On some finishing lines, coatings are air dried rather than baked. On others, auxiliary energy sources raise ambient temperatures, enhancing air drying in cold weather. Most factory-applied liquid paint is converted to solid films by several curing mechanisms, including solvent or water evaporation and polymerization. Heat accelerates curing.
In conventional paint-baking ovens, organic solvents and water evaporate and paint films dry and cure under carefully controlled conditions. These ovens use gas, oil, steam or electricity to heat the air. Up to 45 min dwell time may be required in thermal ovens, depending on the chemistry of the coatings, oven temperature, configuration of the product and curing equipment.
Direct-fired gas ovens may have the burners inside the enclosure, where combustion products contact the coating film. On the other hand, the burners may be in separate combustion chambers equipped with recirculating blowers to push oven air through the burner box and back to all places where heat must be supplied. They also have blowers to exhaust fumes.
In other cases, hot air containing solvent evaporated from paint is exhausted to heat exchangers, providing heat for other manufacturing processes to comply with air-pollution-control regulations, vaporized solvent may be removed from the air before discharge to the atmosphere, by incineration, using carbon filtration or other means.
Direct-fired oil ovens are possible because of redesigned burners. They can bake a variety of colored paints without discoloration and soot accumulation.
Indirect-fired ovens are similar in design and operation to direct-fired ovens, except that air is heated as it passes over heat exchangers. Since combustion products do not contact the coating film, they cause no darkening, no discoloration, and no haze.
Electrically heated ovens use resistance heaters. Otherwise, they are similar to indirect-fired ovens and are used where other fuels are unavailable.
Steam-heated ovens can be used in plants having a sufficient supply of high-temperature steam. Super-heated steam is used to heat ovens having the same configurations as indirect-fired ovens.
Electric andgas infrared ovens cure coatings requiring a rapid temperature rise at varying film temperatures and varying distances from the radiation source. These ovens use specially designed elements to produce infrared radiation. Some units have lightweight elements and reflectors that can be moved from one location to another or erected and suspended in various ways. They all require make-up and exhaust air.
Induction-curing ovens cure coatings on wire, rod or continuous strips. High-frequency induction coils excite the molecules in the substrate, generating heat that cures a variety of liquid and powder coatings from the inside out.
High-speed curing ovens are used in coil-coating lines where strip or tape travels through coating and curing processes at several hundred feet per minute. These ovens can cure coatings in extremely short times. In some cases, hot air jets support the long catenary curve of strip passing through the oven. In others, opposing air jets alternating with high-intensity heat sources float the strip in a horizontal position.
Radiation curing uses ultraviolet light or electron-beam radiation to cure coatings specially formulated to respond to these energy sources. These coatings are essentially free of volatile organic compounds (VOCs). Owing to their rapid cure rate and negligible surface heating, they can be used to cure coatings on temperature-sensitive substrates such as plastics.
Vapor curing is not one, but two technologies. In the first, called vapor- permeation curing, parts coated with specially formulated paint pass through a chamber containing air permeated with catalyst vapor. In the second, called vapor-injection curing, the catalyst requires no curing chamber. Instead, it is sprayed, along with the coating, using a specially designed spray gun.
Unlike bake-off ovens—which typically operate at 250-500°F—burn-off ovens usually run at 700-900°F or greater, causing the coating to burn without flame, which is called pyrolysis. An integral water system suppresses the generation of any open flame. Once the coatings have been burned, any resulting ash can be removed with a pressure washer.
High-temperature oven strippers pyrolyze organic portions of the coatings in an oven heated to 700-800°F in a low-oxygen atmosphere. In the first phrase, the resultant volatiles are driven off, leaving carbon and inorganic compounds. In the second phase, the carbon remaining on the substrate is burned at higher temperatures in excess oxygen to form carbon dioxide. Afterwards, the inert pigments and fillers remaining on the substrate are removed mechanically. To comply with air-quality standards, an afterburner in the oven exhaust burns the remaining organic volatiles to form carbon dioxide and water. Heat exchangers, in more efficient systems, recycle the energy.