Part 5 of the seven-part series on material selection for chemical process equipment focuses on selection of paint and powder coatings for process equipment and process system/area materials. A broad range of paints and powder coatings are available to protect materials and equipment from damage, corrosion, and degradation from other environmental factors such as UV light. Deciding which paint or coating is most appropriate requires careful consideration. While generally applied at the factory under carefully controlled conditions, it is common to require field touchup or final coatings after installation is complete. This can be a key consideration in deciding what paint or coating is applied to each piece of equipment or material.
The type of paint selected can make all the difference in the expected life of the equipment; it is very important to understand the differences so that you can recognize potential problems and avoid costly mistakes.
Types of Paint
There are many types of paints but they generally fall into two categories; solvent based and water based. Most paints are comprised of a binder or resin, such as epoxy or urethane, combined with pigments for color and a liquifying agent, either water or a chemical solvent. In some cases, special additives are used to give the paint specific properties, e.g., resistance to UV light, added flexibility, or increased hardness to name a few. Both solvent- and water-based paints can be either one- or two-part systems and have a variety of requirements for application and curing or drying. Some paints dry simply by the water or solvents evaporating out of the film, some chemically react and cure when mixed, while some require atmospheric moisture (humidity) or heat to properly cure. In all cases, manufacturer’s instructions for application and drying/curing environment should be carefully followed for best results.
Water-based paints are often preferred, when practical, as they typically contain far less VOCs. This can reduce health and environmental hazards as well as flammability of the paint fumes in the work space. This is particularly important in confined spaces. Solvent-based paints tend to be less susceptible to environmental effects on the drying cycle due to the fact that humidity directly affects water evaporation, but not necessarily solvent evaporation or catalyzed chemical reactions.
Types of Powder Coatings
Powder coating is an excellent option for finishing industrial equipment; it is highly durable and can be formulated for superior corrosion resistance. There are two main types of powder coatings – thermoplastic and thermosetting. Thermoplastic powders melt when baked to form a continuous coating that can be re-melted after cooling. Alternatively, thermosetting powders chemically react when heated to form a polymer network that is more resistant to breakdown though cannot be re-melted. The most common coatings include epoxy, polyester, nylon, acrylics, polyurethane, and hybrids of epoxy and polyester.
The success of any paint or powder coating system hinges on proper surface preparation. There are two general categories for surface preparation – chemical and mechanical cleaning. At a minimum, all materials will require a chemical cleaning to remove oils and other surface contaminants that would reduce coating adhesion. In addition, if mechanical cleaning is required, chemical cleaning should be performed first so that oils and other contaminants do not become embedded in the surface of the base material.
Mechanical cleaning is commonly required for metals with heavy surface scaling and corrosion products, as is usually found on most hot-rolled carbon steel products. The most common mechanical cleaning methods include media blasting, grinding, and sanding. The type of media used will depend on the scale that needs to be removed, the base material, and thickness of the part. Blasting thin sheet metal for instance can warp the metal if an aggressive media is used, especially at high pressure. In all cases, recycled media should be avoided as scale dusts can be embedded in the material surface if they become part of the blasting media. Additionally, in preparation for coatings, edges and corners typically need to be eased or radiused as paints and powder coatings have a tendency to thin out at edges which can lead to formation of cracks and chips.
Plastics that are to be painted may require a light sanding to roughen the surface in order to promote adhesion. There are chemical adhesion promoters that may be used in some cases instead or as a supplement to mechanical preparation. For any coating system, the surface preparation should be done in accordance with widely recognized industry standards, such as the joint Society for Protective Coatings (SSPC) and the National Association for Corrosion Engineers (NACE).
Pretreatment generally refers to chemical processes used to prepare the cleaned base metal for the subsequent coatings. This may include acid etching, chemical (chem) film, or phosphating to name a few. The purpose of this step is usually to prevent the formation of corrosion products between cleaning and application of the coating and to promote adhesion of the coating. This is particularly important for metals, such as aluminum, that form oxide layers quickly. Pretreatment is considered essential for some coatings.
Industrial products are generally painted by manual or automated spray operations in multiple layers, starting with a primer coat for best adhesion to the base material. A heated cure cycle is often required before subsequent coats. Paints may also be applied by brush, roller, or even dipping. For certain products, particularly in high volume manufacturing, electrostatic spray deposition (ESD) painting can be used to significantly reduce overspray and paint waste. In all cases, parts and assemblies must be masked to avoid applying paint to surfaces not meant to be coated, such as threads and close tolerance mating surfaces.
Powder coatings are most commonly applied with ESD by a special gun that applies an electrostatic charge to the powder particles (attracting them to the grounded metal part) or by submerging parts in a fluidized bed of powder. Due to the requirement for baking, the option to powder coat is limited to products that can withstand elevated temperatures and can fit in the required curing oven.
Next week, watch for Part 6: Material Selection for Chemical Process Equipment – Selection & Evaluation Considerations
ITI is a global consulting, engineering, and design-build firm based in Burlington, Vermont. We specialize in manufacturing processes, water and wastewater treatment, recycling, and ventilation applications for the metal and surface finishing industry.
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