Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 66 of 331

Disadvantages are the following: (1) High-energy usage. They must be baked at elevated temperatures with schedules such as 45 minutes at 110°C (230°F) or 10 minutes at 176°C (350°F). (2) Some formulations remain tacky at ambient temperatures and leave walls and floors of spray booths tacky. (3) High viscosi- ties of some compliant formulations require special spray application equip- ment. Alternatively, apply at fluid temperatures of 100–110°F. (4) They are not for plastic or other heat-sensitive substrates because of the high baking tem- peratures. (5) Stains caused by the spray washer cleaning process are often "pho- tographed" through the coating finish. (6) As with many high-solids coatings, smooth finishes, free of orange peel, may be difficult to achieve. (7) They may re- quire close application controls. (8) An operator learning curve required. (9) Ap- plied costs are greater than for conventional-solids baked enamels. EPOXY ESTERS Epoxy esters are coatings that in many ways resemble alkyds in that they are sin- gle component and require no mixing of multicomponents prior to application; however, they are harder and more chemically resistant. In addition, they are available in solvent- or waterborne formulations. Epoxy esters are air- or force- dried at temperatures less than 90°C (194°F). The similarity between epoxy esters and alkyds lies in the fact that they are the products of reactions between moderate equivalent weight (800–1,500) epoxy resins and fatty acids. The properties of the resulting epoxy ester polymer resins are related to the actual equivalent weight of the original epoxy resin and the type of fatty acid with which it was esterified. Consequently, some epoxy esters are softer, more flexible and slower drying than other formulations that may be harder and faster drying. They also tend to have better chemical resistance and are harder than alkyds. These resins require metallic driers, as do alkyds, to start and maintain the dry- ing sequence. These coatings are used in situations where alkyds would normally be selected but where a harder and more chemically resistant finish is required. The advantages of epoxy esters are the following: (1) Coatings are single-com- ponent materials and therefore maintain a constant viscosity, provided that temperature remains constant. (2) They are available in high-solids formula- tions. (3) They can be formulated into VOC-compliant water-reducible formu- lations at very low VOC contents. (4) Storage stability is excellent for the sol- vent-solution types, with long-term stability for water-thinnable systems. (5) They can be easily pigmented with normally available mixing equipment. (6) The solvent-borne types are very similar to medium- oil-length alkyds in most char- acteristics. (7) The water-reducible coatings resemble their alkyd counterparts. (8) They can be applied using the full spectrum of available spray equipment. (9) Some are FDA approved and are used for applications in which such approval is important. Disadvantages of epoxy esters are the following: (1) The major disadvan- tage for nearly all epoxy derivatives is their very poor resistance to chalking on exterior exposure. They chalk so heavily and so soon after exposure that they have poor color retention. (2) For exterior service, they can be successfully used only as primers and must be top-coated as soon as possible after being ap- plied on an exterior exposed surface. (3) Because of their poor exterior durability they should only be used as top coats for interior exposure. (4) Yellowing can be a problem depending on the epoxy and fatty acid from which the epoxy es- 65

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