Topping It All Off: Coatings on Paper and Printing
When deciding on the paper to use for marketing materials such as a brochure or sell sheet, many of our customers tell us, “I’d like a shiny paper; it looks so professional.”
We’re not sure how this association between shiny paper – which we printers refer to as coated paper – got started, but we have a theory. Full color printing requires a smooth, uniform paper surface and so is almost always done on a sheet that has had a coating applied during the manufacturing process. The purpose of the coating is to improve the way the surface of the sheet receives the ink, and it works! Full color printing on a coated sheet looks sharp and bright – in a word, professional.
The importance of paper surface
The surface of paper has a significant effect on the final appearance of a printed piece. For an image to appear sharp and true to color, the ink needs to stay on the surface of the sheet with minimal absorption into the fibers. Since ink dries by a chemical reaction, not by losing moisture, there is no advantage to its being absorbed – in fact, absorption can interfere with ink maintaining the proper size and shape of the dots used to create images and maintain color consistency (a process called dot gain).
The paper surface can be altered during manufacturing by sizing. Internal sizing is achieved by adding a solution of rosin, glue, gelatin, starch or modified cellulose to the paper pulp to make the paper less absorbent. External sizing treats the surface of the paper after it has dried.
Paper coatings
After paper is formed, it may have a coating applied to the surface. This is done to improve the paper’s properties – both decorative (such as whiteness and opacity) and functional (smoothness and absorption).
Paper coatings are a mixture of a pigment and a binder. The pigment is a substance suspended in water that dries to form a hard coating. (In paper coatings, the pigment color is white.) China clay, known as kaolin, is a traditional pigment; calcium carbonite is now more common. Other pigments include talc (for smoothness); silica (to improve water absorption for inkjet papers); titanium dioxide (for whiteness and opacity); and resins (to control the ink absorption rate).
The binder is a substance that holds the pigment particles in suspension and attaches them to the paper. Resins such as starch or latex are examples of binders.
Paper coatings come in three degrees of smoothness and hardness: dull, matte and gloss, with gloss being the smoothest and hardest. All three control dot gain (the tendency of images to spread, affecting the sharpness of the image and color consistency), so the main difference between them is how light is reflected. By reflecting the most light back to the reader, a gloss coating makes all images appear more distinct. However, gloss coating does have a disadvantage – it can provide a glare that tires the eyes.
Paper can be coated on one or both sides; this is easiest to see on cover papers. C1S is the designation for coated one side, while C2S means coated two sides. One type of coating, known as cast coating, creates a high-gloss, enamel-like finish by casting the coating paper against a highly polished, heated steel drum.
Coating the printed sheet
In addition to using a coated sheet to print, a second coating can be applied after the ink has been laid down on the sheet. The purpose of this coating is different than a paper surface coating. Now the gloss provides a contrast to the paper – it reflects back more light than the other areas of the printed sheet and so appears brighter. An after-printing coating can also be used to provide some protection to the printed surface against wear, fingerprinting and scuffing that can occur with repeated handling of the printed piece.
There are three main types of coatings used after printing:
- Overprint varnish acts like a solvent-based ink. It is usually colorless (though may be tinted for a desired effect). Varnish can be applied over the entire printed sheet, or only in spots to highlight a specific area by making it reflect more light. It can also be printed as a half tone (i.e., a series of dots) to provide subtle effects such as the appearance of dimension.
Varnish is available in satin, dull or gloss finish (indicating the amount of light it reflects back to the reader). Varnish made with tung or linseed oil may yellow over time and all varnishes emit volatile organic compounds (VOCs) during application. - Aqueous coating is, as its name implies, water-based. As with overprint varnish, it can be applied to the entire sheet (called all over aqueous coating) or just to specific locations. Aqueous coatings are available in satin, matte and gloss finishes. Aqueous coatings dry quickly, meaning the printed sheet can be handled almost immediately.
A distinct environmental advantage of aqueous coating is that it emits very low or negligible VOCs and can be recycled easily. Aqueous coating does not yellow on the sheet and has no residual odor. - UV coating is a liquid or paste applied to the printed sheet that remains in this form until exposed to ultraviolet light. UV coating contains photo initiators which react immediately to UV light to create a hard, protective finish that has high gloss and hardness. UV coating offers the greatest protection for the printed surface and because it reflects back the most light of all the post-press coatings, creates deeper, more vibrant colors and sharper images. UV coating does not emit VOCs.
Things to remember about coatings
When selecting the kind of coated paper to use for a job or post-press coating to apply, it is useful to know some of the drawbacks of the choices. Here are the ones we encounter most often:
- The resin binder typically used to surface-coat paper is sensitive to heat. So if you are printing shells or master sheets that will be imprinted at a later date, be aware that the high heat of a laser printer could cause the coating to soften and release the image from the sheet. Either ask us to test the sheet you are considering, or specify a laser-safe paper, where the surface of the sheet has been treated with UV light after being applied.
- If you are printing a direct mail marketing piece, be aware that not all mailing houses can address on UV or aqueous coating. Inkjet addressing systems require a special ink formulation and a dryer to address on aqueous and a separate ink-and-dryer to address on UV coatings. If we are handling your mailing, we will advise you, or check with your mailing house.
- All post-press coatings require contrast with the printed sheet to be visible. If you are using spot varnish, aqueous or UV coating – that is, adding the coating to selected areas of the printed sheet – we advise you to make the spots big enough to be visible. A hairline spot coating, for example, may provide too little contrast with the rest of the sheet to be noticed.
You can count on us
As always, we hope this discussion has provided a useful overview. If you have a specific idea in mind for using a post-press coating or aren’t sure whether to use a matte, dull or gloss coated sheet, give us a call. We’ll be glad to help.