Paint & Coatings Industry - April 2008 - (Page 42)

The Future of UV Coatings is Now! lengths triggers a free-radical reaction among chemical groups that, within seconds, results in crosslinking (curing). Specifically, this chemical process (chain reaction) forms a polymer from monomers and oligomers. UV cure may be accomplished in enclosed chambers saturated with high-intensity electrically generated UV light. The enclosed chamber is required because the presence of oxygen inhibits the curing of free-radical-initiated systems. The oxygen must be overcome with nitrogen or another inert gas. Due to the high cost of enclosed chambers, special formulations are commonly used to overcome the effects of oxygen that are typically present during curing. Today, UV systems rarely have enclosed chambers. With regard to coating shrinkage, thermoset coatings are crosslinked with urea and/or melamine resin, which co-react resulting in only about a 1% shrink. In comparison, the reaction of free radical polymerization in UV coatings results in about a 20% shrink. When formulating, the chemist must compensate for this shrinkage. In UV coatings, these accelerators or catalysts are called photoinitiators. For total curing to take place, the UV light must activate or “see” all of the photoinitiators. As a result, this imposes some limitations on the dry film thickness (~1 mil DFT) of the UV coatings, especially those containing pigments that can be UV-cured by a single pass. The pigment molecules act to absorb and reflect or block the UV light from reaching some of the photoinitiators. As expected, black-pigmented coatings as well as dark colors are the most difficult to cure because of their characteristic to absorb light waves. The lighter, brighter pigments, in general, can be more readily cured. FIGURE 2 | Free-radical curing. UV light energy decreases with the square of the distance between the light source and the surface receiving the light. Therefore, the UV light source is kept as close to the coated part as possible. For this reason, UV cure is easier on flat surfaces. However, highly polished parabolic reflectors, strategically positioned light sources and/or rotation of the substrate (part) enable certain three-dimensional parts to be successfully UV coated, such as bicycle frames and hand tools. UV curing occurs in a matter of seconds, which permits compact paint systems. The quick cure also reduces foreign particles on the surface and minimizes substrate heating, a great advantage for heat sensitive substrates. Energy Sources Wavelengths UV light used in curing is in the spectral range between 240 and 450 nm (nanometers). This requires using photoinitiators to initiate polymerization within these wavelengths. Typically three distinct ranges of wavelengths can be achieved with the various light sources. A range of 240 to 320 nm (UVB and UVC) is obtained using an “H” bulb. This is a Mercury (Hg) “doped” bulb, which emits wavelengths that are shorter and primarily provide surface cure. These bulbs are typically used for nonpigmented, clear UV coatings. A range from 320 to 400 nm (UVA) is obtained using a “D” bulb. This is an Iron (Fe) “doped” bulb, which emits medium wavelengths. A range of 400 to 450 nm is obtained using a “V” bulb. This Gallium (Ga) “doped” bulb emits longer wavelengths that can penetrate below the surface of the coating, down to the substrate. This provides superior adhesion and is necessary to fully cure pigmented coatings. UV Lamps Free Radical (UV) Curing UV Radiation Source Molecular Chains Propagate Initiation of the Reactive System Chain Reaction (Curing) Absorption Highly Cross Linked Film UV lamps are controlled discharge devices that generate electromagnetic energy, giving off infrared and visible light, as well as ultraviolet radiation. The primary types of UV lamps are (a) electrode (medium pressure mercury arc lamp) and (b) electrode-less (microwave powered). Other UV lamps that are available include low-pressure mercury arc lamps (fluorescent) and high-pressure xenon arc lamps (both electrode type) and lasers, which are electrode-less. Photo initiator (Catalyst) Photochemical Reaction 42 Free Radicals Formed Measurement Parameters Every UV-curable coating specification should include parameters for the amount of illumination onto a surface and a timeframe.2 The key parameters are as follows. APRIL 2008 | W W W . P C I M A G . C O M http://WWW.PCIMAG.COM

Table of Contents Feed for the Digital Edition of Paint & Coatings Industry - April 2008

Paint & Coatings Industry - April 2008 Contents Viewpoint Industry News Calendar of Events Company News Mergers and Acquisitions Names in the News Snapshots of Success – Automotive Applications Designing UV-Curable Materials for High-Temperature Optical Fiber Applications The Future of UV Coatings is Now! Milling Media Review Part 1 – Bead Density Effect Preparation and Properties of UV-Curable Polyurethane Acrylate Resins for Metal Surfaces Conquering Color Harmony High-Performance PU Crosslinkers for Two-Component Waterborne Applications Decorative Film Laminates How Long Will the Gilding Glitter? Gaining a Competitive Advantage Using Benchmarking Supplier Showcase Materials Watch Products and Literature Classifieds Advertiser Index Automotive Color Trends New England Museum Solves Century-Old Vanderbilt Mansion Mystery

Paint & Coatings Industry - April 2008

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