdout of the low viscosity coating. The primer is usually a water-based OPV specifically designed to be used as a UV primer: no waxes, lower glass transition temperature, and higher surface energy.

To prepare the extremely porous Teslin substrate surface for a low viscosity, UV curable topcoat; a wax-free, oleoresinous varnish primer is applied on press via the printing unit's ink train. Braden-Sutphin # 201V11259 varnish/primer has been used successfully with Pierce-Stevens UV topcoat J-9387D. In addition, INX # C19325 as a UV primer followed by Pierce-Stevens J9328D UV coating has also been successful.

The printed substrate can be primed with an aqueous primer such as NiCoat 9200 UV primer or Cork Industries CKK 1250 primer. Mira-Glos ® 4400 UV topcoat has been applied over either of these primers. Primers for low viscosity UV top coats should be applied at a coverage of > 3.0 lb./ 25X38 in. ream (>4.45 g/m2) to achieve proper holdout of the UV varnish. High levels of photo-initiator in the UV OPV assist in raising the gloss level and lowering the brittleness of the OPV.

The machine direction/cross direction (MD/CD) orientation of Teslin substrate plays a role in the end use of the UV coated product. The product should be evaluated for proper orientation of the MD/CD prior to actual production. The key concern is the possible brittleness of a UV coating creating cracking and tearing with the finished piece.