Metal Finishing Guide Book


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nal properties, such as hardness, gloss retention, color retention and sunlight resistance. [Eq. (2)] Acid + Resin, phenol formaldehyde, vinyl toluene, styrene, acrylic, or silicone= Modified alkyd (2) Typical modifying resins include styrene, vinyl toluene, acrylics, silicone and others. The modified resins are more commonly known as modified alkyds. Alkyd and modified alkyd enamels are available in VOC-compliant formulations as high-solids coatings with VOCs of 340���420 g/L (2.8���3.5 lb/gal). The most important advantages of high-solids alkyds and modified alkyd [air- or force-dried, with VOCs less than 420 g/L (3.5 lb/gal)] are the following: (1) They are single-component coatings with performance properties similar to those of conventional solids alkyds. (2) They are available at VOC levels of 3.5 lb/gal, with a few at 2.8 lb/gal. (3) They can be formulated as primers and top coats and can be air-dried at ambient (room) temperature, although they should preferably be force-dried below 90��C (194��F). (4) They can be spray applied with conventional air-atomizing spray, airless, air-assisted airless and HVLP and the full range of electrostatic spray guns. (5) They are available in a wide range of colors and all gloss levels and are easy to self-touch-up. (6) They can be applied to most substrates, although they are not recommended for application directly to zinc or zinc-coated surfaces; a nonalkyd primer should be used instead. (7) They are less sensitive to the surface cleanliness of substrates than most other coatings. (8) They are the preferred choice of coating for many low-to-medium cost items or for large machinery that cannot be subjected to high-temperature ovens. Disadvantages are the following: (1) High-solids formulations generally have long ambient air-drying times (approximately 6 to 8 hours). (2) It is often difficult to maintain film thicknesses less than 1.5 mils. This is particularly evident on complex geometries, such as weldments and assemblies. Therefore, by default more coating is applied than is actually required. (3) They tend to exhibit higher viscosities than high-solids polyurethanes of similar VOC content. (4) Some formulations require the coatings to be heated during spray application to adequately lower the viscosity for application. (5) Gloss and color variations can occur from one surface to the next, owing to uneven film thicknesses. (6) Long recoating times, sometimes several hours or overnight, are not uncommon. This is aggravated if the film thickness is too high. (7) Some modified alkyds have a ���critical��� recoating period. The coating cannot be recoated during a certain window, sometimes 2���10 hours. (8) They are not generally used for texture finishing. (9) They tend to be relatively soft coatings initially. Hardness improves over a period of days to a final pencil hardness value of approximately HB (compare this with a pencil hardness of 3H���6H for epoxies and polyurethanes). (10) They tend to have limited resistance to longterm ultraviolet (UV) (sunlight) exposure; chalking and color fading are prevalent. (11) They exhibit poor resistance to alkalinity, chemicals, solvents, and immersion in water. Alkyds and modified alkyds are commonly used as general-purpose shop primers for steel and other substrates; however, they are not recommended for direct application to zinc or zinc-coated substrates. Because alkyd resins can be modified in so many ways, they are still among 170

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