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An understanding of the process is required to develop a coating is useful when considering the composition and properties of any coating. The following explanation is from the viewpoint of the coatings development chemist. It is a consideration of the types of basic raw materials available, required properties, formulation variables and finally, coatings performance.
A coating can be defined as a fluid that forms a solid, continuous, adherent film or barrier by some physical or chemical means when applied to a surface. This film is designed to enhance, beautify, or protect the surface to which it has been applied.
Whether a coating is an inhibitive primer or glossy topcoat, its formula consists of six basic components: binders, pigments, extenders or fillers, solvents, rheology modifiers, and special additives. The coatings chemist selects the types and quantities of each of these components, and combines them into a liquid form that changes to a solid film when applied to a substrate. We will examine each of these types of components.
Binders are the polymer components that form the continuous film. They hold all other components together, adhere to the substrate, and provide the paint with its basic generic type. Binders may be classified by their method of film formation: thermoplastic or thermosetting. Those that function by simply drying (solvent evaporation) are thermoplastic. Vinyls and chlorinated rubbers are examples of thermoplastic binders. They are generally softened by heat and re-dissolved by the appropriate solvents. In the case of thermosetting polymers, the binder is chemically changed by either an atmospheric component, such as water (moisture cure urethanes) or oxygen (alkyds), or with another type of chemical usually in the form of a co-reactant (two-component epoxies and urethanes). The reaction can also take place upon baking (baked enamels) or with ultraviolet or electron beam radiation.
The major types of binders are:
Pigments are used in coatings to impart color and opacity (hiding) to the final film. There are two general types of pigments: inorganic such as metal oxides or combinations of oxides, and organic such as phthalocyanurates or quinacridones. Pigment selection can be quite complicated. The pH stability of the pigment, its UV resistance, its chemical resistance, particle size, and oil absorption must all be carefully selected to provide the desired properties. Improper selection can result in a coating that degrades or changes color as the pigments are attacked by acid rain, sunlight or heat. Pigments can provide protection for the binder and stability for the film.
Extenders are generally inert solids used for special purposes. These materials are often classified as a subset of pigments. Extenders are usually transparent in the film and, therefore, do not interfere with the color pigments. They are used to obtain hardness, mar resistance, strength, abrasion resistance, non-skid properties, or to control gloss, and are usually chemically inert. Some of the flake varieties, such as mica and aluminum, can protect the binder by horizontal flake orientation, often called leafing, to provide a barrier to the atmosphere. In addition, aluminum flake is used to reflect light, and thereby protect the binder from UV degradation. Fibers such as fiberglass, cellulosics, and plastics, impart tensile strength and tear resistance to the paint film. Aside from the flakes and fibers, the generic types most commonly used are clay, talc, and silica.
Solvents are organic liquids (or water) used to change the viscosity of the coating, so that it can be properly mixed and applied. The solvent dissolves or dilutes the binder or polymeric resin. In fact, the binder generally determines which solvents can be used in a coating. Several properties must be balanced when a solvent system is selected. Solvency or solvent power must match the resin in the selected binder. The best solvent is generally the one that provides the lowest viscosity of the resin solution. Evaporation rate must be such that the coating does not “cobweb” (dry too quickly) nor pick up dirt (dry too slowly), nor remain in the film too long and cause performance problems. Usually two or more solvents are used in a coating to obtain the proper balance of these parameters. Solvents usually determine the coating’s flash point, which is a safety and shipping concern. Since the implementation of VOC (Volatile Organic Content) regulations, the amount of solvent per gallon of paint has been important (depending on local laws).
The types of solvents are classified by their molecular structure:
Some chlorinated solvents are also used but may be hazardous, because they can react with metals under pressure. Therefore it’s recommended to use them as advised. Thinners are part of the solvent system and are designed to be added just prior to application. Manufacturers’ recommendations for thinning must be followed for good product performance.
Raw materials can change the flow characteristics of a liquid coating. Rheology is the study of how a coating responds to shear. Shear is applied to a coating during application (brush, spray or roller), but also at other times such as pumping, mixing, and shipping. The coating must not settle in the can, and yet it must be fluid enough to spray. It can’t run off a vertical surface; that is, it must recover after high shear.
The most common types of rheological modifiers are fumed silicas, treated clays, polyamides, associative thickeners, cellulosics, alkali swellables, and castor oil derivatives. Certain solvents may affect rheological modifiers. This fact requires a careful balance of solvents in the coating.
There are several types of miscellaneous coating additives to provide for a variety of special needs and conditions. There are waxes, surfactants, wetting aids, fungicides, ultraviolet light absorbers, antioxidants, catalysts, and anti-mar agents to name a few. Additives are chosen and used carefully.
With these components in mind, we begin the development process. Product development starts with a definition of a kind of work the coating must do. Frequently, the coating must protect valuable equipment or structures. To design a proper coating, the coating formulator must know the type of environment that will be encountered. Will the coated structure be exposed to acids as in a chemical processing plant, or alkalies as in the pulp and paper industry? Must the coating withstand UV radiation (sunlight) or will it be indoors? Will it be applied in Arizona or Alaska? If the coating is a tank lining, its chemical resistance, especially immersion, must be known. It is necessary to know the specific chemicals involved.
Physical properties are also important. If the substrate will be flexed or bent in any way, the coating must be flexible. If it is likely to be hit with rocks or gravel or to have a heavy object dropped on it, the coating must be impact resistant. Application methods are considered. Not all coatings that can be brushed can be sprayed. The development chemist will want to know the intended methods of application. Finally, we must be aware of the true cost of the coating in terms of the protection it provides.
As described elsewhere, corrosion prevention can be provided by several different mechanisms. The sacrificial type is the preferential rusting of a non-structural metal so that the structure will remain intact. This is accomplished by placing the sacrificing metal in contact with the structure. Magnesium and aluminum anodes are used, and of course, several kinds of zinc-rich coatings. Certain inhibitive pigments can be incorporated into a coating to provide corrosion protection. The more common ones are chromates, phosphates or borates.
A simple barrier coat can prevent corrosion. This lowers the diffusion rate of the corrosion agent or electrolyte through the paint film and thereby protects the substrate. The coating must resist the environmental conditions. If maintaining gloss and color is important, UV absorbers or hindered amine light stabilizers and antioxidants may be added. The coating must certainly be chemically inert to its intended environment.
Certain physical properties are often required. Hardness is a common requirement. Some applications require flexibility such as expansion joints, hairline cracks in concrete, or moving railroad stock. The coating must adhere to its substrate or primer. Temperature resistance is important for coatings on smoke stacks, near or on engines, or around furnaces. Abrasion resistance is important for any coating that will be walked on, driven on (parking garages), or have objects dragged across it. Finally, the coating must be able to be pigmented so that the primer or substrate is hidden at a reasonable thickness.
There are several important application variables. The kind of substrate and the substrate conditions available will influence a successful coating application. These substrates include steel, concrete, wood, brick, plastic, etc. The substrate can be new and in excellent condition to accept a coating or it can be old and rusted or deteriorated. In certain areas there are special safety requirements for applying coatings. The most common of these are restricted flash points of the coating and its thinners, and the amount of solvent in the coating or VOC. Coatings can be applied by several methods: conventional, airless, plural component, air-assisted airless, electrostatic spray, squeegee, and of course, brush and roller. Transfer efficiency will become more and more important as an attempt is made to manage our air quality. Sometimes there are special cure conditions such as humidity, temperature or ventilation restrictions. All of the above parameters provide the guidelines for a coating’s design. Raw materials are chosen based on these criteria.
In compounding all of the above raw materials into a protective coating, several variables must be considered:
PVC – Pigment volume concentration (PVC) is the volume percent of pigment and extender in a dry paint film. PVC is one aspect that governs film density, porosity, gloss and to some extent, weathering characteristics.
Stoichiometry – Stoichiometry is important in two-component coatings, such as epoxies and urethanes. Stoichiometry is the chemical ratio of the two components, that is, the amount of Part A and the amount of Part B. It determines the amount of epoxy resin to be mixed with the curing agent, or the amount of polyol to be mixed with a polyisocyanate for a urethane. Therefore, the correct mix ratio is essential for a properly performing paint.
Solids by Weight / Solids by Volume – These are calculations of the amount of solids or non-volatiles in the coatings. Volatiles by weight give the amount of solvent for determining the VOC; solids by volume will allow us to determine the theoretical coverage in dry mil square feet per gallon.
Film Formation – Film formation involves the sum of the binder, solvent, and pigment characteristics that form the film. It must be a continuous film at the designed temperatures and environmental conditions. Ultimately, the coating formula must be balanced to provide the most economical material for the intended protection ($ per dry mil square foot).
Once we have our coating formula, it is necessary to predict performance based on testing properties versus the environment. The coating will be tested for those characteristics that were important in the design criteria. These can include pot life, dry times (to touch, to handle, to recoat), cure under various conditions of temperature and humidity, bend tests, impact, elongation, tensile strength, adhesion, application characteristics, stability, etc.
Test panels may be weathered outside in the Midwest, in Florida or other test areas. In addition, accelerated testing, such as QUV (accelerated exposure to UV light and condensation), Weather-Ometer (accelerated exposure to light and water fog), salt fog, water fog, and aerated brine, is commonly used. Newer equipment, such as KTA Envirotest, combines two or more exposures cycled in one test chamber.
As of yet, there is as not a laboratory test that can accurately and infallibly predict how a coating will perform in the real world. However, an attempt is made to correlate the test results with actual conditions to predict the service life of the coating and fairly calculate the cost per year of service life. A newly formulated coating may be evaluated in the field for several years before it is released. It must meet design criteria and provide customer satisfaction. In summary, a successful coating is a carefully chosen blend of chemical raw materials that when manufactured, applied and properly cured will provide the kind of protection, physical properties, and application parameters required within the design criteria.