— Paper Coating Types —
Paper Coatings – UV, Aqueous and Varnish Coatings Used in Commercial Printing
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Whether it’s applied by a printer or a finisher, the right protective coating or laminate can keep your printing project looking professional.
Even though protection may be necessary on a marketing piece it is often overlooked. While a great deal of thought may have gone into a project for the printing press, much less attention has been paid to preserving the artwork after it leaves the printer. Aside from using a varnish to highlight a photo, designers often give little thought to the use of protective coatings or laminates. And clients may not ask about them at all.
Liquid coatings can be applied in-line by the printer as part of the printing process or off-line after the project leaves the press. Some coatings, such as varnish, can be spot applied to a precise point or points on the page such as just to the photos, for example. Other coatings, including aqueous coatings, are usually flooded across the entire sheet. Different coatings are available in different finishes, tints, textures and thicknesses, which may be used to adjust the level of protection or achieve different visual effects. Areas that are heavily covered with black ink or other dark colors often receive a protective coating to guard against fingerprints, which stand out against a dark background. Coatings are also used on magazine and report covers and on other publications that are subject to rough or frequent handling.
Liquid coatings are by far the most common way to protect print publications. They provide light to medium protection at a relatively low cost. Three major types of coatings are used:
Varnish coatings are available in gloss, satin or dull finishes, with or without tints. Varnishes offer a relatively low degree of protection compared to other coatings and laminates, but they are used widely, thanks to their low cost, flexibility and ease of application. Varnishes are applied just like an ink, using one of the units on the press. Varnish can either be flooded across the entire sheet or spot applied precisely where desired, to add extra gloss to photos, for example, or to protect black backgrounds. Although varnishes must be handled carefully to prevent the release of harmful volatile organic compounds into the atmosphere, when dry they are odorless and inert.
In addition to providing relatively little protection, varnishes have other drawbacks too. One problem is that over time, they tend to yellow. Yellowing is not a big concern when the varnish is used over process colors, but it is noticeable when the varnish is applied over unprinted paper, especially today’s high-brightness blue-white papers. And for some reason, silk, dull and matte coated papers tend to reveal yellowed varnish much more than their gloss-coated counterparts. In an attempt to counteract the yellowing, some printers will add a small amount of opaque optical whitener to the varnish that will be applied over white paper, but it is a less than perfect solution.
Varnishes also require the use of printers’ offset spray powder to keep the printed sheets from sticking together before the varnish is completely cured. The powder that is left behind can affect the look and feel of the finished piece, an especially important concern in fashion catalogs and other publications where appearance is everything.
Low cost water based aqueous coatings are among the most commonly used coatings available today and provide good protection from fingerprints and other blemishes. Like varnishes, aqueous coatings are applied in-line on press, but they are shinier and smoother than varnish, have higher abrasion and rub resistance, are less likely to yellow and are more environmentally friendly. Aqueous coatings dry faster than varnishes too, which means faster turnaround times on press.
Available in gloss or dull finishes, water based coatings offer other advantages as well. Because they seal the ink from exposture to the air they can help prevent metallic inks from tarnishing. Specially formulated aqueous coatings can be written on with a number two pencil, or overprinted using a laser jet printer, a key consideration in mass mail projects.
Since they are less likely to yellow, aqueous coatings provide a good alternative to varnish, especially when it comes to protecting projects that feature large amounts of white space. However, it is difficult to apply a spot aqueous coating with the same degree of precision that is possible with a varnish, which is why aqueous coatings typically are flooded across the entire sheet. Because the coatings are typically applied over the entire sheet and are water based, most experts recommend using 80# text weight or heavier paper stocks to keep the paper from becoming curled or wrinkled. Aqueous coatings can be used in conjunction with either varnish or UV coatings, but doing so can be costly, and unless production is managed carefully, the coatings may not dry or lay right.
Aqueous coatings and UV coatings are also susceptible to chemical burning. In a very small percentage of projects, for reasons not fully understood, certain reds, blues and yellows, such as Reflex Blue, Rhodamine Violet, Purple and PMS Warm Red, have been known to change color, bleed or burn out. Heat, exposure to light, and the passage of time can all contribute to the problem of these fugitive colors, which may change at any point from immediately after the job leaves the press to months or years later. Light tints of colors, made using a 25% screen or less, are especially prone to burning.
To help combat the problem, ink companies now offer more stable, substitute inks that are close in color to ones that tend to burn, and these inks are often used to print light tints or bright colors. Even so, burning can still occur and dramatically affect the look of the project.
Extremely high gloss UV, or ultraviolet, coatings offer more protection than either varnish or aqueous coatings. UV coatings are applied in-line by printers or, more frequently, off-line by printers, finishers or converters. UV coatings are applied as a liquid, using a roller, screen or blanket, and then exposed to ultraviolet light to polymerize and harden the coating, with zero emissions. The coatings can either be applied across the entire page or, while lacking the precision of a varnish, on a spot basis. The coatings are available in a high gloss as well as matte, satin and a wide variety of specialty finishes, including glitter and tints, and even different scents.
When first introduced, UV coatings were often perceived as murky and yellow. Those problems have been corrected, but the coatings still have other drawbacks. Like aqueous coatings, UV coatings are susceptible to chemical burning. UV coatings also are more likely to show fingerprints than either aqueous coatings or varnish, and some UV coatings can make paper difficult to fold. Dwell time, viscosity, temperature, the intensity of the UV light source and the interaction between the coating and the paper can all cause problems too. If conditions aren’t right, the coating will not have a chance to spread evenly across the page before the coating is cured, and the finish can develop an “orange peel” look.
UV coatings also tend to accentuate the roughness of, or any defects in, the surface of the paper. Some printers insist that UV coatings require the use of coated paper stocks because uncoated papers allow the coating to sink into the sheet, leaving little of it on the surface. And in the past, most printers said that UV coatings should be used in conjunction with UV inks and that if conventional inks were used, they had to be wax free and allowed to dry completely before the coating was applied. However, new hybrid inks help reduce the potential for drying and surface problems.
Film laminates are usually applied by finishers or converters that also offer die cutting, embossing, foil stamping and other such services. The film may be applied using either a wet method, which relies on solvents, water or both, or the more environmentally friendly thermal method, which uses heat to iron the film and paper together. Either way, the entire sheet is generally laminated there is no practical way to spot laminate a project.
Lamination films are available in a variety of tints and textures, and there’s even a lenticular film designed to help create a holographic effect. The films are classified by thickness, which is measured in mils, or thousandths of an inch. The thinnest films, typically around 1.2 mils, are used on items that are rolled or folded. Heavier films, of up to 10 mils, leave a heavy plastic coating on the sheet that can stand up to anything short of a small but determined Rottweiler. The laminates can be applied on one side or both sides of the paper, and with a sealed edge, which makes the sheet virtually waterproof.
Film laminates offer much more protection than liquid coatings in exchange for longer production times and higher costs. Heavily textured stocks, however, may be difficult to laminate because the film cannot reach down into the valleys of the paper.
The most popular and least expensive laminate, polypropylene is available in gloss, satin and matte finishes. Polypropylene provides a softer finish than other laminates, which makes it the best choice for projects that will be folded, but it is more prone to scratching than the other laminates. It’s possible to write on polypropylene films using dry erase and some other types of markers. Polypropylene is a good bet when you are looking for increased strength and good protection at a reasonable cost.
The stuff of leisure suits and the most durable of all laminates, polyester offers the greatest strength and abrasion resistance, at a cost that is usually higher than polypropylene. Polyester provides a hard coating, extremely resistant to scuffing and tearing, in gloss, satin or matte finishes. When a polyester laminate is used to produce folders or other materials that need to be foil stamped or glued, a special glueable laminate should be used.
The most expensive and most stable of the laminates, nylon offers a unique advantage when thermal laminating is used. Unlike polypropylene and polyester, nylon does not stretch when it is heated, which means that it will not later shrink as it cools and cause the paper to curl. (This phenomenon can often be seen in the covers of perfect bound paperback books.) Nylon laminates are available in a variety of finishes.
The Thicker They Come…
The two major families of protective finishes both have their strengths and weaknesses. But the key to remember, as Tony Soprano might say, is that “protection costs yez.” Whether you’re talking about bodyguards or protective coatings, the thicker and stronger they are, the greater their price. Coatings that are applied in-line, while the job is running, are generally the least expensive, and have the least impact on finishing and shipping or mailing costs. However, they can be hard pressed to withstand a thumbnail drawn across the page or other common assaults.
Laminating provides what is literally a whole new layer of protection, but it requires extra time, a larger budget and careful coordination between the printer and the laminator. If sealed edge lamination is used, sheets must already be trimmed to final size before they are laminated. Conversely, die cutting, embossing, folding and scoring should all be done after the paper is laminated.
Taking it on the lam presents other challenges too. Heavier laminates can dramatically increase mailing costs, cannot be recycled like conventional paper, and have a big impact on the look and feel of the publication. Ultra high gloss laminates can make any text that appears beneath them as difficult to look at as Waterworld.
What’s more, inks and laminates sometimes don’t mix. Inks and varnishes used beneath lamination must be free of waxes, silicone and other agents that are often used to make ink flow more smoothly. If an aqueous coating will be laminated, an aqueous primer should be used. Conventional metallic inks offer very limited adhesion to film lamination. Digital printing may require the use of a special film to handle the oil content of some of the toners used in digital printing.
What’s Right for You?
Laminates offer the greatest protection and are unbeatable in a variety of applications, from tourist maps to the menus in your local pancake house. But with their greater weight, time, complexity and expense, laminates are typically not suited for projects with extremely large press runs, limited life spans or short deadlines. And if laminates are used, there may be more than one way to achieve the desired results. Combining a 5mil film with a heavier paper stock produces the same stiff, heavy duty feel as 10mil laminates over a lighter stock, at a lower cost.
If you can’t decide, remember that the two types of finishes can be used together. A spot matte UV coating, for example, could be applied over a gloss laminate, although the smoothness of the laminated surface could make it difficult to maintain register as the project moves through the coater or press. The key is to work with your printer and finisher. Discuss the effect you’re after and the level of protection you need to provide. Then consider how to best apply your budget to reach those objectives. If the project will be laminated, make sure to factor in additional time and, often, additional costs in shipping and mailing.
Use Coated Paper
No matter what coating you use, the results will look best on coated paper. That’s because the hard, nonporous surface of coated paper holds the liquid coating or film on the top of the paper, without allowing it to run into the valleys found in the surface of uncoated stocks. This superior holdout helps ensure that the protective finish will go on smoothly. The smoother the surface, the better the quality.