Industrial coatings are thin films deposited on materials to add or enhance specific properties such as corrosion resistance, wear resistance, conductivity, etc. Industrial coatings are used in production plants and by qualified applicator shops to coat discrete parts, finished assemblies (automobiles, trucks, aircraft, vessels), tanks, piping, metal sheet, continuous webs, wood panels, paper, and paperboard.

The use of industrial coatings involves two steps: application of the resin on the substrate and formation of the film to its surface.

Application

There are a number of methods for applying industrial coatings to the surfaces of parts. This is important to keep in mind when selecting a coating, since some resins may be limited to certain application methods.

Dip coating or immersion coating involves immersing the part in a bath filled with the coating material in order to effectively coat all surfaces on the workpiece.

Fluidized bed coating is a simple dipping process that can be either conventional or electrostatic.

In conventional fluidized beds, products that are preheated above the melt temperatures of the powder are dipped in the bed, where the powder melts and fuses into a continuous coating. High transfer efficiency results from little drag out and no dripping.

An electrostatic fluidized bed is essentially a fluidized bed with a high voltage dc grid installed above the porous plate to charge the finely divided particles. The advantages of electrostatic fluidized bed coating is that preheating of parts is generally not necessary and small products, such as electrical components, can be coated uniformly and quickly. The disadvantages are that the product size is limited and inside corners have low film thickness owing to the well-known Faraday cage effect.

Electrophoretic deposition includes a broad range of coating processes that use an electric field to deposit colloidal particles onto a substrate acting as an electrode.

Electrostatic spraying is a subcategory of electrophoretic deposition which involves atomized paint droplets which are charged at the tip of the spray gun by an electrode. It allows the workpiece to be completely coated with a film of uniform thickness, including normally inaccessible areas.

Spin or centrifugal coating is an effective way to cover small metal parts and flat surfaces with a protective coating. After immersion in excess coating material, the mechanism uses centrifugal force to bleed the excess from the parts, leaving only an even thin layer covering the surface. Spin coating is commonly used in microfabrication for creating very thin films.

Trickle impregnation is a process in which surface tension effects allow resins to be wicked or soaked up by the internal porosity of a part (like a sponge soaking up water). The process involves preheating the substrate, dispensing a measured amount of resin onto its surface, and (after adequate impregnation) heat-curing the resin. 

Vacuum-pressure impregnation (VPI) involves using a vacuum or other means to create pressure to drive resins or coatings into a surface after a dip or trickle coating application. In vacuum applications, a part is submerged in a resin bath under a vacuum; the vacuum is released to force resin into the substrate. When withdrawn from the resin bath, the part is drained, cleaned, and then cured. 

Wet spraying or atomization coating involves all types of sprays which break up a bulk liquid resin into smaller droplets. This process usually involves spray guns or special nozzles.

Film Formation

Part of the coating selection process involves an understanding of the process by which the resin forms the film on the substrate it is applied to.

Cross-Linking

Cross-linking, by definition, is the process by which polymers react to form long chains in a "linked" network. This linkage (commonly known as curing) results in the formation of larger polymers which effectively form a sheet or film on the substrate. There are a number of ways in which cause curing in a resin, including radiation, heat, moisture, and chemicals.

Evaporation

Some coatings consist of solids with the desired properties suspended in a liquid. After application to the substrate, the solvent evaporates and leaves behind a purely solid coating. This process is based on low solids content and large amounts of organic solvents. It is used commonly in many spray paints and lacquers, but is highly regulated and limited due to the high levels of solvent released into the atmosphere. Because of this, the evaporation method is often used in conjunction with cross-linking - inhibitors which prevent premature curing evaporate with the solvent and allow cross-linking to occur.

Coalescence

Coalescence in a coating is the flowing together or coming together of polymer particles in suspension, creating a clear polymer film. This occurs when small polymer particles are formed as a dispersion in water or organic solvent above the polymer's glass transition temperature. If the polymer is below its glass transition temperature, a coalescent (coalescing agent) can be added to lower this property and cause film formation. Coalescence takes place mainly in latex based polymers. Limitations on the use of organic solvents have made water the predominant carrier solvent for coalescent coating technologies.

Fusion Bonding

Fusion bonding is similar to coalescence but involves purely solid particles, most commonly powders (powder coatings). First, these solids are coated on the substrate using an electrostatic spray or fluidized bed and then the particles are fused by heating to form a continuous film. Technologies which use fusion bonding include thermoplastic coatings, thermoset coatings, and moisture-cured coatings.

Types

Coatings can be classified into a number of categories based on their composition and function.

Lacquers - coatings made from the clear sap of the lacquer tree (rhus verniciflura) that produce a hard, durable finish to decorate and protect wood, metal, and other surfaces. In modern product terminology, lacquer-based finishes are known as shellac, and lacquer itself refers to other polymers dissolved in VOCs. Lacquer is typically more durable than shellac.

Paints - a large category of coatings including pigmented liquids or powders used to protect and/or beautify substrates. The two general classifications of paint are enamels and lacquers.

• Enamel paints both dry and cure once applied to a surface. They are extremely stable, and can last for decades if stored properly. They also have lower levels of volatile organic compounds (VOCs) and are more environmentally-friendly. Enamels dry/cure times are typically much longer than lacquers, but newer catalyst additions are reducing that time difference.
• Lacquer paints dry but do not cure. They dry harder, smoother, and more quickly than enamel paints and are easier to sand since enamels can be gummy; however, lacquer paints are not as tough and will chip or crack more easily than enamels. Lacquer paints often use more aggressive solvents which can degrade some plastics and brush materials. They also contain higher levels of VOCs which can be harmful to people and the environment. In some states, only businesses are allowed to apply lacquer paints.

Selection Tip: With the advent of aqueous acrylics, many differences between traditional paints have merged. Aqueous acrylic lacquers are becoming a popular choice, combining properties of lacquers and enamels. They are more sensitive to humidity and temperature, but eliminate the more environmentally dangerous chemicals associated with traditional lacquers.

Powder coatings - coatings which are applied as free-flowing, dry powders. These coatings are dry, meaning they don't require a solvent to keep the components together. Powder coatings are applied using either an electrostatic spray (see image right) or a fluidized bed. Parts are heated before or after application to fuse the particles together and bind them to the surface. Powder coatings are used mostly for coating metals, and may be thermoplastics or thermosets.

Thermoplastic coatings are powder coatings which do not undergo any additional reactions during the fusion bonding process, but only melt and solidify out into the final form.

Thermoset coatings are powder coatings that incorporate a cross-linker into the formulation. During the cure cycle (melting) it reacts with other chemical groups to develop and improve its performance properties.

Primers - preparatory coatings that are applied to treat a surface before the application of another coating. They are primarily designed to increase coating adhesion to the substrate surface. Some primers also lend uniformity to the topcoat, inhibit corrosion of the substrate, and/or stop coating discoloration. For more information about primers, visit the Primers and Adhesion Promoters area on Engineering360.

Sealers - coatings defined as surface sealants used to seal the substrate surface, providing protection from contamination and corrosion. They are applied to porous castings, powder metal parts, stator windings, and transformers to seal the surfaces and/or internal porosity by soaking into or impregnation through the open pores. Sealing or impregnation processes use vacuum, pressure and/or wicking action techniques to drive resins or coating materials into parts. Visit the Industrial Sealants product area on Engineering360 for more information.

Stains - semitransparent or semisolid coatings used to accent wood grains and add protection. Semitransparent stains penetrate the wood without forming a film, allowing much of the wood grain to show through the finish. Semisolid stains act more like paints, forming a protective film and not deeply penetrating the wood. The properties of the pigment, preservative, water repellant, and resin in the stain determine its durability and performance.

Varnishes - transparent, hard, protective finish consisting typically of a drying oil, resin, and thinner or solvent. They are applied as liquids to wood or other materials to provide a colorless coating that protects from abrasion, chemical attack, water damage, and in some cases UV light. The drying and curing of varnishes depends on the type of its vehicle it uses. Some varnishes are mixed with stains to provide color and added protection to the surface. The lifespan of varnish can be extended by first staining the surface before applying many coats of varnish. 

Coating Chemistry

Industrial coatings can consist of a variety of different chemicals and resins, each with different properties that need to be considered in the selection process.

Acrylic is a synthetic resin used in high-performance latex or water-based paints. Acrylic resins form the paint's binder and enable the coating to last longer and retain its color. Acrylic coatings are recommended for bonding metals. They can also be used with oily surfaces, glass, ferrite, plastics, and fiber-reinforced plastics (FRP).

Alkyd resins are used mainly in interior and exterior trim paints. Some medium-duty equipment and marine enamels use alkyd resins as binders.

Aluminum coatings contain aluminum, a silver-white, metallic element used to make hard, light, corrosion-resistant products.

Ceramic coatings consisting of oxides, carbides, nitrides, carbon, and other non-metals with high melting points. Refractory ceramic coatings are sprayed onto the insides of kilns, and on elements to lengthen life and improve efficiency. Coated elements are more efficient and may last up to 10 times longer than uncoated elements. Although ceramic coatings are expensive, they provide long-term savings.

Conversion coatings cause chemical conversions on metal surfaces to produce thin, adherent, compound coatings. Conversion coatings include oxide, phosphate, and chromate coatings.

Glaze or glass enamel coatings are fused onto ceramics, metal, or porcelain. Glazes are inorganic enamels based on a fused silicate composition. Glass or porcelain coatings have glass-like properties, such as high heat and electrical insulation.

Epoxy resins are a large, high-performance group of resins. Epoxies generally out-perform most other types of resins in terms of mechanical properties and resistance to environmental and chemical degradation. Epoxy coatings make excellent sealants and in aircraft components are used almost exclusively.

Fluoropolymers are a family of engineering plastics characterized by high-thermal stability, low friction, and almost universal chemical stability. PTFE, one example, contains fluorine and recurring tetrafluoroethylene monomer units. Teflon®, a popular type of PTFE, is a registered trademark of DuPont.

Formaldehyde resins are thermosetting molding compounds and adhesives that provide strong bonds and good resistance to high temperatures. Phenolic or phenol formaldehyde, urea formaldehyde, furan, and melamine resins are all part of this category. Generally, the most durable resins are made from chemicals of the phenol group and formaldehyde.

Polyurethane is a tough, rubber-like elastomer based on the condensation of organic isocyanates with resins containing hydroxyl groups. It provides flexibility, impact resistance, and durability. Polyurethane is also referred to as urethane.

Rubber bases and polymer binders are translucent or transparent and solid or semi-solid. They contain synthetic and/or natural materials. Examples of resin bases and polymer binders include acrylic, alkyd, copal ester, epoxy, polyurethane, polyvinyl chloride, and silicone coatings.

Silicone coatings have excellent UV resistance, high gloss durability, low weathering, and high temperature resistance. They are also water-repellant, resistant to corrosion, and have fairly good chemical resistance, though not as reliable as polyurethanes and epoxies. Silicone contains a unique polymer system that can be a very effective release coating.

Silver coatings are commonly applied to metals to increase their conductivity or reflectance properties.

Vinyl resins are used in coatings as the major binder component. Vinyl resins are used in both polyvinyl chloride (PVC) and polyvinyl acetate coatings.

Zinc coatings contain zinc, a blue-white metallic element that is malleable and ductile even at ordinary temperatures. Zinc can be electrodeposited and is used extensively as a coating for steel and iron to prevent corrosion and rusting. Metals with a zinc protective layer are known as "galvanized" metals.

Properties

The main purpose of industrial coatings is to add or enhance a material's properties. Buyers should select coatings based on what properties are important to their specific application. Properties involving coatings include:

• Anti-static / ESD control - coatings used to minimize static electricity in sensitive environments.
• Chemical resistant - coatings which resist acids, alkalis, oils, and general chemicals.
• Conductive - coatings used to form an electrically-conductive layer.
• Dielectric - coatings made from nonconducting materials used in optical applications. High-reflection coatings consist of a stack of alternating layers of high-and-low refractive-index materials. Each layer in the stack has an optical thickness of a quarter-wave at the design wavelength.
• Corrosion inhibiting - coatings which prevent moisture from reaching the metal or underlying substrate, or provide a sacrificial layer.
• EMI / RFI shielding - coatings provide shielding from electromagnetic interference (EMI) or radio frequency interference (RFI).
• Flame retardant - coatings are flame-retardant in accordance to Underwriters Laboratories, Inc. (UL) Flame Class 94V-0, or other equivalent ISO standards. These materials reduce the spread of flame or resist ignition when exposed to high temperatures. They also insulate the substrate and delay damage to the substrate.
• Heat resistant - coatings resist damage from heat, or are formulated for use in high-temperature environments.
• Protective - coatings are designed to protect substrates and surfaces.
• Touch-up - coatings are used to repair and match the original coating where it has been damaged by scratching, corrosion, abrasion, erosion, scuffing, denting, chipping, delaminating, or other processes.

Selection Tip: The touch-up coating material should have good adhesion to damaged and prepped substrate and undamaged paint surfaces. The color and functional protective properties should match those of the existing paint or coating on the substrate.

• Waterproof / water repellant - coatings are clear, exterior finishes that cause water to bead-up on the surface. They also minimize the penetration of water into the substrate.
• Wear resistant - coatings are designed to resist wear or erosion. Wear is caused by a sliding action between two or more components. Erosion is surface damage or material removal caused by the impact of particles or slurries.
• Weather resistant - coatings are weather-resistant or protect against damage from UV radiation.

Surface Properties

The surface properties that should be considered when selecting a coating include:

Temperature sensitivity - surfaces like wood and plastic may not allow for certain application processes such as powder coating which require high temperatures to cure the material to the substrate.

Profile - some coatings are more compatible on rough profiles than others. Rough surfaces may require more coating than smooth surfaces for sufficient coverage due to the increased surface area. Some coatings may even require roughening or smoothing of the surface before they can be applied effectively.

Contamination - surfaces must typically be clean of all surface contaminants, oil, grease, and oxides to prevent impairment of the coating's adhesion to the substrate.

Application Tip: The important of a chemically clean substrate cannot be overemphasized. Some chemical contaminants are not even readily visible on the surface, but most will cause adverse effects (e.g. attracting moisture through the coating) which result in premature failure.

Chemical compatibility- certain coating chemicals may have adverse effects on certain substrates, or vice-versa. Compatibility between the surface material, the coating, and the outside environment should be considered before selecting a coating.

Performance Specifications

After considering the chemistry and properties of the coating and substrate, industrial buyers should assess the coatings performance specifications. There are a number of specifications that may be important to consider.

Coverage - the theoretical area of a substrate that a coating can cover at a given thickness, usually given in square feet (ft²). Coatings with higher solids content may provide greater adequate coverage per gallon, possibly contributing to a significant cost savings in industrial-scale applications.

Wet thickness - the thickness or range of thicknesses by which a coating can be applied.

Cure/dry temperature - the temperature is the minimum temperature at which coatings dry or cure effectively.

Operating temperature - the temperature or temperature range that a coating can be used at or exposed to without degrading structural or other required end-use properties.

Resistivity - a measurement of the coating's ability to resist the flow of electrical current. This is important for surfaces with the potential to be exposed to electrical components.

Dielectric strength - the maximum voltage that a dielectric material can withstand under specified rupturing. It is usually expressed as volts per unit thickness and is also called disruptive gradient or electric strength.

Lifespan - the average life of the coating after its application. When coatings degrade, they require reapplication or touch up to remain functional. Longer lifespan can thus save on labor and material costs over time.

Resources

DuPont

Special Chem - Powder Coating Center

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