Paint Curing Methods

Curing and hardening may be either physical or chemical. With a suitable combination of binders, both types of curing methods may proceed in parallel or overlap. 

Curing methods may be divided into three groups  

2) Curing with radiation (IR, UV, electron beams, laser beams, plasma arc) 

3) Curing by means of electrical processes (inductive curing, resistance, high-frequency and microwave curing) 

Curing with Radiation. See also Section 3.7. Radiation curing methods have become increasingly important in the last few years. Paint curing proceeds more rapidly than in circulating air curing since the whole workpiece does not have to be heated. However, only large flat parts can be satisfactorily treated. 

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The volatiles emitted from white amide-cured epoxy powder paints, cured and manufactured by the solution method are listed in Table 9.17. The most abundant compounds detected were isobutyl methyl ketone (4-methylpentan-2-one) and the various isomers of xylene which are typical solvents used in the production of resins. The volatiles characteristic of all three amide-cured epoxy powders studied were an unidentified compound and melamine. 

To cure at the point where the assembly is made can incur additional process costs which may not be necessary. However, if the assembly is dimensionally unstable before curing or there are other factors which preclude the use of the paint shop curing cycle, then local curing methods are used. Curing at this stage of the process can either produce a fully or partially cured assembly dependent upon the local requirements. 

Although rolling and dipping are sometimes used, power spray painting is the usual method of paint application. Among the coatings used are polyurethane, epoxy, acrylic, alkyd and vinyl based paints. With paints that are oven cured, products must have sufficient heat resistance to survive without distortion, etc. 

Radiation-induced modification or processing of a polymer is a relatively sophisticated method than conventional thermal and chemical processes. The radiation-induced changes in polymer materials such as plastics or elastomers provide some desirable combinations of physical and chemical properties in the end product. Radiation can be applied to various industrial processes involving polymerization, cross-linking, graft copolymerization, curing of paints and coatings, etc. 

When the trouble is black thread, planters use paints, coal-tar washes, asphalt, and other treatments with varying degrees of success. New tests have shown that Fylomac 90, applied in a 0.25% solution, effects a cure in eight successive applications. In one of the most recent studies on this, Carpenter (8) tested 22 fungicides on Phytophthora panel trouble in Costa Rica. His work was under ideal conditions for the disease and with a well tested invariably successful, artificial method of inoculation. He found only one fungicide sufficiently good for his purpose—Ortho-cide 50 [captan, n-(trichloromethylthio)-4-cyclohexene-l,2-dicarboximide], wettable, mixed with Filmfast sticker. 

The method of preparation for the second type of paint consisted of mechanical polishing, cleaning, phosphatlng for 5 minutes with Oxy-Plus 84 DRS solution, drying for 30 minutes without rinsing, application of one layer of precoat followed by air drying for 4 hours at room temperature and application of one layer of Marlnox SR-2 paint followed by curing for 8 days at room temperature. 

Following the application of paint, the item is passed through a drying or curing oven. The methods employed include convection, infrared or ultraviolet heating or combinations thereof. The particular methods depend on the types of paints being used, economics, and production requirements. Once dried, the items are sent for inspection and final packaging or assembly. 

When thermoplastics, as mouldings or in other forms, are exposed to temperatures in the range 80 to 130°C there may be some release of strain, softening, and distortion—which makes it impossible to employ with them conventional painting methods that require higher temperatures for curing. 

Disk brake pads and drum linings are finish ground to the final dimensions after the oven cure process. Sometimes, the surface of disk pad products are scorched or seared under the influence of heat applied in the 250-700" C range. This burns off the organics in the pad surface and helps the initial friction properties of brake pads. Scorching can be accomplished by a variety of methods—direct gas flame, contact with an electrically heated hot plate, or radiant heat from a heated surface. Subsequently, the parts are painted and stenciled with product information prior to packaging. 

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