An alkyd is a polyester modified by the addition of fatty acids and other
components. They are derived from polyols and a dicarboxylic acid 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 acids. The inclusion of the fatty acid confers a tendency to form
flexible coating. Alkyds are used in paints 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. The
original alkyds were compounds of glycerol and phthalic acid sold under
the name Glyptal. These were sold as substitutes for the darker colored Copal
resins, thus creating alkyd 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 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 also used in items such as resins and paints.
For the "drying" resins, triglycerides are derived from polyunsaturated fatty
acids. 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 and use of
metal salts, the so-called oil drying agents. These metal complex that
catalyze crosslinking of the unsaturated sites.
Alkyd coatings are produced in two processes; fatty acid 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 transesterification of the triglycerides
into a mixture of mono- and diglyceride oils. To this resulting mixture acid,
anhydride is added to build the 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 anhydride 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
desired. 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 paint and varnish makers. Metalcasting
Alkyd or oil-urethane binders are used in metalcasting for the creation of
sand-based moulds. The alkyd resin is mixed with a polymeric isocyanate 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 oil length in the resin. Alkyds are also modified with
phenolic resin, styrene, vinyl toluene, acrylic monomers, and polyurethanes. By
adding certain modifying 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 acid and now
para-tert-butylbenzoic acid, 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
petroleum, which is the predominant raw material source of most other coatings
such as vinyls, acrylics, epoxies, and polyurethanes. Typical sources of drying
oils for alkyd coatings are: linseed, tung, sunflower oil, safflower oil,
walnut oil, soybean oil, fish oil, corn oil, DCO., and tall oil. Non drying/
plasticizer resins are made from castor, palm, coconut oils and cardura.
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.
References ^ Frank N. Jones, "Alkyd Resins",
Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH,
doi:10.1002/14356007.a01_409 . Published online: 15 January 2003.
^ "Additives for Coatings" J. H. Bielman, Ed. Wiley-VCH, 2000, Weinheim.
ISBN 3-527-29785-5. ^ C. W. Ammen. Metalcasting p. 36. ISBN
978-0-07-134246-9