By definition these are condensation polymers of alcohols and acids. They are linear polymers and have two double bonds.

Made from a combination of oil and glycerol, reacted by heating (or cooking) with phthalic anhydride to from the alkyd resin.

Drying oils such as linseed, soya bean, sunflower seed or tung produce alkyds that dry by oxidation. Non-drying oils such as castor, coconut or cotton seed give plasticizing alkyds used to give flexibility to lacquer films.

Alkyds have an oil length expressed as a percentage – from below 45% as short oil, soluble only in aromatic hydrocarbons e.g. Xylol to medium oil up to 60%, and long oil over 60%, soluble in aliphatic hydrocarbons e.g. White Spirit.

They can be modified with various other polymers to give a wide range of drying and hardness characteristics and can be used in air drying and stoving finishes.

Long oil alkyds feature in household paints, they are relatively soft, but remain flexible. Medium oil alkyds are used in fast air dry enamels. Short oil alkyds are used with nitrogen resins (melamine) in industrial stoving finishes and in the vehicle industry – amino/alkyds types.

All are classed as convertible coatings, depending mainly upon oxidation and, therefore, not re-soluble when dry.

Because of the low molecular weight (compared with say nitrocellulose), less thinner is required to reduce them to application viscosity. They may, therefore, be applied at higher total solids content, and with less shrinkage occurring during the drying, they exhibit better build and filling.

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Alkyds dry by the action of the oxygen in the air, catalyzed by metal compounds called driers. These are additives, and present in quantities of less than 1%.

During the drying process there is a considerable uptake of oxygen, linseed oil can take up about 12% of its own weight. The oxygen up take produces free radicals within the resin. These free radicals bond to other chains, finally producing a cross-linked resin.

Cobalt and Manganese are surface driers or primary driers, and cause the film to set up. Lead is a well-known through drier or secondary drier, but due now to health restrictions on its use, it is being replace by calcium or Zirconium.

The driers are balanced to allow through drying occurring without causing wrinkling of the surface. Recoating cannot take place until full through dry is obtained – usually overnight.

Vehicle paints such as Permobel (medium oil alkyd) and Transport Finish (long oil alkyd) have the following general properties:

Special alkyds that may be encountered in the vehicle finishes are:

Styrenated

Alkyds modified with between 10-60% additions of styrene. The advantage of fast dry, due to the rapid solvent release of styrene, but a limitation on recoat, requiring a full oxidation period of up to 24 hours. Useful in QD primers in wet on wet systems.

Vinyl Toluene

Use to speed up the drying of alkyds used in brush applied paints.

CHEMICAL PROCESS FOR PRODUCTION OF ALKYDS RESIN

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Alkyd Gloss Finish

Ready for brushing at 72.5% solids.