Coating
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Description
A thin layer deposited or applied on the surface of an object (substrate). The primary purpose of applying a coating is to improve an objects critical properties and to create a protective barrier against deterioration of the surface. Secondary purposes may be aesthetic. Coatings may be applied as liquids, gases or solids e.g. powder coatings.
Examples of the types of materials are used for coatings include:
- Fabrics, Metals, Paints, Varnishes and Polymer films.
Often coatings are used to achieve multiple goals. For example, coatings can be formulated to:
- Seal a surface, form a barrier, change color, produce or minimize texture, maintain surface appearance, be anti-corrosive, be anti-reflective, decrease friction, increase wearability, minimize light transmission, provide adhesion, prevent adhesion (non-stick), provide flame resistance, and to provide identification.
A major consideration for most coating processes is to control coating thickness and adhesion. Methods of achieving this range from a simple brush to expensive precision machinery.
- Application processes include: draping, brushing, rolling, dipping, spraying, baking, shrink-wrapping, vapor deposition, electroplating, anodizing, printing and laminating.
| Synthetic coatings | Process | Application | Advantages | Disadvantages |
|---|---|---|---|---|
| Acrylic | Premixed solutions of acrylic polymer bits dispersed in water; Cures by drying; Multiple options for clear or colored coatings. | May emit acetic acid vapors (allow 4 weeks cure); cleans with soap and water while wet | Cost effective, dry quickly easy to use; Resists weathering and fading; Environmentally friendly | Surface must be well-prepared for optimal adhesion |
| Alkyd | Oil-modified polyester that cures through oxidation | May release peroxides and carboxylic acids; cleans with solvent based products | Glossy finish and robust; Resists weathering and wear; Glossy finish | Slow drying with strong odor during application |
| Ceramic coating | Resin containing silica and titanium dioxide that chemically bonds to paint; Applied using a spray gun | Primarily used as a high gloss top coat for autos | Improves durability and wear resistance; Has low surface tension; | Cannot be used directly on a substrate; Provides little protection from tensile or compressive stress |
| Epoxy | Two component solution (epoxy resin and hardener) carefully mixed prior to used; Water-based options available | Careful mixing and curing required | Excellent for metal surfaces; High moisture resistance; Resists corrosion and damage | Surface must be well-prepared; Coatings are sensitive to UV light and temperature; Low flexibility |
| Laminate | A multilayer material or a film used to provide a protective layer | Plastic or aluminum sheets adhered to the substrate | Provide a barrier to moisture or volatile organic | The film must be uniformly sealed without any holes or gaps |
| Polyaspartic | Polyurea coatings formed by reaction with polyisocyanates and aspartic esters | Buildings, bridges, automobiles | Cures rapidly; Highly durable; Resists weathering and wear; Maintains high gloss and does not yellow | Surface must be well-prepared to ensure adhesion |
| Polyester | Synthetic resin that hardens with a catalyst | Strong, durable surface that withstands abrasion, weathering and wear; Flexibility minimizes cracking | Application requires precise control over temperature and humidity to fully cure. | |
| Polyurethane (aliphatic) | Pre-mixed solution of a polyol with an isocyanate; cures with moisture in air; water-based options available | Must be cured completely at optimal conditions of moisture and temperature; isocyanate is a carcinogen | Often used as a top coat; Highly durable; Resistant to wear and weather; Excellent gloss and color retention; Low VOC formulations available | |
| Powder Coating | An electrostatic process to adhere powder-based substance onto a charged metal surface followed by heat curing. | Metals, household appliances, automobile parts | Coatings are durable with many color options; No solvents are used thus no VOCs emissions; Typically used on metals | Requires spray booths and ovens; Difficult to achieve thin coatings |
| Silicone | Oligosiloxanes dispersed in water that dry into a flexible silicone resin | Spray, dip or paint; Clean with soap and water; Does not need primer; Non-toxic | Heat and moisture resistant; Good adhesion; Highly flexible; May be used on multiple surfaces even fabric and concrete | Requires up to 48 hours to fully cure; Heat and/or UV light can speed curing |
| Polysiloxane | Option that combines silicone and polyurethane properties, without using isocyanates | Applied directly to primed steel or previously painted substrates by brush, roller or spray; Low VOCs | Durable, glossy finish; Highly resistant to UV, weather, and chemicals; Retains color and gloss | Surface must be well-prepared to ensure adhesion; Low flexibility |
| Natural coatings | Examples | Application | Advantages | Disadvantages |
|---|---|---|---|---|
| Drying oil (unbaked) | Linseed oil, Tung oil, Tall oil, Poppyseed oil, Perilla oil; Sunflower oil; Hempseed oil; Safflower oil; Lumbang oil; Oiticica oil; Stillingia oil; Walnut oil | After application to a substrate, drying oils auto-oxide in ambient condition forming a thin non-tacky tough film. The curing time for each type of oil is depentent on its degree of unsaturation. Additives can speed the drying time. A solvent is required for cleanup. | Efficient provides antifungal and antibacterial properties, water repellency, and protection from UV; Typically used for wood protection and preservation | Known to release peroxides for the first few days and carboxylic acids for many weeks or many months; Many oils will discolor with age, especially with exposure to UV light; dDifficult to remove |
| Carbohydrate | Starch, Cellulose, Pectin, Gums, Agar, Carrageenan, | Most materials swell in water to form a gel that dries by releasing water; Additives, such as lignin, can decrease biological attacks | Most are typically used as adhesives, consolidants, emulsifiers, and thickeners; Films are thin, flexible and matte; Non-toxic; Biodegradable. | Films usually shrink on drying and are not optically transparent; they are susceptible to biological attack; Rarely used as a durable coating |
| Mineral | Whitewash (Calcium carbonate/Calcium hydroxide); Water glass (Potassium silicate, Sodium silicate, or Calcium silicate) | Appropriate for concrete, stucco, brick, plaster or any adobe-type of surfaces; Some have been used on glass, porcelain, leather and textiles; Brushes and rollers, while still wet, clean easily with water. | Coatings penetrate and fill pores and cracks; good durability; Non-flammable providing passive fire protection | Process is not reversible |
| Protein | Casein, Collagen, Gelatin, Albumin (egg white), Keratin, Whey, Soy protein, Zein, Animal glue | Ensure formula does not contain a drying oil in the formula, Brushes and rollers, while still wet, should clean easily with water. | Suitable for most objects and outside of case; Non toxic; Biodegradable; Most often used as adhesive and not a coating | Most are hydrophilic; Hot water will cause breakdown to smaller components; Additives are need to produce a durable film; |
| Resin (Varnish) | Shellac, Dammar, Mastic, Sandarac, Rosin, Amber, Copal, Myrrh | Soluble in hydrocarbon solvents (such as turpentine) and sometimes in alcohol; Drying occurs with solvent evaporation; Solvent required for clean-up | Suitable in enclosures after complete drying; Avoid direct contact with object in a humid environment as surface may become sticky; Safe for most collections; | Poor vapor barrier unless many layers are used; Dry film becomes sticky in moist environments; Solvent is the main VOC; May discolor with age especially with exposure to UV light. |
| Wax (plant or mineral) | Beeswax, Lanolin, Spermaceti, Lac wax, Carnauba, Candelilla, Bayberry, Japan wax, Paraffin, Ceresin, Ozocerite, Montan, Microcrystalline | Waxes are thermoplastic and melt at low temperatures (between 40 and 100C); Soluble in organic solvents; As a coating, they often need repeated depositions for layers to build up. | Easy to apply; produces a smooth, glossy surface; Hydrophobic and are often used on wood |
| Metal Coatings | Process | Substrates | Advantages | Disadvantages |
|---|---|---|---|---|
| Anodized | Formation of protective oxide layer; immersion in electrolytic solution | Non-ferrous metals, primarily aluminum | Long-lasting, chemically stable,, non-toxic, no dangerous by-products | Unsuitable for ferrous metals; limited options for colors |
| Galvanized | Immersion of metal in molten zinc bath to produce protective zinc carbonate layer | Steel, bolts and nuts, air ducts, trash cans | Highly stable, adheres tightly; zinc coating actively covers scratches, cuts and dents after they occur | Expensive for large items; color is dull gray |
| Electroplated (electrodeposition) | Deposition of a thin layer of one metal on the surface of another; both metal are placed in electrolytic solution | Copper, nickel, gold, silver, chrome, zinc tin, jewelry, ornaments; | Excellent corrosion resistance; often increases in mechanical properties, can produce aesthetic appearance | Expensive; produces non-uniform coating thicknesses; potentially toxic; solutions contain environmentally harmful chemicals |
Synonyms and Related Terms
coat; surface coat; Anstrich (Deut.); enduit (Fr.); revêtement (Fr.); recubrimiento (Esp.); rivestimento (It.); revestimento (Port.)
Collection Risks
(from Tetreault 2017) "Coatings formed by oxidative polymerization, such as oil-based paints, oil-based urethanes, alkyds, melamine and epoxy esters (one-component or one-part systems), should be avoided in museums because they release acids and peroxides that can harm objects, especially papers, metal objects (lead, copper and their alloys) and black and white photographs. Fortunately, with new volatile organic compound (VOC) regulations in Canada and the United States, these coatings are becoming less and less available. The coatings that are generally acceptable in archival and museum contexts are acrylic or acrylic-urethane emulsion paints (latex) and, for special applications, two-part epoxy and two-part urethane paints."
Resources and Citations
- Jean Tétreault, Products Used in Preventive Conservation CCI, December 2017. Link
- Corrosionpedia: Coatinghttps://chansmachining.com/types-of-industrial-coating/ Accessed October 2024
- Wikipedia: Coating Accessed Oct 2024
- Chans Machining: Industrial coatings Accessed October 2024
- Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
- ASTM, "Standard Terminology Relating to Paint, Varnish, Lacquer and Related Products", Annual Book of ASTM Standards, Section 6, Paints, Related Coatings and Aromatics, ASTM, D16, 7-Jan, Jul-96
- Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
- The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998
- Art and Architecture Thesaurus Online, http://www.getty.edu/research/tools/vocabulary/aat/, J. Paul Getty Trust, Los Angeles, 2000