Abstract:
A method of powder coating at least one surface of a non-conductive object having a plurality of sequential steps. The first step is pretreating the surface of the non-conductive object to ensure that the surface is suitably cleaned. The second step to apply a sufficient quantity of moisture on the surface of the non-conductive object to facilitate adhesion of a powder coating to the surface. The next step is to spray a polymer coating powder on the moistened surface of the non-conductive object. This spraying step is done immediately following the application of moisture but prior to a complete evaporation of the applied moisture. The fifth step is to evaporate any remaining applied moisture from the non-conductive object prior to curing. The last step is to cure the coating on the surface of the non-conductive object without decomposing the powder and then either cool or allow the non-conductive object to cool naturally.

Description:
This application is a Continuation-in-Part of Ser. No. 08/833,724 filed Apr. 9, 1997 now abandoned. 
    
    
     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&#39;s finger enhances one&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawing, wherein: 
     THE FIGURE is a schematic representation of a method of powder coating in accordance with the teaching of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred method of powder coating will now be described with reference to THE FIGURE. 
     The preferred manner of performing the method steps, which will hereinafter be described, is to position an object  12  on a conveyor  14  and then subject object  12  to the method steps sequentially as conveyor  14  advances. Prior to being subjected to the method steps, object  12  must be suitably cleaned. The preferred manner of doing this is by means of vacuums  16  with vacuum heads  18  disposed about conveyor  14 . It may also be desirable to subject object  12  to other pretreatment, depending upon the materials out of which object  12  is made. It is preferred that wood products be sanded and vacuumed. 
     The preferred method of powder coating, with presently contemplated enhancements, includes the following steps. The first step involves moisturizing object  12 , prior to it being sprayed, by passing object  12  through a steam chamber  20 . When object  12  has completed passing through steam chamber  20  it has a moist application surface  22 . Steam chamber  20  is connected by conduit  24  to a source of steam  26 . Although this method can be used with any material, wood is believed to be best suited for the application of this method, as it tends to temporarily hold moisture on its surface. Moisture can be detrimental to the coating process if excess moisture is released during heating. 
     The second step involves spraying polymer coating powder  28  onto the moist application surface  22  by passing object  12  through the spray chamber  30   a.  Spray chamber  30   a  has a plurality of spray nozzles  32  which generate a low velocity powder mist. The moisture on application surface  22  has been found to facilitate the adhesion of powder  28  to object  12 . The powder is delivered to spray chamber  30   a  by conduit  31 . A spray unit  33  is used to draw powder from a powder reservoir  34  and deliver the powder through conduit  31  to spray nozzles  32 . Although not essential, it is preferred that the powder be applied using a spray application process. Having a moist application surface  22  has been found to enhance the spray application process, with better adherence of powder along edges and in recesses. The spray application can be, but need not be, performed in combination with conventional electrostatic processes. 
     The third step involves curing powder  28  adhering to application surface  22 . The preferred manner of curing powder  28  is by heating. There are alternative curing processes such as through the use of radiation. Powder  28  is brought to a temperature sufficient to melt and react, without decomposing, by passing object  12  through at least a first curing chamber  34   a  in which is disposed a heat source. A heat source is preferred that is capable of heating powder  28 , with the least penetration possible with respect to object  12 . A penetrating heat source is to be avoided, as it is unnecessary and undesirable to heat object  12 . Heating of object  12  results in thermal expansion, with inevitable thermal contraction when object  12  cools. Thermal contraction during cooling can adversely effect the adherence of the coating. 
     It is viewed as being desirable to have flexibility to apply a plurality of thin coats of powder or one thick coat. When a plurality of thin coats are desired, additional steps may be added relating to placing of second, third and perhaps subsequent coats on object  12 . Spraying a second coat of polymer coating powder  28  onto object  12  by passing object  12  through a second spray chamber  30   b.  Heating the second coat of powder  28  adhering to object  12  to a temperature sufficient to melt, without decomposing, powder  28  by passing object  12  through a second curing chamber  34   b.  Spraying a third coat of polymer coating powder  28  onto object  12  by passing object  12  through a third spray chamber  30   c.  Heating powder  28  adhering to object  12  to a temperature sufficient to melt, without decomposing, the powder  28  by passing object  12  through a third curing chamber  34   c.    
     It is preferred that a further and final step be taken of passing object  12  through a cooling chamber  36 . Cooling chamber  36  is kept cool by means of a refrigeration unit, generally indicated by reference numeral  38 . 
     It is preferred that spray chambers  30   a,    30   b,    30   c  have powder recovery and recirculation means. The powder recovery and recirculation means include a plurality of powder recovery sumps  40  connected to recovery tanks  42  and to recirculation conduit  44 . 
     Each chamber described above, must be isolated from the other chambers. It is preferred that this be accomplished by means of by air generated curtains  46 . A source of inert air  48  is connected by means of air conduit  50  to each air curtain  46 . Each chamber is isolated from outside air and from the other chambers. 
     As the application of moisture to an object surface during a powder coating process runs contrary to current practices, the moisture application will now be further described. The process has particular utility for obtaining adhesion of powder to non-conductive substrates, where there would otherwise be no adhesion, poor adhesion of irregular adhesion of the powder; such as wood, plastic, and cardboard. When wood is being coated, the wooden object is prepared by standard procedures for wood working. The wood is cut to the desired design, sanded, and vacuumed to produce a uniform clean surface free of oils and dirt. 
     The steam chamber consists of a cabinet into which wood panels or other types of non-conductive objects can be rapidly introduced and retrieved. An example of a way in which this can be done is a chamber with a roller conveyor system, in which wood parts are carried rapidly, tunnel style, through the chamber on rollers. The chamber is equipped with an array of nozzles through which saturated steam is directed onto the object. The arrangement of nozzles can be adjusted for uniform application of steam to the object. The nozzles direct steam at the part, but also ensure that the steam chamber is completely filled with saturated steam. Care must be taken to ensure that drops of condensed water cannot fall on the workpiece as it passes through the chamber, as this is highly detrimental to resulting coating quality. 
     Steam is preferably generated by heating water to boiling and directing the resulting saturated vapour through steam lines into the steam chamber. The steam lines are, preferably, well insulated so that the temperature of the steam does not drop significantly. A small amount of moisture that does condense, should be drawn off separately so that is not introduced into the steam chamber. The steam is supersaturated, and is visible as a “cloud of steam”. The process works best when hot steam (near the boiling point of water) is used. It would be possible to carry out the process at low temperatures, including room temperatures. At room temperature “supersaturated steam” can be created with an atomizer by passing liquid water through a nozzle to produce a cloud of droplets an vapour. The atomized water is projected onto the wood surface, resulting in a moisturized surface which can be powder sprayed. The process will not be as effective at lower temperatures as it is when hot steam is used. The reason for this is that the moisture must be evaporated before cure. With hot steam this evaporation occurs very rapidly. At room temperature additional time or a heating step may be required after spraying, but before curing. Humidity, that is moisture at a relative humidity of less than 100%, (not visible as a cloud) would not be as effective in this process and possibly would not be effective at all. 
     The process works best when moisture application, powder spraying, and evaporating occur rapidly in succession. Timing and co-ordination of these processes is essential to successful operation of the process. This co-ordination will now be described with reference to a pilot plant that was developed to prove this process. In the pilot plant, the moisturizing process takes about 15 to 20 seconds, which is the time required for the object to pass through the steam chamber. The steam temperature is slightly below the boiling temperature of water. The moisturized wood immediately passes into the powder spray chamber, for powder application. It takes about 25 seconds for the panel to pass through the spray chamber, at which time much of the moisture has evaporated. Another 25 to 30 seconds pass before the coated panel enters the cure chamber, the additional seconds help ensure complete evaporation of moisture prior to curing. Moisture application and evaporation must occur rapidly. It is highly undesirable for the moisture to penetrate into the wood, as release of water vapour from the wood would create problems during curing. It is very important that the moisturized wood be transferred immediately from the humidity cabinet to the spray booth, as moisture rapidly evaporates and the benefit to powder adhesion are lost. It should be noted that the rapid nature of the process will provide a commercial advantage. 
     The following guidelines are provided to assist in determining an appropriate amount of moisture. An appropriate amount of moisture has been applied to the wood surface when the powder coating uniformly adheres to the surface, and the moisture fully evaporates prior to cure. 
     Currently moisturized pieces. When the wood object emerges from the steam chamber, the wood has darkened considerably, due to moisture on the surface. The surface of such pieces will feel moist to the touch. If passed immediately through the spray chamber, without spraying, the pieces lose the dark colour (ie. their moisture) by the time they leave the spray chamber. 
     Inadequately moisturized pieces. The pieces are not adequately moisturized if the powder coating will not stick to the surface. 
     Over moisturized pieces. The pieces are over moisturized if free water (droplets or films) can be observed on the surface. Excessive moisture interfaces with the uniform application of powder to the surface, causing it to clump or flow. 
     In summary, the correct amount of moisture is that amount which rapidly absorbs onto the surface of the wood without deep penetration or without leaving a thin film of moisture on the surface. The correct amount of moisture can be applied to the wood surface by varying the following parameters to optimize the process; steam temperature, steam delivery rate, residence time of the object in the steam chamber, residence time of the object in the spray chamber. The correct amount of moisture will vary with the type of substrate being coated. For example, some woods absorb little water, and with such woods a small amount of moisture must be applied very rapidly followed rapidly by spraying. Other woods are highly absorbent and with such woods larger amounts of moisture must be applied to ensure wetness. Consequently, additional time is required for evaporation of the moisture prior to cure. 
     It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the Claims.