Alkyd Coatings-Oil Based Coatings
Oxidative curing alkyd coatings sometimes referred to as “oil-based coatings” or oleoresins have been in use in one form or another for thousands of years. They cure by air oxidation when oxygen from the air reacts with unsaturated fatty acids in their drying oils. By this reaction, liquid resins are converted to solid films. Modern day alkyds are typically a combination of natural drying oils and synthetic resins. A beneficial feature of alkyds is that they are single package and are easy to apply with low cost equipment. A negative feature is that they can be very slow to cure.
Urethane alkyd coatings are alkyd drying oils reacted with an isocyanate to produce a hard and durable film. These materials have been used in the food processing industry and have proven to stand up well to the necessary and constant cleaning required in those facilities. Urethane alkyds do not normally have better UV resistance than a standard alkyd but do have better chemical resistance and a harder more abrasion-resistant finish. A small amount of an epoxy resin can be added to an alkyd coating to make an epoxy ester. This single package coating has better moisture and chemical resistance than a plain alkyd, but it should not be confused with a two-component epoxy which has far higher levels of water and chemical resistance. Another common combination of alkyd is with a silicone when the material is used as an exterior coating. The US Navy uses silicone alkyds as their exterior topcoat on all surface ships.
Air-oxidizing coatings have limited solvent resistance. They continue to oxidize and cross-link after initial drying and curing. Thus, with time, they become harder, more brittle, and less soluble in solvent. That is, they become more like thermosetting coatings and are harder to recoat and maintain.
Areas of concern on the application of alkyd coatings:
- Need to know the specific type of alkyd resin being used and follow the manufacturer’s recommended curing times and over-coat information
- Very important to watch for excessive film build
- When an alkyd coatings are used as a shop primer for industrial equipment it may be necessary to remove them prior to applying a two component overcoat
- Alkyd coatings are not normally compatible with concrete or zinc-based materials
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