Paint & Coatings Industry - April 2008 - (Page 41)

Coatings is Now! Pigments Pigments are solid particles that provide color and gloss control and remain insoluble in the binder. Typical pigments include: inorganic, organic and metallics. FIGURE 1 | Curing illustration. 100% Solids Layer of Paint Application A B Flash Period C Curing D Additives Chemical additives are included in the paint formulation to provide unique characteristics and to improve the paint performance. Typical additives include catalysts, anti-blocking agents, defoamers, flow-control agents, gloss modifiers (high or low), storage stabilizers, UV-blocking agents, dispersants and rust inhibitors. Thermal vs. Radiation Curing Formulated UV-cured coatings can be looked at similarly, by replacing the words resin, solvent and catalyst with the words oligomer, monomer and photoinitiator. Oligomers provide bonding, pigment encapsulation, flow-out and the physical properties of the coating. Like solvent, the monomer reduces the viscosity, but also provides some properties similar to the oligomer, plus flexibility and crosslinking. Monomer addition in most cases reduces the viscosity to a level that makes roll coating or rotary atomization an effective application method. For easier application or better flow out, UV coatings can be formulated at less than 100% solids by adding solvent or water. In this way, a conventional or HVLP spray gun can be used for application. The photoinitiator acts as a catalyst to initiate the chemical reaction. Monomers and oligomers are converted to polymers, and crosslinking occurs during this free radical reaction. A B C D W et paint Wet P aint s prayed on parts P arts enter oven P arts exit oven 35% s olids 55% s olids 80% s olids 100% s olids • Chemical curing – In this case, the system contains more than one polymer. One is a backbone polymer such as an alkyd or an acrylic and the other a crosslinking resin, such as melamine. Heat and a catalyst (internal or external) create a reaction, which causes the backbone polymer and the crosslinking agent to crosslink into a paint film. • Emulsion-type film formers – These are similar to lacquers in that evaporation of the water (and coalescing solvent if present) results in coalescence of the polymer particles to form a paint film. These emulsions are usually acrylics; however, polyesters, polyurethanes, alkyds and epoxies are also available. All of these polymers and the way they cure have one thing in common: upon curing, the solvent or water evaporates and the resin, additives and pigment remains, which can be defined as a 100% solids layer of paint. Graphically, this can be represented, as shown in Figure 1.1 In comparison, the UV-cured coatings use a different method to form a cured paint film. When exposed to ultraviolet light, free radical-curable film formers crosslink into a paint film within seconds. These formulations are typically 100% solids, but also can be made with 5-50% solvent or water (see Figure 2). Curing Another way to distinguish between UV-cured coatings and solvent/water-based coatings is by the different types of polymers and how they cure. Solvent or waterborne polymers are often characterized based on how they form a paint film. • Lacquer – A lacquer is based on a synthetic thermoplastic film-forming polymer dissolved in organic solvent. It forms a paint film solely by solvent evaporation. • Auto-oxidation – Polymers containing carbon-carbon double bonds that add molecular oxygen across the double bond to form a crosslinked paint film. Alkyds are typical of this type of reaction. Radiation Curing Radiation-curable coatings contain various accelerators or catalysts that are dormant, until exposed to UV light. The absorption of UV wavePA I N T & C O A T I N G S I N D U S T R Y 41

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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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