Basic Coatings Technology

Basic Coatings Technology - excerpted from GENERIC COATING...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: excerpted from GENERIC COATING TYPES: An Introduction to Industrial Maintenance Coating Materials BASIC COATINGS TECHNOLOGY Lloyd M. Smith, Ph.D. Vice President Corrosion Control Consultants and Labs Herndon, Virginia, USA Coatings have many uses in industrial situations. They are used for corrosion control, chemical resistance, heat resistance, temperature control, identification, decoration, camouflage, fire retardation, noise control, anti-fouling protection, and many other reasons. Corrosion protection is one of the primary uses of coatings in industrial applications. Corrosion is the destruction of material by chemical or electrochemical reaction on exposure to the environment. All materials, including metals, concretes, and plastics, will eventually corrode. However, coatings are a cost-effective method of controlling corrosion. Alternatives include the use of alloys or composite materials that may be more corrosionresistant but not as cost-effective. This book is organized by generic types of industrial protective coatings rather than by enduses. Different generic types of coatings may be used for the same application. Alternatively, a generic coating type may have many end-use applications. The user or specifier must determine the most appropriate or cost-effective coating system for particular structures and exposure environments, and weigh the advantages and disadvantages of each type of paint or coating. Terminology used in the industry can be confusing. The terms paint, coating, and lining sometimes are used interchangeably, but there are differences in their meanings. The Paint/Coatings Dictionary1 defines paint and coating as follows: • Paint - Any pigmented liquid, liquefiable or 1 mastic composition designed for application to a substrate in a thin layer which is converted to an opaque solid film after application. Used for protection, decoration, or identification, or to serve some functional purpose such as the filling or concealing of surface irregularities, the modification of light and heat radiation characteristics, etc. • Coating - A liquid, liquefiable, or mastic composition which is converted to a solid protective, decorative, or functional adherent film after application as a thin layer. Based on these definitions, the major difference between paint and coating is that paint is pigmented, while no such requirement is mentioned for coating. They both are liquid, liquefiable, or mastic compositions that are converted to a film after application as a thin layer. Therefore, varnishes and clear coats are coatings but not paints. Processes such as galvanizing and metallizing also meet the definition of coating. With galvanizing, for example, zinc is heated to a liquefiable state and applied as a thin layer. It is converted to a solid film with a decorative or functional purpose. While the distinctions between paint and coating appear to be minor, it is common in the industry to distinguish between them, even though most materials used are pigmented and meet the definition of paint. Coating generally refers to materials used for protective or functional purposes, while paint refers to materials used for aesthetic or decorative purposes. Thus, a structure is coated while a room is painted. This Basic Coatings Technology by Lloyd M. Smith, PhD. differentiation is emphasized further by some who refer to the materials used in industrial situations as protective coatings. The definition of lining from the Industrial Maintenance Coatings Glossary2 is “a material used to protect a container against corrosion and/ or to protect the contents of a container from contamination by the container shell material.” Liners commonly are thought of as thick, builtup systems containing matting or similar reinforcing material. However, the definition does not exclude coatings from use as linings. In fact, coatings are the standard material for lining the inside of water storage tanks. There are many similarities in composition, handling, and use of paints, coatings, and linings. There also are some differences. The basic concepts and terminology of the coatings industry are presented here to help the reader better understand the information presented about the various generic types of coatings in this book. COATING COMPONENTS A coating can contain as few as three or four ingredients or as many as twenty or thirty ingredients, depending on the formulation. The three main components of a coating are the resin, pigment, and solvent. Resin and solvent comprise the liquid portion of a coating. Together, they are called the vehicle. Resin and pigment are referred to as the film solids, since they are the materials left after the coating has dried. Resins Resin is the binder that holds the pigment particles together and provides adhesion of the coating to the surface. Most coatings are named by the generic type of resin (i.e., vinyl, epoxy, acrylic, polyurethane, etc.). The resin component of most coatings is a mixture or chemical blend of materials. There are different types of epoxy resin, for example, and many combinations of 2 epoxy resin and hardener. So, the resin composition of an epoxy coating may be different for epoxy coatings from different suppliers or even for different products from the same manufacturer. In addition, other resin types can be modified with epoxy resin. Therefore, referring to a coating as an epoxy—or any other type of resin— provides only limited information. The resin or binder is responsible for most of a coating’s physical and chemical properties, including hardness, abrasion resistance, chemical resistance, weather resistance, adhesion, and cohesion. The type of resin system also determines a coating’s curing mechanism. Resins can be classified as thermoplastic or thermosetting. Thermoplastic resins can be repeatedly softened by heating and hardened by cooling. They also can be dissolved by the original solvent used in the coating. Vinyls are such a resin. Coatings based on thermoplastic resins usually are packaged in one container. Thermosetting resins, however, undergo a chemical reaction by the action of heat, catalysts, ultraviolet light, etc., that makes them relatively infusible. They do not harden and soften by heating and cooling or redissolve in solvent. Epoxies are such a resin. Coatings based on thermosetting resins usually are packaged in two or more containers, although thermosetting resins that cure by such methods as heating, ultraviolet light, or reaction with constituents in the atmosphere come in one container. Pigments Most pigments are inorganic compounds, although some bright color pigments are insoluble organic compounds. Prime pigments provide opacity, a term for hiding power. They also can provide improved durability, weathering, and protection to light-sensitive resins. Titanium dioxide is a commonly used prime pigment for white and light tints. Other pigments are added for color. excerpted from GENERIC COATING TYPES: An Introduction to Industrial Maintenance Coating Materials Some coating types contain anti-corrosive pigments for corrosion protection. Lead- and chromium-based compounds historically used for this purpose are being replaced by nontoxic metal compounds such as phosphates, chromates, phosphosilicates, phosphites, some organic compounds, and other compounds. Flat, platelike pigments, such as mica and aluminum flake, are used to decrease moisture permeability. Still another group of pigments, known as extender or filler pigments, add mechanical strength to the film, control viscosity, reduce settling, reduce gloss, and improve film-build. Examples of these pigments are talc, silica, and clays. A critical parameter in a coating film is the pigment-to-resin ratio, known as the pigment volume concentration (PVC). Properties of a coating film change dramatically near the critical pigment volume concentration (CPVC), the point at which there is just sufficient binder to coat the pigment particles and to fill the voids between them. Coatings formulated below the CPVC (i.e., those containing an excess of resin) have high gloss, low moisture permeability, and a potential to blister. Adding more pigment reduces gloss, increases moisture permeability, and decreases the tendency to blister. When the CPVC is exceeded so there is insufficient binder to wet the pigment particles and fill in the void spaces, the cohesive and adhesive strength may be reduced. Some coatings, such as inorganic zinc-rich primers, are intentionally formulated above the CPVC. This is necessary because of the mechanism by which they provide corrosion protection. The coatings formulator determines the acceptable PVC for a particular product. The properties of the dry film depend upon maintaining the proper ratio of pigment to resin. This is why properly mixing a coating prior to use is so important. If settled pigments are not redispersed in the mixing process, the dry film will not have the properties or performance designed by the formulator. 3 Solvents The main function of the solvent is to provide ease of coating application. Solvents dissolve or disperse the resin, provide flow-out and leveling during application, and control adhesion and durability of the dry film. A coating formulation usually contains a blend of solvents. The resin is dissolved or dispersed in the primary solvent. However, if one of the resin constituents is not soluble in the primary solvent, a co-solvent may be needed. Other solvents may be added to control the evaporation rate or to provide adequate flow-out and leveling. The solvent can control the rate of chemical reaction in some coatings. Solvents are not part of the dry film. They evaporate during the drying or curing process. Organic solvents contribute to the production of photochemical smog. As a result, federal, state, and local air quality regulatory agencies set limits on the volatile organic compound (VOC) content allowed in coatings. The maximum VOC content allowed varies by locale and by coating type or use. For example, zinc-rich primers, heatresistance coatings, and swimming pool coatings have a higher VOC limit than industrial maintenance coatings. These variances were obtained due to the unique characteristics or uses of these coatings, with no viable alternatives available. Currently, the VOC content limit for industrial maintenance coatings varies from 2.0 Ibs/gal (240 g/L) to 3.8 Ibs/gal (450 g L), depending on locale. VOC regulations are having a profound effect on coatings formulation. Coating manufacturers are concentrating their research and formulation efforts on developing VOC-compliant coatings. The main technologies currently being used are high-solids and water-borne coatings, because they are low in VOC content. There even are some coatings on the market that contain no volatile organic compounds. High-solids Basic Coatings Technology by Lloyd M. Smith, PhD. coatings contain a high percentage of solids, hence less solvents. Water-borne coatings consist of two classes of materials. There are latex or emulsion coatings, in which water is the main solvent. The other class is water-reducible coatings, which contain a solvent blend that can be thinned with water. Additives In addition to resins, pigments, and solvents, many coating formulations contain additives— specialty materials that vary widely depending on the resin type. Oil-based coatings, for example, contain driers to promote curing. Hard, brittle resins such as vinyls contain plasticizers to produce a more flexible film. Emulsion systems employ a number of additives, including wetting agents, dispersants, freeze-thaw stabilizers, antimicrobial agents, and film-forming aids. Other additives may be incorporated into a coating formulation to control consistency and pigment settling or improve sag resistance. PRODUCT DATA SHEET Users and specifiers need not know coating formulation. However, proper use of a coating requires them to follow information supplied by the manufacturer. Product data sheets contain a wealth of information about the proper selection, use, and application of a particular coating. The amount of information and the form in which it is presented vary by manufacturer. Information on selection and use of the coating is presented mainly for specifiers. This information typically includes the generic type of coating, intended uses of the product (i.e., primer, intermediate coat, or topcoat), acceptable substrates, and recommended exposure environments. The product data sheet also may indicate where the coating should not be used. If it is unclear whether the intended use falls within the recommended uses of the material, the manufacturer should be consulted. 4 Performance data—information about chemical resistance and physical properties of the coating— may be presented on a data sheet either as subjective ratings or as the results of test methods developed by consensus standards organizations such as ASTM. One such common test is for salt fog resistance, which is the topic of ASTM B 117, Method of Salt Spray (Fog) Testing. Interpreting these test results requires knowledge of the test methods and the performance of coating materials of similar generic type. Weldon3 discussed interpretation of the results of some of these tests. Note there are no standard definitions in the industry for subjective ratings such as excellent, good, and fair, and, therefore, they should be interpreted with caution. Data sheet information on compatible coatings indicates acceptable primers (or whether the coating can be applied directly to a substrate) and topcoats. Some manufacturers present information on recommended coating systems. Others include this information in a separate document such as a coating system selection guide or specifier’s guide. Other categories of product data sheet information include the color, gloss, and basic physical characteristics of the coating, such as density, percent solids by weight, percent solids by volume, flash point, viscosity, and recommended dry film thickness. Many manufacturers also report the VOC content of the product, including the VOC content when the coating is thinned with the recommended thinner. Density, percent solids by weight, and viscosity are useful if there is a need to quantitatively verify that a batch of paint was manufactured to tolerance. These measurements are determined by simple laboratory tests. The percent solids by volume can be used to calculate how much coating material to apply to achieve the desired dry film thickness or to determine the expected dry film thickness for the applied wet film thickness. Multiply the percent solids by volume by the wet film thickness to determine the expected dry film thickness. excerpted from GENERIC COATING TYPES: An Introduction to Industrial Maintenance Coating Materials Coatings must be applied within or near the manufacturer’s recommended thickness or thickness range, or problems may occur. Specifying a coating too thin may affect the life of the coating system. Rust may form sooner than expected on steel substrates, for example. On the other hand, specifying a coating too thick may require two coats, adding significantly to the cost of application. Likewise, applying a coating in one coat at a dry film thickness significantly above the manufacturer’s recommendation can result in lack of curing, solvent entrapment, blistering, mudcracking, delamination, or other defects. Product data sheet information on surface preparation, storage, mixing, application, and drying all relates to the use of the coating. This information is as important to the user and specifier as it is to the applicator. The cleanliness of the surface to which the coating will be applied is usually presented with reference to surface preparation standards from consensus organizations such as SSPC: The Society for Protective Coatings, NACE International, or ASTM. Surface preparation specifications may range from solvent cleaning for application of a topcoat to white metal blast cleaning for complete removal of all previously applied coating material. The requirements for primers applied to steel will include both cleanliness and surface roughness or anchor profile. The user/specifier must determine that the minimum surface cleanliness required is achievable before using/specifying a particular product. The surface cleanliness level indicated on the data sheet is the minimum. Higher levels of cleanliness are acceptable, lower levels are not. Storage conditions are important because coatings are complex mixtures of chemicals. The quality of a coating can deteriorate with time due to heat, cold, or moisture. Like other perishables, coatings have a shelf life, which is their storage limit in an unopened can. Depending on the coating, shelf life can be as 5 long as a few years or as short as several weeks. The product data sheet should indicate storage conditions, such as a specific temperature range Proper mixing of a coating is required to make it homogeneous. The way a coating is mixed can affect its performance. Some coatings can be mixed by stirring, shaking, or mechanical means. Others can be mixed by mechanical means only. Even then, mixing must be done carefully to blend the materials without overagitation, which can introduce air or moisture into the coating and result in application or curing problems. Coatings that are supplied in multiple containers must be mixed in the proper order and in the proper proportions after the individual components are mixed. Multicomponent coatings are packaged so correct chemical proportions will result if all contents of each container are combined. If not, the result may be a substandard film. Thinning instructions generally are included with mixing instructions. Both the type and amount of thinner to be used are important, because the thinner must maintain a proper balance of properties in the coating. The recommended thinner should not be replaced with another thinner without the manufacturer’s approval. Using the wrong type or amount of thinner can cause a variety of application or curing problems. Successful use of a coating requires adhering to any relevant induction time and pot life requirements listed on the product data sheet. Induction time is how long a mixed coating must pre-react in the can before it can be applied. Pot life is the maximum amount of time a coating can be applied after it is mixed. Induction time and pot life are temperature-dependent—shorter in warm temperatures, longer in cool temperatures. Users and specifiers should check these requirements to determine if they are reasonable for expected conditions or if careful planning of the timing of mixing and application is needed. Basic Coatings Technology by Lloyd M. Smith, PhD. Application conditions generally listed on data sheets include materials temperature, ambient temperature, surface temperature, relative humidity, and dew point. Temperature and relative humidity requirements are product specific. Good painting practice requires that surface temperature be at least 5˚F (3˚C) above the dew point. Site ambient conditions can limit the choice of coatings. Site conditions that fall outside the specified range may be altered, such as by dehumidifying a contained area or warming it with indirect heat in cold weather. If a coating requires a specific type of application equipment, that information should be on the product data sheet as well. Some coatings can only be applied by spray methods, so they would not be suitable where only manual methods can be used. Likewise, factors such as site restrictions on spray painting, skill of the work force, and availability of specialized equipment may mean that certain types of coatings would be unsuitable in particular circumstances. Product data sheets may include a drying schedule, which should be checked before selecting a coating. Different types of drying times may be presented, including dry-to-touch (the time for a coating to be tack-free), dry-to-handle (the time when a coated piece can be moved carefully), dry-torecoat (the time when another coating layer can be applied), and final cure. Drying times and cure generally are presented as a minimum amount of time at a specific temperature. This information should be reviewed in light of any site and project limitations. There also may be a maximum dry-to-recoat time that may require special attention or planning. CURING MECHANISMS Cure refers to the length of time before a coating can be put into service. There are four curing mechanisms for coatings: air oxidation, solvent evaporation, chemical reaction, and hydrolysis. This section explains the characteristics and 6 differences of each method. Air Oxidation Coatings that cure by air oxidation contain drying oils. These include oilbased coatings and some hybrid coatings. Crosslinking of the resin polymers occurs by reaction with oxygen in the air. The solvent evaporates when the coating is applied, but it takes longer for oxygen to permeate the film. Therefore, recoat times are relatively long—days for some formulations. The slow-drying characteristic of oil-based primers can be an advantage since the coating can flow into the surface. Air-oxidizing coatings generally are applied at low film builds, such as 2 to 3 mils (50 to 75 micrometers) dry film thickness per coat. They can surfacecure if applied too thickly in one coat. When this occurs, the top of the coating cures, limiting oxygen permeation to the rest of the coating. As a result, the coating is hard on top and soft on the bottom. A thickly applied layer may cure through with additional time, but an excessively thick coating layer will not. Oxidation continues after a coating has cured. This causes the coating to become brittle with time. Air-oxidizing coatings are easily recoatable, but sooner or later the system will become too brittle, too thick, or lose adhesion. Then, it must be totally removed and replaced. Coating films that contain oils may undergo a process called saponification, which is the hydrolysis of an ester by an alkali with the formation of an alcohol and a salt of the acid portion. The oil-containing portion of the coating is an ester. The alkali may come from the substrate. Portland cement concrete is an alkaline surface, as are reaction products of some active metals, notably zinc. The salt formed is a metallic salt of a fatty acid, better known as soap. In severe cases of delamination resulting from saponification, it actually may be possible to wet excerpted from GENERIC COATING TYPES: An Introduction to Industrial Maintenance Coating Materials a surface and form a lather. Because of saponification, oil-containing coatings should not be applied to concrete, galvanized surfaces, or zinc-rich primers. Air-oxidizing coatings have moderate moisture permeability. Therefore, anticorrosive pigments usually are added to the primer formulation for products applied to steel. Air-oxidizing coatings generally are formulated in one component and have unlimited pot life. They are easy to apply by brush, roller, or spray. Solvent Evaporation Coatings that cure by this method only require that the solvent evaporate from the film. These coatings are made by dissolving the resin in an appropriate solvent. No crosslinking or chemical reaction occurs during film formation, which involves attraction and entanglement of the resin molecules to the point where their movement is restricted. These types of coatings contain thermoplastic resins. Vinyls, chlorinated rubbers, and asphaltics are examples of coatings in this class. Solvent-evaporating coatings have relatively low solids. VOC-compliant formulations are difficult to make. Solvent-evaporating coatings have low moisture permeability and protect by a barrier mechanism. They have good water and sunlight resistance but poor solvent resistance. They are easy to repair because the topcoat solvent softens the existing film, giving a good bond. Solvent-evaporating coatings are applied by spray methods only. They become viscous quickly, so they cannot be worked with a brush or roller. The curing mechanism does allow low temperature application, although drying is retarded. Latex and other water-borne coatings also cure by solvent evaporation. In these cases, the solvent is water. The resin is present as emulsified particles, which coalesce to form a 7 film as the water evaporates. The coalescing reaction is temperature-dependent, and there is a proper temperature range in which they can be applied. Application methods include brush, roller, or spray. Latex films have relatively high permeability. Therefore, they are useful on substrates such as concrete and wood that are porous, or “breathe.” Anticorrosive pigments are used in latex coating systems designed for steel. Although water is the main solvent in waterborne coatings, organic cosolvents usually are included in the formulation so the emulsified resin particles are compatible with the water. The amount of organic solvent used is relatively small, so these coatings generally have reduced VOC levels. A number of resins that cure by other mechanisms can be made compatible with water. However, a distinction must be made between emulsion coatings and water-reducible coatings. Waterreducible coatings contain a solvent blend that can be thinned with water, but the curing mechanism does not change. Therefore, water-reducible alkyds still cure by air oxidation, and waterreducible epoxies still cure by chemical reaction. Chemical Reaction Coatings that cure by chemical reaction are packaged in two or more containers. One can contains the resin. The other can contains the crosslinking agent, referred to as the hardener or curing agent. The resin, after mixing, becomes thermosetting, which means the components form a film by a chemical crosslinking reaction. Epoxies, polyurethanes, and polyesters are common coatings that cure by this mechanism. Chemically cured coatings have a pot life and may have an induction time. The chemical reactions are temperature-dependent, and the materials can only be applied and cured above Basic Coatings Technology by Lloyd M. Smith, PhD. the minimum application temperature. The crosslinking reaction that occurs usually results in a smooth, hard film, which is likely to have a maximum recoat time. Most chemically cured coatings can be applied by brush, roller, or spray equipment. These films have low permeability and protect by barrier formation. The properties of the barrier film provide these coatings with good chemical and Solvent resistance. There are a number of resins and hardeners that can be combined, which gives the formulator great latitude in designing VOCcompliant systems. Hydrolysis Hydrolysis means reaction with water. Coatings that cure by hydrolysis require a sufficient amount of moisture in the air to react with the chemical groupS to form the film. Self-curing, solvent-borne inorganic zinc-rich primers and moisture-curing polyurethanes are the two main coating types that cure by this mechanism Humidity is as important—if not more important— than temperature in the cure of these coatings. Warm air can hold more moisture than cool air. For example, air at 70˚F (21˚C) will have a higher absolute humidity (grains of water per unit volume of air) than air at 50˚F (10˚C) when the relative humidity is the same at both temperatures. Moisture-cure coatings have a pot life. Since they are exposed to air during mixing and application, moisture in the air will react with the resin, causing it to polymerize. PROTECTION MECHANISMS The three mechanisms by which coatings can protect a substrate are barrier protection, inhibitive pigment protection, and sacrificial protection. All coatings protect partially or solely by barrier 8 protection. They separate the substrate from the environment, especially sunlight and moisture. One indicator of a coating’s ability to act as a barrier is its moisture vapor transmission rate, which is the rate at which moisture vapor permeates a coating and reaches the substrate. Good barrier coatings have very low moisture vapor transmission rates. Coatings with high moisture vapor transmission rates incorporate anticorrosive pigments for application to steel substrates. These pigments are slightly soluble, and a small amount dissolves as moisture permeates the film. They are carried to the substrate where they passivate the steel. Some pigments react with certain species that accelerate corrosion, forming an insoluble compound to protect the substrate from those species. Corrosion theory states that when two dissimilar metals are in contact and corrosion conditions exist, the more active metal will corrode to protect the less active metal. Zinc metal is more active than steel. Therefore, sacrificial coatings contain zinc in electrical continuity with the steel. Zincrich primers, for instance, contain elemental zinc dust as the major pigment. Zinc metal also can be spray-applied by a process called metallizing, moltenapplied by hot-dip galvanizing, or mechanically applied by a method known as sherardizing. Understanding protection mechanisms is an asset in the proper design of coating systems. DESIGN OF A COATING SYSTEM A coating system consists of surface preparation and one or more coats of material applied in a specific order. Surface preparation includes both level of cleanliness and roughness (anchor profile). The first coat of material applied to a surface is the primer. The function of the primer is to provide adhesion to the surface for subsequent coats. Some primers for steel contain anti- excerpted from GENERIC COATING TYPES: An Introduction to Industrial Maintenance Coating Materials corrosive pigments, but this is a secondary function. An important concept is that the entire coating system provides corrosion protection. The second coat in a multicoat system is the intermediate coat. An intermediate coat provides thickness for increased barrier protection as well as specific chemical resistances. An intermediate coat also can serve as a tie coat between an incompatible primer and topcoat. The last coat of material applied is the topcoat. Its main function is resistance to weather, sunlight, and chemicals. It also adds to the barrier protection of the coating system. Aesthetics, such as color and gloss, are important properties of topcoats in many situations. However, a coating system does not always consist of three coats of material. It may include one coat, two coats, or three or more, depending on such factors as severity of the exposure environment, expected life of service, and appearance requirements. There is a relationship between coating life and total thickness of a coating system. Designing a coating system should take into account the curing and protection mechanism of the coating layers. It make no sense to place an inhibitive pigment system on top of a barrier coating, for example. It is rare that only one coating system is available for a specific use. In most cases, there are alternatives that can be used. A wealth of information is available to assist the user/specifier in selecting a coating system. This information can be found in technical literature about industrial maintenance coatings as well as in government-sponsored research or evaluations. Also, many coatings manufacturers recommend coating systems in their data sheets, specification guides, or industry-oriented literature. Designing a coating system starts with identifying desired properties for a particular 9 service and identifying limitations that may exist. Desired properties may include one or more of the following: corrosion resistance, color and gloss retention, abrasion resistance, water resistance, chemical (including fuel) resistance, heat resistance, cleanability, and ease of maintenance. Corrosion Resistance Corrosion resistance is a desired property of many coating systems. The life of a corrosionresistant coating system depends on surface preparation, the coating material selected for each layer, and the thickness of each layer. In selecting coating material, the major decision is the type of primer, which must be compatible with the substrate. Color and Gloss Retention Color and gloss are functions of the topcoat. Important considerations in specifying color and gloss are the initial gloss desired and the exposure environment. Deterioration of color and gloss will result from exposure to sunlight, with direct exposure being the most aggressive. Coatings protected from sunlight, on the other hand, will have very little deterioration. Abrasion Resistance Abrasion resistance in a coating may be needed for a variety of reasons. For instance, a floor coating may need resistance to vehicle traffic, such as forklifts or carts. Alternatively, the source of abrasion may be air-borne particulates. For heavy exposures, such as vehicular traffic, abrasion resistance is a function of the complete coating system. For light exposures, such as airborne particles, abrasion resistance is primarily a function of the topcoat. Water Resistance Water resistance can involve many different situations, including immersed structures such as Basic Coatings Technology by Lloyd M. Smith, PhD. the inside of water storage tanks, locks and dams, or ship bottoms. It also involves structures that are constantly wet because of location, such as being downwind from a cooling tower. Wetting can be intermittent. This can include structures that are exposed to water for periods of time or long times of wetness due to condensation. Water resistance is a function of the total coating system. Chemical Resistance Chemical resistance is a primary consideration of linings that may be exposed to liquids, such as for primary or secondary containment structures, although chemical contaminants can also be in the air. For instance, structures downwind from a chemical plant, refinery, pulp and paper mill, or power plant may need to be protected with coating materials that are resistant to a specific type or class of contaminants, such as acid. A chemically resistant coating also may be needed for splash and spillage areas that receive intermittent exposure to a chemical. (There is no uniform definition of splash and spillage. To some manufacturers, it refers to constant intermittent exposure; to others, it refers to situations when an upset occurs and is cleaned up in a timely manner.) Chemical resistance is a function of the total coating system for primary linings. In other exposures, it may be a function mainly of the topcoat and intermediate coat. Heat Resistance Heat resistance may be needed for constant or intermittent heating conditions. Most coatings are based on organic chemicals, which limits constant heat resistance to the range of about 150 to 250˚F (66 to 121˚C), depending on generic type and formulation. Special coatings are needed for higher temperature exposures. Heat resistance is a function of each layer of a coating system. Cleanability Cleanability of a coating system refers to the 10 removal of dirt, grime, and other contaminants. Cleanability of the coating system on fuel storage tanks is important, for example, because a light color must be maintained for temperature control to reduce evaporation of the stored product. Cleanability may be necessary for sanitary reasons in some circumstances, and the ability to decontaminate a coated surface is a primary concern in certain nuclear and military applications. Cleanability should not be confused with scrub resistance, which is resistance to coating thickness erosion from the cleaning process. Cleanability and scrub resistance are mainly functions of the topcoat. Ease of Maintenance Maintenance is the process of making repairs to a coating system, either with or without application of a full topcoat. Some coating systems are intended to be replaced once degraded, while other coating systems are intended to be maintained one or more times during their life cycle. Some situations may require almost constant maintenance to a coating system. Limited opportunities to perform the work or adverse working conditions may make maintenance a high priority in the selection of a coating system. Other Factors A variety of other factors also may influence the choice of coating materials and limit the number of coating systems to consider. Some common limitations include surface preparation, access to the surface, drying times, weather, equipment, applicator skills, safety and environmental regulations, and budget. • Surface preparation — It may not be possible to achieve the desired level of substrate cleanliness for certain types of coatings. For example, it may only be possible to perform hand-tool cleaning. Therefore, only primers intended for surfaces cleaned with hand tools can be used. excerpted from GENERIC COATING TYPES: An Introduction to Industrial Maintenance Coating Materials • Access to the surface — Access for coating maintenance may be limited due to physical constraints of construction or layout. Limited access usually relates to the ability to perform surface preparation, although limited access also can relate to coating application methods. • Drying times — Drying time is important when coating projects are under time constraints. Time pressures may result in a coating being applied before the previous coat has dried sufficiently or a unit being put into service before the coating system has cured. Such problems can be avoided by selecting the appropriate coating materials. Drying time also may be important for new construction where coatings are applied in the shop, since the pieces cannot be moved until dryto-handle times are met. • Weather — Weather conditions, including temperature and humidity levels, are other factors to consider. Minimum drying and curing temperature requirements are a more common concern than high temperatures. If the ambient temperature is near the minimum required level, the area might possibly be heated to bring the temperature within an acceptable range. High humidity can be a limitation with certain coating types. Polyurethanes, for example, will not produce the expected gloss when applied at relative humidity levels above the maximum listed on the data sheet. Low humidity is a limitation with moisture-cured coatings, although it may be possible to introduce moisture or humidity by other means, such as spraying with water. • Equipment — Available equipment can affect surface preparation as well as coating application. For example, spray-applied coatings should not be used when brushes and rollers are the only types of equipment available. Some specialty coatings may require plural-component spray equipment in which the components are mixed at the spray gun. Such coatings should not be applied with any other type of spray system. Coatings that require agitation should not be used 11 if an agitation pot is not available. If the proper equipment is not available, consideration should be given to acquiring it. • Applicator skills — Certain high-performance coatings require skilled applicators to apply the coatings correctly. Coating work to be performed by unskilled labor should involve the “friendliest” materials available. • Safety and environmental regulations — Safety and environmental regulations may limit surface preparation as well as coating selection. For instance, lead paint removal regulations may favor coating maintenance. Therefore, overcoating materials would be preferred to primers that must be applied to bare metal. As another example, flammable solvents may be especially hazardous in a particular work area, so water-borne formulations may be indicated instead. • Budget — Budget is a limitation on every project. However, an expensive coating project might still be cheaper than the alternatives, such as losing use of the structure, constructing the unit or structure of a more resistant material, or using a different type of protective system. Practicalities dictate a limited amount of funds for coating needs. Various options may be available, such as using funds for certain structures while delaying work on other structures, or using an overcoating system rather than total removal and replacement. In other cases, budgetary considerations may dictate a two-coat system rather than a three-coat system. Budgetary influence on the choice of coating materials is usually related to surface preparation and the cost of the entire project rather than just the coating materials themselves. A more informed choice of coating systems can be made once the properties and limitations of various materials have been determined. Even then, there usually are alternatives. It is good practice to use the minimum number of systems possible. This makes it easier to keep track of Basic Coatings Technology by Lloyd M. Smith, PhD. the coatings applied to various surfaces and also makes it simpler for field personnel. In some cases, only a small number of coating systems may be needed because of limited exposure environments and functions. In other cases, twenty or more coating systems may be required due to varying exposure conditions and multiple coating functions. COATINGS SPECIFICATION Once the decision on a particular coating system has been made, the next task is to specify and purchase the materials. The most common specifying methods are formula specification, performance specification, and user experience. Formula Specification Formula specifications are coatings recipes developed by government agencies, especially the federal government, and technical organizations such as SSPC: The Society for Protective Coatings. The advantage of formula specifications is supplying the user with a known, proven coating material that should be identical from each supplier. The disadvantage is that the formula specification may be outdated and not cover all generic types of coatings. The federal government, the largest preparer of formula specifications, has cut back considerably on the maintenance and updating of specifications. Many formulas have been rescinded because they contained hazardous materials. Others have been abandoned due to similarity with other formulations. Only some have been updated to be VOC-compliant. There are no federal government formula specifications for certain types of coatings, such as epoxy mastics. Performance Specification Performance specifications allow more latitude in the formulation of the coating than formula specifications, with performance requirements greatly increased. Performance usually is 12 specified as the minimum requirement for a battery of accelerated laboratory tests. Organizations such as SSPC, which have turned to performance-type specifications, may require a field history as part of the acceptance criteria. Performance specification enables a coating type to be specified, even for VOC compliance, without severely limiting the ingenuity of the formulator in a time of changing technology and regulations. However, with performance specification, there is no correlation between hours of exposure in an accelerated laboratory test and years of exposure in the field, or exposure in one field environment and successful performance in another. A properly written performance specification will eliminate unacceptable coatings, but will not differentiate among acceptable coatings. Coatings that pass the requirements of a performance specification are often included on a qualified products list (QPL). A test program is developed and coatings from different manufacturers are evaluated. Those that meet the criteria or minimum levels of the test program are placed on the QPL. The federal government has QPLs associated with some specifications. Some government agencies, such as the National Aeronautics and Space Administration (NASA), have evaluated specific coating systems from different suppliers and reported the results in a form that can be used as a QPL. Other large users also have developed QPLs for coating systems they use. Michigan Department of Transportation, for example, has a QPL for a system consisting of an epoxy polyamide zincrich primer/epoxy polyamide intermediate coat/ aliphatic polyurethane topcoat based on a battery of accelerated laboratory tests. Private companies also have developed QPLs based on exposure tests at their facilities. As an alternative to QPLs, some users have established lists of qualified suppliers or vendors based on criteria such as technical service, product availability, and limited performance testing. excerpted from GENERIC COATING TYPES: An Introduction to Industrial Maintenance Coating Materials User Experience Another method for specifying paints is selecting products from manufacturers with which the user/specifier has had good experience. This method is not as formalized as a QPL test, but it does provide a level of confidence in the quality of the product. It is important that coating performance and not applicator performance be evaluated. Even a test patch program can be useful. Test patches of coatings from different suppliers can be applied to a structure and evaluated regularly. Unacceptable coatings will be evident relatively quickly, so useful information can be generated within a few years. Another evaluation method is to contact references obtained from the coating manufacturer or supplier for similar projects where the recommended coating system was used. COST There are a number of methods for evaluating the cost of coating materials. All too often, the choice of coating material is based on the cost per gallon. However, this cost can be misleading. Coatings are applied to achieve a certain dry film thickness. Therefore, the percent solids by volume affects the true cost of the material. For example, a coating that costs $20 per gallon ($5.28 per liter) at 70 percent solids and is applied at 3 mils (75 micrometers) dry film thickness has a theoretical materials cost (100 percent transfer efficiency on a flat surface) of $0.053 per square foot ($0.57 per square meter). However, a coating that costs $18 per gallon ($4.76 per liter) and is applied at the same thickness but has 50 percent solids has a theoretical materials cost of $0.067 per square foot ($0.72 per square meter). The lower solids formulation is about 25 percent more expensive than the higher solids formulation. Actually, the cost of the coating material is usually a small percentage of the cost of the 13 overall project. Surface preparation and coating application are much larger cost factors. Breevort and Roebuck4 have gathered information on the costs of coating projects from around the United States. For example, using a zinc-rich primer/ epoxy polyamide intermediate coat/polyurethane topcoat over a near-white blast-cleaned surface, the average costs per square foot (square meter) in 1992 dollars were Surface preparation (field) Coating application Coating materials Total $1.00 $0.85 $0.34 $2.19 ($10.76) ($9.15) ($3.66) ($23.57) 46% 38% 16% 100% The coating materials represent about 16 percent of the cost. If the project had included the removal of a lead-based coating system, environmental and worker protection items would have increased the cost of the project by a factor of 2 or 3. The cost of the coating materials would then be about 5 to 7 percent of the cost of the project. The preferred method for selecting a coating material is not based on cost per gallon (or cost per liter). Rather, it is based on the life cycle cost of the installed coating system per year of service (cost/ft2/yr or cost/m2/yr). While it is possible to obtain a fairly accurate estimate of the cost per square foot (cost/ft2) or cost per square meter (cost/m2), determining the years of service can be more difficult. In many cases, records about years of service are not kept, the coatings technology currently in place is out of date, or newer coatings have not been on the market long enough to determine their life. To compensate for lack of data, estimates have been made of the expected life of coating systems. These estimates include information gathered by Breevort and Roebuck4 for industrial exposures and estimates made by Hare5 for bridges. CONCLUSION Basic knowledge of coatings technology will assist the user and specifier in implementing a Basic Coatings Technology by Lloyd M. Smith, PhD. cost-effective coatings program. This appendix has presented important concepts used in coatings. Generalizations have been presented on classes of coatings materials. These were meant for illustrative purposes. The rest of this book contains more detailed information about the major generic types of coatings commonly used in industrial maintenance painting. NOTES 1. Paint/Coatings Dictionary (Philadelphia, PA: Federation of Societies for Coatings Technology, 1978). 2. Joseph A. Bruno Jr., ed., Industrial Maintenance Coatings Glossary (Pittsburgh, PA: Technology Publishing Company, 1994). 3. Dwight Weldon, “Understanding Test Data from Coatings Manufacturers’ Product Data Sheets,” Journal of Protective Coatings and Linings (Pittsburgh, PA: Technology Publishing Company, May 1993), 52-59. 4. Gordon H. Brevoort and A. H. Roebuck, “A Review and Update of ‘The Paint and Coatings Cost and Selection Guide,’” Materials Performance (Houston, TX: NACE International, April 1993), 31-45. 5. Clive H. Hare, “Protective Coatings for Bridge Steel,” NCHRP Synthesis of Highway Practice 136 (Washington, DC: Transportation Research Board, December 1987). ACKNOWLEDGMENTS The editors gratefully acknowledge Stephen G. Pinney and Kenneth B. Tator for their time and effort in reviewing this chapter. 14 ...
View Full Document

This note was uploaded on 03/17/2010 for the course ME ME78212 taught by Professor Prof.sulis during the Spring '10 term at Institut Teknologi Bandung.

Ask a homework question - tutors are online