Paint and Varnish Coatings
Paint and Varnish Coatings
coatings formed as the result of film formation (drying) of paints and varnishes applied to the surface of various articles. The main purposes of paint and varnish coatings are protection of materials from damage, such as corrosion of metals and rotting of wood, and decorative finishing of articles. There are also special-purpose coatings (electrical-insulation, fluorescent, temperature-indication, and heat-resistant, gasoline-resistant, and oil-resistant coatings). Paint and varnish coatings are used in all areas of the national economy and everyday life. With proper use, the service life of paint and varnish coatings may be as long as several years. Such coatings are inexpensive and easy to apply and repair, and they impart an attractive appearance to the surface being coated. The world consumption of paints and varnishes was about 16.6 million tons in 1974.
The properties of paint and varnish coatings are determined by the composition of the materials (the type of film-forming materials, pigments, and other components), as well as by the structure of the coating, which most often consists of several layers. The most important requirements made of paint and varnish coatings are strong bonding (cohesion) of the layers and adhesion of the bottom layer to the substrate, hardness, bending and impact strength, moisture impermeability, atmospheric resistance, and a combination of decorative properties (such as transparency or covering power, color, degree of luster, and pattern).
Among the materials used in the production of multilayer paint and varnish coatings are primer coats, which are applied directly to the substrate for protection from corrosion and to provide adhesion of the material to the substrate; sealers, which are applied to the ground coat when it is necessary to fill pores or small cracks and to eliminate small surface defects; paints, which impart the required decorative properties to the surface and provide the required resistance of the coatings to external effects; and varnishes, which are applied over the layer of paint to increase the luster of the coatings (varnish is applied directly to the surface to be protected when transparent coatings are desired). The total thickness of multilayer paint and varnish coatings is usually 40–300 microns (μ).
The industrial process for producing paint and varnish coatings includes preparation of the surface, application of the individual layers, and drying and finishing of the coatings.
The quality of preparation of the surface before the application of paint to a great extent determines the adhesion of the coatings to the substrate. Effective methods for the preparation of metal surfaces are roughening by blasting with shot or by the hydraulic-abrasive method or the creation of a microporous sublayer by oxide coating or parkerizing.
In the 1920’s and 1930’s the technology of application of paints and varnishes began to undergo appreciable changes in connection with the development of production of synthetic film-forming materials, and also as a result of the development of effective means of mechanization and automation of the production processes. The manual methods for applying paints and varnishes with a brush or trowel, which have long been known, are used on a limited scale in modern industry because of their low productivity and difficulties in working with quick-drying paints and varnishes. The most widespread method of applying paints and varnishes in machine building is with manual or automatic spray guns. The use of this high-productivity method makes possible the production of high-quality coatings on surfaces of various shapes. With automatic spray-painting units, the paint or varnish and the air used for spraying may be preheated to 55°-70°C. This makes possible the application of high-viscosity materials and thus a reduction in the number of layers required to produce coatings of the desired thickness. A disadvantage of the method is the high waste of paint or varnish (up to 50 percent) as a result of atomization in the surrounding atmosphere (“fogging”). This also produces difficult working conditions. Therefore, paints and varnishes are usually atomized in isolated, well-ventilated chambers. The fogging losses may be substantially reduced (to 15–30 percent) by atomizing the materials under high pressure generated by a pump (4–25 mega-newtons per sq m, or 40–250 kilograms-force per sq cm).
A sharp reduction in fogging losses (to 5–10 percent) may be achieved by atomizing paints and varnishes in a high-voltage DC electric field (about 100 kilovolts). The corona discharge generated on the sharp edge of the spray device imparts an electric charge (usually negative) to the particles of the material, which leads to their atomization and deposition on the oppositely charged and grounded substrate. Multilayer paint and varnish coatings are applied to both metallic and nonmetallic surfaces (particularly to wood, with a moisture content not less than 8 percent) in an electric field. Electrical spraying, which is widely used for painting parts on conveyor belts, is performed automatically. Parts of unique or various shapes are painted with manual electric atomizers; articles of complex shape are painted with pneumatic-electric and hydraulic-electric atomizers, which make possible the coating of recessed areas of a surface. Aerosol spraying with cans filled with paint or varnish diluted with liquefied Freon is used in touch-up painting and restoration of the exterior appearance of articles (such as motor vehicles and furniture).
Identical mass-produced articles of streamlined shape may be painted by dipping and jet-spraying methods; the latter method involves lower consumption of materials. A defect of coatings produced by these methods—running and beading—may be avoided by passing the painted articles through a tunnel containing vapors of a solvent. This retards the evaporation of the solvent from the applied layer, thus preventing premature hardening of the paint or varnish.
Varnish-casting machines are used for applying polyester paints and varnishes to the wooden exterior stock.
Anode electrodeposition (DC, 30–500 V) in baths is used for the production of paint and varnish coatings from water-thinned primers and varnishes on automated conveyor lines. Electrophoresis of the particles of the paint or varnish leads to their discharge and precipitation on the anode and their conversion to a water-insoluble form. This method may be used to apply only a single layer of a coating (20–25 μ), since the insulating effect of the material prevents the electrodeposition of subsequent layers. The use of rollers is a high-productivity method for the application of paints and varnishes to sheets and rolls of materials, such as metal ribbons and strips.
Paint and varnish coatings may be dried when cold (natural or air drying) or hot (artificial drying or oven-baking). Cold drying is used for quick-drying materials, as well as for slow-drying coatings applied to materials that cannot be subjected to high temperatures. Hot drying makes possible not only accelerated evaporation of the solvent but also the hardening of coatings based on thermosetting film-forming materials. One of the oldest hot-drying methods is the convection method, which is performed in drying chambers; in this method the drying of each layer requires one to three hours. The thermal-radiation drying method, using infrared radiation, is three to six times more productive. The radiation sources are incandescent lamps or dark emitters, consisting of metal panels or ceramic plates that are heated to 400°-700°C by tubular electric heaters or gas burners. Dark emitters are more durable, efficient, and economical than lamp heaters. Rapid drying of paint and varnish coatings is achieved in the induction method of drying by heating the substrate with eddy currents. The action of ultraviolet light or a beam of fast electrons leads to the drying (hardening) of polyester paint and varnish coatings within fractions of a second.
Finishing operations include sanding the dried bottom layers of paint and varnish coatings to remove extraneous inclusions and to improve the cohesion of the layers. If necessary, the top layer of a coating may be polished with pastes or slurries. The sanding and polishing operations may be performed manually or with pneumatic tools or automated devices.
The quality control of paint and varnish coatings consists of visual inspection and determination (on samples) of hardness, elasticity, bending strength, anticorrosion properties, and atmospheric resistance.
REFERENCESLiubimov, B. V. Spetsial’nye zashchitnye pokrytiia v mashinostroenii, 2nd ed. Moscow-Leningrad, 1965.
Aronov, N. V. Oborudovanie i mekhanizatsiia tsekhov nemetallicheskikh zashchitnykh pokrytii Moscow, 1969.
Spravochnik po lakokrasochnym pokrytiiam v mashinostroenii. Moscow, 1964.
Al’bom oborudovaniia okrasochnykh tsekhov. Moscow, 1970.