206
Fundamentals of Corrosion
heat and actinic rays of the sun. Mildew inhibitors (phenylmercury, zinc,
and cuprous compounds) are commonly added to oil-based and latex paints.
Latex paints (water emulsion) invariably have a number of additives acting as
surfactants, coalescing aids, emulsion stabilizers, etc. Vinyl paints often have
a 1% carboxylic acid (generally maleic acid) modification to the vinyl resin
to promote adhesion to metals. Conversely, hydroxyl modification (generally
an alcohol) aids in adhesion of vinyls to organic primers. The use of a particular additive can be critical to the performance of the paint; and because
additives are usually added in trace amounts, they may be most difficult to
detect upon analysis of the paint.
7.2.5 Fillers (Extenders)
The primary function of fillers in organic coatings is to increase the volume
of the coating through the incorporation of low-cost materials such as chalk
or wood dust. They may also be used to improve coating properties such as
impact and abrasion resistance and water permeability.
In addition to lowering the cost, extenders also provide sag resistance
to the liquid paint so that the edges remain covered. When the paint has
dried, they reduce the permeability to water and oxygen and provide
reinforcing structure within the film. Talc and mica are used extensively
as extenders. Mica is limited to approximately 10% of the total pigment.
Both talc and mica, but particularly mica, reduce permeability through
the film as plate-like particles block permeation, forcing water and oxygen to seek a longer path through the binder around the particle.
7.3 Causes of Bond and Coating Failures
Bond and coating failures can result from any one or a combination of the
following causes:
1. Poor or inadequate surface preparation and/or application of the
paint to the substrate
2. Atmospheric effects
3. Structural defects in the paint film
4. Stresses between the bond and the substrate
5. Corrosion
Fundamentals of Corrosion
heat and actinic rays of the sun. Mildew inhibitors (phenylmercury, zinc,
and cuprous compounds) are commonly added to oil-based and latex paints.
Latex paints (water emulsion) invariably have a number of additives acting as
surfactants, coalescing aids, emulsion stabilizers, etc. Vinyl paints often have
a 1% carboxylic acid (generally maleic acid) modification to the vinyl resin
to promote adhesion to metals. Conversely, hydroxyl modification (generally
an alcohol) aids in adhesion of vinyls to organic primers. The use of a particular additive can be critical to the performance of the paint; and because
additives are usually added in trace amounts, they may be most difficult to
detect upon analysis of the paint.
7.2.5 Fillers (Extenders)
The primary function of fillers in organic coatings is to increase the volume
of the coating through the incorporation of low-cost materials such as chalk
or wood dust. They may also be used to improve coating properties such as
impact and abrasion resistance and water permeability.
In addition to lowering the cost, extenders also provide sag resistance
to the liquid paint so that the edges remain covered. When the paint has
dried, they reduce the permeability to water and oxygen and provide
reinforcing structure within the film. Talc and mica are used extensively
as extenders. Mica is limited to approximately 10% of the total pigment.
Both talc and mica, but particularly mica, reduce permeability through
the film as plate-like particles block permeation, forcing water and oxygen to seek a longer path through the binder around the particle.
7.3 Causes of Bond and Coating Failures
Bond and coating failures can result from any one or a combination of the
following causes:
1. Poor or inadequate surface preparation and/or application of the
paint to the substrate
2. Atmospheric effects
3. Structural defects in the paint film
4. Stresses between the bond and the substrate
5. Corrosion