Powder coatings, which are dry, finely divided, free flowing, solid materials at room temperature, have gained considerable popularity in recent years over liquid coatings for a number of reasons. For one, powder coatings are user and environmentally friendly materials, since they are virtually free of harmful fugitive organic solvent carriers that are normally present in liquid coatings. Powder coatings, therefore, give off little, if any, volatile materials to the environment when cured. This eliminates the solvent emission problems associated with liquid coatings, such as air pollution and dangers to the health of workers employed in coating operations.
Powder coatings are also clean and convenient to use. They are applied in a clean manner over the substrate, since they are in dry, solid form. The powders are easily swept up in the event of a spill and do not require special cleaning and spill containment supplies, as do liquid coatings. Working hygiene is, thus, improved. No messy liquids are used that adhere to worker's clothes and to the coating equipment, which leads to increased machine downtime and clean up costs.
Powder coatings are essentially 100% recyclable. Over sprayed powders can be fully reclaimed and recombined with the powder feed. This provides very high coating efficiencies and also substantially reduces the amount of waste generated. Recycling of liquid coatings during application is not done, which leads to increased waste and hazardous waste disposal costs.
In the past, most powder coating was performed on metals which can withstand high temperatures at which many conventional coating powders fuse and cure. Recently, however, several coating powders have been developed for substrates, such as wood, which require coating powders which fuse (in the case of thermoplastic coating powders) or fuse and cure (in the case of curable coating powders) at relatively low temperatures. Examples of such coating powders are found, for example, in U.S. Pat. Nos. 5,824,373, 5,714,206, 5,721,052, and 5,731,043, the teachings of each of which are incorporated herein by reference. Low temperature coating prevents charring of the substrate and excessive outgassing of moisture.
A frequent problem encountered when coating low-temperature substrates, such as wood, with coating powder is non-uniformity of coating in areas of the substrate which are difficult to coat, such as the edges and corners of kitchen cabinet doors. It has been found that preheating wood substrates, particularly in the 200.degree. F. to 275.degree. F. range, prior to electrostatic application of coating powders, provides more uniform coating of flat surfaces but can dry out sharp edges, making electrostatic coating difficult.
However, preheating to near or above the boiling point of water tends to dry cellulosic products such as wood, fiberboard, particle board, paper, etc. Such materials tend to have a residual water content, wood typically having a water content of between about 3 and about 10 wt %. This residual moisture presents problems in coating cellulosic substrates with coating powder in that if the temperature is too high, significant outgassing causes defects, e.g., pinholes, in the coating. Similar problems have been noticed with fiber-containing plastic. This is one reason why cellulosic substrates must be coated with powders that fuse and cure at relatively low temperatures. On the other hand, the residual moisture in cellulosic materials is necessary for the material to hold sufficient electrical charge to be electrostatically coated with coating powder. Preheating of cellulosic substrates for the purpose of achieving uniform, continuous coatings may reduce the water content to where the charge-carrying capacity of the substrate is so reduced that electrostatic application of the coating powder is inefficient. Accordingly, it is a general object of the present invention to be able to preheat a substrate, such as a cellulosic substrate, for the purpose of achieving a uniform, continuous coating and at the same time maintaining sufficient moisture level of the substrate for electrical charge-carrying purposes.
In this regard, it was proposed to moisten the surface of lignocellulosic substrates prior to the pre-heating step so that the substrates would retain sufficient moisture and charge-carrying capacity at the point of electrostatic coating powder application. An example of this approach is found in above-referenced U.S. Pat. No. 5,824,373 which teaches maintaining substrates in a high humidity environment prior to pre-heating and optional humidity control through application of the coating powder. This approach, however, was discarded because it tended to warp the surface of the substrate.