Cure Terms Glossary
Air drying can cover a range of phenomena (including simple evaporation) but, in the context of cure, the term applies to crosslinking initiated by oxidation. Oxidation of a (suitably active) C-H bond creates a hydroperoxide. Cleavage of the peroxide link creates the initiator radicals.
Hydroperoxide decomposition can be effected by heat or by the action of redox metal catalysts. In the terminology of air-drying resins, a hydroperoxide decomposition catalyst is called a drier.
The role of oxygen in this initiation can be contrasted with that in cure inhibition. Two different mechanisms are involved. Oxygen inhibition occurs when the oxygen acts as a co-monomer in the cure and forms a relatively inactive peroxy radical (ROO⋅).
For successful air drying, the resin formulation must be relatively insensitive to oxygen inhibition. The presence of allylic (C=CCH2) hydrogen helps destroy the peroxy radicals, and the familiar air-drying alkyds utilise components which are rich in allylic H. These components are unsaturated fatty acids obtained from a range of natural oils. Examples include:
Alkyd resins are created
by polyesterification where these fatty acids are reacted together with
polyhydric alcohols (e.g. glycerol) and cyclic anhydrides. The fatty acid
is monofunctional in CO2H and is therefore a chain terminator in resin
formation. This ensures a manageable molecular weight for solubility and
ease of compounding into coating formulations. Generally increasing the
fatty acid content provides for better brushing properties at the expense
of cured film durability.
Depending on the weight of fatty acid in the resin, alkyds are referred to as: short oil (<45%), medium oil (45-55%) and long oil (>55%). Resins are also classified according to their iodine value as: drying (>140), semi-drying (125-140) and non-drying (<125).