Alkyd

Alkyd refers to alkyd resins, a family of polyester resins modified with fatty oils to improve drying and film-forming properties. They are typically described as oil-modified polyesters and are widely used in paints, varnishes, and enamels. The backbone is a polyester formed by reacting polyhydric alcohols (such as glycerol or pentaerythritol) with polybasic acids or anhydrides (for example phthalic anhydride). A fatty oil component, derived from drying oils like linseed or tall oil, is incorporated to impart drying characteristics and flexibility.

Cure mechanism and modifiers: The resin cures by autoxidation of the fatty acid residues in air, often accelerated by metallic driers such as cobalt, manganese, or zirconium compounds. The oil length—the proportion of drying oil in the resin—classifies alkyds as short-, medium-, or long-oil; short-oil resins dry quickly and form hard films, whereas long-oil resins yield slower drying but greater flexibility and gloss.

Applications and trends: Alkyds are the workhorse of solvent-borne coatings, used as base resins for exterior and interior enamels, varnishes, and industrial finishes. They offer good adhesion, pigment binding, and a broad balance of hardness and impact resistance. Limitations include relatively moderate UV resistance and VOC content, leading to development of modified alkyds—such as acrylic- or polyurethane-modified alkyds—and waterborne alkyd emulsions to reduce solvent emissions.

History: alkyd resins were developed in the mid-20th century and became popular due to improved drying speed and film properties compared with unmodified polyesters.

---