Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
1998-11-19
2001-04-10
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S477000, C427S478000, C427S485000, C427S195000, C427S315000, C427S317000, C427S421100
Reexamination Certificate
active
06214421
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of powder coating, applicable to powder coating any type of material and, in particular, wood and other non-metallic surfaces.
BACKGROUND OF THE INVENTION
Conventional powder coating processes involve spraying a coating of polymer plastic powder onto an object, and then applying heat to the coating. The heat applied must be sufficient to cure the powder and, if applicable, to enable it to chemically react, but not so extreme in either time or duration as to cause the polymer to start to decompose. When heat is removed, the powder hardens onto the object.
In theory, powder coating will work effectively with any object that can withstand the application of the heat necessary to melt the powder. In practice, however, problems are often encountered in getting the powder to adhere to the object. In an effort to improve adherence of powder, the object being coated is commonly heated or electrostatically charged.
A particularly difficult material to get a polymer plastic powder to adhere to is wood. Notwithstanding the heating of the wood and the use of electrostatic application methods, the quality of powder coating finishes on wood have generally been unsatisfactory.
SUMMARY OF THE INVENTION
What is required is a method of powder coating that will improve the adherence of the powder to the object being coated, particularly when that object is made of wood or other non-metallic materials.
According to the present invention there is provided a method of powder coating. A first step involves moisturizing a surface of an object which is to be coated with supersaturated steam to provide moisture on the surface just sufficient to cause powder to adhere to the surface. A second step involves spraying polymer coating powder onto the surface before the moisture evaporates, whereby moisture on the surface aids in the adhesion of the powder to the object. A third step involves curing the powder adhering to the surface after the moisture has evaporated from the surface, without decomposing the powder.
The method, as described above, has resulted in a greatly improved quality of coating. Having moisture on the application surface greatly enhances the ability of the powder to adhere to the surface, much as the licking one's finger enhances one's ability to pick up sugar. The teaching in the prior art of heating the object and immersing it in a fluidized bed of powder, is believed to be counter-productive, especially when coating objects made of wood which have limitations on the temperature to which they can be heated. When working with metal, care must be taken to avoid excessive moisture, as excessive moisture will adversely effect the quality of the coating. Wood and other non-metallic materials are believed to be best suited for the application of this method.
Although beneficial results may be obtained through the use of the method, as described above, even more beneficial results may be obtained when a plurality of spray nozzles are used to apply the powder which generate a low velocity powder mist. In the prior art, the spray nozzles utilized were generally high volume/high velocity jets which bombarded the object with powder. It is believed that such high volume/high velocity sprays are counter-productive. When the object is dry such sprays tend to polish the application surface. This is especially the case with wood. This polishing effect actually clears powder from the application surface. It is, therefore, preferred that the spray nozzles generate a low velocity powder mist onto the moist application surface.
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patent: 3908036 (1975-09-01), Milleson
patent: 3919437 (1975-11-01), Brown et al.
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patent: 5364657 (1994-11-01), Throne
patent: 5753302 (1998-05-01), Sun et al.
patent: 5824373 (1998-10-01), Biller et al.
Thin Films Science & Technology 6 “Coatings On Glass”, HK Pulker 2nd Ed., pp. 52-55, 1984.*
“Power Coating” The Complete Finishers Handbook, N. Liberto, ed. p. 44, 1994.
Davis and Bujold
Parker Fred J.
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