Paints and other architectural coatings (stains, varnishes, lacquers, sealers) protect and beautify the surfaces of buildings. A good coating job begins with thorough surface preparation to make the surface (called the substrate) ready to receive the coating. The coating materials must be carefully chosen and skillfully applied using the proper tools and techniques. To finish the job, environmental conditions must be right for the drying or curing of the coating.
材料成分 Material Ingredients
Most architectural coating materials are formulated with four basic types of ingredients: vehicles, solvents, pigments, and additives.
The vehicle or binder provides adhesion to the substrate and forms a film over it; it is often referred to as a filmformer.
Solvents are volatile liquids used to improve the working properties of the paint or coating. The most common solvents used in coating materials are water and hydrocarbons, but turpentine, alcohols, ketones, esters, and ethers are also used.
Pigments are finely divided solids that add color, opacity, and gloss control to the coating material. They also impart hardness, abrasion resistance, and weatherability to the coating.
Additives modify various properties of the coating material. Driers, for example, are additives that hasten the curing of the coating. Other additives may relate to ease of application, resistance to fading, and other functions.
溶剂型涂料和水性涂料 Solvent-Based Coatings and Water-Based Coatings
Coatings fall into two major groups, solvent-based andwater-based. The most common solvents in solvent-basedcoatings are alkyd or polyurethane resins, or natural oils.These coatings cure by evaporation of the solvent, oxidationof the vehicle, or moisture curing from reaction ofthe vehicle with humidity in the air. Cleanup after paintingis usually done with mineral spirits. Water-based coatingsuse water as the solvent. Most vehicles in water-basedcoatings are vinyl or acrylic resins. Cleanup is done withsoap and water. In everyday use, solvent-based paints aremost often referred to as “oil paints” or “alkyd paints” andthe water-based paints as “latex paints.”
Historically, water-based and solvent-based coatingseach had uses for which they were preferred. Alkyd paintsgave a smoother, harder surface than water-based paintsand were favored for interior painting, especially where adurable, high-gloss finish was desired. Alkyd primers alsooffered superior covering ability and reliable adhesion toproblematic substrates. In comparison, latex paints generatedfewer odors during application than alkyd paints,an important consideration for renovation work wherebuilding occupants could be exposed to paint fumes.Where used for exterior finishing, latex paints produceda more flexible coating able to stretch without breakingwhen subjected to expansion and contraction of the substratewith changes in temperature and humidity. Latexcoatings are also more breathable, lessening the chanceof moisture being trapped behind painted siding or trim.
However, many solvent-based coatings also emit significantlymore volatile organic compounds (VOCs) thanwater-based coatings. As pollution regulations and sustainability-influenced indoor air quality standards haveplaced increasingly strict limits on VOC emissions, theuse of solvent-based coatings has decreased markedly.In response, manufacturers have worked to improve theformulation of water-based coatings to the extent that,today, water-based coatings with low VOC or even no VOCemissions are available that can equal the performance ofsolvent-based coatings in almost all applications.
VOC emissions from solvent-based coatings can alsobe lowered by increasing the solids content and loweringthe solvent content of the coating or using solventsthat are exempt from low-VOC regulations. Also new tomarket are hybrid alkyd-water emulsion coatings in whichalkyd resins are suspended in water. These products offermany of the advantages of traditional alkyds, such as easeof application, excellent coverage, and durability, but inlow-VOC formulations that can be cleaned up with water.
油漆和涂料的种类 Types of Paints and Coatings
The various types of architectural coatings can be defined by the relative proportions of vehicle, solvent, pigment,and additives in each.
Paints contain relatively high amounts of pigment.The highest pigment content is in flat paints, those that dry to a completely matte surface texture. Flat paints contain a relatively low proportion of film-forming vehicle.
Paints that produce glossier surfaces are referred to as enamels. A high-gloss enamel contains a very high proportion of vehicle and a relatively low proportion of pigment.The vehicle cures to form a hard, shiny film in which the pigment is fully submerged. A semigloss enamel has a somewhat lower proportion of vehicle, though still more than a flat paint.
Stains range from transparent stains to semitransparent and solid stains. Transparent stains are intended only to change the color of the substrate, usually wood and sometimes concrete. They contain little or no vehicle or pigment, a very high proportion of solvent, and a dye additive. Excess stain is wiped off with a rag a few minutes after application, leaving only the stain that has penetrated the substrate. Usually, a surface stained with a transparent stain is subsequently coated with a clear finish such as varnish to bring out the color and figure of the wood and produce a durable, easily cleaned surface.
Semitransparent stains have more pigment and vehicle than transparent ones. They are not wiped after application. They are intended for exterior application in two coats and do not require a clear topcoat. Also selfsufficientare the solid stains, which are usually water based. These contain much more pigment and vehicle than the other two types of stains and resemble a dilute paint more than they do a true stain. They are intended for exterior use.
The clear coatings are high in vehicle and solvent content and contain little or no pigment. Their purpose is to protect the substrate, make it easier to keep clean,and bring out its inherent beauty, whether it be wood,metal, stone, or brick. Lacquers are clear coatings that dry extremely rapidly by solvent evaporation. They are based on nitrocellulose or acrylics and are employed chiefly in factories and shops for rapid finishing of cabinets and millwork. A slower-drying clear coating is known as a varnish.Varnishes may be either solvent or water based. Most of them harden either by oxidation of an oil vehicle or by moisture curing. Varnishes are useful for on-site finishing.Varnishes and lacquers are available in gloss, semigloss,and flat formulations.
Shellac is a clear coating for interior use that is made from secretions of a particular Asian insect that have been dissolved in alcohol.Shellac dries rapidly and gives a very fine finish, but it is highly susceptible to damage by water or alcohol.
There are many finishes, intended primarily for furniture and indoor woodwork, that are based on simple formulations of natural oils and waxes. A mixture of boiled linseed oil and turpentine, rubbed into wood in many successive coats, gives a soft, water-resistant finish that is attractive to sight, smell, and touch. Waxes such as beeswax and carnauba can be rubbed over sealed (and sometimes unsealed) surfaces of wood and masonry to give apleasingly lustrous finish. Finishes based on waxes and oils usually require periodic reapplication during service to maintain the character of the surface.
There are countless specialized coatings formulated for particular purposes. In tumescent coatings add fire resistance to steel. Industrial highperformance coatings, such as epoxies and urethanes, provide greater resistance to physical wear, chemical attack,and corrosion than are possible with ordinary paints. Asphaltic coatings may be applied to some roofing materials.Coatings based on portland cement may be applied over masonry or concrete.
Many of the newest and most durable architectural coatings are designed to be applied in the factory, where controlled environmental conditions and customized machines permit the use of many types of materials and techniques that would be difficult or impossible to use in the field. These include powder coatings, which are sprayed on dry and fused into continuous films by the application of heat; highly durable, multicomponent coating formulations such as fluoropolymer finishes , and many others.
建筑涂料的施工 Field Application of Architectural Coatings
No aspect of painting and finishing work is more importantthan surface preparation. Unless the substrate isclean, dry, smooth, and sound, no paint or clear coatingwill perform satisfactorily. Normal preparation of woodsurfaces involves scraping and sanding to remove anyprevious coatings, patching and filling holes and cracks,and sanding the surface to make it smooth. Preparationof metals may involve cleaning with solvents to remove oiland grease; scraping and wire brushing to remove corrosionand mill scale; sandblasting if the corrosion and scaleare tenacious; and, on some metals, chemical etching ofthe bare metal to improve the paint bond. New masonry,concrete, and plaster surfaces generally require someaging before coating to ensure that the chemical curingreactions within the materials themselves are completeand that excess water has evaporated from the material.
A number of materials are designed specifically toprepare a surface to receive paints or clear coatings. Pastefillers are used to fill the small pores in open-grainedwoods such as oak, walnut, and mahogany prior to finishing.Various patching and caulking compounds serve tofill larger holes in substrates. A primer is a pigmented coatingespecially formulated to make a surface more paintable.A wood primer, for example, improves the adhesionof paint to wood. It also hardens the surface fibers ofthe wood so that it can be sanded smooth after priming.Other primers are designed as first coats for various metals,masonry materials, plaster, and gypsum board. Woodtrim and casings in high-quality work are back primed byapplying primer to their back surfaces before they areinstalled; this helps equalize the rate of moisture changeon both sides of the wood during periods of changinghumidity, which reduces cupping and other distortions.A sealer is a thin, unpigmented liquid that can be thoughtof as a primer for a clear coating. It seals the pores in thesubstrate so that the clear coating will not be absorbed.
To prevent premature drying, surfaces to be paintedshould not be exposed to direct sunlight, extreme temperatures,or high-speed winds. The paint materials themselvesshould also be at normal room temperature.
Paint and other coatings may be applied by brush,roller, pad, or spray. Brushing is the slowest and mostexpensive method; it is best for detailed work and forapplying many types of stains and varnishes. Spraying isthe fastest and least expensive, but also the most difficultto control. Roller application is economical and effectivefor large expanses of flat surface. Many painters preferto apply transparent stains to smooth surfaces by rubbingwith a rag that has been saturated with stain.
A single coat of paint or varnish is usually insufficientto cover the substrate and build the required thickness offilm over it. A typical requirement for a satisfactory paintcoating is one coat of primer plus two coats of finish material.Two coats of varnish are generally required over rawwood. The surface is lightly sanded after the first coat hasdried to produce a smooth surface to which the final coatis applied.
Different coatings require curing periods that rangefrom minutes for lacquers to days for some paints. Duringthis time, environmental conditions similar to thoserequired for the application of the coating must be maintainedto ensure that the coating cures properly.
油漆和饰面变质 Deterioration of Paints and Finishes
Coatings are the parts of a building that are exposed tothe most wear and weathering, and they deteriorate withtime, requiring recoating. The ultraviolet (UV) componentof sunlight is particularly damaging, causing fadingof paint colors and chemical decomposition of paintfilms. Clear coatings are especially susceptible to UV damage,often lasting no more than a year before discoloringand peeling, which is why they are generally avoided inexterior locations. Some clear coatings are manufacturedwith special UV-blocking ingredients so that they maybe used on exterior surfaces. The other major force ofdestruction for paints and other coatings is water. Mostpeeling of paint is caused by water getting behind thepaint film and lifting it off. The most common sourcesof this water in wood sidings are lumber that is dampat the time it is coated, rainwater leakage at joints, andwater vapor migrating from damp interior spaces to theoutdoors during the winter. Good construction practicesand proper design of air barriers and vapor retarders canminimize these problems.
Other major forces that cause deterioration of architecturalcoatings are oxygen, air pollutants, fungi, dirt,degradation of the substrate through rust or decay, andmechanical wear. Most exterior paints are designed to“chalk” slowly in response to these forces, allowing therain to wash the surface clean at frequent intervals.
典型建筑涂料系统 Typical Architectural Coating Systems
The accompanying table summarizes some typical specificationsof coating systems for new surfaces of buildings.Especially where alkyd coatings are indicated, conformanceof product VOC emissions to applicable limitsshould be verified.
建筑涂料系统标准 Standards for Architectural Coating Systems
The Master Painters Institute (MPI) sets standards forpaints and painting methods. Its manual of coating systemsstandardizes and simplifies the specification of completecoating systems, that is, combinations of surfacepreparation, primer, and topcoats appropriate for anygiven substrate and meeting a variety of performancelevels.
Sustainability standards for paints and other architecturalcoatings continue to evolve. Green Seal’s GS-11 Standardfor Paints and Coatings sets performance standardsfor hiding power (opacity) and cleanability, limits VOCemissions, and restricts the inclusion of certain toxic orharmful ingredients in paints. Compliance with GS-11 is,at the time of this writing, recognized by LEED for creditfor low-emitting architectural paints and coatings. MPI’sGreen Performance Standards establish criteria in the samecategories of performance, emissions, and restrictedingredients, but in a manner that more closely coordinateswith that organization’s other product standards.Emissions limits set by the California Air Resources boardare referenced in many sustainability guidelines, includingthe LEED rating system.