Natural and synthetic resins are used in a large variety of air-drying and baked finishes. The natural resins include both fossil resins, which are harder and usually superior in quality, and recent resins tapped from a variety of resin-exuding trees.
The most important fossil resins are amber (semiprecious jewelry), Kauri, Congo, Boea Manila, and Pontianak. Recent resins include Damar, East India, Batu, Manila, and rosin. Shellac, the product of the lac insect, may be considered to be in this class of resins.
The synthetic resins, in addition to the ones discussed in Art. 4.90, are used for applications requiring maximum durability. Among them are phenol formaldehyde, melamine formaldehyde, urea formaldehyde, silicones, fluorocarbons, and cellulose acetate-butyrate.
Phenolics in varnishes are used for outdoor and other severe applications on wood and metals. They are especially durable when baked.
Melamine and urea find their way into a large variety of industrial finishes, such as automobile and refrigerator finishes.
Silicones are used when higher temperatures are encountered that can be borne by the other finishes.
Fluorocarbons are costly but provide high-performance coatings, industrial siding, and curtain walls with excellent gloss retention, stain resistance, and weather resistance. Cellulose acetate-butyrate provides shop-applied, high-gloss finishes.
