Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 77 of 331

dry. The characteristics of hardness, flexibility, chemical resistance, abrasion re- sistance, physical, and chemical attributes are derived from the basic latex poly- mer and specific modifications. Acrylic latexes are known for their good exterior durability and excellent re- sistance to ultraviolet (UV) degradation. When used for outdoor exposure they retain their original gloss and color over longer periods. In this regard they are superior to unmodified alkyds, which tend to have poorer gloss and color re- tention. Acrylic epoxy hybrids are less commonly specified. They comprise two- or three- package systems in which emulsified epoxies are used to cross-link aqueous acrylics. Properly formulated coatings are corrosion resistant and can produce finishes that have very good gloss, hardness, alkali, and abrasion resistance. Un- like conventional solvent-based epoxies, some mixed waterborne coatings have pot lives up to 36 hours at reasonable ambient temperatures. Acrylic epoxy hybrids are used in applications where the hardness, flexibility, and chemical resistance of an epoxy is desired. These coatings will be used for gen- eral metal finishing where high performance in terms of physical and chemical properties are not required, but improvements over the acrylic latex is preferred. Epoxy water reducible coatings that can be air- or force-dried below 90°C (194°F) with VOC levels at or below 340 g/L (2.8 lb/gal) are available from a few ven- dors. Generally, they are supplied as two- or three-package systems. In the later case, neither components A nor B contain water. However, after they have been mixed in prescribed proportions water is added in fairly large quantities to ad- just viscosity to sprayable levels, usually in the range of 20 seconds on a Zahn #2 cup. The most commonly available water reducible epoxies are formulated as primers complying with military specifications MIL-P-53030 (lead- and chromate- free) and MIL-P-85582 (containing chromates). They can be top-coated with most other coatings systems, in particular ,polyurethanes, and are used when good corrosion resistance is required. As with all high-performance coatings, proper- ly prepared surfaces are mandatory. You might consider specifying the water-re- ducible epoxies when you require better corrosion resistance than can general- ly be obtained from alkyd and alkyd-modified primers. Because epoxies tend to chalk when exposed to weather and sunlight (whether water or solvent borne), they are usually not used as topcoats. However, vendors do formulate these epoxies as topcoats where high performance is a require- ment. As primers, they are commonly specified for military hardware, steel and alu- minum frames and weldments, cold-rolled steel panels and cabinets, aerospace, and electronic components. As with all other resin systems, particularly water- bornes, proper pretreatment of the substrate is essential. Polyurethane dispersions are waterborne systems that can air/force dry at tem- peratures below 194°F (90°C). Essentially, they are polyurethane lacquers dis- persed in water, which implies that as the water evaporates, the coating film is formed. No other curing mechanisms take place. In fact, they are completely re- acted products with no free isocyanate groups, and after the water has evaporated, the film is as hard as it ever will be. Apparently, very low VOC contents are achievable; however, the technology is relatively new and is still in the process of being tested by various companies. While the polyurethane dispersions can be used on metal parts, much like the con- 76

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