Building Design and Construction Handbook - Merritt - Ventech!

BUILDING MATERIALS 4.103
Epoxy and Epoxy-Polyester. Catalyzed two-part, all-epoxy coatings are formed
by addition of a catalyst to the liquid epoxy just before application (pot life a few
minutes to a day). Films are as hard as many baked-on coatings and are resistant
to solvents and traffic. Oil-modified epoxy esters, in contrast, harden on oxidation
without a catalyst. They are less hard and chemically resistant than catalyzed epoxies,
but dry fast and are easily applied. Epoxy-polyesters mixed just before use
produce smooth finishes suitable for many interior surfaces and are chemically
resistant.
Polyurethanes. These produce especially abrasion-treatment, fast-hardening coatings.
Two-component formulations, of variable pot life, are mixed just before use.
One-component formulations cure by evaporation and reaction with moisture in air
(30 to 90% relative humidity). Oils and alkyds may be added.
Vinyl Solutions. Solutions of polyvinyl chloride and vinyl esters dry rapidly and
are built up by successive, sprayed thin coatings. They characteristically have low
gloss, high flexibility, and inertness to water but are sensitive to some solvents.
Adhesion may be a problem. Weather resistance is excellent.
Dryers. These are catalysts that hasten the hardening of drying oils. Most dryers
are salts of heavy metals, especially cobalt, manganese, and lead, to which salts of
zinc and calcium may be added. Iron salts, usable only in dark coatings, accelerate
hardening at high temperatures. Dryers are normally added to paints to hasten
hardening, but they must not be used too liberally or they cause rapid deterioration
of the oil by overoxidation.
Thinners. These are volatile constituents added to coatings to promote their
spreading qualities by reducing viscosity. They should not react with the other
constituents and should evaporate completely. Commonly used thinners are turpentine
and mineral spirits, i.e., derivatives of petroleum and coal tar.
4.91 PIGMENTS FOR PAINTS
Pigments may be classified as white and colored, or as opaque and extender pigments.
The hiding power of pigments depends on the difference in index of refraction
of the pigment and the surrounding medium—usually the vehicle of a protective
coating. In opaque pigments, these indexes are markedly different from those
of the vehicles (oil or other); in extender pigments, they are nearly the same. The
comparative hiding efficiencies of various pigments must be evaluated on the basis
of hiding power per pound and cost per pound.
Principal white pigments, in descending order of relative hiding power per
pound, are approximately as follows: rutile titanium dioxide, anatase titanium dioxide,
zinc sulfide, titanium-calcium, titanium-barium, zinc sulfide-barium, titanated
lithopone, lithopone, antimony oxide, zinc oxide.
Zinc oxide is widely used by itself or in combination with other pigments. Its
color is unaffected by many industrial and chemical atmospheres. It imparts gloss
and reduces chalking but tends to crack and alligator instead.
Zinc sulfide is a highly opaque pigment widely used in combination with other
pigments.