Driers in Coatings
Driers, also referred to as siccatives, are organometallic compounds soluble in organic solvents and binders1.
Chemically, driers (e.g. cobalt-ethylhexanoate) belong to the class of metal soaps and they are added to air-drying coating systems to accelerate or promote the drying process after application.
The transformation from the liquid film into the solid stage within an appropriate time occurs by oxidative cross-linking of the binder system, a process which is catalyzed by the metallic cation of the drier.
The Drying Process
After application of the paint onto the substrate the solvent is released, which is the physical part of the drying process. In order to obtain a hard paint film, further polymerization of the binder is necessary.
The chemical part of the drying process involves oxidative cross-linking and starts with the reaction between unsaturated parts in the binder molecule and oxygen (air). This process is known as the "autoxidation process" and similar to the drying process in solvent-rich paints (figure below)2
. This part of the drying process significantly affects film hardness.2
Fig: The autoxidation film formation process of alkyd resins
High solids alkyd paints differ from the classical alkyd paints in having lower solvents contents as well as in resin composition. In order to reduce solution viscosity and enable higher solids contents, high solids resins typically are composed of lower molecular weight polymers and demonstrate high reactivity towards autoxidation. The high chemical reactivity is required to compensate for the low molecular weight of the resin. Waterborne alkyd paints are based on alkyd resins, either emulsified in water or aqueous (typically colloidal) solutions.
Effect of Driers on Film Hardness