Chemistry of the drying process
– Drying oils harden through a chemical reaction called autoxidation.
– Oxygen molecules from the air insert into carbon-hydrogen bonds in unsaturated fatty acids.
– Hydroperoxides are formed and undergo crosslinking reactions.
– Polymerization occurs, resulting in a stable film.
– Linseed oil can increase in weight by 17% as it absorbs oxygen during the drying process.
– Metal salts, especially cobalt, manganese, and iron derivatives, accelerate the drying process.
– These salts function as homogeneous catalysts.
– They speed up the reduction of hydroperoxide intermediates.
– Addition reactions occur, producing free radicals that engage in crosslinking.
– The process terminates when pairs of free radicals combine, rendering the film dry.
Constituents and properties
– Drying oils consist of glycerol triesters of fatty acids.
– They contain high levels of polyunsaturated fatty acids, such as alpha-linolenic acid.
– Iodine number is used to measure the drying property of oils.
– Oils with an iodine number greater than 130 are considered drying.
– Oils with an iodine number of less than 115 are non-drying.
– Non-drying waxes and resins do not form covalent bonds like drying oils.
– Waxes and resins remain soluble in their original solvent.
– Cured oil paint or varnish does not dissolve in its original solvent.
– Some paint removers chemically change drying oils to dissolve them.
– Waxes and resins can be recovered unchanged after solvent evaporation.
– Drying oils have high viscosity, slow drying time, and form a hard film when exposed to air.
– They enhance the depth and richness of colors and provide protection against weathering.
Types of drying oils
– Linseed oil
– Tung oil
– Poppy seed oil
– Walnut oil
– Safflower oil
Applications of drying oils
– Used as a binder in oil paints
– Used in wood finishing and restoration
– Used in varnishes and coatings
– Used in the production of linoleum
– Used in the manufacturing of printing inks
Safety considerations
– Rags, cloth, and paper saturated with drying oils can spontaneously combust.
– Heat released during the curing process can ignite oil-soaked materials.
– Precautions include wetting rags, storing them in fireproof metal containers, or immersing them in water or solvents.
– Leaving linseed-oil-soaked rags in a pile caused a major fire in a Philadelphia office building in 1991.
– Proper storage and handling of drying oil-soaked materials is crucial to prevent accidents.
– Drying oils are flammable.
– Proper ventilation is necessary when using drying oils.
– Avoid prolonged skin contact.
– Store in a cool, dry place away from direct sunlight.
– Dispose of rags and brushes properly to prevent spontaneous combustion. Source: https://en.wikipedia.org/wiki/Drying_oil
A drying oil is an oil that hardens to a tough, solid film after a period of exposure to air, at room temperature. The oil hardens through a chemical reaction in which the components crosslink (and hence, polymerize) by the action of oxygen (not through the evaporation of water or other solvents). Drying oils are a key component of oil paint and some varnishes. Some commonly used drying oils include linseed oil, tung oil, poppy seed oil, perilla oil, and walnut oil. Their use has declined over the past several decades, as they have been replaced by alkyd resins and other binders.
Since oxidation is the key to curing in these oils, those that are susceptible to chemical drying are often unsuitable for cooking, and are also highly susceptible to becoming rancid through autoxidation, the process by which fatty foods develop off-flavors. Rags, cloth, and paper saturated with drying oils may spontaneously combust (ignite) after a few hours as heat is released during the oxidation process.
