An
alkyd is a
polyesterPolyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
modified by the addition of
fatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s and other components. They are derived from
polyolA polyol is an alcohol containing multiple hydroxyl groups. In two technological disciplines the term "polyol" has a special meaning: food science and polymer chemistry.- Polyols in food science :...
s and a
dicarboxylic acidDicarboxylic acids are organic compounds that contain two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R may be an alkyl, alkenyl, alkynyl, or aryl group...
or carboxylic acid anhydride. The term alkyd is a modification of the original name "alcid", reflecting the fact that they are derived from
alcohol and organic ac
ids. The inclusion of the fatty acid confers a tendency to form flexible coating. Alkyds are used in
paintPaint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
s and in moulds for
castingIn metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
. They are the dominant
resinResin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
or "binder" in most commercial "oil-based" coatings. Approximately 200,000 tons of alkyd resins are produced each year.. The original alkyds were compounds of
glycerolGlycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...
and
phthalic acidPhthalic acid is an aromatic dicarboxylic acid, with formula C6H42. It is an isomer of isophthalic acid and terephthalic acid. Although phthalic acid is of modest commercial importance, the closely related derivative phthalic anhydride is a commodity chemical produced on a large...
sold under the name Glyptal. These were sold as substitutes for the
CopalCopal is a name given to tree resin that is particularly identified with the aromatic resins used by the cultures of pre-Columbian Mesoamerica as ceremonially burned incense and other purposes...
resins and made varnishes which were much paler in colour. From these, the alkyds we know today were developed.
Manufacture
There are two types of alkyd resins, drying (including semi drying) and nondrying. Both types are typically produced from dicarboxylic acids or anhydrides, such as
phthalic anhydridePhthalic anhydride is the organic compound with the formula C6H42O. It is the anhydride of phthalic acid. This colourless solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics. In 2002, approximately 4.6 billion kilograms were...
or
maleic anhydrideMaleic anhydride is an organic compound with the formula C2H22O. It is the acid anhydride of maleic acid and in its pure state it is a colourless or white solid with an acrid odour....
, and
polyolA polyol is an alcohol containing multiple hydroxyl groups. In two technological disciplines the term "polyol" has a special meaning: food science and polymer chemistry.- Polyols in food science :...
s, such as
trimethylolpropaneTrimethylolpropane is the organic compound with the formula CH3CH2C3. This colorless solid is a triol. Containing three hydroxy functional groups, TMP is a widely used building block in the polymer industry.-Production:...
, glycerine, or
pentaerythritolPentaerythritol is the organic compound with the formula C5H12O4. This white, crystalline polyol with the neopentane backbone is a versatile building block for the preparation of many polyfunctionalized compounds such as the explosive PETN and pentaerythritol tetraacrylate...
.
Alkyds are also used in items such as resins and paints.
For the "drying" resins, triglycerides derived from polyunsaturated fatty acids (often derived from plant and vegetable oils, e.g.
linseed oilLinseed oil, also known as flaxseed oil, is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant . The oil is obtained by cold pressing, sometimes followed by solvent extraction...
). These drying alkyds are cured 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 use of metal salts, the so-called
oil drying agentAn oil drying agent is a coordination compound that accelerates the hardening of drying oils through chemical crosslinking. The catalysts affect the autoxidation of the oils with air...
s. These metal complex that
catalyzeIn chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.-Acid catalysis:...
crosslinking of the unsaturated sites.
Alkyd coatings are produced in two processes;
fatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
process and the alcoholysis or mono-glyceride process. Higher quality higher performance alkyds are produced in the fatty acid process where the composition of the resulting resin can be more precisely controlled. In this process an acid anhydride, a polyol and an unsaturated fatty acid are combined and cooked together until the product has achieved a predetermined level of viscosity, penta alkyds are made this way. More economical alkyd resins are produced from the alcoholysis or glyceride process where 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
transesterificationIn organic chemistry, transesterification is the process of exchanging the organic group R″ of an ester with the organic group R′ of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst...
of the triglycerides into a mixture of mono- and diglyceride oils. To this resulting mixture acid anhydride is added to build molecular weight of the resin into roughly the same product as in the fatty acid process. However, the alcoholysis process, also known as the glyceride process, produces a more randomly oriented structure. To remove the water produced as a by-product and to increase the reaction rate, surplus
phthalic anhydridePhthalic anhydride is the organic compound with the formula C6H42O. It is the anhydride of phthalic acid. This colourless solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics. In 2002, approximately 4.6 billion kilograms were...
was added. Water was thus removed with the unreacted acid by heating the bulk to a temperature needed to do this. It meant that the reaction was not as controllable as would be liked. A new process was introduced, in which Xylene was added to produce an azeotrope with the water. This gave greater control at a lower temperature, this also produced resins at a lower viscosity, useful in making high solids paints,this process was 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 both processes the resin is purified, diluted in solvent and sold to
paintPaint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
and
varnishVarnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss...
makers.
Metalcasting
Alkyd or oil-urethane binders are used in metalcasting for the creation of
sandSand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
-based moulds. The alkyd resin is mixed with a
polymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
ic
isocyanateIsocyanate is the functional group of elements –N=C=O , not to be confused with the cyanate functional group which is arranged as –O–C≡N or with isocyanide, R-N≡C. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more...
and a metallic drier, which speeds the reaction. Unlike other no-bake mould technologies, the process yields no toxic fumes but the moulds need a longer air curing.
Coating Types
Alkyd coatings are typically sold in three classes: long, medium, and short. These terms represent the relative fraction of drying oil component in the resin. Long oil alkyds have a high percentage of drying oil content and are generally sold as medium duty coatings for the consumer market. Medium oil alkyds have less drying oil and have a higher percentage of large molecular weight polyester backbone. They dry slower and are employed as high gloss coatings and wood finishes. At the bottom end are short oil alkyds where the percentage of drying oil is very low in relation to the base polyester polymer or backbone chain. These coatings will not air dry or harden unless heated. Short oil alkyds are employed as baking
enamelsEnamel paint is paint that air dries to a hard, usually glossy, finish, used for coating surfaces that are outdoors or otherwise subject to hard wear or variations in temperature; it should not be confused with decorated objects in "painted enamel", where vitreous enamel is applied with brushes and...
for finished metal products, in conjunction with amino /formaldehyde resins. Alkyds are also modified with phenolic resin,
styrene, vinyl toluene, acrylic monomers (to make them dry more quickly), and polyurethanes. By adding certain modifiying resins, it is possible to produce thixotropic alkyds for decorative use. The latest alkyds are short oil A/D resins where the oil length is shortened by chain stopping with
benzoic acidBenzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...
and now Para Tertiary Butyl Benzoic 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, this has kept the cost of alkyd coatings very low despite ever increasing cost of
petroleumPetroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
, which is the predominant raw material source of most other coatings such as
vinylA vinyl compound is any organic compound that contains a vinyl group ,which are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group...
s,
acrylicsAcrylic paint is fast drying paint containing pigment suspension in acrylic polymer emulsion. Acrylic paints can be diluted with water, but become water-resistant when dry...
,
epoxiesEpoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
, and
polyurethaneA polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...
s. Typical sources of drying oils for alkyd coatings are: linseed,
tungTung can refer to:* Tung tree, a deciduous tree native to China.** Tung oil, a furniture finish made from the seeds of the tung tree.* Tung, a Cantonese Romanization of Chinese family names 董, commonly use in Hong Kong....
,
sunflower oilSunflower oil is the non-volatile oil expressed from sunflower seeds. Sunflower oil is commonly used in food as a frying oil, and in cosmetic formulations as an emollient. Sunflower oil was first industrially produced in 1835 in the Russian Empire.- Composition :Sunflower oil is mainly a...
, safflower oil,
walnut oilWalnut oil is oil extracted from English walnuts . It is about 50% linoleic acid, an essential omega-6 fatty acid. Walnut oil is also a good source of omega-3 fatty acids which are also essential to human nutrition....
,
soybean oilSoybean oil is a vegetable oil extracted from the seeds of the soybean . It is one of the most widely consumed cooking oils. As a drying oil, processed soybean oil is also used as a base for printing inks and oil paints...
,
fish oilFish oil is oil derived from the tissues of oily fish. Fish oils contain the omega-3 fatty acids eicosapentaenoic acid , and docosahexaenoic acid , precursors of certain eicosanoids that are known to reduce inflammation throughout the body, and are thought to have many health benefits.Fish do not...
,
corn oilCorn oil is oil extracted from the germ of corn . Its main use is in cooking, where its high smoke point makes refined corn oil a valuable frying oil. It is also a key ingredient in some margarines. Corn oil is generally less expensive than most other types of vegetable oils. One bushel of corn...
, DCO. (made by dehydrating castor oil, which creates a semi drying, conjugated, oil/fatty acid), and
tall oilTall oil, also called "liquid rosin" or tallol, is a viscous yellow-black odorous liquid obtained as a by-product of the Kraft process of wood pulp manufacture when pulping mainly coniferous trees. The name originated as an anglicization of the Swedish "tallolja"...
(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 allowed to be used in resin manufacture in India, no edible oils were allowed to be used.