Organic Coatings

Organic coatings have a wide variety of properties, but their primary uses require corrosion resistance combined with a pleasing colored appearance. An organic-base film is often resistant to certain environmental substances but not to others, and so must be chosen for a specific set of well-defined service conditions. For example, a vinyl paint might be used on a pump casing that must operate in contact with acidic industrial waters. However, if the same casing is expected to contact hydrocarbons such as gasoline or solvents, a styrene, epoxy, or phenolic coating would most likely provide superior protection.

Types of Organic Coatings. The term paint was once commonly used to designate all liquid organic coatings, but it is considered inadequate to describe modern liquid organic coatings, which in general are subdivided into enamels, lacquers, aqueous mixtures, suspensions, bituminous substances, and rubber-base products. Resins dispersed in a vehicle--for example, enamels or lacquers--cure to relatively hard gels by polymerization, oxidation, or solvent evaporation. A comparison of the chemical and environmental resistance of common resins is given in Table 12.

Table 12 Properties of organic coatings on iron castings

Resin

Resistance to chemicals and environment(a)

Application method

Curing Method

Typical applications

Hydrocarbons

Solvents

Acids

Alkalies

Salts

Water

Weathering

Heat

Cold

Abrasion

Need for primer

Spray

Dip

Fluidized bed

Air dry

Bake

Low cost

Phenolic

E

E

E

F

E

E

E

G

E

E

no

yes

yes

no

yes

yes

Appliances

Urea

E

G

E

E

E

G

G

G

G

E

no

yes

yes

no

no

yes

Appliances

Polyester

G

G

G

F

E

G

G

G

G

G

no

yes

yes

no

yes

yes

Thick coatings

Alkyd

G

P

F

F

E

G

E

F

G

G

yes

yes

yes

no

yes

yes

General purpose

Epoxy

E

E

E

E

E

G

G

E

E

E

no

yes

yes

yes

yes

yes

Scratch-resistant finish

Polyethylene

G

E

E

E

E

E

G

F

E

F

no

no

no

yes

no

yes

Coatings

Styrene-butadiene

E

G

E

E

E

E

G

G

E

G

no

yes

yes

no

yes

yes

General purpose

Urethane

G

E

G

G

E

E

E

E

G

G

yes

yes

yes

no

yes

chemical and marine finishes

Moderate cost

Vinyl chloride

G

F

E

E

E

E

E

G

G

E

yes

yes

yes

yes

yes

yes

Chemical equipment

Melamine

E

G

E

E

E

G

E

G

G

E

no

yes

yes

no

no

yes

Appliances

Polyamide

F

G

F

E

E

F

P

G

G

E

no

yes

yes

yes

yes

yes

Abrasion-resistant coatings

Vinyl butyral

G

F

F

G

G

G

E

G

E

E

no

yes

yes

no

yes

yes

General purpose, primers

Cellulose nitrate

F

F

G

F

E

E

E

P

G

F

yes

yes

yes

no

yes

yes

High-gloss lacquer

Acrylic

F

P

F

G

E

E

E

P

F

F

yes

yes

yes

no

yes

yes

Water-resistant finishes

Vinyl acetate

F

P

F

F

F

G

E

F

F

E

no

yes

yes

no

yes

yes

Decorative

Cellulose acetate butyrate

F

F

F

F

G

F

E

P

G

F

yes

yes

yes

no

yes

yes

Decorative

High cost

Chlorinated polyether

G

E

E

E

E

E

E

G

E

F

no

no

no

yes

no

yes

Chemical equipment

Fluorocarbon

E

E

E

E

E

E

E

E

G

P

yes

yes

no

yes

no

yes

Chemical equipment, nonstick surfaces

Silicone

G

F

G

G

G

E

E

E

G

yes

yes

yes

no

yes

yes

Heat-resistant

finishes

Enamels consist of milled pigments and other additives dispersed in resins and solvents and are converted from liquids to hard films by oxidation or polymerization. Lacquers are thermoplastic resins dissolved in organic solvents that dry rapidly by evaporation. In aqueous coatings, water is the principal vehicle or reducer. The advantages of water-base paints are nominal cost, nonflammability, true odorlessness, and nontoxicity. The disadvantages are difficulties in wettability, flow, and drying. Rubber-base coatings are noted for their mechanical properties and corrosion resistance rather than their decorative effects. Bituminous paints are black materials in which coal tar is dissolved in a solvent that evaporates. The major uses of bituminous paints are those that require extremely low permeability and high resistance to water. Unusual protection against chemical solutions, or special decorative effects, can be obtained by the use of asphaltic coatings or those produced by japanning, both of which are also considered bituminous coatings.

Fluorocarbon coatings produce an unusual combination of properties. They are tough, stain resistant, and nonsticking, and have a very low coefficient of friction. Fluorocarbon coatings resist all common industrial acids and temperatures to 300 °C (570 °F). Domestic cookware and chemical-processing equipment are two major applications of fluorocarbon coatings on iron castings. Fluorocarbon coatings are sprayed as emulsions of proprietary products onto a primed surface and then fused at temperatures of 385 to 425 °C (725 to 800 °F).

Methods of Application. Organic coatings are applied by spraying, dipping, flow coating, fluidized bed coating, electrostatic deposition, and electrophoresis (electrocoating).

Spraying is adaptable to both low-volume and high-volume workloads. It is done by propelling the coating material toward the workpiece by compressed air, hot spraying, hydraulic-airless, and airless-electrostatic methods. Overspraying is most troublesome with compressed air methods and least troublesome with electrostatic methods.

Dipping has been used for centuries; modern refinements include flow coating and electrophoresis. Not all shapes can be painted by dipping. Pockets can exclude paint from surfaces. The shape of the casting should allow easy draining after dipping. The coating should be selected to inhibit sagging or the formation of droplets on edges. The dipping process is easily automated and can be very efficient in use of materials. A thorough review of organic coatings and their applications, advantages, and limitations is available in Ref 25 and in the articles "Painting" and "Elastomeric Coatings and Linings" in this Volume.

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