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This article will take an in-depth look at industrial coatings. It will bring more understanding on topics such as:
This chapter will discuss the principle behind industrial coatings, as well as their various uses.
An industrial coating is a layer of a substance added to a surface to provide protection and to enhance the appearance of the surface. The main function of an industrial coating is to protect a surface from the elements and other forms of damage. They are applied to a wide variety of surfaces, including concrete, timber, metal, plastic, glass, rubber, leather, mortar, and asbestos.
There is an endless variety of industrial coatings, each of which offers different characteristics and provides unique and beneficial properties. The type of environment and the nature of the surface to be coated determines the specific coating that should be used. Coating manufacturers provide consultation and guidance regarding how coatings should be used and applied.
Industrial coatings are applied in a way designed to be both aesthetic and protective. There are several types of industrial coatings, including Xylan-dry film lubricants, resins, xylene, and others.
The purposes and uses of industrial coatings are broad, with the main purpose being the protection of equipment from different types of corrosion. Aesthetics are also an important factor for certain types of equipment and conditions. Industrial coatings are commonly used to prevent concrete or steel from damage.
A secondary use of industrial coatings is to improve the resistance of materials to fire or other potential hazards. They are applied to the linings of water tanks and reservoirs to create a safe environment for potable water that protects against corrosion.
The most frequently used industrial coatings are polymers. Some examples of polymers that are used as industrial coatings are epoxy, moisture cure urethane, polyurethane, and fluoropolymer.
There are various considerations in industrial coating applications. These include:
In order for the industrial coating to take hold, the substrate, or base material to receive the protective layer, must be clean. Most industrial coatings depend on a mechanical or physical bond to stick tightly to the surface of a substrate. Chemical bonding with the substrate surface creates an impenetrable layer of protection. If the surface is not completely clean (i.e. if the surface contains dust particles, chemicals, or other contaminants), the industrial coating is likely to fail prematurely.
The key to the success of the application of an industrial coating is preparing the surface before coating. Before starting the process, the surface of the substrate must be cleared of moisture, dry contaminants, and salts using the correct techniques, like heating processes, blasting procedures, and chemical cleaners. Proper cleaning of surfaces before coating prevents fisheyes, blistering, adhesion failure, alligatoring, bubbling, and cissing.
Priming must be included in every industrial coating procedure. Priming helps the coating and sealant adhere to the surface of the substrate for long-lasting performance. When there are incompatible substrates and coating materials, primers help them to work together. It also aids the part&#;s final aesthetic by removing imperfections at the surface level.
Once the application of the primer is complete, the substrate is ready for the coating process. There are different types of coating processes. Each coating method is designed to totally coat the substrate in the protective coating material. The substrate&#;s size and complexity affect the coating application method. The most common industrial coating processes are dip coating, brush coating, roll coating, spray coating, spin coating, and flow coating.
The performance of industrial coatings is affected by the curing and drying procedures. If the curing and drying procedures are inadequately completed, then the industrial coatings are likely to underperform. Every industrial coating must have a product data sheet that provides the specifications on the right drying and cure-through procedures for optimal coating results.
Industrial coatings require the curing process to be done at the correct temperature for the appropriate duration. The temperature specifications apply to the substrate&#;s temperature but not the oven&#;s temperature. For this reason, bake times vary for parts of varying thicknesses.
After the coating process is complete, parts must be inspected to ensure that the coating is in line with the acceptable parameters. Most industrial coatings have certain degrees of thickness that they must fall within to ensure that they cover the part and don&#;t fail to express the small details or complexities. Paint lines that are well run have quality inspection standards in position to ensure the compliance of every coating project with the acceptable thickness averages.
When opting for the right industrial coating method, both the equipment and the coating material are important. An optimal industrial coating line will need strong pump seals, clean filters and spray tips, and regularly flushed air lines. If equipment is not maintained regularly, the results can be substandard or flawed.
Suppliers of industrial coatings should work with their customers to ensure the auditing of processes with the feedback that they get from applicators, the cleaning of the working areas, and the regular maintenance of equipment.
Considerations in selecting a custom industrial coating formulation include:
The ideal industrial coating formulation depends on the substrate. The formulation may require the following materials: urethane, acrylic, or epoxy. Manufacturers must consider whether parts are made from a variety of materials and when the industrial coating will be needed during the manufacturing process.
Certain substrate and coating pairs can also be bridged by the application of the correct primer or the incorporation of additives to aid in the binding or other properties. Substrates require unique protections such as corrosion protection, food safety regulation compliance, and many more. This must be considered by the manufacturers.
Different methods of application and materials may offer results that are superior to others, depending on the conditions of the surface of the substrate. Some substrates are vulnerable to the changes that occur in composition through different methods of application and curing procedures. For instance, if a manufacturer is working with a substrate made of plastic and paired with a coating, they need a heat cure to ensure the endurance of the prescribed bake temperature without warping.
When selecting a type of coating, the application&#;s environmental factors must be considered. Industrial coatings need an environment that is stable and clean within specific temperature and humidity conditions. If there are inconsistent conditions in the environment, there are more likely to be inconsistent finish results. For instance, uncontrolled humidity and temperature conditions with industrially active, warmer days and cooler nights will result in thermal shock.
During the application or drying period, the coating can be chemically altered by exposure to splashes, chemical fumes and contaminants. Any contamination before curing the coating will reduce its effectiveness. Manufacturers must ensure the protection of their treated parts from abrasion, UV radiation, and physical impact before the completion of the curing process.
The final result is affected by the details of the application process. The precise method of application (such as spraying, brushing, dipping, etc.) must be carefully planned by the manufacturers, and they must also ensure whether the planned process ensures accurate thickness control. Other processes to consider include substrate preparation, post-cure cleaning and treatments, baking and curing equipment, space availability, and drying conditions in controlled environments.
Different coatings exhibit different industrial coating properties. For example, epoxy and urethane coatings can withstand a great diversity of chemicals and physical impact, though epoxies also struggle with outdoor exposure. Meanwhile, both coatings can be damaged by nitric acid. During the application process, water-based coatings are safer for technicians, but in vulnerable substrates, they may increase the risk of corrosion. Ultimately, many of the aesthetic and functional properties are largely dependent on the material of the coating and the application process. Other properties are coating flexibility and elongation; smooth texture, gloss, and film; color quality, retention, and specificity; moisture permeability; solderability and conductivity for further processing. During every stage of a custom industrial coating formulation, it&#;s important to consider the part&#;s application use. Application use is one of the key factors in determining the coating properties needed to perform its end function.
The various coating techniques include:
A cost-effective process provides high quality coating that is uniform on substrates of different shapes or sizes including large surface areas. Dip coating can be carried out manually or automatically for application productions of high volume.
Parts are sunk deep in a tank full of liquid polymer and then withdrawn at a constant rate at temperature or atmospheric conditions that are controlled. The viscosity of the coating, rate of withdrawal from the tank, number of dipping cycles, and length of immersion determine the thickness of the coating. Solidification often occurs at high temperatures inside an oven.
In order for surfaces to be free of contaminants, it is essential prior to dipping them. For process control and to meet standards of consistent quality control, flow and run off properties are important. The internal and external surfaces are coated at the same time with deep coating.
For long lasting protection against abrasion, water, corrosion, humidity, heat or cold, wind, and UV light, master bond eco-friendly liquid polymeric formulations must be selected. These formulations are available in a wide range of colors and they also enhance appearance. Special coating grades are optically clear, tough, guard against solvents or acids, have low friction, vibration/ impact resistance and exhibit exceptional electrical insulation characteristics.
E Coat is a cross between plating and painting and involves submerging a part in a water-based solution that contains a paint emulsion. The solution and part are subjected to an electric current that condenses the solution and forms it over the part. The process allows a part to be coated on the inside and outside. Any metal surface that can be reached by the solution or is exposed is coated.
The E Coat process provides excellent protection against corrosion and rust as well as enhances the appearance of a part. Additionally, E Coat is exceptionally durable with protection against salt spray and offers dielectric and acid resistance. When compared to powdered coatings and solvent based spraying, E Coat provides complete coverage, consistent thickness, and parts may be processed wet or dry.
Since E Coat is so durable, it is widely used by the oil and gas industry, HVAC systems, water and wastewater management, and aerospace. It can be applied to any type of substrate including castings, stampings, structural iron, pipe, springs, and aluminum castings, to name a few.
For outstanding leveling and liquid polymer distribution, experienced personnel and proper brush or bristles are required. To achieve the desired film thickness, multiple coats may be needed.
The synthetic or natural bristles of the brush must be compatible with the product being applied. For covering substrate surfaces that are irregular such as corners, edges, bolt heads, piping, welds, the shape, size, and brush angle must be considered. Brushes must be clean before use. Brushing is slow compared to other techniques used for coating though it has a short set up time. Benefits of brushing include low capital cost, low wastage, and economic suitability for runs of production.
Roll coating is used to apply a base, intermediate, and topcoat to flat substrate using rollers. One method of roll coating uses three rollers, which are soft application roll, polished steel roll, and metering or doctor roll. The material to be coated travels through the three rolls, with each roll performing a specific application. Roll coating can be performed manually or using an automatic conveying system.
For high volume productions, automated roll coating equipment like gravure coating, direct roller coaters can effectively apply liquid polymeric coatings in uniform thicknesses to flat surfaces. In selecting the proper machine, there are some key parameters that include the thickness or width of substrate, the type of substrate, speed of operation, and the full or partial cost.
Roll coaters are found in a wide range of configurations. They offer reliability, cost effectiveness, and high quality finishes comply with a diverse set of requirements. This technique is a popular and continuous energy efficient process that requires less labor. It also offers reduced waste, excellent weight control, efficient coating transfer, and design flexibility.
A special form of roll coating is gravure roll coating, which is a process where adhesives are coated on the substrate. Gravure roll coating uses a roll or applicator ball that is etched to control the amount of coating being applied. The size of the gravure determines the dimensions of the finished coating.
There are several methods used for gravure coating with direct and offset being the most common. With direct, the steel roll has the pattern that is to be engraved or coated and has a blade that eliminates access adhesive. Offset roll coating is a variation of direct and uses an offset roll placed between the gravure and coating material to apply the coating that is placed between the applicator and the gravure roll.
This is an economical, versatile, fast coating process for parts of different types, sizes and shapes including large surface areas. Spraying can be done manually or automatically for viscous or non-viscous liquid coatings with high transfer efficiency for uniform quality finishes.
The desired film thickness is achieved by using different types of spray guns and other equipment, such as air atomized conventional sprays, airless sprays, and air assisted airless sprays. This equipment provides optimal use of material and addresses individual requirements such as quality of finish, desired film thickness, edge build-ups, reduction of overspray, reliable performance, and wasted product concerns.
A key factor in the use of spraying as an application method is the quality of the compressed air, which is measured by size and concentration of solid particles; water vapor content measured by pressure dew point temperature; and concentration of oil with the contamination level influenced by the compressor, dryer, and filtration.
When using a compressed air system, it is necessary to determine the type of air needed for the spraying application. Since all air has water in it, the concentration of water increases as the air is compressed. If dry air is preferred, drying equipment should be used to remove the water. With oil lubricated air compressors, filters are used to reduce the amount of oil with different grades of filters available to produce different qualities of air.
Spraying has a key advantage: it can be achieved from a vertical angle and helps provide a safe work environment. Common drawbacks include uneven spraying, sagging, orange peel, pinholes, overspray, and spluttering. For good optimization and flow, careful features for control must be followed.
Defect-free coatings set high standards for spray application techniques and facilitate transfer efficiency methods. For smooth finishes, it&#;s necessary to find the best spray distance, angle and pattern.
Thermal spray coating uses heat to melt metallic or ceramic materials to apply them to the surface of a workpiece. It is used to improve the appearance of a new workpiece or restore a component that has been damaged. The process adds protection and enhances the appearance of parts and components. The amount of heat used is dependent on the properties and characteristics of the coating material.
The process of thermal spray coating is used to apply coatings to a wide variety of materials, parts, and components. It provides resistance to wear, erosion, cavitation, corrosion, abrasion, and heat. Thermal spray coating improves the properties of products by providing conductivity, insulation, lubricity, and chemical resistance.
All forms of thermal spray coating include spraying small particles of a metal or ceramic material onto a clean, prepared surface where they adhere to form a layer or coating. The thermal process and kinetic energy flattens particles onto the surface and onto each other to create a cohesive coating and layer. Although all thermal spray coating methods appear to be the same, they differ in certain aspects of the process.
The factors that differentiate thermal spray coating processes include the type of heat, temperatures, velocities, coating materials, and the type of bonding. Thermal spray coating processes use unique ways to produce heat with some using arc welding heat while others use gas flames. Included in the type of heat is the temperature reached by the gas stream and the velocity of the coating process. As with all forms of plating and coating methods, the types of materials to be coated on a substrate vary according to their properties, which has an impact on the thermal spray coating method.
Thermal spray coating applies a molten material that has been exposed to heat to the surface of a metal to form a firm and secure bond. Although the purpose of all forms of thermal spray coatings is the same, the processes used to complete coating differ in several ways. Plasma and oxy fuel are two basic forms of fuel used to provide heat.
Flame spray was developed over 100 years ago and uses a welding torch with a high velocity spray to place molten metal on the substrate. The coating feedstock is in the form of a wire or powder. Flame thermal spraying is limited to materials with low melting points and do not strongly adhere to the substrate
HVOF is a flame thermal spray process that creates a combustion jet using a gas, such as hydrogen, propane, or propylene, to reach temperatures of °C up to °C (°F up to °F). The produced coating is very dense with low porosity that is hardened and wear and corrosion resistant.
Two wire electric arc spray uses electric conductivity created between two wires to heat and melt the material for deposition. It has similar combustion as flame spraying. Once the material is heated by the two wires, compressed air distributes it. Two wire electric arc spray is a cost effective method that uses aluminum or zinc as the coating material and has the highest rate of deposition of thermal spray methods.
The plasma spraying process uses an inert gas, such as argon or an argon hydrogen mixture, to heat a DC current arc. The interaction between the DC electric arc and the gas forms a plasma jet. Powder material is injected into the flaming plasma at the nozzle. The rapid expansion of the gas and powder pushes the powder particles onto the surface of the workpiece. The key to the process is the velocity at which the molten material strikes the substrate to form a tight bond.
VPS is a form of plasma spraying that takes place in a low pressure atmosphere chamber with a pressure level below 0.1 mbar to prevent oxygen and hydrogen from affecting the coating process. Under the unique conditions, spraying is completed in the same way as regular plasma spraying. The VPS atmosphere prevents metal particles from oxidizing. The plasma spray has a larger diameter and longer length as convergent and divergent nozzles create greater speed. The coatings from the process are denser and more adherent due to lower oxide in the atmosphere.
With detonation spraying, a long water cooled barrel that has inlet valves for gases and powder has oxygen and fuel fed into the barrel with the powder. The mixture of gas and oxygen is ignited to reach a temperature of ° C (° F) by a spark that heats and accelerates the powder to supersonic speeds. The kinetic energy of the powder particles hits the substrate to form a dense coating. Once the barrel is purged by nitrogen, the process repeats every few seconds.
Several types of coating materials can be applied using the thermal spray coating method. Each type of coating offers various properties and characteristics to enhance the quality and performance of a product. The majority of thermal coatings are abrasion, corrosion, erosion, and rust resistant.
The types of coating materials applied using thermal spray coating include over 80 types of metals and include:
In this technique, a coating is applied in the center of the substrate and then spun by centrifugal force at high speed. The spin coater revolves until the desired film thickness is achieved. The key parameters that help to meet the specific application requirements are the spinning rate, resin materials, substrates, surface tension, acceleration, and viscosity.
Spin coating is the most effective technique in providing thin coatings, thickness uniformity and consistency, and a quality finish that is repeatable without variation. The spin coating process is simple, can be carried out rapidly, and is utilized in the coating across substrates of small or large size. The problems that can be encountered during processing include uncoated areas, streaks, comets, swirl patterns, and too thin or too thick films.
This technique is an easy, fast, reliable manual, or automatic process for the application of liquid coatings. It is mostly recommended to be used on large, flat horizontal sheets and panels that can&#;t be dip coated easily. In flow coating, high coating thicknesses can be achieved with only a single coat.
Flow coating requires little space, minimizes waste, is economical, and has high transfer efficiency. In the design, coatings are to be dispensed on the upper part of the workpiece and flow down to completely cover the flat surface areas. This controlled flow of gravity is a function of the compound&#;s viscosity. The finish or uniformity is affected by curing conditions such as temperature or humidity. Flow coating is not suitable for parts containing holes or pultrusions.
Slot die coating is a pre-metered coating method where the thickness of the final coating is dependent on the rate at which the coating is applied. The material in slot die coating is delivered through a narrow slot positioned over the surface to be coated. The process places a thin film onto the substrate and can be integrated into several types of operations.
The mechanism for slot die coating includes metering, distribution, head positioning, and substrate movement systems. The metering system controls the flow while the distribution system ensures that the coating is spread evenly. The head positioning system keeps the head correctly oriented relative to the substrate. The movement system keeps the substrate moving past the slotted head.
PVD coating, also known as thin film coating, vaporizes solid materials in a vacuum and applies them to the substrate. The material is applied atom by atom to form a thin bond of metal or metal ceramic layer that enhances the appearance, durability, and function of a component. The result of the process is an extremely thin pure coating that is environmentally friendly.
The four steps of PVD coating are ablation, transport, reaction, and deposition. The main types of PVD coating are sputtering and thermal deposition where thermal deposition uses high temperature and vacuum pressure for vaporization while sputtering uses a vacuum chamber with a high energy source.
Sputtering is a slow process and complex but produces a pure uniform coating. Thermal deposition has high throughput and efficiency but requires extra tooling.
PVD requires precision timing for the vaporized atoms to bond with the target material. Reactive or inert gasses, such as oxygen or nitrogen, assist in forming a strong bond between the coating and substrate. The final step in the process involves rotating the substrate while monitoring the rate of deposition using various tools. The process can be automated using conveyor belts placed before the vaporized arc for a specified length of time.
The process for PVD coating is used by a wide assortment of industries, including food packaging, medicine, optics, cooking tools, and plumbing fixtures. It is suitable for use with inorganic solid materials that have a high melting point.
The various types of industrial coatings include:
These types of coatings offer properties that are balanced and unbeatable by any other material. Teflon coatings exhibit excellent dielectric stability, low coefficient of friction, almost total chemical inertness, and heat resistance.
Teflon coatings are the original non-stick finishes. Industrial products made out of Teflon coatings or fluoropolymer resins exhibit exceptional high temperature resistance and are also resistant to chemical reactions, stress cracking, and corrosion. These industrial coatings can be used on aluminum, carbon steel, stainless steel, brass, steel alloys and magnesium as well as on non-metallic like glass, plastics, and fiberglass.
Characteristics of Teflon coatings include:
There are only a few solid substances that can adhere to a Teflon finish. Almost all substances release easily on Teflon finishes, even though tacky materials may show some adhesion.
The range of the coefficient of friction of Teflon is 0.05 &#; 0.20 and it depends on the sliding speed, load, and particular Teflon coating used.
Teflon surfaces are not easily wet since they are both oleophilic and hydrophobic. Cleaning up is easy and more thorough &#; surfaces are self-cleaning in many cases.
Teflon industrial coatings can continuously operate at temperatures of up to 500 °F (260 °C) or °F( 600 °C) with adequate ventilation.
Most Teflon industrial coatings can withstand severe temperature extremes without losing their physical properties. Teflon industrial coatings can be utilized at low temperatures like -454 °F (-270 °C).
Teflon industrial coatings are unaffected by chemical environments.
Teflon has a low dissipation factor, high dielectric strength, and very high surface resistivity over a broad range of frequencies.
As beneficial as Teflon coatings are, there are aspects of it that have to be considered when working with Teflon, especially during the curing process. When it is being cured, Teflon coatings produce a toxic out gas, Phosgene, which is very dangerous. Manufacturers of coatings have data and information regarding the use of Teflon coatings and should be consulted for how to deal with the dangers involved with its use.
This type of coating is considered by many to be the toughest, most durable, and longest-lasting non-stick coating. The difference between Excalibur and other coatings is that Excalibur is a full coating system, rather than simply an applied coating over an existing substrate.
When stainless steel is arc-sprayed onto a component, becoming one with it, the Excalibur&#;s system begins. Next, the stainless steel matrix is impregnated with non-stick coatings that are premium. The result is a coating system with the toughness of stainless steel together with the release properties of the non-stick.
The application of Excalibur Coating includes steps such as:
When the surface properties of an ideal material in engineering construction are wrong, these types of coatings come into play. Xylan is capable of binding strongly to surfaces that do not readily accept other PTFE coatings and it is applied as a thin film. These coatings offer controlled friction, wear resistance, non-stick and release properties, and lubrication and at the same time can also prevent corrosion.
These types of coatings were developed to deliver a broad range of performance attributes that are valuable. These single coat thin films offer excellent corrosion and resistance to chemicals. Other benefits of fluoropolymer include resistance to galling, electrical resistance, abrasion resistance, non-stick, non-wetting, and reduced friction. OEM components that are coated with fluoropolymer have a longer life before replacement.
These types of coatings provide a unique combination of properties that are surface enhancing, unlike any in the industry. They are resistant to extreme temperatures, exhibit superior surface hardness, are anti-galling, and exhibit anti-friction properties. In the high-performance field, Nitro coat is the coating of choice.
Nitro coat barrier coatings are applied to the surface chemically, utilizing some of the most advanced application technology present in industries. To most metallic substrates, very thin, extremely uniform, and dense coatings are applied easily. High performance components that are coated using the nitro coat process demonstrate significant performance consistently under severe extreme conditions in the field and in the laboratory.
These types of coatings are mostly used to improve the performance of a material to increase its operating temperature, load carrying capacity and coefficient of friction. This coating offers effective lubrication in a broad range of loads, in most cases exceeding 250,000 psi. By transferring lubricant between the two surfaces that are to be mated, moly coatings lubricate sacrificially. This helps in the reduction of both the coefficient of friction and wear.
Molybdenum coatings are a combination of high-performance resins and molybdenum disulphide lubricant. The curing of the coating is done thermally for the thorough bonding of the base metal of the part coated.
These are high molecular weight coatings that are specially blended to offer remarkable corrosion resistance across a diversity of environments. These types of coatings effectively perform as abrasion-resistant coatings while offering outstanding barrier protection from alkaline compounds, harsh chemicals and solvents, and caustic solutions.
These types of coatings are developed to protect against oxidation produced by high heats naturally, while simultaneously protecting metal surfaces against acid, water and other corrosive agents. They are applied in the offshore, military, and chemical processes. They offer significant performance benefits. These heat/corrosion coatings can extend the life span of steel engine components.
These types of coatings are two-coat non-stick systems in which there is a primer and a topcoat. They exhibit the highest operating temperature of any fluoropolymer, good abrasion resistance, good chemical resistance, and extremely low coefficient of friction. They are able to withstand a maximum temperature of use which is 600 degrees Fahrenheit. The thickness of application of this coating is 1-3 mm.
These types of coatings are resin bonded polymer coatings with remarkable resistance to chemical reactions and thermal degradation. This coating is virtually unaffected by any solvent up to 500 °F (260 °C), making it a famous alternative for chemical processing industry use. For outstanding corrosion and chemical resistance, PPS coatings may be used by themselves. PPS also serves as an effective primer with the use of a topcoat. On top of its thermal and chemical advantages, PPS also provides outstanding abrasion and wear resistance.
PVDF is a pure fluoropolymer that is highly reactive and utilized in applications that require the highest strength, purity, resistance to acids, solvents, heat and bases, and low smoke generation during a fire event. At high temperatures, PVDF can be dissolved in polar solvents that include amines and organic esters, making them practical for use in coatings that are resistant to corrosion and architectural finishes that are durable on building panels.
PVDF can be readily melted for use in extrusion or injection molding equipment. Components that are coated with PVDF are extensively used in high purity semiconductor markets, in the paper and pulp industry, nuclear waste processing, water treatment, and chemical processing. PVDF is able to meet the food and pharmaceutical processing industries&#; specifications.
These types of coatings provide outstanding chemical resistance and good electrical use properties. ECTFE coatings suit applications that must exceed the capabilities of PVDF for thermal resistance and chemical resistance.
These are coatings that are applied as free-flowing dry powders. These types of coatings are dry, and they don&#;t need a solvent to keep the components together. Powder coatings are applied using a fluidized bed or an electrostatic spray. To fuse the particles together and make them adhere to the surface, parts are heated before and after application. These types of coatings are mainly used to coat metals and sometimes thermoplastics or thermosets.
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Metallized coatings, known as thermal spray coatings, are applied to surfaces in a molten or semi-molten form as a method of corrosion protection that includes spraying metals onto steel or concrete surfaces. Metals commonly used for metallized coatings are aluminum, zinc, and their alloys.
Metallized coatings are used due to their long service life, instantaneous curing, and absence of volatile organic compounds (VOCs). They can be applied under any conditions, with the metal being heated before application. Metallized coatings are used outdoors due to their greater resistance to impact and UV rays.
Hard coat refers to a fast setting urethane or polyurea elastomer. It works well on wood or other materials, with some forms having UL-tested fire ratings. Hard coat forms a solid finish on surfaces such as styrofoam and urethane foam. They contain no solvents and are volatile organic compounds that can serve as vapor barriers.
The formulations of hard coat have varying properties, including soft and rubbery or hard and rigid. Hard coat can be tinted to a base color, which is not UV stable unless it has an aliphatic formulation. In most cases, hard coat is applied without color and painted. Primer is necessary to achieve stable adhesion, especially in extreme and harsh conditions. As with many other types of industrial coatings, surface preparation is required for best results when applying hard coat.
Low friction coatings have a friction coefficient from 0.2 down to 0.05, with the lowest coefficient being offered by polytetrafluoroethylene. There are several types of low friction coatings with the application determining the type of coating being used. Each of the types has different values with varying degrees of wear resistance and hardness.
High static friction coefficients can be generated between mating parts the forces of which can be reduced with the application of a low friction coating. A further benefit of low friction is the minimal chance of foreign substances sticking to treated surfaces. Aside from their use with metals, low friction coatings can be used on elastomers and rubber to reduce their friction values.
This chapter will discuss the applications and benefits of industrial coatings. When working with any coating or coating material, it is essential that workers strictly follow manufacturer&#;s guidelines for the success of the process and the safety of workers. Every manufacturer publishes specific and concise directions regarding the safe handling of coating materials. Additionally, they provide contact information when questions arise regarding application.
There are many diverse types of industrial coatings with many different characteristics. For instance, the PVDF coating is highly reactive and is used in applications where highest strength is required. Each coating provides many benefits to the equipment on which it is applied; for instance, by improving the equipment&#;s wear resistance or strength. Therefore, when selecting a coating for equipment, one must be cautious of the properties of the coating material and the environment in which the equipment is going to be used. The bottom line is that industrial coatings are there to protect equipment from harsh weather or environmental conditions that end up damaging the equipment, ultimately prolonging its lifespan.
Paint is the most commonly used material to protect steel. Paint systems for steel structures have developed over the years to comply with industrial environmental legislation and in response to demands from bridge and building owners for improved durability performance. Previous five and six coat systems have been replaced by typically three coat alternatives, and the latest formulations have focussed on application in even fewer numbers of coats, but with increasing individual film thickness. Examples of this are epoxy and polyester glass flake coatings that are designed for high build thickness in one or two coat applications, and single coat high build elastomeric urethane coatings, up to μm thick.
Modern specifications usually comprise a sequential coating application of paints or alternatively paints applied over metal coatings to form a &#;duplex&#; coating system. The protective paint systems usually consist of primer, undercoat(s) and finish coats. Each coating &#;layer&#; in any protective system has a specific function, and the different types are applied in a particular sequence of primer followed by intermediate / build coats in the shop, and finally the finish or top coat either in the shop or on site.
Composition of paints and film formation
Paints are made by mixing and blending three main components:
(a) The pigments
Pigments are finely ground inorganic or organic powders which provide colour, opacity, film cohesion and sometimes corrosion inhibition.
(b) The binder
Binders are usually resins or oils but can be inorganic compounds such as soluble silicates. The binder is the film forming component in the paint.
(c) The solvent
Solvents are used to dissolve the binder and to facilitate application at the paint. Solvents are usually organic liquids or water.
Paints are applied to steel surfaces by many methods but in all cases this produces a 'wet film'. The thickness of the 'wet film' can be measured, before the solvent evaporates, using a comb-gauge. As the solvent evaporates, film formation occurs, leaving the binder and pigments on the surface as a 'dry film'. The thickness of the 'dry film' can be measured, usually with an electro-magnetic induction gauge. The relationship between the applied 'wet film' thickness and the final 'dry film' thicknesses (dft) is determined by the percentage volume solids of the paint, i.e. dft = 'wet film' thickness multiplied by the % vol. solids.
In general the corrosion protection afforded by a paint film is directly proportional to its dry film thickness.
Paint constituents and their function
Schematic diagram of a paint system
Classification of paints
Multi-coat paint system
Since, in the broadest terms, a paint consists of a particular pigment, dispersed in a particular binder, dissolved in a particular solvent then the number of generic types of paint is limited. The most common methods of classifying paints are either by their pigmentation or by their binder type.
Primers for steel are usually classified according to the main corrosion inhibitive pigments used in their formulation, e.g. zinc phosphate primers and metallic zinc primers, etc. Each of these inhibitive pigments can be incorporated into a range of binder resins giving for example, zinc phosphate alkyd primers, zinc phosphate epoxy primers, etc.
Intermediate and finish coats are usually classified according to their binders, e.g. epoxies, vinyls, urethanes, etc.
Paint coatings
Paints are usually applied one coat on top of another and each coat has a specific function / purpose. These are described as follows.
Primers
The primer is applied directly onto the cleaned steel surface or, in the case of duplex systems, the sealed metal coating. Its purpose is to wet the surface and to provide good adhesion for subsequently applied coats. For primers applied directly to steel surfaces, these are also usually required to provide corrosion inhibition. There are two basic types of primer.
(a) Primers pigmented with metallic elements anodic to steel
These primers are formulated so that, when a break in the coating (due to damage or local corrosion) exposes the steel substrate, the anodic metal corrodes sacrificially in preference to the steel. This effectively stifles steel corrosion and under-rusting of the primer until the anodic metal is exhausted. Zinc-rich primers are the most commonly used of this type.
(b) Primers relying on the high adhesion and chemical-resistance properties of the binder
With these primers, good adhesion is obtained (provided that the surface is very thoroughly cleaned) and it is sufficient to prevent under-rusting at any break in the coating (due to damage). Two-pack epoxy primers are typical of this type. These primers may contain inhibitive pigments to interfere with the corrosion process. Zinc phosphate, for example, is a mildly inhibitive pigment and is widely used in modern primer formulations.
Intermediate coats
Intermediate coats are applied to &#;build&#; the total film thickness of the system. Generally, the thicker the coating the longer the life. Intermediate coats are specially designed to enhance the overall protection and, when highly pigmented, decrease permeability to oxygen and water. The incorporation of laminar pigments, such as micaceous iron oxide (MIO), reduces or delays moisture penetration in humid atmospheres and improves tensile strength. Modern specifications now include inert pigments such as glass flakes to act as laminar pigments. Undercoats must remain compatible with finishing coats when there are unavoidable delays in applying them.
Finish coat
The finish coat provides the required appearance and surface resistance of the system. Depending on the conditions of exposure, it must also provide the first line of defence against weather and sunlight, open exposure, and condensation.
Stripe coats
Stripe coats are additional coats of paint that are applied locally to welds, fasteners and external corners. Their function is to build a satisfactory coating thickness at edges and corners where paint has a tendency to contract and thin upon drying. Specifications should indicate the type and number of stripe coats required and state when they are to be applied.
The paint system
The various superimposed coats within a painting system have, of course, to be compatible with one another. They may be all of the same generic type or may be different, e.g. chemical resistant types, such as a recoatable polyurethane finish coat, may be applied onto epoxy primer and intermediate coats. However, as a first precaution, all paints within a system should normally be obtained from the same manufacturer and used in accordance with the manufacturer&#;s recommendations.
An important factor in the coating system is the definition and measurement of the dry film thickness (dft). Dry film thicknesses are generally checked on the complete paint system, although individual films may be checked separately. Usually, nominal dry film thicknesses are specified but sometimes minimum values are quoted.
For nominal dry film thicknesses, individual values less than 80% of the nominal thickness are not acceptable. Values between 80% and 100% are acceptable provided that the overall average (mean value) is equal to or greater than the nominal.
Specifications for minimum dry film thicknesses require careful paint application to avoid excessive film thickness. The &#;over application&#; of paints can result in the formation of high stresses and may cause premature failure of the system. Wet film thickness (wft) checks may also be required during the application of the coating to check that a subsequent satisfactory dry film thickness will be achieved.
Main generic types of paint and their properties
(a) Air drying paints
For example alkyds
These materials dry and form a film by an oxidative process, which involves absorption of oxygen from the atmosphere. They are therefore limited to relatively thin films. Once the film has formed it has limited solvent resistance and usually poor chemical resistance.
(b) One pack chemical resistant paints
For example acrylated rubbers, vinyls
For these materials, film formation requires only solvent evaporation and no oxidative process is involved. They can be applied as moderately thick films though retention of solvent in the film can be a problem at the upper end of this range. The formed film remains relatively soft and has poor solvent resistance but good chemical resistance. Bituminous paints also dry by solvent evaporation. They are essentially solutions of either asphaltic bitumen or coal-tar pitch in organic solvents.
(c) Two pack chemical resistant paints
For example epoxy, urethane
These materials are supplied as two separate components, usually referred to as the base and the curing agent. When these two components are mixed, immediately before use, a chemical reaction occurs. These materials therefore have a limited 'pot life' before which the mixed coating must be applied. The polymerisation reaction continues after the paint has been applied and after the solvent has evaporated to produce a densely cross linked film which can be very hard and has good solvent and chemical resistance. Liquid resins of low viscosity can be used in the formulation thereby avoiding the need for a solvent. Such coatings are referred to as 'solvent less' or 'solvent free' and can be applied as very thick films.
Prefabrication primers
Also referred to as blast primers, shop primers, temporary primers, holding primers, etc., these primers are sometimes used on structural steelwork, immediately after blast cleaning, to maintain the reactive blast cleaned surface in a rust free condition until final painting can be undertaken. They are mainly applied to steel plates and sections before fabrication. The main requirements of a prefabrication primer are as follows:
Etch primers
These are based on polyvinyl butyral resin reinforced with a phenolic resin to increase water resistance. These primers can be supplied in a single pack or two pack form, the latter providing better durability.
Epoxy primers
These are two pack materials utilising epoxy resins and usually have either polyamide or polyamine curing agents. They are pigmented with a variety of inhibitive and non-inhibitive pigments. Zinc phosphate epoxy primers are the most frequently encountered and give the best durability within the group.
Zinc epoxy primers
These primers can be either zinc rich or reduced zinc types. Zinc rich primers produce films which contain about 85% by weight of metallic zinc powder and the reduced zinc type as low as 55% by weight. When exposed in either marine or highly industrial environments, zinc epoxy primers are prone to the formation of insoluble white zinc corrosion products which must be removed from the surface before subsequent overcoating.
Zinc silicate primers
Zinc silicate primers produce a level of protection which is comparable with the zinc rich epoxy types and they suffer from the same drawbacks, e.g. formation of zinc salts and production of zinc oxide fume during welding. There are currently different categories of zinc silicate primers based upon the binder (organic or inorganic) and the zinc content. Low zinc primers in this group have been developed to improve their weldability and to minimise weld porosity, however their durability is also reduced. The organic silicate primers are the most suitable as prefabrication primers.
Application of paint coatings
Methods
The method of application and the conditions under which paints are applied have a significant effect on the quality and durability of the coating. Standard methods used to apply paints to structural steelwork include application by brush, roller, conventional air spray and airless spray.
Airless spraying has become the most commonly used method of applying paint coatings to structural steelwork under controlled shop conditions. Brush and roller application are more commonly used for site application, though spraying methods are also used.
Brushing
Brush application of paint
This is the simplest method and also the slowest and therefore most expensive. Nevertheless it has certain advantages over the other methods, e.g. better wetting of the surface and can be used in restricted spaces, be useful for small areas, with less wastage and contamination of surroundings.
Brush application of paint
(Video courtesy of Corrodere/MPI)
Roller
Roller application of paint
This process is much quicker than brushing and is used for large flat areas, but demands suitable rheological properties of the paint.
Roller application of paint
(Video courtesy of Corrodere/MPI)
Air spray
Paint coatings for structural steelwork are usually spray applied. The paint is atomised into fine droplets and projected onto the surface to be protected where the droplets join together to form a continuous film. The atomisation can be accomplished in a number of ways.
In air spraying, the paint is atomised by mixing it with a stream of compressed air in a conventional spray gun. The paint can be either sucked into the air stream (as in the simple suction-cup gun used for application to small areas) or fed to the spray gun under pressure from a pressure pot. For ideal application, careful adjustments of the spray nozzle and air pressures must be made by a skilled operator, according to the consistency and composition of the paint product and the film thickness required. The application rates for air spray are quicker than for brushing, or rolling, however paint wastage by overspray is high.
Airless spray
Airless spray application of paint
(Image courtesy of Mabey Bridge Ltd.)
For airless spraying, the paint is hydraulically compressed and, on release through a small orifice in an airless spray gun, it is atomised and projected onto the surface. By changing the orifice size and shape and by varying the hydraulic pressure, atomisation can be accomplished for a wide range of paint consistencies from thin to thick, to give a wide range of rates of deposition. The equipment required is much more expensive than for conventional air spraying, because it must withstand the much higher pressures involved. However, the application rates are higher than for air spray with overspray wastage greatly reduced. For conventional air spraying, the maximum air pressure will normally not exceed 100 psi (6.9 bar); for airless spraying, hydraulic pressures up to 4,000 psi (280 bar) may be required.
A variant of the above involves heating to reduce the consistency of the paint rather than adding diluents. In this way greater film thickness per application is achieved. This method can be used for the application of solvent-free materials such as two-pack products, which can be mixed at the spray gun nozzle at the moment of application. The use of expensive equipment and highly skilled labour is necessary for the achievement of optimum results but may be justified for the protection of large and important structures.
Airless paint spraying
(Video courtesy of Corrodere/MPI)
Conditions of application
The principal conditions that affect the application of paint coatings are temperature and humidity. These can be more easily controlled under shop conditions than on site.
(a) Temperature
Air temperature and steel temperature affect solvent evaporation, brushing and spraying properties, drying and curing times and the pot life of two-pack materials, etc. Where heating is required, this should only be by indirect methods.
(b) Humidity
Paints should not be applied when there is condensation present on the steel surface or the relative humidity of the atmosphere is such that it will affect the application or drying of the coating. Normal practice is to measure the steel temperature with a contact thermometer and to ensure that it is maintained at least 3°C above the dew point.
However, moisture cured paints are available. These paints are specifically formulated for application in damp and humid conditions; reference should be made to the manufacturer&#;s data sheets for details of limiting conditions of application.
Coating applicator training and certification (ICATS)
ICATS Training Programme
With modern high performance coatings, correct application has become increasingly important to achieve the intended performance. Industry has recognised this and established a training and certification scheme for paint applicators (ICATS &#; Industrial Coating Applicator Training Scheme). ICATS registration (or to an equivalent scheme) has subsequently become a mandatory requirement for work on National Highways and Network Rail bridges.
ICATS is owned by the Institute of Corrosion and operated through Correx Ltd., a wholly owned subsidiary of the Institute. The scheme has a mandatory Basic Unit 'Industrial Coating Applicator' that comprises 6 modules:
There are also 2 optional specialist modules:
ICATS is a registered training scheme that meets the requirements of NHSS 19A[1] and enables contractors to provide 'best value' service to maximise coating performance over structure life and to reduce whole life costs.
Additional education
For engineers and inspectors wishing to further their knowledge of corrosion control in the context of structural steelwork, there are web based training programmes available from a specialist training provider, Corrodere:
Trainthepainter is an internationally recognised and certified training programme that provides trainee coating applicators with a comprehensive package of training modules that ensure professionalism and give an insight into methods of surface treatment and application of protective coatings. As with ICATS, this is a registered training scheme that meets the requirements of NHSS 19A[1].
This website is maintained by Steel for Life with funding provided by a number of BCSA&#;s Industry Members. Those sponsors relevant to this article are as follows:
Silver
References
Resources
Further reading
See also
CPD
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