Alkyds
With their initial development in 1921, alkyd resins quickly became the polymer of choice for paint. Varnishes that were previously manufactured with a combination of hard resins and oils were changed to alkyd/oil polymers that improved durability and versatility. The alkyd varnishes were flexible and more consistent in their properties than the older resin varnishes. These versatile resins essentially replaced the natural resin component in a varnish with excellent toughness and clarity. The term “enamel” was first applied to alkyd based paints, due to their hard, vitreous-like finish, compared to the oil/rosin paints.
Alkyds are defined as oil modified polyesters. Variations in the type of oils, acids or alcohols used in the manufacture of alkyds affect the performance and type of coating in which it will be used. The most commonly used types of oil modifiers for general purpose paints are soya and linseed. Alkyds containing these oils have all around good properties and are quite economical. Oils such as tung, safflower, fish, and castor produce more specialized products ranging from polyesters to marine varnishes.
Alkyds also generally have a very high VOC, although recent developments in technology may enable higher solids, lower VOC alkyds as well as water-borne alkyd technology. Waterborne acrylics and acrylic blends, however, have advanced to take the place of many alkyd coatings.
In air dry trade sales (aka ‘architectural’) and industrial alkyd coatings, curing or drying of the film is initiated through the use of metallic acid complexes that catalyze functional groups on the polymer and cross-link the molecules together.
One form of sub-classification of alkyd resins is by the oil content or oil length. The long oil types are generally thinned with mineral spirits and as oil length is shortened the alkyd requires stronger solvents such as ketones, esters, aromatic hydrocarbons and acetates. The synthetic portion of the polymer is less soluble than the oil portion so the less oil, the stronger the solvent must be to dissolve or reduce it. The higher functionality of esters of pentarythyrol make resins with greater amounts of functional groups which in turn speeds up the curing. Generally, the longer the oil length, the longer the dry time of the alkyd.