Page 656 - Corrosion Engineering Principles and Practice

610 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 611

Topcoats
A topcoat may be required to extend the life of the preceding coats.
When designed as a topcoat, the film is normally more dense and
hydrophobic than the remainder of the system to reduce the rate of
moisture permeation to the underlying coats. A topcoat also may be
selected to confer reflectivity, reduce photodegradation, or to affect
color, among other reasons.

14.5.2 Basic Components
Coating materials consist, fundamentally of a binding resin, pigments,
additives and a solvent or dispersant (except in the case of 100 percent
solids coatings, where the latter two items are not used). The resinous
material, plus the pigments, nonvolatile additives, and fillers, comprise
the solids of the coating. These are carried in a vehicle (liquid solvent
or dispersing medium) which may be varied in viscosity by the addition
of a thinner. The vehicle also may ultimately be the binder of the coating
(binding the pigment particles together and to the substrate).
Only the solids remain on a surface after the final coating has
formed. However, a portion of, or the total vehicle may be reacted
with the resin (100 percent solids coating) to produce the final film.
Pigments are added to provide color or as inhibitors in a prime (first)
coat. Fillers are selected to increase the bulk of the coating, improve
the density, increase abrasion resistance, and increase the opacity of
the film. Selection of the wrong filler or addition of excess filler can
downgrade a coating by increasing the permeability or decreasing
the cohesion in the film. Obviously, the solids content of a liquid
coating determines the final film thickness of the coating, depending
on the amount applied over a specific area:

Vol liquid coating (mL) × % solids
L
Film thickness ( m) = (14.5)
m
2
Surface area covered (m )
This theoretical coverage will not be precisely obtained because
of the roughness of the surface and losses occurring during
application of the wet film. Some 15 percent loss is to be anticipated
for spray applications.
Binders
To perform in a practical environment, a coating must convert, after
its application, into a dense, solid, and adherent membrane that has
all the properties discussed previously. The binder is the material that
makes this possible. It provides uniformity and coherence to the
coating system. Not all binders are corrosion resistant, so only a few
serve in the formulation of protective coatings. The binder’s ability to
form a dense, tight film is directly related to its molecular size and
complexity. Binders that have the highest molecular weight will form
films by the evaporation of the vehicle, whereas binders with smaller