GUIDE TO COATINGS TECHNOLOGY
It is not always necessary to paint or coat structural steel; e.g., when the structure is hidden and protected from moisture, it is protected with spray-applied fire protection or aesthetics do not require it. These specific conditions will be clearly explained in this section.
There are many times, however, when the steel structure must be protected against corrosion; e.g., when it is architecturally exposed. Over the past few years, great strides have been made in the development of high-performance coatings leading to the increased use of exposed steel as a means of architectural expression. Steel's high strength-to-weight ratio allows thin and elegant forms to support large loads and span long distances. The ability to have long-term protection on exposed structural steel has allowed many of today's innovative architects to express a wide variety of ideas through the structure itself. Properly specified and applied coating systems can be expected to give 20 to 25 years of initial service life that can be extended almost indefinitely and with subsequent maintenance painting.
Coatings technology continues to evolve with paint systems being developed to meet more and more stringent requirements. This is a blessing in the sense that owners and architects can expect continually improving performance, but it also means that developing a proper specification for a given project requires keeping up with the most recent product developments.
Paint specifications for building structures should be performance-based to allow competition within a performance standard. Paint specifications should also be project specific and take into account the following three factors:
Building end-use—Is it a factory where the structure will be exposed to corrosive processes or high humidity? Is it a public facility subject to abrasion and vandalism (graffiti)? Is it a swimming pool with high humidity and heat? Or, is it an office building that is well-protected and subject to benign usage?
Environment—Is the building located on the coast in a saline atmosphere, at an inland location surrounded by industrial plants, or is it in a desert-dry climate but subjected to relentless attack by the ultraviolet rays of the sun?
Is the structure to be exposed on the exterior, interior or both?
This portion of the guide is intended to inform architects of issues that should be considered in the development of a proper paint specification for building structures. In addition, there is considerable background information intended to help specifiers understand coating systems in general so that they can make informed and intelligent choices. Several coating references are provided at the end of this section.
BASICS OF PROTECTIVE COATINGS
The Corrosion Process
A clear understanding of the corrosion process is essential to understand the steps to inhibit corrosion with protective coatings.
Oxygen combines with iron, the major element in steel, to form rust. This electrochemical process returns the iron metal to the state that it existed in nature—iron oxide. The most common form of iron oxide or iron ore found in nature is hematite (Fe203), which is equivalent to what we call rust. Iron in iron ore is separated from the oxide to yield usable forms of iron, steel and various other alloys through rigorous electrochemical reduction processes. Because the iron has a strong affinity for oxygen, it is necessary to deal with the ever-present tendency to form the more electrochemically stable iron oxides.
The process of combining iron and oxygen, called oxidation, is accompanied by the production of a measurable quantity of electrical current, which is why this is called an electrochemical reaction. For the reaction to proceed, an anode, a cathode and an electrolyte must be present. This is termed a corrosion cell. In a corrosion cell, the anode is the negative electrode where corrosion occurs (oxidation), the cathode is the positive electrode end, and the electrolyte is the medium through which an electrical current flows.
Coatings in Corrosion Control
A coating may be defined as a material which is applied to a surface as a fluid and which forms, by chemical and/or physical processes, a solid continuous film bonded to the surface.
Eliminating any of the reactants in the process can interrupt corrosion. If a barrier is put on to the iron that prevents oxygen and/or water from coming in contact with steel, the corrosion process can be prevented. Steel is not the only surface protected by such barriers. Other alloys and metals such as stainless steel, brass, aluminum and other materials such as concrete, wood, paper, and plastic are also protected from the environment with coatings. Protective coatings that serve as barriers are the principal means of protecting structures.
COMPOSITION OF COATINGS
Most coatings are made up of four principal parts: pigments; non-volatile vehicles (resins or binders); volatile vehicles (organic solvents, water or the combination of both); and additives (specialty chemicals which make the coating function). All of the components of a coating interact to accomplish the purpose for which the coating was designed.
Pigments
Pigments are included in coatings to perform any of the following functions:
- Add color
- Adjust the flow properties of wet coatings
- Resist light, heat, moisture, chemicals
- Inhibit corrosion
- Reflect light for opacity or hiding
- Contribute mechanical strength
Pigments whose prime function is to contribute opacity to coatings are called hiding or prime pigments. The principle white-hiding pigment is titanium dioxide. There are hundreds of colored-hiding pigments which, when used alone or combination with other pigments, give coatings their variety of colors. Hiding pigments can be very expensive. In order to make the paint less costly, non-hiding or extender pigments are used. Certain colors, such as light-stable reds, are more expensive. Determine costs from your coating supplier prior to writing the project specification.
Pigments are used to adjust the viscosity and flow properties of the paint in order to obtain paint that won't sag at high film builds. Using pigments with low oil absorption can decrease the amount of solvents in the paints. Pigments used to reduce or prevent corrosion of a coated surface are called inhibitive pigments.
Pigments help protect the resin in the film from degradation of solar radiation. Hiding pigments do the best job of protecting the resin from the harmful portion of solar radiation by blocking its penetration into a film. Pigments in the film also inhibit penetration of corrosive elements, thus protecting the substrate. Pigments also can add mechanical reinforcement to a film, adding strength, flexibility, and abrasion resistance.
Non-Volatile Vehicles (Binders)
The binder or resin portion (polyurethane, epoxy, etc.) of the coating is the "glue" that holds the coating together
and onto the substrate. The physical properties of the coating are mainly derived from the physical properties of the solid resin, but pigments and additives can affect the final properties. Coatings are generally named after the type of resin used as the coating binder.
Resin binders change from the liquid to the solid state by several different dying curing mechanisms:
Lacquer, dispersion and latex paints dry through the evaporation of solvent and/or water.
Vegetable oil and alkyd paints harden through oxidative cure.
Two-component chemically reactive paints harden through chemical cure, i.e., two components are mixed prior to application and polymerize on the substrate, e.g., epoxy or polyurethane.
One-component chemically reactive paints harden through the reaction of a resin that has an active chemical group, with atmospheric moisture releasing a new chemical group that causes the resin to crosslink.
The simplest drying mechanism is evaporation of the volatile vehicle. Solventborne lacquers generally have very high solvent content because very hard resins needed for good film protection require a lot of solvent to reduce the paint viscosity to application consistency. Vinyl and chlorinated rubber coatings are examples of resins relying on solvent evaporation.
Another type of paint that dries through simple evaporation of the volatile vehicle is waterborne paint. Here a major portion of the volatile vehicle is water which acts to lower the viscosity of the paint. Acrylic and vinyl latexes, water-based epoxies and polyurethane dispersions are examples of this technology.
Coatings based on natural oils or alkyd binders modified with drying oils develop their film properties principally through oxidative curing. Atmospheric oxygen creates active crosslinking sites on vegetable oil or the drying oil portion of the synthetic resin. These sites connect to form a three dimensional, chemically bonded network. Linseed, alkyd and epoxy ester binders are examples of systems that cure by a combination of solvent evaporation and oxidation.
Two-component chemically reactive paint is manufactured and sold in two separate containers. The two multifunctional reactive resinous materials are mixed together just prior to use. The two resins immediately begin to react together to form a polymeric matrix. During polymerization, the paint viscosity will increase. This means that the paint has a specific use life before the paint will gel. Polyurethane and epoxy are examples of these coatings.
One-component chemically reactive paint utilizes polyisocyanate chemistry. The isocyanate group reacts with atmospheric moisture to yield an amine group. The amine reacts very rapidly with additional isocyanate to form a urea crosslink. This paint offers the ease of use of other one-component technologies with the performance of a two-component paint. Moisture-cured polyurethane technology is a rapidly growing example of this technology.
Volatile Vehicles (Solvents)
A solvent is used to dissolve the resins and additives in order to reduce the viscosity of the mixture to provide application consistency and allow the paint to flow out properly. In every case, it is designed to evaporate from the film during or after application.
Solvents are also used in waterborne dispersions and latexes. At some point in either the manufacture of the resin or the paint, solvents are added to soften the resin. During the drying of the paint film, the water evaporates. The dispersion of latex particles come into contact and flow together to form a continuous film. Finally the solvent evaporates from the film. This process, called coalescence, would not take place without the solvent. Resins that are hard enough to produce through tough films are too hard to coalesce without the solvent. Waterborne coatings are gaining interest by specifiers because they are perceived as being environmentally friendly. Although many waterborne coatings do have low levels of solvents, some waterborne paints contain solvent in amounts equivalent to those in high-solid, solventborne coatings.
Environmental concerns are forcing raw material suppliers and paint producers to lower the solvent content of the products they supply in order to reduce the amount of volatile organic compounds (VOCs) released into the atmosphere.
Coatings suppliers select the type of solvent suitable for each type of coating formulation. The choice of solvents is made based on the optimum paint viscosity and evaporation rate that result in proper paint flow and thus, the intended appearance and adhesion. Coating applicators may need to add solvents during application to control viscosity over the various temperature ranges encountered in the field.
The wrong choice of solvents can jeopardize an application. If the chosen solvent evaporates too fast, bubbles caused by the vapor pressure of the solvent may appear in the surface. If the coating is spray applied, the solvent may "flash out" of the spray mist before it reaches the surface, and the spray may become too dry for the paint particles to flow together. This effect is called dry spray. A solvent that is too slow to evaporate may remain in the film too long, causing sags and runs and resulting in a film that is soft and has other altered performance properties.
The applicator must also take care not to add thinning solvent beyond that recommended by the manufacturer, because the paint viscosity may be so slow that the wet films will sag and run. Over-thinned paint that is applied at too low a film build may result in films that are too thin and have no hiding power.
Additives
Additives make up only a small proportion of any paint. Yet without these chemicals the paint could not deliver all of its potential performance.
Paint additives are used to aid pigment grinding, stabilize resin and pigment dispersions, break foams, aid flow, prevent film surface defects, catalyze chemical reactions, prevent oxidation, enhance adhesion, provide slip and abrasion resistance to the film surface, prevent corrosion, and to improve weathering resistance and enhance color retention.
These additives can be inexpensive or can be the most expensive component on a per pound basis of any ingredient. In these days of cost competition, it is not unusual for a paint manufacturer to cut costs by leaving out one of these vital ingredients. Sometimes the effects may not be known until years after the paint application. For example, in a high performance polyurethane topcoat, it is usual practice to add antioxidants and UV absorbers to enhance the weathering resistance. If theses additives are left out of the formulation to lower cost, instead of the ten years of gloss and color retention, only one or two years might be expected. It is imperative that expected paint performance be listed in the job specification.
(Design with Structural Steel - A Guide for Architects)
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