Types of Paints for Structural Steel

Paints are generally described by their resin (binder) designation and sometimes their pigment composition, such as red lead alkyd or epoxy polyamide. Paints may be applied in single or multiple layers. When multiple layers are used, each layer has a special purpose.

The primer is the first layer to be applied. The main purpose of the primer is to wet the surface of the substrate and to provide adhesion and corrosion protection. Pigments such as red lead, zinc chromate, or zinc molybdate are active corrosion inhibitors. Titanium dioxide, chromium oxide, and ferric oxide pigments--while important pigments for color hiding, strength, and barrier reinforcement--are not corrosion inhibitors.

Intermediate coatings are sometimes applied to improve adhesion and impact strength between primer and topcoat and to provide a barrier layer between coats, as well as surfacers and sealers. Intermediate film also inhibits light penetration, reducing actinic degradation.

The final coating is described as the top or finish coat enamel, which provides environmental and chemical resistance. Also, the finish coat provides aesthetics of color and gloss, as well as film characteristics such as hardness and abrasion resistance. Table 14 shows the compatibility of paints applied as multiple layers.

Table 14 Paint compatibility

Primer or weathered paint

Topcoat

Solvent thinned

(latex)

Chemically reactive

Oleoresinous

Alkyd

Silicone alkyd

Phenolic oleoresinous

Vinyl

Chlorinated rubber

Styrene-butadiene/ styrene-acrylate

Acrylic

Polyvinyl acetate

Epoxy

Coal tar epoxy

Polyester

Urethane

Solvent thinned

Oleoresinous

C

C

C

C

NR

NR

NR

C

C

NR

NR

NR

NR

Alkyd

C

C

C

C

NR

NR

NR

C

C

NR

NR

NR

NR

Silicone alkyd

C

C

C

C

NR

NR

NR

C

C

NR

NR

NR

NR

Phenolic oleoresinous

C

C

C

C

NR

NR

NR

C

C

NR

NR

NR

NR

Lacquer

Vinyl

CT

NR

NR

NR

CT

CT

CT

CT

CT

NR

NR

NR

NR

Chlorinated rubber

CT

NR

NR

NR

CT

CT

NR

CT

CT

NR

NR

NR

NR

Styrene-butadiene/styrene -

CT

CT

CT

NR

CT

NR

CT

CT

CT

NR

NR

NR

NR

Bituminous

NR

NR

NR

NR

CT

CT

CT

CT

NR

NR

NR

NR

NR

Water thinned (latex)

Acrylic

C

C

C

NR

CT

CT

CT

CT

CT

NR

NR

NR

NR

Polyvinyl acetate

C

C

C

NR

CT

CT

CT

CT

CT

NR

NR

NR

NR

Chemically reactive

Catalyzed epoxy

NR

NR

NR

NR

CT

CT

CT

CT

CT

CT

CT

CT

CT

Coal tar epoxy

NR

NR

NR

NR

NR

NR

NR

NR

NR

CT

CT

NR

NR

Zinc-rich epoxy

NR

NR

NR

NR

CT

CT

CT

C

NR

CT

CT

NR

NR

Polyester

NR

NR

NR

NR

CT

CT

CT

CT

CT

CT

CT

CT

CT

Inorganic zinc

NR

NR

NR

NR

CT

CT

CT

CT

NR

C

C

NR

CT

Cementitious

NR

NR

NR

NR

CT

CT

CT

CT

NR

C

C

CT

C

Urethane

NR

NR

NR

NR

NR

CT

CT

CT

CT

NR

CT

C

CT

Note: C: normally compatible. CT: compatible with special surface preparation and/or application. NR: not recommended because of known or suspected problems. Certain combinations

Note: C: normally compatible. CT: compatible with special surface preparation and/or application. NR: not recommended because of known or suspected problems. Certain combinations

Enamels may be cured by air drying or oven baking. Air dry enamels are cured essentially by a combination of solvent evaporation and oxidation. Baking enamels incorporate catalysts and cross-linking agents that require heat for polymerization. Coatings may be classified according to curing method, as is shown in Table 15. Each generic type is discussed separately in the following sections.

Table 15 Classification of coatings according to methods of cure

Method of curing

Generic type

Comments

Air oxidation of drying oils (solvent thinned)

Oleoresinous

Good wetting, slow curing, soft film recommended in normal environments only

Alkyd

Good wetting and appearance, poor in alkaline or solvent environments

Silicone alkyd

Improved durability, gloss, and chemical resistance compared to alkyds, but still poor in alkaline or solvent environments

Phenolic oleoresinous

Good resistance to abrasion and mild chemical environments; however, dark color of binder precludes use in white or light tints

Solvent evaporation (lacquers)

Vinyl (polyvinyl chloride-acetate)

Good water resistance, limited solvent resistance, poor adhesion unless surface has been properly prepared with abrasive blast cleaning

Chlorinated rubber

Good water resistance, limited solvent resistance

Styrene-butadiene, styrene-acrylate

Good water resistance, limited solvent resistance

Coal tar

Soft, black only; of limited use, mostly on mechanically cleaned surfaces

Polyvinyl-butyral

Exclusively used in pretreatment (wash) primers

Evaporation of water (latex, emulsion, water-thinned)

Acrylic

Recommended in normal environments only

Chemical reaction

Epoxy

Good water, chemical, abrasion, and solvent resistance, chalks freely on exterior exposure, difficult to topcoat

Coal tar epoxy

Improved water resistance and lower raw material costs compared to epoxies, black only. Difficult to topcoat

Polyester

Frequently used with glass fibers to give abrasion- and water-resistant coating. Only fair alkali resistance

Zinc inorganic

Requires adequate surface preparation (SSPC No. 10, Near White Blast Cleaning); adequate curing time required; excellent corrosion protection; good abrasion, solvent, and high-temperature resistance; must be topcoated in aggressive environments; reacts with alkali-sensitive topcoats

Cementitious

Inexpensive, requires adequate curing for best performance, and tends to chalk with aging, poor corrosion resistance

Urethane

Good water, chemical, abrasion, and solvent resistance. Difficult to topcoat

Air-Oxidizing Coatings. Oleoresinous (oil-drying) coatings were among the earliest to be used to provide protection from environmental deterioration. These coatings contained natural vegetable or fish oils that cure to a solid by reacting with oxygen from the air. This usually slow reaction is accelerated by using driers in the coating formulation. Paints using these oils have excellent wetting properties and are used on poorly prepared surfaces such as SSPC-2 or 3.

Alkyd coatings are developed by reacting drying oils with phthalic anhydride or other polybasic acids that increase durability and hardness. Alkyd coatings are currently the most widely used coatings, because of their good wetting, flexibility, curing, and application properties. Alkyd coatings are among the most suitable for general atmospheric exposure.

Silicone alkyd coatings have a silicone resin reacted with the alkyd resin to form the resin binder. When compared to alkyd coatings, silicone alkyd coatings have superior chemical resistance, color retention, and gloss.

Phenolic coatings are prepared using resin binders that incorporate phenol-formaldehyde in dry oils. The use of phenolformaldehyde improves water resistance, but lowers exterior durability.

Epoxy resins combined with drying oils improve the corrosion resistance of epoxy ester coatings. Epoxy ester coatings are single-component coatings and should not be confused with two-component chemically reacting epoxies.

Lacquers are coatings that dry by solvent evaporation. The major resins used are acrylics, cellulosics, and vinyl. Lacquer coatings contain dissolved solid resins that form a continuous hard film after the solvent evaporates from the coating. Because coatings are not chemically active during curing or weathering, they can be redissolved in the same solvent. This allows lacquers to be readily overcoated, resulting in excellent intercoat adhesion. Lacquer coatings have poor solvent resistance, but have excellent chemical and water resistance.

Acrylic resins are frequently used in lacquers, because of their fast drying properties and chemical and water resistance. Chlorinated rubber coatings are fast drying and are being used increasingly in this country. Chlorinated rubber coatings have excellent moisture resistance. Coal tar and asphaltic coatings are used on below-grade structural steel because of their moisture resistance and abrasion resistance. Polyvinyl-butyral coatings, such as those conforming to military specification DoD-P-15328D, are used as pretreatments for alkyd and vinyl coatings applied to steel and galvanized structures.

Two-Component Coatings. In general, coatings that cure by chemical reaction have the best combination of durability and water, solvent, and chemical resistance. Chemically cured coatings are packaged in two separate containers, and the chemical reaction is initiated after the two components are combined. Epoxy coatings have either an amine or polyamide curing agent although polyester and polyurethane constituents have been used. Amino-cured epoxies tend to have better chemical and solvent resistance. Polyamide-cured epoxies have better flexibility and water resistance. Because epoxy coatings cure to a hard, smooth, solvent-resistant finish, they are difficult to overcoat. To ensure good bonding, topcoats are applied to incompletely cured undercoats, allowing the topcoats to chemically react with the undercoats. If the solvent has not evaporated by the time the topcoat is applied the solvent can be entrapped, resulting in the blistering of the topcoat. If the undercoat is completely cured before the topcoat is applied, a fog or mist coat (thinned topcoat) is first applied to improve intercoat adhesion. Epoxy coatings chalk during weathering, although chalking occurs to a smaller extent with newer materials. When chalking occurs, it can be removed by sanding or brush-off abrasive blasting. Epoxies are exothermic when they cure; they cure slowly when applied below 8 °C (50 °F) and extremely rapidly above 32 °C (90 °F). Coal tar epoxies have coal tar pitch added to the epoxy resin. This combination increases the water resistance and makes the coating more tolerant to poor surface preparation. A coal tar epoxy cured with a low molecular weight amine is especially resistant to an alkaline environment, such as occurs on a cathodically protected structure. Some coal tar epoxy systems become brittle when exposed to the sun and must be protected. Coal tar epoxies are more difficult to overcoat than epoxies and come in colors ranging from tan to black.

Urethane coatings provide the tough, durable, smooth finish that is typical of chemically cured coatings. Aliphatic urethanes provide bright, chalk-resistant finishes with exceptional physical properties. Urethane coatings may be air dried or heat cured.

Polyester coatings are used most frequently with glass fibers or flakes for reinforcement. Polyester provides a thick coating that is tough and durable, with good resistance to abrasion.

Inorganic zinc coatings are available as primers or complete coating systems. Water-borne inorganic zinc-rich coatings are based on post- or self-curing alkali silicates (sodium, lithium, potassium), alone or in combination. Solvent-borne inorganic zinc coatings are based on an ethyl silicate vehicle. Finely divided zinc dust is added to the water- or solventborne binder just before application. An inorganic zinc coating applied to a well-prepared ferrous substrate can provide good protection without being topcoated at film thicknesses from 0.75 to 1.25 mm (0.03 to 0.05 in.). All topcoats applied to inorganic zinc must be alkaline resistant, or the topcoat can saponify and become water soluble. Zinc coatings should not be allowed to come in contact with gasoline, because the zinc is slightly soluble and will be leached from the film.

Organic zinc-rich coatings may be formulated with a number of resins, but the epoxy resins are most widely used. Zinc-rich coatings are not abrasion resistant and should be overcoated when required with a coating system containing a compatible binder similar to that used in the original coating. Zinc-rich organic coatings are more tolerant to poor surface preparation and are easier to topcoat.

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