Although alkyds are no longer the largest volume resin type used in coatings, they still play a significant role in the coatings industry, not only because of their versatility, but also because they employ a significant amount of renewable material. The term alkyd is derived from alcohol and acid. Alkyds are prepared from the condensation reaction between polyols, dibasic acids and fatty acids. The fatty acid portion is derived from vegetable matter and thus is a renewable resource. Key performance features of alkyds include their ability to offer improved surface wetting (from the bio-based fatty acid portion of substrates and pigments) and lower cost (also primarily from the fatty acid portion). The most widely used polyols include glycerol, pentaerythritol and trimethyol propane whereas the most widely used dibasic acids are phthalic anhydride and isophthalic acid.
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Naturally-occurring oils are in the form of triglcerides. Triglycerides are triesters of glycerol and fatty acids. Triglycerides can be drying oils, but many are not. The reactivity of drying oils with oxygen results in 1,4 –dienes. The naturally-occurring oils are comprised of mixtures of mixed triglycerides with different fatty acids as part of the glyceride molecules.
Some of these glyceride molecules are comprised of a higher percentage of fatty acids with a greater amount of non-conjugated unsaturation with diallylic methylene groups and result in improved drying capability. For example, linoleic acid has one active diallylic group (-CH=CH – CH2 – CH=CH -), whereas linolenic has two active methylene groups. Also, to increase drying speed, alkyds can be modified with vinyl toluene or styrene to increase the Tg and thus reduce the time required to reach a given hardness. If the amount of oil in an alkyd is over 60%, it is called a long oil alkyd. If it’s between 40 and 60%, it’s known as a medium oil alkyd, and those with less than 40 are considered short oil alkyds. The formula for calculating the percent oil length based on the amount of fatty acid is as follows:
In addition to the amount of oil as well as the selection of the alcohol and acid functional components, the type of oil has a profound effect on the dry time and performance.
Fatty acids are further categorized into drying, semidrying and non-drying. Non-conjugated oils are considered drying oils if their drying index, as calculated as follows, is more than 70. The higher the amount of Linolenic and Linoleic content, the higher the drying index:
Although drying speed is improved as the % linolenic increases, the rate of yellowing for exterior white coatings is also greater. Accordingly, alkyds using safflower and sunflower oils which provide improved resistance to yellowing as a result of their lower linolenic content.
In addition to classifying alkyds by their oil length and the type of fatty acid present, alkyds are also classified into oxidizing and non-oxidizing categories. Oxidizing alkyds crosslink through a complex multistage auto-oxidation mechanism, whereas Non-oxidizing alkyds do not crosslink and are thus thermoplastic unless their available hydroxyl groups are crosslinked with an aminoplast (heat cured) or isocyanate crosslinker (ambient cured).
Alkyd resins are produced by three main processes¹:
In the fatty acid process where the composition of the resulting resin can be more precisely controlled, 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. The monoglyceride process is a two step process where an excess of glycerol is used in conjunction with an unsaturated raw vegetable oil to first transesterify the oil and then a dibasic acid is added in the second step to form an alkyd. In the acidolysis process a triglyceride is first reacted with a dibasic acid to replace one of the fatty acid groups, and then in the second step a polyol is added to form the alkyd resin.
In a formulated paint, autooxidation of oxidizing alkyds is slow unless catalyzed, accordingly metal salts are added to accelerate cure. Historically the most widely used driers are oil soluble salts of 2-ethylhexanoic acid or naphthenic acid with lead, cobalt, manganese, zirconium and calcium. Cobalt and manganese salts promote surface dry, whereas lead and zirconium promote through dry. Lead and cobalt based driers have toxicity issues and have been largely replaced by less toxic driers. Calcium salts do not show much catalytic activity, but are used to reduce the amount of surface and through driers as well as to assist pigment wetting. In general, a ladder study should be undertaken to optimize the level of driers, as, in addition to promoting cure, they also contribute to long-term embrittlement, discoloration and moisture sensitivity.
Alkyds are versatile and can be modified to fit multiple applications for use in coatings to provide low VOC or enhanced performance. For example, higher solids alkyds can be prepared by decreasing the dibasic acid to polyol ratio, using a higher percentage of oil or decreasing the molecular weight distribution. Water-reducible alkyds are available in the US and Europe that contain very little or no VOC. Some of these alkyds utilize surfactants to provide improved stability in water, while others are made by emulsifying the alkyd in hot water with the aid of an emulsifier. Waterborne alkyds can be more sensitive to hydrolysis of the ester linkage. Hydrolytically stability can be improved by employing an acrylic shell with an alkyd core. Lastly, uralkyds are also known as oil-modified urethanes as they are comprised of a urethane linkage and oil. These are made through transesterification of a polyol and an oil to form a monoglyceride and then reacted with a less than stoicheometric amount of diisocyanate. Uralkyds provide excellent mar, abrasion, chemical and saponification resistance and are used as maintenance paints and for wood application requiring these properties.
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 Encyclopedia of Polymeric Nanomaterials, University of Akron, Springer-Verlag, Berlin Heidelberg, 2014
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