Application of Organic Coatings
Before applying organic coatings, a primer (or primary coating) is applied on the metal surfaces to improve bonding properties of the metal surface, so that the later applied coating of paints may adhere well to the metal surface. Primer also fills small tiny pores on the surface, making it smoother and even. It also gives a corrosion resistant layer on metal surface. A primer should not absorb the paint coating given on it. Lead oxide is the most widely used primer for steel surfaces.
Zine and lead chromate primers are used on steel, zinc and aluminium surfaces. In order to achieve good results, the surface prepared by a primer coating is finished by sanding before applying organic finishes. Following common methods are used for applying organic finishes or coatings.
1. Brushing is applying paint with a brush and is a slow process being manual and calls for good skill on the part of a painter for high finish on the job. The process has flexibility of use of finishes in difficult situations as overhead painting, corners. interiors, etc.
2. Spray painting comprises atomizing (or breaking up) of liquid coating into fine. spray and throwing it to the work surface with high velocity, using a spray gun operated by compressed air. Other methods used for spray painting are hydraulic spraying (called airless spraying), steam spraying and electrostatic spraying (wherein the sprayed coating is collected on the job surface without wasting). Hydraulic or airless spraying reduces fog or rebound (as happens in compressed air spraying). Both hot and cold airless spraying are used. A typical hot airless spraying is shown in Fig.
3. Dip coating is useful for parts which are to be painted all over by dipping into a paint bath. The process can be mechanized also using conveyors.
Illustrating the hot airless paint-spraying system. Adding heat to airless spray permits spraying at lower pressures, thereby reducing overspray and rebound, Airless spraying does not use compressed air to atomize paint. Instead, hydraulic pressure (34 to 306 kn/mm) is used to atomize the paint by pumping the paint at high pressure through an orifice in the spray nozzle. The paint released at such high pressures is separated Into droplets forming a fine atomized spray.
4. Flow coating consists of a conveyor with hanging parts (to be painted) passing through an enclosure (tunnel), where coating is showered on the parts hanging with the conveyor.
5. Roller coating is used for coating large flat surfaces, mostly sheets. Coating is applied on both the sides of sheet which is later passed through a set of rollers (up and below the sheet) dipped in coating.
6.Electro-coating is an efficient method of providing a thin film of coating. The system comprises a tank in which emulsion is contained, workpiece works as anode and tank as cathode when DC supply is used. The high DC voltage applied gives an electrostatic charge to the electrolyte (or emulsion) whose particles are attracted towards the workpiece and get deposited there in the form of thin uniform film.
7. Powder coating:
The use of dry powders in coating has increased considerably, because not only are the thicker coats applied in one go, the powder coatings also have better performance with higher chemical resistance than paints and the coatings do not drip or sag. Thermosetting powders of epoxies, polyesters and acrylics are used. In case of thermoplastic powders, the plastics used are PVC, cellulose acetate butyrate, polyethylene, nylon, chlorinated polyether and fluorocarbons. Most widely used powders are the epoxies as they are formulated to be tough, highly durable and highly decorative. PVC is also the most commonly used thermoplastic powder, relatively low cost, good chemical, abrasion and impact resistance and often used for electrical insulation. The two more popular techniques of powdered coatings are (a) fluidized bed and (b) electrostatic spray.
The essentials of fluidized bed are shown in Fig.(a) . Pre-heated parts (up to above the melting point of powder) are immersed in a fluidized bed consisting of a tank filled with dry plastic powder. Particles of powder contacting the job surface melt and adhere there. Immersion time is generally 1 to 30 seconds. After dipping, excess powder is removed immediately by tapping or air blasting. Parts coated with thermosetting powders such as epoxies are post-heated for fusing and curing the resin. The fluidized method gives a thick coating.
Fluidized-bed dipping method for applying powder coatings to metals.
Electrostatic spraying utilizes the basic principle of electrostatics that the opposite electrical charges attract each other. A typical set-up for air-electrostatic spray is shown in Fig .18(b) . The set-up consists of an atomizer (or air spray gun),
Air-electrostatic spray system for water-reducible coatings. Paint is atomized by the conventional compressed air method. The charger creates electrostatic field between the nozzle (spray gun) and the workpiece, charging the passing paint particles.
electrostatic voltage pack and power supply, paint supply and compressed air supply. As the atomized paint particles leave the air spray gun, they are directed under an electrode (charger) that is charged to about 62.000pi negative. The paint particles pick up a negative surface charge and proceed or seek out the nearest grounded or earthed surface (obviously the job surface) at zero potential and attach themselves to it. The electrical charge of the particles quickly dissipates upon contact with the job surface. Thus, other paint particles tend to seek out unexposed surfaces, providing a uniform build-up. As the particles approach the job surface, they rapidly lose velocity and become almost floating as they reach the job surface The electrostatic spraying is good for large surfaces and thin coatings. After spraying, the coating is fused in an oven.
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