The Chemistry of Alkyd Paints
Alkyd paints belong to a family of coatings derived from the reaction between a polyol, typically glycerol or pentaerythritol, and a dibasic acid such as phthalic anhydride, modified with drying oils or fatty acids. The name "alkyd" itself is a portmanteau of "alcohol" and "acid." These polymers form a crosslinked network through oxidative curing, a fundamentally different mechanism from the simple water evaporation that dries latex paints.
The curing process begins when unsaturated fatty acid chains in the alkyd resin react with atmospheric oxygen. Metal driers, usually cobalt, manganese, or zirconium salts, catalyze this reaction. The result is a densely crosslinked polymer film that is harder and more resistant to abrasion than most water-based coatings.
Traditional alkyd paints use mineral spirits, also known as white spirits or Stoddard solvent, as the carrier. Mineral spirits is a petroleum distillate composed primarily of aliphatic and alicyclic hydrocarbons in the C7 to C12 range, with a boiling point between 150 and 200 degrees Celsius.
Superior Leveling and Film Formation
Alkyd paints have historically been valued for their exceptional leveling characteristics. Because they dry more slowly than water-based paints, the liquid film has more time to flow out brush marks and surface irregularities before curing. The result is a smooth, glossy finish that is difficult to match with fast-drying latex formulations.
The film builds to a thickness of roughly 2 to 3 mils dry film thickness per coat compared to roughly 1.5 to 2 mils for typical latex paints. This higher build contributes to the perception of depth and richness in oil-based finishes, particularly on wood trim and cabinetry.
Alkyd paints also exhibit better adhesion to chalky or poorly prepared surfaces. The smaller molecular size of the alkyd resin allows it to penetrate and lock into porous substrates more effectively than the larger polymer particles in latex emulsions.
Yellowing Over Time
The most significant drawback of oil-based alkyd paints is their tendency to yellow when used indoors. This yellowing results from the ongoing oxidation of unsaturated fatty acid residues in the cured film. Areas deprived of natural light, such as the back of a door or behind furniture, yellow more severely because UV exposure can partially bleach the chromophores responsible for the discoloration.
Soy-based and safflower-based alkyds yellow less than linseed-based formulations, but all oil-modified alkyds will eventually yellow in low-light interior conditions. This makes them problematic for use in closets, kitchens with minimal natural light, and on white trim that is expected to remain bright for years.
Manufacturers have responded with waterborne alkyd technology. These hybrid products emulsify alkyd resin in water with acrylic or polyurethane modifiers. Sherwin-Williams ProClassic Waterborne Alkyd and Benjamin Moore Advance are leading products in this category. They deliver much of the leveling and hardness of traditional alkyds while dramatically reducing both VOCs and yellowing potential.
VOC Regulations: EU and US
The European Union's Directive 2004/42/CE, implemented in phases beginning in 2007, established mandatory VOC limits for decorative paints and varnishes. Phase 2 limits, effective from 2010, restrict solvent-borne interior trim and cladding paints to a maximum of 300 grams of VOCs per liter of product ready for use. Exterior solvent-borne coatings are limited to 400 grams per liter.
In the United States, the EPA's Architectural and Industrial Maintenance Coatings Rule, commonly called the AIM Rule, sets national VOC limits. For alkyd interior trim enamels, the national limit is 350 grams per liter. Several Ozone Transport Commission states and California have adopted more stringent limits, with some categories as low as 250 grams per liter.
These regulations have accelerated the shift away from solvent-borne alkyds. Many traditional oil-based products have been reformulated or discontinued outright. In Canada, federal VOC regulations phased in between 2009 and 2021 have similarly restricted alkyd product availability.
Drying Oil Chemistry and Reactive Diluents
Drying oils are triglycerides containing a high proportion of polyunsaturated fatty acids. Linseed oil, with approximately 52 percent linolenic acid and 17 percent linoleic acid, remains the most important drying oil for alkyd manufacturing. Tung oil contains roughly 80 percent eleostearic acid, a conjugated triene that dries faster and forms a more water-resistant film than linseed oil.
Long oil alkyds contain 60 to 70 percent oil by weight and are used in exterior trim paints where flexibility is important. Medium oil alkyds, with 45 to 60 percent oil, balance hardness and flexibility for interior trim. Short oil alkyds, below 45 percent oil, cure to a harder film and are used in industrial baking enamels.
Some modern formulations use reactive diluents such as alkyd esters or vegetable oil methyl esters to reduce solvent demand while maintaining application viscosity. These materials participate in the oxidative crosslinking reaction rather than simply evaporating, contributing to film formation while reducing VOC emissions.
Declining Market Share
In North America, oil-based architectural coatings represented approximately 25 percent of interior paint sales in 2000, but this has fallen below 5 percent as of 2024. Water-based technologies have improved dramatically in hardness, block resistance, and application feel, closing the performance gap that once justified solvent-based products.
The professional painter market, which was the strongest holdout for alkyd trim paints, has increasingly adopted waterborne alkyd hybrids and high-performance acrylics. These products eliminate the need for mineral spirits cleanup, reduce odor complaints from building occupants, and comply with increasingly strict VOC limits globally.