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What Does UV Protection Really Mean?
When we discuss the qualities of overlaminate films the characteristic most often cited is the ability to protect printed graphics from ultraviolet (UV) radiation. Certainly, UV radiation is a major factor in causing graphics to fade, but it is far from the only one.
For one thing, all wavelengths of light -- including visible light -- affect laminating films, print media top coatings and printed images to some degree. Of course, there is nothing we can do about the effects of visible light. And optical clarity is clearly a desirable characteristic of an overlaminate film.
Image fade is also caused by industrial pollutants, oxygen, heat and water. Laminates provide some degree of protection from all of these elements, but that particular discussion is bigger than we have room for.
Consequently, we will limit our discussion to the surprisingly confusing subject of what UV protection means with reference to overlaminate films, specifically the pressure-sensitive or heat-activated materials used to shield wide-format graphics from the elements. A challenge arises in part because there is a considerable degree of ambiguous information currently in circulation pertaining to UV protection. (A less polite word for ambiguous would be erroneous.) Further confusion arises due to the terms used to discuss the topic. For example: What exactly is being protected from UV radiation, and where is the protective component located in the film?
Let's look at some of the claims first. If we were to analyze statements made by manufacturers and suppliers of overlaminate films, we would probably discern that the claimants fall into three basic groups.
1. The first group claims that "all" their laminates provide UV protection, based on the assumption that if you place anything over a digital image, it will offer UV protection "to some degree." Whether glass, animal glue, Scotch tape or whatever, they all impose a barrier between the image and the harmful UV radiation, or so the argument goes.
At times this misinformation is due to an "accidental" misinterpretation of the terminology used by manufacturers of base materials. Manufacturers of vinyls, polyesters and adhesives use terms like "natural UV inhibitors," "UV stable film" and "UV curing" in describing their products and manufacturing processes. But rather than ask awkward questions about what those terms really mean, the members of our first group remember only the term "UV" and use it wherever possible.
2. Conversely, group two has a UV sophistication level almost beyond belief. For me, personally, it actually is beyond belief, but you can make your own decisions. This group is found primarily in media-ink-laminate matched component system circles. Most of us accept the fact that some colors and inks are more susceptible to fading than others, but the matched component scientists take these claims a little further. They insist that their laminate UV protection package has been formulated in such a way that it will offer selective additional protection for designated "easy-fade" colorants. These colorants are found in their proprietary matched component system.
Furthermore, customizing UV protection to a particular media and/or ink would seem an almost unthinkable task given the many variables that are difficult, if not impossible, to control and predict. A few examples of these variables are color saturation, color combinations, varying amount of ink absorption due to humidity, adhesive coat weight differences, printer performance, etc.
The practice that verges on the irrational, however, involves a series of data measurements regarding UV inhibitive qualities. In this scenario, the measured data ranges from rather approximate to very precise, carried out, in fact, to several decimal places. I'm sorry, but if you add three values, and any one of them is approximate, then the total is approximate. It doesn't acquire additional credibility because one of the values is carried out to 10 decimal places.
I am not categorically dismissing the value of matched component systems. It is possible to enhance the degree of UV protection across particular light wavelengths ranges by selecting specific UV absorbers. Unfortunately, there are simply too many variables that come into play to make the kind of exaggerated claims we get in the marketplace. Until digital imaging technology matures and we can get some real history, it will be hard to beat group three's approach.
3.The third group looks for a good balance of science and common sense. They recognize that there are in fact two critical elements for comprehensive UV protection, and not just the UV "blocking" property we usually hear about. UV Absorbers. Absorption is the property we most often discuss. Ultraviolet absorbers such as Benzotriazole UVAs convert UV energy into harmless heat energy, which is dispersed through the film coatings. They protect the underlying media and image (inkjet, e-stat, photographic) from fading. This transformation process is regenerative and can be repeated indefinitely.
Well then, we might inquire, why not load the film full of absorbers for maximum protection? Simply because the UV absorber additive will cause discoloration if used in higher concentrations. It will also affect the adhesion/cohesion properties of the adhesive if added in sufficiently high quantities.
A principle we derive from this information is there are limitations on the amount of UV wavelength inhibition that an overlaminate can provide. UV absorbers are also expensive and the process of adding them to the adhesive can be complex and difficult, especially in aqueous adhesive (also known as water-based or dispersion coating).
UV Stabilizers. The less-often discussed factor in any UV protection scenario, UV stabilizers protect the individual components of the laminating film from degradation. Hindered Amine Light (HAL) stabilizers do not absorb UV light, but rather neutralize free radicals generated by UV light exposure. In a plastic film, free radicals can cause yellowing, embrittlement and loss of plasticity and clarity. Stabilizers can also aid in preventing resultant delamination of the laminate and have the ability to regenerate themselves in most circumstances.
Stabilizers offer no or little protection to the underlying media or the image printed on it. However, if the laminate itself breaks down, it most certainly loses its value as a protective barrier.
There are two components in a laminate film to which UV stabilizers and absorbers can be added: the adhesive and the base film. Because stabilizers and absorbers can only be added to the base film during manufacturing, a coater/manufacturer of over-laminating films has to carefully select the right grade of vinyl, polyester or polycarbonate base film. Adding stabilizers and absorbers to the adhesive component of the laminate can then further enhance UV protection. You might have some fun directing questions on these subjects to your film supplier.
Most Calendared PVC vinyls currently used in overlaminates for the digital imaging industry are monomeric vinyls. These vinyls are suitable for indoor and short-term outdoor use. Most monomeric vinyls contain some UV stabilizers, but rarely do they contain UV absorbers. The absorbers must then be added to the adhesive of the laminate for meaningful UV protection of the underlying media (image). Try getting a clear answer on that one.
Polymeric Calendared vinyl films utilize technically advanced higher molecular weight, branched chain plasticizer compounds. These are what you want for extended/long-term outdoor jobs. Polymeric plastic materials are more resistant to migration and leaching that are caused by external environmental conditions such as UV radiation, rain, pollution and temperature extremes. They also achieve flexibility over a wider temperature range and their stability prevents embrittlement, loss of clarity and other degradation effects. Because of the greater demands of outdoor applications, polymeric vinyl films usually already include some stabilizers and absorbers. But find out first.
Question: Does a laminate with a good UV protection package (absorbers and stabilizers) render a previously fade-prone image (thermal dye-based inkjet) fade proof?
No. It will certainly slow the fade process, but it is very difficult to predict a time frame.
Question: If I want an image to last at least one year outdoors, should I select a two-year outdoor laminate to compensate for a six-month outdoor media?
No. If your image needs to last one year outdoors, select a media, ink and overlaminate that are recommended for one year outdoors. The chain is only as strong as the weakest link.
Question: My digital output is guaranteed for six months outdoors without lamination. Will laminating this output offer additional protection?
Yes. As long as the stabilizers in the laminate render it suitable for outdoor use for at least six months, the film laminate will offer additional protection from rain, abrasion, temperature fluctuations, etc. Any UV absorbers present in the laminate will add to the fade protection.
Marc Oosterhuis is president of Drytac Group of companies in the United States, Canada and Europe. For comments and suggestions, he can be contacted by email at email@example.com. You can visit Drytac's web site at www.drytac.com.
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