Architectural coatings include paints, sealers, and specialty coatings for building and construction applications (e.g. floors, roofs, decks, pavement, walkways, etc.). They are designed to provide a protective and/or decorative layer on the surface of an architectural feature or fixture in both indoor and outdoor applications.

Coatings - How They Work

Most coatings are applied to surfaces using sprayers, rollers, brushes and other applicators. Powder particles or liquid film coatings are converted into solid film coatings through one of several film drying, curing or forming processes: cross-linking, evaporation, coalescence, or fusion bonding.

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. A number of coating technologies rely on polymer cross-links for film formation and adhesion to the surface. These include multi-component coatings, moisture-cured coating, and radiation-cured coatings.

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 an 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.

Specific coating technologies that fall into the above categories are as follows:

• Multi-component coatings use a polymeric protective film that applies a prime coat, intermediate coat, and/or finish coat. Multicomponent coatings consist of multiple liquids (A+B or A+B+C) that are blended together just before an end-use application. Some of the components can be catalysts, curing agents, retarders, accelerators, fillers, reinforcements, colorants, or specific property enhancers.
• Moisture-cured coatings are single-component adhesives which, after fusion and solidification, undergo further curing via a reaction with moisture. This reaction crosslinks and polymerizes the material. Polyurethane reactives (PUR), certain silicones, and cyanoacrylates are examples of materials which react with moisture or water to cure the coating.
• 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.
• Radiation-cured coatings incorporate reactive liquid vehicles which are crosslinked by high-intensity ultraviolet (UV) or electron beam (EB) radiation. These coatings are favorable for substrates which are heat-sensitive (e.g. wood, plastic).

Types of Coatings

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

Ceramics - coatings consisting of oxides, carbides, nitrides, carbon, and other non-metals with high melting points. They provide resistance to thermal shock, oxidation, and chemical corrosion. Although ceramic coatings are expensive, they provide long-term savings.

Inks - liquids, powders, or transfer tapes that are printed, sprayed, rolled, or transferred onto substrates or media for printing, marking, and creating graphics and patterns. Inks are generally thicker and more viscous than paints. Visit the Industrial Inks product area on Engineering360 for more information.

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 considered 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.

For more information on paints, visit the Industrial Paints product area on Engineering360.

Powder coatings - coatings which are applied as free-flowing, dry powders. The powder may be thermoplastic or thermoset polymer. 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.

Primers - complete, preparatory coatings that are applied before painting. They are designed to provide adequate adhesion between the surface and subsequent topcoats of other coatings. Some primers lend uniformity to the topcoat, inhibit corrosion of the substrate, and/or stop topcoat discoloration. For more information, visit the Primers and Adhesion Promotors product area on Engineering360.

Image Credit: LOD Offroad

Sealers - coatings which form a barrier to seal surfaces from chemicals, UV rays, and/or water. Sealants are defined as substances used to seal joints or gaps between surfaces, or to form a barrier between them - sealers are specifically surface sealants. Visit the Sealers and Seal Coatings product area on Engineering360 for more information on sealers.

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 Composition

There are endless varieties and mixtures of substances that coatings can consist of. In addition to performance, a coating's chemical makeup determines its compatibility with the substrate (surface), with the surrounding environment, and to any corresponding codes or regulations. The chemical makeup of most coatings includes a body, binder, and/or a vehicle.

• The body of the coating is the solid base portion of the coating. It is also responsible for making the film harder and more abrasion resistant and reducing shrinkage cracks during drying. Some coatings and paints are named after the body, (e.g. lead paint, zinc paint, aluminum paint).
• The binder or resin is the film forming component of the coating. It imparts adhesion, binds the pigments together, and influences properties like gloss potential, durability, flexibility, and toughness. Binders include synthetic or natural resins such as acrylics, urethanes, polyurethanes, fluoropolymers, polyesters, melamine resins, epoxies, silicones, or oils.
  • Acrylic coatings provide excellent environmental resistance and fast setting-times.
  • Alkyds are wear-resistant products for high-traffic areas, but take longer to dry.
  • Epoxy coatings exhibit high strength and low shrinkage. These architectural coatings also resist chemical and environmental damage.
  • Urethane coatings provide flexibility, impact resistance, and durability.
• The vehicle or solvent is the liquid part of the coating in which the body and binder are dissolved. It carries the film forming parts of the coating and adjusts its viscosity and flow characteristics. Vehicles include water; various solvents such as aliphatics, aromatics, alcohols, ketones, and alkyds; and various oils such as linseed, soya bean, fish, and dehydrated castor.

Here are some basic differences between oil-based and water-based coatings:

Table Credit: Glidden Professional and Devoe Coatings

For more information on the properties of coating resins and substances, visit the Industrial Coatings Selection Guide on Engineering360.

Performance Specifications

There are a number of specifications that describe the performance of architectural coatings.

• Coverage - the theoretical area of a substrate that a coating can cover at a given thickness. Coatings with higher solids content may provide greater adequate coverage per gallon, possibly contributing to a significant cost savings.
• 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. Coatings with a shorter lifespan will require reapplication sooner than those which last longer.

Features

Often coatings are classified and selected based on the specific features and characteristics they exhibit. These features 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.

Surfaces

When selecting a coating, the composition of the substrate is just as important to consider as the substrate itself.

Material

In order to work effectively, the coating must be compatible with the surface it is being applied to. Substrate types include:

• Asphalt
• Ceramic
• Concrete
• Glass
• Metal
• Plastic
• Wood

Type

Architectural coatings may be designed or specified for a certain type of building surface or structure. Surface and structure types include:

• Basements
• Bridges
• Ceilings
• Decks
• Floors
• Foundations
• Roofs
• Tunnels
• Walkways
• Walls

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.
• 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.

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.

Application Form

Related to surfaces is the form by which the coating is delivered. Certain applications may limited the selection of coatings to those which can be applied in a certain way.

• Aerosol - coating is dispensed as an aerosol or spray.
• Cartridge - ink, coating, or marking material is packaged in a cartridge for controlled delivery via a printer or other type of precision dispensing equipment.
• Film or laminate - coating is adhered to the substrate as a thin sheet of plastic.
• Liquid - coating is applied as a liquid using rollers or brushes.
• Marker - coating, ink, paint, or marking material is packaged and dispensed as a marker or pen.
• Paste - coating is a paste or moldable solid.
• Transfer Tape - ink, coating, or marking material is packaged as transfer tape for controlled delivery as an alternative to direct coating.

References

Britannica - Surface Coatings

Image credit:

Grainger Industrial Supply