An alkyd is a polyester

resin In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on n ...

modified by the addition of

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

s and other components. Alkyds are derived from polyols and organic acids including dicarboxylic acids or carboxylic acid anhydride and triglyceride oils. The term ''alkyd'' is a modification of the original name "alcid", reflecting the fact that they are derived from ''alc''ohol and organic ac''id''s. The inclusion of a fatty acid confers a tendency to form flexible coatings. Alkyds are used in

paint Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...

varnish Varnish is a clear transparent hard protective coating or film. It is not a stain. It usually has a yellowish shade from the manufacturing process and materials used, but it may also be pigmented as desired, and is sold commercially in vario ...

es and in moulds for casting. They are the dominant resin or binder in most commercial oil-based coatings. Approximately 200,000 tons of alkyd resins are produced each year.. Published online: 15 January 2003. The original alkyds were compounds of

glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids know ...

and

phthalic acid Phthalic acid is an aromatic dicarboxylic acid, with formula C6H4(CO2H)2. Although phthalic acid is of modest commercial importance, the closely related derivative phthalic anhydride is a commodity chemical produced on a large scale. Phthalic ac ...

sold under the name Glyptal. These were sold as substitutes for the darker-colored

copal Copal is tree resin, particularly the aromatic resins from the copal tree '' Protium copal'' ( Burseraceae) used by the cultures of pre-Columbian Mesoamerica as ceremonially burned incense and for other purposes. More generally, copal include ...

resins, thus creating alkyd

es that were much paler in colour. From these, the alkyds that are known today were developed. Glyptal

Manufacture

Alkyd resins may be classified as drying (including semi drying) and nondrying. Both types are typically produced from dicarboxylic acids or anhydrides, such as phthalic anhydride or maleic anhydride, and polyols, such as trimethylolpropane, glycerine, or pentaerythritol. Alkyds are a synthetic resin and used in items such as paints. They are not the same as

derived from natural sources such as plants. For the drying resins, triglycerides are derived from poly unsaturated fatty acids (often derived from plant and

vegetable oil Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of fruits. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, ...

s, e.g. linseed oil). These drying alkyds are cured by the

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

in air. The drying speed and the nature of the coatings depends on the amount and type of drying oil employed (more polyunsaturated oil means faster reaction in air) and presence of catalysts, the so-called oil drying agents. These catalysts are metal complexes that catalyze crosslinking of the unsaturated sites.

Cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, ...

salts are particularly effective and widely used. Alkyd resins are produced in two processes: the

process and the alcoholysis or monoglyceride process. Higher-quality, higher-performance alkyds are produced in the fatty acid process, in which the composition of the resulting resin may be more precisely controlled. In this process, an

acid anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid. In organic chemistry, organic acid anhydrides contain the functional group R(CO)O(CO)R'. Organic acid anhydrides often form when one equiva ...

, a polyol and an unsaturated fatty acid are combined and cooked together until the product has achieved a predetermined level of viscosity. Pentaerythritol based alkyds are made this way. More economical alkyd resins are produced from the alcoholysis or glyceride process, in which end-product quality control is not as paramount. In this process, raw vegetable oil, high in unsaturated component, is combined with additional polyol and heated to cause transesterification of the triglycerides into a mixture of mono- and diglyceride oils. Soybean oil is often used. Acid anhydride is added to the resulting mixture to build the molecular weight of the resin into roughly the same product as with the fatty acid process. However, the alcoholysis process produces a more randomly oriented structure. To remove the water produced as a by-product and to increase the reaction rate, surplus phthalic anhydride is added. Water is thus removed with the unreacted acid by heating the bulk to a specific temperature. The reaction is not as controllable as would be desired, so a new process was introduced in which xylene is added to produce an azeotrope with the water. This gives greater control at a lower temperature and also produces resins at a lower viscosity, useful in making high-solids paints. This is known as the AZO process. In both cases, the resulting product is a polyester resin to which pendant drying oil groups are attached. At the conclusion of each process the resin is purified, diluted in solvent and sold to

and

makers.

Metal casting

Alkyd or oil-urethane binders are used in casting for the creation of

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a soil texture, textur ...

-based moulds. The alkyd resin is mixed with a

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

ic isocyanate and a metallic drier, which speeds up the reaction. Unlike other no-bake mould technologies, the process yields no toxic fumes, but the moulds need more air-curing time.

Coating types

Alkyd resins are usually classed as long oil, medium oil and short oil. These terms represent the oil length in the resin. Alkyds are also modified with

phenolic resin Phenol formaldehyde resins (PF) or phenolic resins (also infrequently called phenoplasts) are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Used as the basis for Bakelite, PFs were the first comm ...

, styrene, vinyl toluene, acrylic monomers (to make them dry more quickly) and isocyanates to produce a polyurethane modified alkyd. Urethane alkyds are manufactured by reacting the OH groups residual on the alkyd with NCO groups from an isocyanate often TDI toluene diisocyanate. By adding certain modifying resins, it is possible to produce thixotropic alkyds for decorative use such as non-drip paints. The latest alkyds are short oil resins in which the oil length is shortened by use of a polymeric chain stopper usually a monobasic acid such as benzoic acid or ''para''-''tert''-butylbenzoic acid (Alkydal M 48). These have a better controlled molecular weight distribution and better durability. Alkyds for decorative use have extra oil cooked in to lengthen them and to make them more durable. Short oil resins used in stoving enamels are made from non-drying saturated oils or fatty acids. These usually have much higher hydroxyl and acid values to be able to react with the hydroxyl groups of the amino resins. These mixtures are usually stabilized with amines to prevent gelling on storage. Because the major components of an alkyd coating, i.e. fatty acids and triglyceride oils, are derived from low cost renewable resources, the cost of alkyd coatings has remained very low despite the ever-increasing cost of

petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...

, which is the predominant raw-material source of most other coatings such as vinyls, acrylics, epoxies and

polyurethane Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...

s. Typical sources of drying oils for alkyd coatings are

tung oil Tung oil or China wood oil is a drying oil obtained by pressing the seed from the nut of the tung tree (''Vernicia fordii''). The oil and its use are believed to have originated in ancient China and appear in the writings of Confucius from ab ...

, linseed oil,

sunflower oil Sunflower oil is the non-volatile oil pressed from the seeds of the sunflower (''Helianthus annuus''). Sunflower oil is commonly used in food as a frying oil, and in cosmetic formulations as an emollient. Sunflower oil is primarily composed ...

, safflower oil, walnut oil, soybean oil, fish oil, corn oil, DCO (made by dehydrating castor oil, which creates a semi-drying, conjugated, oil/fatty acid) and tall oil (resinous oil by-product from pulp and paper manufacturing). Non-drying/plasticizer resins are made from castor, palm, coconut oils and cardura (a synthetic fatty, versatic acid). Dehydrated castor oil was at one time the only oil permitted in resin manufacture in India; no edible oils were allowed.

Waterborne alkyds

There has been a general trend worldwide to formulate resins and coatings that are waterborne rather than formulated with solvent as waterbased materials are perceived to be environmentally friendly. Waterborne alkyds have thus been made available too.US Patent 5,137,965 1992

References

External links

*{{Commonscatinline Polyesters Coatings Painting materials Polymers