Most paints can be classified according to their vehicles or binders which are described below in alphabetical order:
Alkyd
Alkyds are coatings produced by reacting a drying oil acid with alcohol. Drying of the surface occurs by the evaporation of a solvent; curing of the resin occurs by oxidation. The more oil there is in the formula, the longer it takes to dry, the better the wetting properties, and the better the elasticity. Alkyds can be used as interior or exterior trims paints, machinery enamels, or durable wall finishes. To avoid saponification, alkyd coating should not be used directly on masonry, galvanized metal or other alkaline surfaces except over an alkali resisting primer or sealer.
Epoxy-Catalyzed
Catalyzed epoxies are coating produced by combing an epoxy resin with a curing agent. Solvent evaporation causes the surfaces to dry while a chemical cross – linking process, called copolymerization, is the curing mechanism. The mixture has a limited time of workability, referred to as “pot life” which may vary from a few minutes to several hours depending on the formulation. When properly cured, catalyzed epoxy coatings have excellent solvent and chemical resistance. They are excellent coating for walls, producing a surface that is highly resistant to abrasion, chemicals, and cleaning. Many epoxy coating can be used on floors in high traffic areas. Most epoxies develop a non-progressive chalk face on exterior exposure, but otherwise have good durability. Water based acrylic epoxies approach the durability and performance of their solvent based counterpart. They offer the added advantage of low odor and can be used over conventional paints on interior applications.
Epoxy-Esters
Epoxy esters are coatings produced by reacting a drying oil, (as in the alkyds above) with an epoxy resin. This type of coating dries by solvent evaporation and cures by oxidation. Neither as hard nor as chemically resistant as catalyzed epoxies, epoxy esters have a good intermediate degree of toughness and chemical resistance and can be used in areas subject to occasional spills of aggressive liquids. They are easy to apply and are available as conventional coating in a single package and do not require the use of a hardener or catalyst.
Latex
Latex paints are complex composition of synthetic resins (usually acrylic or vinyl acrylic) and pigments kept dispersed in water by surfactant. They also contain small amount of coalescing solvents. Latex paints dry by evaporation of the water. As water evaporates from the film, the coalescing solvent allow the particles of resin to fuse together (coalesce) forming a continuous coating. Latexes have excellent adhesion, color and gloss retention, long term flexibility, and toughness. Their advantages also include ease of application and clean up, safety, and VOC compliance. Most latex paints must be protected from freezing and applied at a minimum temperature of 50 0 F/10 0 C. Some specialty products may be applied at lower temperatures.
Polyurethane
Polyurethane (urethanes) coatings are those containing an isocyanate complex. They from tough, hard, flexible, chemical resistant films by one two methods:
Moisture curing – moisture cured urethane dry by solvent evaporation and cure by reacting with moisture /water vapor in the air. Generally, for this to occur, relative humidity levels must exceed 20%.
Copolymerization – often called catalyzed or two component urethanes, these cure by the addition of a co-reactant (catalyst) to the isocyanate – containing component. Mixing, induction time, and pot life vary according to the type of isocyanate and catalyst used.
Urethanes are light stable, gloss retentive, and non-yellowing. For maximum performance, they are often used over epoxy primers or zinc- rich primer with epoxy intermediate coats to protect chemical plants, bridges, water and waste water facilities, and other industrial sites.
Zinc Rich Primers
Zinc – rich are those containing zinc particles (at least 80% by volume) in the dried film. Because zinc is a more “Active” metal than steel, when exposed to aggressive chemicals or corrosive agents, the zinc will “corrode” to protect the steel substrate. This the same protective mechanism that allows hot dipped galvanizing to minimize corrosion on steel. In order for this cathodic protection to take place, there must be direct contact between the zinc pigment particles and the steel substrate. Therefore, it is imperative that the steel be sandblasted to provide both a clean surface and a textured / roughened surface. These zinc – rich primers are available in two types:
Organic – this variety uses an organic (carbon containing) binder to “hold” the pigment in the film. An organic zinc – rich primer will provide the benefits of the organic resin used (for example, epoxy) as well as the benefits of the zinc pigment.
Inorganic – this variety creates an inorganic zinc/silicate matrix on the surface of the steel. It is inorganic because there are no carbon – containing materials in the adherent film. The properties of inorganic zinc closely resemble those of the zinc metal itself: high temperature resistance and resistance to immersion in non – potable water or water with mild solutions of chemicals (pH 6-8).