Industrial & Specialty Printing - July/August 2012 - 24

Figure 4 Use a UV radiometer regularly when printing UV-cured PSAs.

adjusted accordingly. A UV radiometer (Figure 3) should be used frequently to read the irradiation energy of the UV lamp(s) (Figure 4), or a UV integrator can be installed inline. This ability to control the cross-linking process is an advantage for the printer, who now has flexibility to select from a range of PSA performance characteristic, depending on the dose given. The printer can choose a low dose, creating a soft, tacky PSA or a high dose, producing a more cohesive PSA, resulting in higher shear strength. Familiarity with the available printable PSA systems enables confidence in pairing adhesive to application. paIrIng applIcatIon and adhesIve The broad range of industrial graphics supplied to consumer markets is roughly represented as follows: • Automotive and electronic instrument control panels • Membrane-switch keypads • Home appliance • Aviation panels • Automotive interior trim • Cell phones • Foam padding • Audio equipment • In-store marketing • Permanent/temporary displays • Ink transfer for sporting equipment and vehicle decoration • Labels and stickers for indoor and outdoor use. Pressure-sensitive finishes can be applied to a huge variety of substrates. Cardboard, metal foils, coated films, and all sorts of plastics, formed in a myriad of shapes. Depending on the end use requirements, an adhesive is matched to the substrate and the specifications. The industrial manufacturer is seeking the best bond to the substrates required by a given application. The adhesive supplier will usually provide adhesion values which vary by substrate for a given adhesive (Figure 5). Printable PSAs are commonly used as alternatives to 2-5 mil transfer tapes. The

materials, newer printable water-based adhesives are making production easier. This new generation adhesive prints smoothly, almost without air bubbles. Though they still require longer drying times with good air movement compared to solvent-based formulations, the new generations of water-based PSAs are faster drying. Solvent based acrylics typically range from: -20/-40°C up to +60/+80°C (or -4 /-40°F to +140 /+176°F) in temperature tolerance. Their light (ambient and UV) resistance makes them ideal for exterior applications. Initial bond strength is not as high as with the rubber based adhesives, but within a few hours it exceeds the values of the rubber-based adhesives. Rubber Adhesives (typically synthetic rubber) adhere to almost anything. They have very high initial tack and peel strength, combined with excellent humidity/moisture resistance. The range of temperature tolerance is medium, from (-20/-30°C to 82°C) or -4/-22°F to +180°F. They are economical, and near transparent, though darkening when exposed to oxygen and light. Solvent-based printable PSAs are typically available in rubber or in acrylic resin. They differ in property profile, as indicated above, and price. Both product groups have very high water and steam resistance. They

both have good printability, very good surface flow and extraordinarily short drying times. After a short time, release liners can be applied or other processing steps can be carried out. Since they contain solvents, regulatory issues with employee exposure and VOC emissions may be a factor. UV-curable printable PSAs achieve film formation via chemical reaction (crosslinking), when exposed to UV light after printing. They have a temperature resistance of approximately -30°C to +85°C or -22°F to +195°F. Industrial manufacturers appreciate UV curable PSAs for their relatively problemfree application. Higher production speeds and shorter cycle times are possible. They have great printability, excellent flow, fast curing, and environmentally friendly processing. Even with the finest mesh, there is no danger of the adhesive drying in the screen. These formulations exhibit very good water and aging resistance and high resistance to yellowing. They are compatible with most printing inks. Improved process control is required to ensure product quality. It’s critical to coordinate intensity and the spectrum of the UV light source with the absorption of the photo initiator, to reach optimum cross-linking. The UV light source has to be tested at regular intervals to detect falloff /performance loss and the line speed

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Industrial & Specialty Printing - July/August 2012

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