Lesson 2-1 PURPOSE, COMPOSITION, AND TYPES OF PROTECTIVE-COATING MATERIALS
2-1. Painting Terms
The following are some of the common terms found in protective-coating materials that you will encounter in this lesson as well as throughout the ACCP. They will assist you with your understanding of protective coatings.
2-2. Protective-Coating Composition
To select the best protective coating for a given surface, it is important that you understand the composition of coatings. All coatings–whether used for corrosion protection, to provide good aesthetics or a pleasing appearance, or for any other purpose–will contain a film-forming material. This material may be organic or inorganic and after it has been applied, it may form a hard, impervious film, a soft porous film, or combinations in between. The film-forming material may be clear (unpigmented) or filled with a variety of different pigments, depending on its function. When the film-forming material (resin) contains pigments, it is called a binder. The binder will hold the pigment particles together and hold them to the surface they have coated. When the binder (resin plus pigment) is dissolved in a solvent to make a liquid, the combination (solvent, binder, and pigment) is considered to be a vehicle. The term vehicle comes from the ability to transport and apply the liquid to the surface being coated. Once on the surface, the solvent evaporates and the vehicle becomes a pigment-binder system. Paint application properties (such as spreadability) are usually characterized by the vehicle in its liquid condition. The viscosity, rate of solvent evaporation, and consistency of the wet coating are most important during application. After application, the pigments determine the corrosion-inhibitive properties and, generally, the color and some flow-control properties of the applied coating. The binder determines the weatherability of the coating, its environmental resistance, and its ability to function in a given environment. The required surface preparation, and often the application equipment and techniques, are determined by the binder.
2-3. Purpose and Function of Protective-Coating Materials
The use of and demand for protective coatings have grown until, today, over 5 billion dollars each year is spent on them. Commercial establishments, uses a considerable quantity of these coatings for protection, sanitation, cleanliness, illumination, visibility, safety, efficiency, appearance, camouflage, and fire retardance.
a. Protection. Protection of the surface is the most important purpose for paint. If properly chosen and applied, your painting efforts will protect wood buildings from moisture and sunlight, metal structures from corrosion, and concrete and masonry surfaces from the weather. Paint, when regularly programmed, offers long-range protection that extends the useful life of a structure.
b. Sanitation and Cleanliness. The use of paint and proper painting operations will promote sanitation and cleanliness. Paint provides smooth, nonabsorptive surfaces that are easily washed and kept fee of dirt. Such surfaces tend to keep foodstuffs from adhering. Adhering foodstuff harbors germs and causes disease. The coating of rough or porous areas seals out dust and grease that would otherwise be difficult to remove. Paint coatings will reveal buildup of foreign substances, indicating that better housekeeping practices are in order. Paint coating, therefore, is an essential part of general maintenance programs that is performed for hospitals, mess halls, offices, warehouses, and living quarters.
c. Illumination and Visibility. When white or light-tinted paints are applied to ceilings and walls, they reflect both natural and artificial light, help brighten rooms, and increase visibility. On the other hand, darker colors reduce the amount of reflected light. Flat paints diffuse, soften, and evenly distribute illumination; whereas gloss finishes reflect more like mirrors and may create glare. Color contrasts improve visibility of the painted surface, especially when paint is applied in distinctive patterns. For example, white on black, white on orange, or yellow on black can be seen at greater distances than single colors or other combinations of colors.
d. Safety and Efficiency. A recent innovation is to use paint for the identification of pipes carrying different material, such as oil, steam, water, and air, through a hangar or an industrial plant. Paint is also used to indicate hazards or danger areas by using various colors. This area usage of paint is expanding at a very rapid pace, especially in schools, hospitals, other institutions, and industrial plants.
e. Appearance. The primary use of paint is to provide maintenance and improve safety and efficiency. Decorative painting is sometimes considered to be secondary in importance and is therefore kept at a minimum. However, the functional use of color creates comfortable living and working conditions and more pleasant surroundings that result in improved morale and increased efficiency.
f. Fire Retardance. Application of fire-retardant paint will delay the spread of fire and assist in confining it to its origin. This allows more time during which fire-fighting equipment can arrive to extinguish the blaze before it gets out of control. The use of fire-retardant paint is restricted to appreciable areas of highly combustible surfaces and for selected uses. Use of this paint type is further restricted to interior surfaces except for Arctic areas. You should not consider fire-retardant paint as a substitute for conventional paints because its cost is much higher.
2-4. Principle Ingredients of Protective-Coating Materials
The principal paint ingredients are the pigment and the vehicle. The pigment is the part of the paint that constitutes the solids, and the vehicle is the fluid. The many types of materials used in the formulation of paint are far too numerous to describe in this lesson; however, in order to acquaint you with the most common paint pigments and vehicles, they are discussed as a class, and in some instances, the more important materials in a class are described.
Pigments are fine, solid particles used in the preparation of paint, which are insoluble in the paint vehicle. Pigments are available in two forms: paste and powder. Powder forms a stiff consistency when combined with oil, lacquer, or an other vehicle. There are five main types of pigments to use: white, color, metallic, black and extender.
a. White Pigments. White pigments are of great importance to you and the paint industry because white is a widely used color and serves as the base for popular pastel or tinted colors. The most common white pigments used are titanium dioxide and antimony oxide. Due to regulations, lead and zinc-oxide pigments are no longer in use or are restricted to 0.06 percent or less of the paint's content.
(1) Titanium dioxide. This white pigment has been available since 1932. It has a definite advantage where maximum hiding is required. Titanium dioxide has overshadowed all other white pigments because of its chalking ability. This means the surface becomes chalky, and you can rub or wash it off. This is an obvious advantage when white or light-colored house paint is used because rain will wash away the chalk, and along with it goes the dirt and the grime, leaving a clean, bright surface. Its disadvantage is with dark colors because it gives a fading appearance to the color.
(2) Antimony oxide. This pigment is used chiefly in the manufacture of fire-retardant paint, where it is used in combination with chlorinated materials and calcium carbonate. The Navy uses paints with this type of pigment on interior surfaces of its ships.
(3) Zinc oxide. This is one of the finest of all white pigments and actually one of the whitest. It is opaque to ultraviolet light and, when incorporated in a paint, protects the film from the destructive action of the ultraviolet rays in sunshine. While zinc oxide contributes to the hiding power of paint, some of its other properties are more important. It is not discolored by sulfur gases in the atmosphere, and it is nonpoisonous. Zinc oxide is used as a preventive for mildewing or yellowing and also adds hardness to a paint film. It is a valuable pigment for controlling checking, chalking, and dirt retention of exterior paint films. It is also used in metal-protective paint for rust inhibition and in other paints for greater resistance to scrubbing. There is a current trend to use latex primers in place of zinc-oxide paint for rust-inhibition work.
b. Color Pigments. Color pigments can be obtained from deposits in the earth's crust or through chemical processes.
(1) Earth colors. The most widely known earth colors in the painting industry are yellow ocher, raw and burnt sienna, and raw and burnt umber:
Yellow ocher. This color contains hydrated iron oxide as a coloring matter. Various ochers contain from 10 to 60 percent hydrated iron oxide, the remainder being siliceous matter or clay. In color, yellow ochers vary from pale to very dark yellow, almost olive. When of good quality, they are excellent pigments, permanent in color, and combine well with all paint vehicles as well as other pigments. When used with white pigments, they will produce fine-cream or buff tints. They are primarily used as tinting colors.
Raw sienna. This color resembles yellow ocher in general composition, since it is colored by hydrated iron oxide. It is brownish-yellow in color. When used with white, it produces cream tints which have greater color strengths than ochers. Raw siennas are valuable for use as staining and graining colors. However, they are undesirable for base colors.
Burnt sienna. This color is obtained by roasting raw sienna, thereby producing a strong, reddish-brown pigment. Burnt sienna is used to a lesser degree than raw sienna. However, it is a very valuable pigment when used for shading and glazing.
Raw umber. This color is similar in composition to sienna. It has a greenish-brown color, is permanent, possesses medium opacity, and mixes well with paint vehicles. Raw umber gives drab tints to white and gives a great variety of other shades when mixed with color pigments, and it is used extensively for these purposes.
Burnt umber. This color is calcined (roasted) raw umber. It possesses a rich, brown color that is darker than raw umber and free of redness. Burnt umber is used in practically all types of paints and stains.
(2) Chemical colors. Chemical colors are very important today and are used more than earth colors because they can be produced cheaper by chemical reaction than from basic raw materials. These colors consist of blue, green, maroon, orange, red, yellow, brown, and violet pigments.
c. Metallic Pigments. Metallic pigments are used to form a metallic film (barrier) in paint. The most common are aluminum, copper brown, and zinc:
(1) Aluminum pigments. These pigments are largely pure metallic aluminum and contain appreciable amounts of a polishing lubricant and a mixture of stearic and other fatty acids. They are purchased in either paste or powder form. An average formula mix for the production of aluminum paint is 2 pounds of aluminum powder or paste to 1 gallon of mixing varnish.
(2) Copper pigments. Copper brown powders are usually alloys of copper with small amounts of zinc and iron. Approximately 1 to 2 pounds of metal per gallon will give good hiding qualities. Copper brown in combination with small amounts of a toxic substance, such as a mercury or arsenic compound, is used for making antifouling ship-bottom paint. Various shades, from a light brass to a dark, antique copper, are used in nitrocellulose lacquer to make the popular brass and bronze finishes.
(3) Zinc pigments. Zinc dust is a metallic-gray powder with about 3 percent zinc oxide on the surface. It has gained importance as a pigment for making metal-protective coatings.
d. Black Pigments. One of the most important black pigments known to the paint industry is lampblack. It is most commonly used for shading paints, since it has exceptionally great shading strength and is extremely permanent.
e. Extender Pigments. Extender pigments are certain white or colorless natural substances that are ground into a fine powder and added to paint. They are used to—
Some of the more important extender pigments are barytes (barium sulfate), china clay (aluminum silicate), mica, silica (glass sand), talc (magnesium silicate), and whiting (calcium carbonate).
In a layman's language, the vehicle is defined as the liquid portion of a paint, enamel, lacquer, or similar substance which holds the pigment in the solution. The vehicle may be any of such liquids as volatiles (thinners or solvents) and such solids as nonvolitiles (drying oils, driers, resins, and varnishes). It is perhaps the most important part of the coating, as it furnishes the desired qualities of adhesion, toughness, flexibility, and resistance to various environments. It gives the paint its strength and life. The vehicle is usually divided into two parts; they are volatiles and nonvolatiles.
a. Volatiles. Volatiles are the nonsolid portions of vehicles that evaporate. They consist of thinners and solvents. Volatiles, more commonly called thinners, are organic liquids that are used to reduce the viscosity (thickness) of a vehicle or paint to a suitable brushing or spraying consistency. Volatiles are also used to control many other properties of paint, such as the ability to penetrate and/or wet the surface being coated, the leveling, the lap time (period between paint coats), the dispersion of pigments, and the ease of brushing or spraying. Some of the more common volatiles are turpentine, mineral spirits, naphtha, xylene, acetone, and various alcohols, ethers, ketones, and esters. Water, of course, is used as a thinner for water-based paints. When selecting volatiles to thin paint for use in the field, you should follow the directions of the paint manufacturer or the specifications, since some paint vehicles will not mix with certain volatiles.
b. Nonvolatiles. Nonvolatiles are the solid, nonevaporating portions of a paint that are left after the solvent evaporates. They are the solid portions of the vehicle that form the actual film on the surface. They consists of film-forming materials, such as drying oils, driers, resins, and varnishes.
(1) Drying Oils. Drying oils are nonvolatile vehicles. A nonvolatile vehicle is defined as the liquid portion of a paint aside from its volatile thinner and water (or other solvent). Not all oils are drying oils. An oil is classified as a drying oil if it will set or harden under normal exposure conditions when applied as a thin film. Some of the most commonly used drying oils are linseed, soybean, tung, oiticica, perilla, fish, and dehydrated castor.
(a) Linseed oil. Linseed oil, the most important of drying oils and the principal nonvolatile paint vehicle, is obtained from flaxseed. It is used as the basic vehicle in paint because it has a natural ability to dry in the open air, forming an elastic, durable, solid film which resists the attack of weather and wear. The addition of a drier will hasten its normally slow hardening rate.
(b) Soybean oil. Soybean oil, a slower drying oil than linseed oil, is generally used with faster drying oils. The main use for it is in varnish vehicles for interior paints and enamels. Paints made with soybean oil resist yellowing. It is used in some of the best interior, white enamels.
(c) Tung oil. This oil is one of the chief oils used in the manufacture of fast-drying, waterproof, oil varnish. Since raw tung oil dries rapidly but not to a smooth film, it is not used in exterior house paint; whereas linseed oil is almost universally used. Its chief usage is in the manufacture of varnish.
(d) Oiticica oil. This oil is extracted from the seeds of certain trees growing in northern Brazil. It is similar to tung oil and is used in the manufacture of paint and varnish.
(e) Perilla oil. This oil is extracted from the seeds of the perilla plant grown in Manchuria. It is extensively used in the manufacture of varnish because of its superior drying and film-forming qualities.
(f) Fish oil. This oil is mostly extracted from the menhaden and the pilchard fish. It is satisfactorily used for covering hot surfaces, such as smoke stacks, where hard-drying paints are not suitable. It is not recommended for interior use.
(g) Castor oil. This oil is produced from the castor-oil plant. Its use has increased since being accepted as a satisfactory replacement for tung oil.
(a) Driers are catalytic agents. They hasten film hardening when added to a drying oil or a paint or varnish that contains oil. The most common driers used in paint are oil-soluble naphthenates of cobalt, zinc, manganese, or lead. Lead driers promote hard drying throughout the thickness of the film and influence the polymerization rate of the vehicle more than they do the oxidation rate. Lead driers are not as strongly catalytic as others in promoting the drying of paint films. Cobalt driers assist oxidation strongly and, if used alone, may cause wrinkling because of rapid drying of the outer layer of the film.
(b) Driers are classified as through driers and surface driers. Through driers are compounds of lead, cadmium, zinc, or zirconium. Surface driers are compounds of cobalt and manganese. Use surface driers in conjunction with through driers to prevent wrinkling, uneven drying, and such. Table 2-1, summarizes individual drier characteristics and their effect on the film if the drier is used alone.
Table 2-1. Characteristics of driers
|Classification||Compound||Characteristics||Effect (If used alone)|
|Through drier||lead, cadmium, zinc, and zirconium||Hard drying throughout||Tough film|
|Surface drier||Manganese||Intermediate drying||Brittle film|
|Cobalt||Rapid drying of outer layer||Wrinkling of film|
(c) When using a given paint or vehicle you may make use of one or both of the drier classifications, depending on the nature of the paint product. If too much drier is used or is improperly combined, the drier mixture may adversely affect the durability of the film or may not allow the paint film to harden. It is safer to purchase finished paint products with driers added whenever practicable, since the addition of driers in the field is readily susceptible to error.
(3) Resins. Natural and synthetic resins available to the paint industry are so numerous and varied in properties that they will only be mentioned. This same complexity and multiplicity make a definition almost impossible. Resins, together with drying oils, comprise the bulk of film-forming materials, or binders, in the surface-coating field. Natural resins, such as resin and shellac, are extracts from trees and insects. Synthetic resins, such as alkyds, alkyd modifiers (vinyls, phenolics, silicones, epoxies), acrylics, and polyurethanes, are manufactured from basic organic material, such as petroleum, alcohol, coal tar, and vegetable oils.
(a) Alkyds. Alkyds are the backbone of modern paint in a great many combinations. The alkyds are characterized by toughness, flexibility, and durability Use them in exterior and interior coatings for utility and decorative purposes. However, they do not have good chemical resistance.
(b) Alkyd modifiers. Other resins (modifiers) are combined with alkyd resins to improve its properties. Examples of alkyd modifiers are vinyls, phenolics, silicones, and epoxies.
Vinyls. Vinyls are used where construction materials, metal, and wood must be protected from high humidity or an acid-caustic environment.
Phenolics. Phenolics appear in varnishes and enamels that require extra hardness and abrasion-resistance.
Silicones. Silicones appear primarily in heat-resistant coatings. They are not particularly hard-surfaced and may craze. They are usually baked.
Epoxies. Epoxies have outstanding adhesion and chemical resistance. In combination with other resins, they appear as baking enamels. Air-drying types are frequently used with converting agents just prior to application to set up films of improved characteristics.
(c) Acrylics. Acrylics have outstanding light resistance and weather durability, and they have moderate chemical resistance. Used with nitrocellulose as a hardener, they have better drying properties and increased hardness. Styrene acrylic and latex emulsions are some examples of acrylics.
(d) Polyurethanes. Polyurethane coatings are characterized by a very high gloss, superior toughness and weather durability, and good chemical resistance. They are also converted or catalyzed in coating applications.
c. Varnishes. Varnishes contain oil, resins, thinners, and driers. They are produced by combining oils and resins in definite proportions, then adding thinners and driers to the mixture. The various types of resins, oils, and other available components make it possible to produce varnishes adaptable to many uses. You may use varnishes as clear coatings or as vehicles for pigmented paints.
2-7. Types of Protective-Coating Materials
You must determine the best type of protective coatings for your use. There are many factors for you to consider, such as the condition of the surface, the method of application, curing conditions, the service expected by the user, the cost, and the length of time the appearance must remain satisfactory. The materials mentioned in the previous sections on pigments (paragraph 2-5) and vehicles (paragraph 2-6) are used to make many different types of organic finishes for decorative and protective purposes; however, some of the most common types are oil-based paint, enamel, lacquer, water-based paint, varnish, shellac, and stain:
a. Oil-Based Paint. Oil-based paint consists principally of a drying oil (usually linseed) mixed with one or more pigments. Oil-based paint may be modified by the addition of varnish. The exterior surface of houses and metal surfaces in atmospheric exposure will usually be coated with oil-based paints. The vehicle in these paints can be a combination of raw and processed oils, or it may be a single oil, depending on the properties required in the paint. The pigments and their quantities are usually selected on the basis of cost and their ability to impart the desired application properties, such as durability, economy, brushability, and color. Oil-based paints are characterized by easy application, slow drying, and a good ability to wet the surface being coated. They normally chalk in such a manner that permits recoating without costly surface preparation.
b. Enamel. Enamel is a paint which is characterized by the ability to form an especially smooth film. By definition if the pigment product is relatively easy to brush and is used on large areas such as walls or structural steel, it is called a paint. If it is relatively fast drying, levels out to a smooth, hard finish, and is used on relatively small areas or smooth substructures such as woodwork, it is called an enamel. Enamels are commonly thought of as pigmented, varnish-vehicle paints that have good flow and leveling properties and dry rapidly (4 to 16 hours) at normal temperatures. In general, the ability of enamel to wet the surface and coat surface irregularities is not as good as oil-based paints. Enamels generally fail by chipping, cracking, blistering, or similar action as a result of a gradual decrease of elasticity that comes with age. Enamel films are generally harder, tougher, and more resistant to abrasion and moisture penetration than oil-based paints. Enamel may be applied by brushing, spraying, or dipping. When applying enamel by brush, flow the paint on the surface rather than brushing it out as with oil-based paints. Enamels of the baking type are widely used in industrial finishing.
c. Lacquer. Lacquer differs from oil-based paint and enamel because it contains some type of resin as a vehicle. Lacquer is normally applied by spray since most types dry too fast for brushing. Lacquer is usually applied by a series of thin films. You can obtain a hard, brittle coating; a tough, elastic coating; a high or low gloss film; and many other variations by the proper choice of lacquer materials. The durability of lacquer finishes on some makes of automobiles is proof that good protection is given by lacquer. Vinyls and rubber-based solutions (paints) are technically lacquers. They have outstanding durability in many types of exposures.
d. Water-Based Paint. Water-based paint is distinctly different from other paint because the vehicle is an emulsion of binder and water. The emulsion technology is promising for VOC compliance because water is not a restricted solvent. Water-based paints have the advantages of easy application, drying by evaporation of water, easy reduction of viscosity with water, and easy cleaning of tools with soap and water. This type of paint was originally used as an interior coating; however, with product advances in water-based paints and the increased concern over fire prevention, its usage has broadened to outside surfaces as well. Water-based paints fall into many categories, but casein, latex, and calcimine are the more important ones.
(1) Casein. In casein paint, casein (protein of milk) is used as the binder. Casein paint consists of powdered casein, hydrated lime, inert and hiding powdered pigments, a preservative, and tinting colors. Small amounts of vegetable oils are added to improve its washability. The principal pigment added to casein paint is titanium. Casein paints usually cover a surface wall in one coat, dry rapidly, and adhere to new, unsealed plaster without blistering the paint. Use casein paints for interior decorative work on plaster, wallboards, fiberboards, stucco, cement blocks, and such. Casein paints are also available for exterior masonry, stucco, brick, and concrete.
(2) Latex (emulsions). This term was originally used to describe raw, latex rubber-emulsion paint (rubber in water). Now, the term is used in connection with all resin and rubber-emulsion paints. Water is used to both thin these paints and clean the brushes. Many types have been made, but vinyl and acrylic emulsions are the most popular. Emulsion paints are very good for use on interior and exterior masonry because they breathe, allowing vapor to slowly pass through the film. Emulsion paints are glossless, and you must ground special colors in a suitable vehicle to color them.
(3) Calcimine. Calcimine is a mixture of powdered pigments, such as whiting and china clay, and glue. You compound the paint by mixing the prepared pigments with water. The paint is an inexpensive way to cover wet (damp) wall and drywall interior surfaces. A compound of glue, a tinted wash, and calcimine may be used on wet walls. The best results, however, are obtained by using a varnish sealer and primer before applying the calcimine.
e. Varnish (Resin). Varnish is a transparent or opaque liquid that provides protection and decoration. It dries to a hard, transparent coating. The kinds and types of varnishes used are numerous. Some manufactures' catalogues list from 100 to 200 varieties. Oil varnishes are the most frequently used and include spar varnish, interior varnish, flat varnish, rubbing varnish, and colored varnish. All of the above varnishes are extensively used to finish and refinish interior and exterior wood surfaces, such as floors, furniture, woodwork, metal fixtures, and so on. They produce a durable, elastic, and tough surface that dries to a high-gloss finish and does not mar easily. Exterior varnishes are specially formulated to resist weathering.
f. Shellac. Shellac comes from the secretions of the scale insect. This insect secretes a resinous material on trees which is gathered, washed, and ground, and then melted and formed into a thin sheet. These thin sheets of shellac are then broken into flakes (one of the forms in which shellac is sold on the market). Shellac is now produced with chemicals in modern factories. Shellac is soluble in alcohol and used extensively on furniture as an undercoat and over knots to prevent bleeding.
g. Stain. Stain is used to change the color of various types of wood and to bring out the beauty of the grain. They are usually applied before the varnish or other finish; however, some oil stains are used as a preservative for shingles and other rough, exterior, wood surfaces. The most commonly used stains are water, spirit, oil, and varnish.
(1) Water stain. Water stain is cheap, penetrates the wood deeply, and is transparent. It produces an even penetration and gives the most even and clearest tone of all stains. A disadvantage in its use is that the water raises the grain of the wood that is being finished, requiring extra sanding. Use water stains for inside woodwork and furniture.
(2) Spirit stain. You make spirit stain by mixing aniline colors in alcohol. They dry quickly, and the colors are brilliant, transparent, and beautiful. When you use a spirit stain, the surface may be stained, filled, and shellacked or varnished the same day. For this reason, spirit stains are useful for repair and touch-up work on floors, woodwork, and furniture and for all sorts of quick jobs. Spirit stains penetrate old varnish surfaces to a greater depth than other stains, but they are not permanent in sunlight or very strong artificial light.
(3) Oil stain. An oil stain is probably the most convenient for finishing woodwork of various kinds. It is easily obtained, easily applied, and does not raise the grain of the wood. It dries slowly enough to permit brushing and rebrushing without showing lap marks. If the color is too dark after the stain is applied, you may wipe off some of it to produce a lighter color. Use Table 2-2 to determine what oil stains to mix, then mix oil stains in a base of 1/2 gallon boiled linseed oil and 12 gallon of turpentine or mineral spirits. For example, to make mahogany, add 1 pint Vandyke brown and 1 1/2 pints rose to the base.
Table 2-2. Mixing oil stains
|Cherry||1 1/2 pints|
|Mahogany||1 pint||1/2 pint|
|Light oak||1 pint||1/4 pint|
|Dark oak||Touch||1 pint||1/2 pint|
|Old maple||1/2 pint||1/2 pint||1/2 pint|
|Walnut||1 pint||1 1/2 pints||1/4 pint|
(4) Varnish stain. Varnish stain is made by adding a stain to a varnish. When you use a varnish stain, it is possible to stain and varnish in one operation.