Alkyd Coatings

For much of the twentieth century, alkyds were the dominant industrial and architectural paint. Higher-performance epoxies, polyurethanes and waterborne acrylics have since taken over many demanding uses, but alkyds remain widely used for machinery enamels, structural steel in mild environments, shop primers and general maintenance painting.

Their advantages are practical: a single can, no mixing, long working time, good flow and the ability to wet and penetrate surfaces that have only been hand- or power-tool cleaned. Their limitations are equally well defined, and most alkyd failures come from using them where they do not belong.

Alkyd chemistry

An alkyd resin is a polyester made by reacting a polyol (commonly glycerol or pentaerythritol) with a dibasic acid or anhydride (commonly phthalic anhydride) and a source of fatty acids from vegetable oils such as soya, linseed or tall oil. The name comes from alcohol plus acid.

The proportion of oil in the resin is described as its oil length:

• Long-oil alkyds are soluble in aliphatic solvents, brush easily, penetrate well and remain flexible, but dry slowly. They are typical of architectural and maintenance paints.
• Medium-oil alkyds balance drying speed, hardness and flexibility and are common in industrial enamels and primers.
• Short-oil alkyds are harder and faster drying and are often used in baking enamels or blended with other resins.

How alkyds cure

Alkyds cure in two overlapping stages. First, the solvent evaporates and the film becomes dry to touch. Second, unsaturated carbon–carbon bonds in the fatty-acid chains react with atmospheric oxygen in a process called autoxidation, forming cross-links that harden the film over days to weeks. This is described in more detail in how coatings cure.

Autoxidation is slow on its own, so alkyds contain driers: metal carboxylates that catalyze the reaction. Cobalt has traditionally been the main surface drier, with manganese, zirconium and calcium used as through-driers and auxiliaries. Regulatory scrutiny of cobalt compounds in some regions has encouraged the development of alternative drier packages.

Because cure depends on oxygen diffusing into the film, thick coats can skin over on the surface while remaining soft underneath, which leads to wrinkling. Cold temperatures slow the reaction considerably.

Types of alkyd coatings

Where alkyds work best

Alkyds are appropriate for interior and mild exterior exposures, roughly corresponding to the lower ISO 12944 corrosivity categories and, with good systems, some moderate ones. Common uses include:

• Shop primers and finish coats on structural steel for dry, enclosed buildings.
• Enamels on machinery, equipment, doors, frames and handrails.
• Maintenance overcoating of existing alkyd systems, where their low stress and good wetting reduce the risk of lifting old coatings.
• Surfaces prepared by hand- or power-tool cleaning, such as SSPC-SP 2 or SSPC-SP 3, where blast cleaning is impractical.

Even on these less demanding jobs, good practice pays off. Remove loose rust, mill scale and flaking paint, wash off dirt and salts, and feather the edges of sound existing coating. Apply alkyds in moderate coats rather than one heavy coat, allow each coat to dry thoroughly before recoating, and avoid painting late in the day when dew may form on a film that is still soft. In cool weather, allow extra drying time between coats and before handling or shipping coated parts.

Limitations and failure modes

Saponification

The ester linkages in alkyds are broken down by alkalis, turning the binder into soap. This saponification makes alkyds unsuitable for direct application to fresh concrete, mortar or plaster, and to zinc surfaces such as galvanizing and zinc-rich primers, where zinc reacts with the oil to form zinc soaps. The typical result is peeling or a sticky, softened film. Alkaline conditions also develop at cathodic sites on corroding steel, which contributes to alkyd failure near rust spots.

Aging, chalking and yellowing

Autoxidation continues after initial cure, so alkyd films gradually become harder and more brittle and can crack as the substrate moves. Exposure to UV causes gloss loss and chalking, and many alkyds yellow with age, particularly white or pale colors kept in the dark.

Water and chemical resistance

Alkyds are relatively permeable and are not suited to immersion, continuous condensation or strong chemical exposure. In these environments epoxy-based systems are the usual choice.

Advantages and limitations

Frequently asked questions

Is alkyd paint the same as oil-based paint?

Most modern “oil-based” paints are alkyds. Traditional oil paints used unmodified drying oils such as linseed oil; alkyds use oil-modified synthetic resins that dry faster and harder.

Why is my alkyd coating wrinkling?

Wrinkling usually happens when a film is applied too thick, so the surface cures before the underlying paint. Cold temperatures and excess drier can make it worse.

Can I put latex paint over an alkyd?

Often yes, but a glossy aged alkyd may need cleaning and light abrasion, or a bonding primer, for good adhesion. Test a small area first.

Can alkyds be used on concrete floors?

Not directly on fresh or damp concrete, because alkalinity attacks the binder. Epoxy, polyurethane, polyaspartic or acrylic products formulated for concrete are better choices.