Types of Driers and their Functions

Drier is one of the major components of an alkyd-based paint. The main function of a paint drier is to accelerate the drying of resin (mainly alkyd) in the paint and in consequence paint film becomes hard. It helps the resin in the absorption of the oxygen from the air, make a cross-linked structure at the molecular level that hardens the film at the visible level. It is available in the form of a solid or a paste in the market.

Features of Paint driers?

1.??????Accelerate drying process

2.??????Easily soluble

3.??????Required in small quantity

4.??????To be added just before use

5.??????Compatible with paint

?Paint Drying Process

?Driers catalyze the uptake of dioxygen and decomposition of hydro-peroxides to free radicals, resulting in hardened cross-linked polymer networks that bind the pigment to the painted surface of the treated object. The quantity of driers added to the paint is limited to 3%. If used in excess; especially in the finishing coat, it has a tendency to destroy the elasticity of the paint and causes a flaking defect in the paint. Thus, they are used after dissolution into volatile solvents that are compatible with the paint. Time taken by the drying process depends upon the nature of the solvent, the thickness of film and additives. The drying of solvent-based resins is a four-stage process. These processes are as followed:

?A.?Induction period:?After the paint is applied to the wall, the solvent is released, which is the physical part of the drying process.?In this stage, the volatile solvents used to bring down viscosity are evaporated and coalescence of the binder and pigment particles happens. This leads to closed film formation.

?B.?Hydroperoxide formation: The film starts uptake of atmospheric oxygen and the auto-oxidation process of unsaturated starts which leads to the formation of hydroperoxide functionalities.

?C.?Decomposition of hydroperoxides: As hydroperoxides are not stable for a long time, so they decompose to give peroxide and alkoxide radicals.

?D.?Cross-linking:?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).?The generated radicals initiate cross-linking which causes the resin to gel which is then followed by drying and hardening the film.?This process is known as "auto-oxidation" and significantly affects film hardness.

Autoxidation Film Formation Process of AlkydResins

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