APPARATUS FOR COATING SUBSTRATES WITH INDUCTIVELY LOADED POWDER RESIN PARTICLES
BACKGROUND OF THE INVENTION 1) Field of the Invention The present invention relates to an apparatus for electrostatically delivering charged particles onto a substrate where subsequently they form a continuous and uniform coating. One aspect of the invention relates to an apparatus useful for coating substrates wherein the resin powders have an improved electrostatic charge. Another aspect of the invention relates to an apparatus that uses an induction / conduction charging system for coating substrates. 2) Previous Art In recent years, there has been much progress in the field of electrostatic powder coating. Powder coating, as a separate technology, results in a number of clear advantages over other coating methods, such as brushing, sinking and conventional spraying. These advantages include the inherent advantages due to the absence of solvents (safer, less harmful to the environment, less expensive, cleaner working environment), as well as the reduction of the time taken for the coating processes to produce an article ready for use Also possible with this method is the thickness control of the coating and the ability to produce a high quality finish of a simple application treatment. Much of the previous work in the field resulted in methods that were developed and that present many of these advantages. However, there are still a number of disadvantages within the technology that need to be resolved. Powder coating technology is based on the principle of electrostatic charging and currently the practical methods available for charging are classified in a corona charging system, electric tribo charging system or a hybrid system. Each system has evolved from the corona charging system which is a hollow barrel through which the powder is transported pneumatically, reaching the charge of powder by means of ionic bonds in the barrel or the outlet of the gun. A brief review of each of the current systems and the reason for the development of the most recent tribo and hybrid systems is given here to serve as background for the present invention. The basic corona charging system involves charging by sputtering using an ion source such as a high-voltage corona electrode or a radioactive element. This method is frequently used to apply the load in highly insulating materials such as plastics. It can be very inefficient when powdered electrostatic charge is applied since many of the ions produced do not contribute to the loading of particles, but instead sit elsewhere, for example, in the workpiece itself in a coating operation of dust. In the worst case, load inefficiencies less than 1% are repeated in the co-dust powder coating equipment. In the corona charging system, dust is transported from a funnel through feed hoses to a spray gun. A sharp electrode in the gun is connected to a high voltage generator and the combination of electrode geometry and high voltage (up to 100 kV in some guns), creates an electric field in excess of the local gas failure force envelope, which is usually air. A corona discharge is generated and releases the ions formed in front of the charging electrode. The dust particles are transported through this region of charge space and charged by means of ionic bonds. The particles follow a flow pattern of air and those that are sufficiently charged, are deposited in the work piece, which is generally maintained at a ground potential. The polarity of the charging electrode can be returned to create either a positive or negative charge on the particle, generally, a negative charge is preferred due to the large number of ions that are produced. The charging efficiency of this system is very poor because only a small fraction (- 0.5%) of the ions produced by the corona contributes to the charge in the powder. Most of the ions produced by the corona gun do not bind to the powder sprayed particles, but travel as "free ions" to the workpiece, where they accumulate rapidly within the deposited dust layer. As more free ions reach the workpiece, the intensity of the charge within the powder layer reaches saturation. At this point, small electrostatic discharges (delayed ionization) can occur resulting in interruptions in the coating and, finally, poor finish quality. The delayed ionization attack essentially limits the useful thickness of the coating that can be applied using a corona equipment for powder loading coating. In addition to requiring a high-voltage power supply, another disadvantage of the corona gun is that it is not appropriate for applications that need penetration into cavities or corners. This is due to all the voltage that appears on the high voltage external electrode falling between the crown head and the work piece to ground with little, or no subsequent penetration of the field associated with this voltage into cavities or recesses. These areas then resemble the enclosed Faraday cages. Under these conditions, the internal coating is only achieved by pneumatically transporting the particles within these areas, which is difficult to achieve while simultaneously ensuring a good coating uniformity everywhere. Probably the most common alternative crown charging system is the triboelectrification or friction load that takes place when two different materials or surfaces that are previously discharged, that is in an electrically neutral state, make contact and are then separated. During this process of electrostatic charging, one of the surfaces that has a charge of positive polarity and the other with a negative charge is also separated. This process is commonly presented daily. Examples are the dust that is transported through a pipe and a person walking along a carpeted room. In this last case, there is friction between the soles of the shoes and the carpet. The magnitude or even the polarity of the electrostatic charge generated in this way are strongly dependent on factors such as surface contamination, moisture content, and the nature of the contact. Although this method of generating charge is used in the re-covering of electrostatic powder, quality problems have been encountered. While a normal corona gun applies a charge of approximately 1 x 10"3 C / kg of dust particles, the friction charge transfers a few hundred electron charges per contact and, therefore, to obtain equal charges. to a corona gun, thousands of contacts are required.The simplest method by which it is reached is a straight tube in which there is a flow of turbulence, which results in a large number of dust / wall collisions. The wall surfaces are ideally insulated arranged with points to ground so that the generation of charge on the surface can decay to ground, usually using PTFE, poly (tetrafluoroethylene), in commercial systems and their place in the triboelectric series ensures that most dust charges with positive polarity make contact with it.With triboelectric guns, the free ion current is eliminated or considerably reduced, and as there is no electric field to When applied, the particles are directed towards the work piece by a combination of air flow and the field produced by the charge dust cloud. Due to these factors, delayed ionization does not occur for 10 to 20 seconds in tri-boelectric systems and it is easier to obtain heavy or thick films with this system. Another advantage is the ability of the system to coat inside the cavities, small complex parts and products with sharp corners, etc. In addition, the friction load not only exceeds the Faraday cage effect and reduces delayed ionization, but also facilitates the design of the gun to accommodate dew heads that accept different types of noses. The main disadvantage with a tribo gun is that there is a decrease in efficient load change, after a prolonged period of operation. Another disadvantage is the particle size distribution of the powder has a significant effect on the tribo charge and its efficiency. A typical coating powder contains a combination of small, medium and large particles, ranging from the size of a submicron to greater than 80 microns in diameter. It is known that within such systems a hi-polar charge can be presented, with smaller particles that charge better with a negative polarity. The efficiency of the load is a function of the diameter of the particle, and as a result the smaller the particle, it is not attracted electrostatically to the work piece, resulting in a deposition of medium sized particles. In this way, the efficiency of the transfer is reduced and therefore all the efficiency of the operation of the system, due to the increase of generation of deposits in the guns and collection of dust and recycling equipment. Fluidification problems can also occur in the feed funnel. Finally, there are also the so-called "hybrid" pistols that contain the aforementioned methods, i.e. Corona charging and triboelectrification in a gun, in an attempt to combine the advantages of both systems. However, this approach does not remove the inherent inherent disadvantages of both guns - low dust loading and transfer efficiency. The efficiency of the coating is about 70-75% if the materials currently available for practical industrial purposes are preferably used. Any dust not deposited will be wasted or must be recovered by means of special recovery equipment and reused by adding it in small portions to the virgin powder or by recycling it in a resin preparation step. Producers of powder coatings claim that it is possible to achieve 97-98% of the use of powders, citing this as an incentive to change wet spray systems where no spray is wasted.
A defect of this argument is that to achieve such high utilization, special recycling equipment must be operated with an exclusive base in each line, where it is not easy to change the type or shade of the coating material. Therefore the cost of installing the recovery apparatus and the difficult operation scheme and the time required for recovery are added to the total cost. Accordingly, one or more of the following objects can be achieved with the practice of the present invention. It is an object of the present invention to provide an apparatus for charging an electrostatic powder for use in powder coating applications, which is free of the aforementioned disadvantages. Another object of the invention is to provide an apparatus for charging powders that allow an electrostatic charge to be developed in the powder in a reliable and repeatable manner. Another objective is to provide an apparatus that can accurately and reliably control the amount and polarity of the developed electrostatic charge and thus ensure the coating of all areas of a workpiece at any required thickness. Another object of the invention is to provide an apparatus for applying a load to thermoplastic and thermoplastic resins that are used in powder coating operations. Another object is to provide an apparatus wherein the electrostatic charge on the powders is improved by incorporating an electrostatic property modifying agent into or onto the surface of the resin. Another object is to provide an apparatus for electrostatically applying charged powders as the coating of solid objects. Another object is to provide an apparatus for coating solid objects by inductive means. Another object is to provide an apparatus for coating solid objects with a powder resin that can be subsequently fused to provide a uniform and continuous coating on such objects. Another object of the present invention is to provide an apparatus for the application of powder coating of solid objects which is efficient and minimizes the waste of the powder. Another object of the invention is to provide a useful system for spraying electrostatically charged powders into solid objects that can be fused to provide a permanent finish. Another objective is to provide a novel system for spraying electrostatically charged powders onto hot solid objects where the melting of the powder in a permanent finish is achieved. These and other objectives will be easily achieved with the light of the previous teachings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram illustrating the basic principle of corona loading. Figure 2 is a schematic diagram illustrating the tribo basic load. Figure 3 (a) is a schematic diagram that re-presents an object resting on a plate between a neutral electric field. Figure 3 (b) is a schematic diagram showing an electric field applied between the plates of Figure 3 (a) when raising the top plate to a high voltage where the load on the surface of the object is induced. Figure 4 is a schematic diagram of a charge gun induction showing the nose. Figure 5 is a schematic diagram showing an induction of fluidized bed coating of car-ga. Figure 6 is a schematic diagram showing the apparatus using the inductive / conductive principle used in the present invention. Figure 7 is a schematic diagram with more details of another nose of induction charging gun. DESCRIPTION OF THE INVENTION In a broad aspect, the present invention is directed to an apparatus for coating a substrate with electrically charged resin powder particles which subsequently form a uniform and continuous coating in such a substrate. The apparatus of the present invention consists of, in combination: (cl) an electrically insulating fluidized bed zone for inductively charging resin dust particles, (u) high voltage mechanisms arranged in a portion of the fluidized bed zone and connected to a power source of high voltage, (c) ground electrode mechanisms located in another portion of the fluidized bed zone, where an electric field can be created between the high voltage and the ground electrode mechanisms to inductively load the particles, () mechanisms of fluidized air to introduce air into the fluidized bed zone, thus establishing the presence of the resin powder particles, a powder cloud electrostatically charged within the fluidized zone, c) transport mechanisms for transporting the particles of powders electrostatically charged from the fluidised bed zone, and (i) tight supply mechanisms in the mechanisms and transportation to direct the electrostatically charged particles on the substrate. The use of the apparatus of this invention imparts an electrostatic charge to organic powders to make them useful for powder coating applications. This involves forming a mixture of the powders and at least one electrostatically active modifying agent, and treating the mixture under electrically inductive / conductive conditions sufficient to impart to the powders a resistivity of about 109 to about 1013 ohm.m. percent relative humidity. Noting that the disadvantages as enumerated previously, are due to the electrostatic of the present systems, the present inventors have conducted extensive and exhaustive investigations to develop an apparatus and method that is based on a completely new approach of the powder charge used for the Electrostatic powder coating. As a result, it was found possible to overcome all of the above disadvantages inherent in the powder coating process as was currently practiced in developing a method of loading powder by influence, having first modified the powder by adding an electrostatically active agent to the resin powder. . The present invention is achieved based on this discovery. The present invention provides an apparatus for electrostatically charging a resin powder by means of influence, known as either an induction or conductive charge and depositing the charged powder on a substrate. The term "induction" or "inductive" as used throughout the specification and the accompanying clauses, encompasses both the electrostatic charge of induction or conduction. The resin powder composition comprises (i) a thermoplastic or thermoplastic resin and (ii) an electrostatically active modifying agent incorporated into or in the resin. The modifying agent used is one that does not alter the mixture or the durability characteristics of the resin powder. The modifying agent is also useful in promoting the ease with which the charge is imparted and retained despite the size of the powder particles. Accordingly, the present invention provides an apparatus for electrostatically charging a powder for use in powder coating, free of the aforementioned conventional disadvantages, which allows an electrostatic charge to be efficiently and uniformly developed in the powder in a It is reliable and repeatable and can also accurately and reliably control the quantity and polarity of the developed electrostatic charge (thus the ability to cover all areas of the workpiece, even at a required thickness). . The invention also provides an apparatus for producing a powder with the intention of coating the surface of solid objects (work pieces) for use with the aforementioned method of electrostatic charging. The objects of the present invention are achieved by placing the modified powder in an area where an electric field is present, in such a way as to allow the electric charge to flow in the powder particles, which by means of the modification with an electrostatically active agent, are sufficiently conductive to facilitate electrical conduction. This property of the dust is characterized by its resistivity (surface or mass) and generally ha-soft, the lower the resistivity, the easier it will be to place an electrostatic charge by means of induction. Once loaded, the powder is pneumatically transported to the work piece. The charge in the powder will decay once deposited with the fall speed increasing with increasing resistivity. It is very important that the powder remains attached to the work piece sufficiently so that the workpiece is transported to the setting furnace. If the load decays very quickly, this can not be guaranteed. In this way, there are two requirements: low resistivity for an efficient load and high resistivity for longevity in the adhesion to the work piece. To achieve these contradictory requirements, a number of different measures are proposed. The first involves a resistivity compromise approach where the powder resistivity is modified to a value of between about 109-1013 ohm / meters, and preferably between about 1010-1012 ohm / meters. With these values, the load of approximately 63% of a limit value (which is a function of the particle size, shape and material, as well as the strength of the electric field to which it is exposed) is reached in approximately 0.2 to 2 seconds. Once the load on the work piece drops to ground at 37% of the value at which it was loaded, it occurs in the same time frame, but the period in which the attractive image force operates is sufficiently long to allow the establishment of the adhesion forces, between the particles and the substrate and between the particles themselves, to develop. These forces are sufficient to keep the dust in the work piece long enough for it to be transported to the furnace for permanent fusion. The setting time is usually from about 5 - 10 minutes to about 275 ° - 450 ° F. It should be noted that dust particles with resistivities lower than the lower limit stated above, are not retained in the workpiece or substrate, enough time to establish adhesion, while it is difficult to control the process with a resistivity over the upper limit. A second method involves the spray of charged powder on a hot work piece to ground. The temperature of the work piece is such as to ensure the partial melting of the dust particles as they settle on it, in this way, the adhesion to the work piece is due to the humidity of the piece due to the melted powder and not to electrostatic forces. A third method involves an application, a little different, but not less important, of electrostatic dust spray: the finishing of electrostatic insulating materials such as plastics or ceramics. In this case, the charge of dust and spray is similar to that of the work pieces to ground with conventional conductive finish, but the electrostatic helps to ensure the position and even the coating is achieved in a different way. Since the workpiece is insulating, no image loading is induced as the charged cloud approaches, so that dust is not attracted to the workpiece unless it is preloaded with the polarity opposite to the powder charge. This is achieved by means of crown loading of the work piece, in this way a deposition field is established between the work piece and the dust cloud. The coating will continue until there is no net charge on the workpiece and adhesion is ensured because no charge relaxation can occur from the insulating workpiece. Other methods can be used, some depend on the geometry of the insulating workpiece, eg, in the case where it is a thin sheet or film, the coating on one side may be possible by placing a conductive substrate on the opposite side and placing a voltage over it, opposite in polarity with respect to the charge on the powder. A fourth method involves a key discovery made during the intensive research that led to this invention. The ideal solution for the low resistivity dichotic requirements to achieve an efficient load and high resistivity for an adequate adhesion can be when designing a powder that has a resistivity that is, in the broadest sense, dependent on the situation, this is , a resistivity that is a function of the prevailing conditions in the charging station and in the work piece. By controlling the conditions in both areas, having first designed the powder to be extremely sensitive to changes in the environment in which it is found, it has been found that it is possible to ensure a low resistivity in the charging station and a high resistivity in the the piece of work. By examining the activity of various modifying agents of electrostatic property (hereinafter referred to as modifying agents) as a function of the temperature, moisture content and electric field strength, a family of modifying agents has been identified. When they are added to the powders currently available for powder spraying, they modify the resistivity of the composite powders and make it dependent on the variables mentioned above such as temperature, moisture content and electric field strength. As already indicated previously in this document, the powder resin composition for the electrostatic coating of the present invention consists of a thermal or thermoplastic setting resin and from 0.01% to 20% by weight of a property modifying agent. electrostatics. This composition may also contain a curing agent, a pigment, a metal powder filler, a flow controlling agent, a plasticizer or a stabilizer. In the present invention, the thermal setting resin may be of a conventional type such as an epoxy resin, a polyester resin or an acrylic resin. In the same way, the thermoplastic resin can be a vinyl chloride re-sine, a polyamide resin, a cellulose resin, a polyolefin resin, a polyethylene resin, a polyester resin or a nylon resin. The resin can be used alone or in combination as a mixture. The modifier of the electrostatic property as an essential component of the present invention can be a polyalkylene ether, a polyethylene glycol, a polyethoxylated stearyl alcohol, a quaternary ammonium salt or a halogenated ammonium salt. These compounds can be used alone or in combination as a mixture of two or more. The quaternary ammonium salt includes, for example, 3-lauramidopropyl trimethylammonium methyl sulfate (Cyostat LS, manufactured by the Cyanamid Company) and (Cyostat SN, Cyastat SP, Cyastat 609 manufactured by the same company) and (ATMER of antistatic range of ICI). The resin powder composition used in the apparatus of the present invention can be easily pre-stopped according to a conventional method. For example, the binder resin and the modifying agent can be heated, melted and kneaded by means of a conventional mixing machine such as a single or multiple screw extruder, a Banbury mixer or heating rods, and then cooled and pulverized to obtain a powder. Any method normally used for the preparation of a powder mixture, as any method for mixing a binder resin powder and a powder of an electrostatic property modifying agent. In some cases it may be necessary to form a film on the surface of the binder resin of the electrostatic property modifying agent by means of the application of mechanical energy to the mixture. In this case, the ratio of the diameters of the particles (average volume) needs to be greater than 10: 1, in which the binder resin is the largest particle. The particle size of the resin powder for the coating according to the present invention should preferably be within the range of about 10 to about 250 microns. The resin powder composition for the coating of the present invention may also contain, in addition to the above components, a hardener, a pigment, a metal powder, a filler, a flow control agent, a plasticizer, a stabilizer and others. additives, as the case requires. The resin powder for the coating of the present invention can be applied to substrates made of metals, ceramics, plastics, etc. by means of an apparatus for powder coating that is also discovered.
Various primers can be applied to such substrates, or other diverse pretreatments can be applied to such substrates. Preferred embodiments of the powder coating apparatus of the present invention will now be described, but the invention is not limited to the described configuration. The invention will be more readily understood with reference to the drawings in which Figures 1 and 2 show the prior art processes for powder coating applications. Figure 1 is a schematic diagram showing the basic principle of corona loading while Figure 2 shows the principle of tribo loading. The induction / conduction load depends essentially on the flow of the electrostatic charge on the surface of the object or material to be charged. For this reason, the object or material to be charged can not be a strong electrical insulator. Figure 3 (a) shows this effect by showing a large particle between two parallel electrodes. In the figure, there is no energy applied to the electrodes and, therefore, there is no charge on the particle. In Figure 3 (b), a potential is applied to the electrodes and the electrostatic charge flows from the lower electrode through the surface of the particle and the particle is charged. If the particle were removed from the lower electrode and removed from the system, it would still retain the charge. Now, the particle is charged by induction. The same situation would occur if the polarity of the electrodes were inverted with the lower electrode trans-formed in the high-voltage electrode and the upper one making ground. In this case, the particle would be charged to a positive polarity. If the particle were constructed, not from an electrically conductive or partially conductive material, but from an insulating material such as Teflon, the electrostatic charge of the lower plate would not be able to flow through the particle surface and, therefore, Therefore, the particle would not acquire a charge. It should be noted that the term "induction" can be applied to cases in which the object to be charged is both in contact with the ground electrode or with the high voltage electrode. For greater precision, "induction" is used when the object is in contact with ground and "conduction" when the object is in contact with the high-voltage source. The situation is symmetric and so is the magnitude of the load obtained. The important parameters with respect to the induction / conduction load are the loading and unloading speeds. These are governed by the electrical conductivity of the material. The more resistive a material is, the longer it takes to achieve the maximum load levels. For example, a metal that is highly conductive will acquire charge by induction in a fraction of a mi-crosecond. A composite polymer may require several seconds. An approximate guide with respect to the speed at which a material will acquire or dissipate charge by induction / conduction is given by the following formula: t = e0erp where p is the resistivity of the material in ohm / meters, e0 is the permissiveness of the space free (8.85 x 10 ~ 12), er is the dielectric constant and t is the time taken for the load to reach 63% of its maximum when it is charging (or 37% of its maximum when it is discharging). Both high voltage power supplies and power supply systems are already established technology. The powder induction / conduction charge will be achieved in the load transfer platform, which is one of the key areas of the invention. The exact design will be changed according to use. To illustrate the above, the platform for coating a large and heavy piece transported by a track would not resemble in any way the platform for fuse boxes suspended from a top conveyor. The loading platform can be incorporated in the head of the gun or stream on top of the gun so that the powder is pre-loaded before it is ejected instead of at the point of ejection. Additionally, it is possible to incorporate two charging stages, the first current of the gun so that the pre-filled powder reaches the ejection point; the second the use of a high-voltage electrode in the nozzle of the gun that essentially places the charge on the powder at this point and using the electric field established between the high nozzle and the work piece to ground to assist in the transfer and dust deposit. Figure 3 (a) is a schematic diagram representing an object (2) resting on a plate (3) between a neutral electric field. Figure 3 (b) is a schematic diagram showing an electric field applied between the plates of Figure 3 (a) when elevating the upper plate (4) at a high voltage where the induced load flows to the surface of the object. Figure 4 illustrates an alternative induction coating system. The powder is pneumatically transferred to a high electric field region in the head (5) of the gun where it acquires the charge by induction. The charged powder (6) is transferred to the workpiece (7) by a combination of electric field and ai-re flow. The introduction of a counter electrode can intensify the field at this point and improve the increased intensity load as required. The effect and the need for said electrode can be determined by means of an analysis of the geometry of the field. Figure 5 illustrates an alternative method of coating objects using an induction / conduction loading technique. In this case, the object (2) to be coated is suspended on a fluidized bed (8). The dust in the bed is charged by means of contact with electrodes (4) of high voltage buried in the mass of the powder. The powder coating is transferred to the workpiece by a combination of fluidized air (9) and electrostatic attraction forces. Figure 6 is directed to the inventive concept of the present invention and shows the preferred representation of the basic design for an induction energy charging apparatus. It shows an electrostatic charger of the fluidized bed type and a powder applicator. The powder is continuously fed to a bed or zone (10) electrically isolated from a powder container (not shown) through the port (12). The entire bed can be placed on a vibrating table (14) that helps to release the powder in the bed. The fluidized air (16) is fed below the air distributing plate (18) and the transport air enters the bed near the top in a radial direction from the point (20) placed directly opposite the port (22) of outlet, towards the nozzle (24) that directs the powder towards the substrate (26). An electric field is established through the bed, in which the electrodes are a high voltage electrode (28) supplied by a source (30) of extremely high voltage. The lower electrode is formed by the upper layers of the fluidized powder, in contact with a concrete grating (32), which makes ground. The load is induced on the powder when it enters the bed and once it is transported up and out of bed by means of fluidized air and transport, this charge is retained in the powder until it reaches the workpiece . An electric field created between the high voltage nozzle of the applicator and the work piece to ground aids in the transport and deposition of the charged powder. Another embodiment of the present invention is directed toward a modification of the above-mentioned apparatus, of Figure 6, which is in the form of a manually held spray gun. This apparatus is shown in Figure 7 and consists of, in combination: (a) a deposit of powdered resin particles (not shown), (b) a rolling gun (42) consisting of a barrel through which the particles (38) can be transported towards a substrate (60) and a mechanism for supporting the gun (42), (c) an electrostatic induction charging zone (44) at one end of the barrel, in which the zone consists of two spaced electrodes between which the charged particles can be passed to the outside, one of the electrodes (46) is a high-voltage electrode and the other an electrode (48) making ground, (d) a high-voltage resistor (50), a mechanism (52) of high-voltage wiring placed in the barrel of the gun and connected to the high-voltage source (not shown), (e) mechanisms (52) and (54) to connect the electrode to ground to an external ground source, and (f) a mechanism (40). ) to introduce the fluidized particles (38) of powder into the other end of the barrel. The powder, with an amplitude of resistivity from about 109 to about 103 ohm / meter is transported pneumatically to the gun by means of a flexible hose (40). The powder is transported along the hollow barrel of the gun until it reaches the charging zone by electrostatic induction at the end of the barrel. The charging region is composed of two electrodes, with the high-voltage electrode shown in the lower part of Figure 7. Electrical energy of up to 60kv is applied to the high-voltage electrode by means of a high voltage cable through a high voltage resistor placed inside the barrel of the gun and that is very close to the electrode. The powder, forced to contact one of the electrodes, acquires a charge due to induction before it is expelled from the barrel. The expelled charged powder (56) is transported to the workpiece or substrate (60) by means of a combination of the transport air and the electric field (62) and will hit the substrate. The powder particles will be supported on the surface (58) of the work piece by means of electrostatic forces. The workpiece is then transferred to the curing oven to allow the powder to melt *. Alternatively, the melting of the powder can begin contacting by preheating the substrate. Both the apparatus of Figure 6 and that of Figure 7 can be manufactured from materials that will be evident to those skilled in the art after knowledge of the present invention. It should also be noted that the apparatus of Figures 6 and 7 can be employed to ensure that all particles are fully charged before being deposited on the substrate.
The invention will be explained more fully by considering the following examples: EXAMPLE 1 Powder Modification Step Evlast 1000 / 1W104, a white powder made of polyester resin manufactured by EVTECH Co. of North Carolina, USA, was used. UU , and commercially available in this test example. It was determined that the resistivity of the powder under a relative humidity of 20% was 1.5 x 1015 ohm / meters. Resistivity was measured using a powder resistivity measuring cell developed by the Electa-trostatics olfson of the University of Southampton, United Kingdom. One kilogram of this powder was mixed with 2% by weight of the Cyostat LS agent. The mixture was melted, extruded, cooled and ground to a fine powder. The resulting powder was also sieved and the portion passing the 150 μm was used in this test example. It was determined that the resistivity of the test powder under a relative humidity of 20% was 1 x 1011 ohm / meters. It was determined that the average volume diameter of the test powder was 40 microns. A feed of 4 g / min "1 of the test powder was supplied to an apparatus similar to that shown in Figure 6. Once there was a sufficient supply of powder present in the bed, the fluidized air and air supplies were opened. and adjusted for stable state conditions, that is, the dust that left the bed through the nozzle is exactly the same as it entered the feed, once these conditions were met, a voltage was applied from 20 kV to the upper electrode The space between the upper electrode and the ground plate was 10 cm, so a minimum electric field of 2 kV cm "was established through the bed. A conductive plate was placed as white (test workpiece) of approximately 100 cm2, directly in front of the nozzle and at a distance of 30 cm. The test plate was ground using an electrometer that was able to measure the amount of charge flowing to the plate. Dust was collected on the plate for 20 seconds, beginning 5 seconds after the voltage was applied. At this time, 1.1 g of powder was collected on the plate, to which 9.4 x 10"8 load Coulombs had flowed, indicating that a load of almost 1 x 10" 4 Coulombs per kilogram had been applied to the powder by of a load by induction. Such specific levels of loading are sufficient to achieve a good adhesion of the powder. All the powder adhered to the plate at least 2 minutes after the spray had finished.
Example 2 Scotchkote 213, a melt bonded epoxy resin manufactured and commercially supplied by 3M of Minnesota, USA, was used in this test example. UU .. One kilogram of this powder was mixed dry with 20 g of antistat. The powders were mixed together in an aring mixer until a homogeneous mixture was obtained. Before and after the modification, it was determined that the re-sistivity of the binder resin and the composite powder was 3 x 1014 ohm / meters and 1.2 x 109 ohm / meters, respectively under a relative humidity of 20%. The average volume diameter of the test powder measured was 25 μm. A feed of 3 g / min "1 of the test powder was supplied to the apparatus in a manner similar to that of Example 1. Again, a voltage of 20 kV was applied to the upper electrode to achieve stable state conditions. White plate was heated to a surface temperature of 115 ° C and powder was sprayed onto the plate for 30 seconds.At this time, 1.35 g of powder was transferred to the plate and a load of 5.5 x 10"7 Coulombs flowed towards the plate. All the powder adhered to the plate and the layer in contact with it was fused. In addition to replacing conventional powder coating systems, the present invention also has applications in other industrial areas that use the coating. As long as the material to be applied can be loaded by means of an induction / conduction process and that the flow characteristics of the material are adequate, the use of induction / conduction as a charging method has advantages in a large number of industrial applications. For example, there is a great interest in applying good quality coatings to electrically insulating materials. An example of the above is the application of decorative coating on glass, such as bottles. There is in fact a problem inherent in achieving the above with conventional electrostatic systems since the corona discharge on the conventional coating equipment produces a high proportion of free ions that charge the surface to be coated to the same polarity as that of the applied material. Since the surface to be coated is electrically insulating, the load can not coat and quickly repels the approaching particles which results in low transfer efficiency and low quality coatings. In the case of a powder loaded by induction / conduction, no free ions are produced and therefore this problem does not arise. There are also several specific industries in which the use of a powder loaded by induction / conduction prior to the application of an object or surface can be advantageous. The application of a coating of good quality to insulators, anti-corrosive coating of pipes and receivers, internal coating of light bulbs, frosting of glass and decorative coatings on wooden or plastic furniture can be achieved with the practice of this invention . It is also known that popular flavorings such as chili or cheese and onion on the snacks distributed by means of packages are currently applied in the form of powder in a relatively coarse manner that is inefficient and allows waste. Many food products have a resistivity of 106 - 1013 ohm / meters which makes them ideal candidates for an electrostatic charge by induction. Also, snacks on which powdered flavorings are applied are often imperfect electrical conductors and this reinforces the advantages of inductive loading due to the absence of free ions. Although the invention has been illustrated by the preceding examples, it should not be considered limited to the materials employed therein, but rather the invention relates to the generic area as disclosed hereinbefore. Several modifications and in-corporations can be made without departing from the spirit and alleance of it.