Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 178 of 331

gen.8 Initially, photoinitiators (UV sensitizers) were not as efficient as those available today. Early UV-curable coatings required the use of nitrogen blan- keting to achieve adequate cure speeds. Current photoinitiator chemistry has rendered nitrogen blanketing obsolete, save for special applications. Advances in curing efficiency coupled with highly UV-reflective materials in curing cham- ber construction have effectively eliminated a primitive UV-curing issue: line- of-sight cure. (See Fig. 2.) Of special note, UV curing is defeated by the addition of solvents to the coat- ing by production line operators. This practice, stemming from historical use of solvents and retarders with solvent-borne coatings, is the cause of such addi- tions being made to UV-curable products. Attempts to extend coverage by adding solvents only causes problems on UV-processing lines. It must be kept in mind that one 1.0-mil coat of solvent-free, 100%-solids coatings is the same as two 1.0-mil coats of 50% solids or four 1.0-mil coats of 25% solids. In UV-curable coatings, solvents, such as mineral spirits, alcohol, glycol (butyl), and the like, confound the advantages of near-instantaneous UV curing by im- posing a flash-off (drying) time. Any such solvent addition, which does not flash- off and remains in the coating during the actual cure, will cause at least one or a combination of the following to happen: 1.Retard/inhibit the UV reaction, which may cause problems that can masquerade as light source/equipment issues; 2.manifest finish flaws, such as orange-peel, bubbles, and so on that may be mistaken for equipment issues; or 3.autoignition of solvent fumes since UV lamps operate above 1,500°C, well above the ignition temperature of most solvents. Rather than using solvents to control coating viscosity, UV-curable coatings should be formulated to be used within an optimal viscosity and temperature range without the need to use solvents as a viscosity control. Properly formulated UV-curable coatings should never need to be diluted with solvents. Reaction Termination As the number of available reactive acrylic sights is consumed during polymerization, the chain reaction shuts down. Once the reactive sites are polymerized the photoinitiator fragments neutralize by recombining. The photoinitiator is either trapped within the polymer or be- comes part of the polymer if it is a functionalized (polymerizable) photoinitiator. COMPONENTS OF UV-CURABLE COATINGS UV-curable coatings are comprised of the following basic components: l.Photoinitiators—Absorb UV light and start the polymerizing free-radical reaction 2.Acrylic resins—Primary resin family of UV-curable materials. Monomer are low-molecular-weight acrylates. This component contributes physical characteristics such as hardness, softness, elasticity, etc. Monomer resins are not to be mistaken for raw material monomers, which are used to make the acrylate resins, i.e., acrylic acid. Oligomers are high-molecular- 177

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