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FLEXO Magazine : July 2009
TECHNOLOGIES & TECHNIQUES Since these basic properties are related to each other, you may need to consider one or more of these characteristics when developing an application. Generally, within a given chemistry, as you design for higher shear, there is a decrease in peel. An application that requires a high shear adhesive may also benefi t from low tack characteristics. Shear is a property that measures the internal strength of an adhesive in the same plane as the bond. A label backed with a low shear adhesive may slide and fall off over time. In contrast, a high shear adhesive stays in place. Shear testing methods fall into one of two basic categories: dynamic or static. Of the two, the static method is the most popular. This often involves applying a fi lm with a specifi ed area to a surface, and then hanging a weight off of the material. Depending on the application, this test is used to examine time to failure or distance moved in a specifi c time. Outdoor power equipment OEMs and converters often report a bubbling beneath logos and warning labels and it has been determined that, more likely than outgassing, the occurrence is either challenges during the label application process or a product performance issue. By exploring both possibilities by observing how OEM workers applied the labels, one can fi nd several inconsistencies in technique, including how the release liner is removed, how the label is placed on the surface and whether it is smoothed out by hand or a squeegee. It should also be observed how the current label material and control products functioned during the same application process. Along with human nature, application success also fl uctuates depending on the size of the label and the thickness of the material it was printed on. The larger the label and the greater the application surface, the more likely air will get trapped and bubbles to occur. While ease of application is essential, aesthetics and durability are critical components when it comes to outdoor equipment nameplate/brand identity, for bad labels project a poor image to the consumer. This kind of application requires an exceptionally high-shear adhesive for bubble-free label graphics. Within the realm of pressure-sensitive adhesives, shear plays a big role when it comes time for mounting nameplates, product identifi cation labels, and safety/hazard/instructional labels. If the fi lm is backed with a low-shear adhesive, the adhesive may fl ow, causing the label to slide or move from its intended position. This can diminish the aesthetic value of the product. If the adhesive fl ows out from under the label, dirt or dust may adhere to the adhesive, creating an unattractive halo. In a worst-case scenario, an identifi cation or warning label may slide off enitrely. Another example: When applying a label to the curved surface of a vial, storage tank or drum, shear again comes into play. Curved surfaces can lend themselves to fl agging, which is usually a low-rate (slow) process. When fl agging occurs over time (versus immediately), it is most often because the level of shear does not meet the application need. When trying to identify a shear-related fl agging failure, look for an adhesive split (where the adhesive remains on both the surface and the label) or a visible legging phenomenon (where the adhesive stretches before snapping back to the label or the surface). While it is not solely a phenomenon related to curved surfaces, a high-shear adhesive will resist fl agging and many other associated types of label failure. And that’s just the type of performance retail or point-of-purchase (POP) applications will need. Other, high-shear, silicone-based adhesives can also resist extreme temperatures. These products are ideal for high and low temperature gasketing and sealing, vibration damping in a wide variety of temperatures, and applications calling for limited fl ammability in the automotive, aerospace, industrial and fl exible electronics industries. The products operate at temperatures from -300°F to +500°F. It is in such applications that tack, another important component of adhesion, comes into play. Tack refers to the value of measuring an instantaneous bond in applications where there is a need to create a bond as quickly as possible. High-speed labeling applications, for example, require excellent initial tack. Without that strong, immediate bond, labels can misapply. In other instances, you may want a low tack adhesive, such as in the case of graphic advertising for fl oors. Safety and warning labels must remain intact for the life of the tool. 28 FLEXO JULY 2009 www. f le xography. org
Sustainable Spring 2009