MXPA96003005A - Triboelectric coating powder and process to cover mat substrates - Google Patents

Abstract

An electrically chargeable dielectric coating powder and a triboelectric coating process for applying the powder to wood substrates are described. The coating powder is a mass of a finely divided thermally fusible dielectric plastic material having an average particle size (Mv) of between 30 and 45 microns, and a particle size distribution (all the percentages defined are in percent by weight). weight) of: from 95 percent to 100 percent less than 88 microns, from 5 percent to 15 percent less than 15.56 microns, and from 0 percent to 6 percent less than 11 microns. Preferably the coating powder has 0 percent greater than 88 microns and an Mv = of about 30-40, preferably 35-40 microns. Preferably, the particle distribution further includes: from 10 percent to 15 percent less than 15.56 microns, and 4 percent to 6 percent less than 11 microns, and an Mv of between about 35 and 36 microns. Particularly preferably, the particle distribution further includes: about 11.5 percent less than 15.56 microns, and about 4.3 percent less than 11 microns, and an Mv of about 35.9. Preferably a powder in a thermosetting resin composition

Description

"TRIBOELECTRIC COATING POWDER AND PROCESS TO COVER WOOD SUBSTRATES" BACKGROUND OF THE INVENTION REFERENCES The present application is a continuation in part of our copending application Serial Number 08 / 169,793 filed on December 20, 1993, which is in turn a continuation of our application serial number 07 / 837,459, filed on February 14, 1992 , abandoned now.

FIELD OF THE INVENTION This invention relates to an electrostatically chargeable dielectric coating powder and a triboelectric coating process for application of the powder to wood substrates. More particularly, the invention relates to a mass of a finely divided thermally fusible insulation resin or plastic material, having a specific average particle size and a specific particle size distribution, and a triboelectric coating process for coating substrates. of wood using this pulverized material by means of which the overrun of dust and the restoration are minimized. This invention relates in particular to the wood triboelectric coating with a coating powder and curing at a low temperature which results in a continuous protective decorative coating having a minimum number of pitting.

DESCRIPTION OF THE PREVIOUS TECHNIQUE A number of solvent-free type coating paint systems have been developed wherein a finely divided, thermally fusible material is deposited on the substrate whose deposit is then melted into a continuous functional or decorative film on the substrate. Representative of these types of processes are flame spraying, fluidized bed, hot plating, electrostatic spraying (ESP) and electrostatic fluidized bed (ESFB). The ESFB is a hybrid of a fluidized bed and ESP, as explained, for example in U.S. Patent Number 4,689,241. Flame spray and fluidized bed coating processes typically apply the thermoplastic resin powders as coatings on metal or other substrates. These techniques, however, proved to have several major disadvantages with one being the general inability to coat at a much lower film thickness of about 250 microns and the other being the need to have on hand a relatively large deposit of each used powder. Subsequently, thermosettable powders, for example epoxides, were used and found to be superior to the thermoplastic resin powders first used in ESP processes. The inherent advantages of thermosetting resin systems in relation to thermoplastics in a powder coating, especially ESP, are well known. The continuous development of ESP technology that provided the necessary discovery to make powder coating an economical alternative to conventional liquid coating. In the typical ESP process, the coating powder is kept in a fluidized bed tank, injected into a stream of air and raised to a spray gun where the powder is charged by passing through a corona discharge stable created by a high voltage source. The charged powder is then transported to a part or substrate to be coated through a combination of electrostatic and aerodynamic forces. The powder is projected onto the substrate so that the aerodynamic forces bring the dust particles as close as possible to the substrate, where the electrostatic forces predominate and cause the particles to be attracted to and deposited on the rectified substrate. The coated substrate is then placed in an oven where the individual powder particles melt, flow and form a continuous film on the substrate. There are several aspects of the process involved in ESP, among which are the dust load, powder transport, adhesion mechanisms, self-limitation, ionization and Faraday cage effect. During spraying, charged dust particles tend to run along the electric field lines. As the thickness of the deposited powder layer grows, the voltage on the surface of the powder will increase so that the charged particles tend to deposit in regions of lesser thickness having a lower surface voltage. The thickness of the powder layer therefore tends to increase in uniformity at least on the flat surfaces, and therefore, is inherently self-limiting or self-regulating. However, as each successive batch of charge particles and free ions generated by the high voltage corona discharge approach the deposited powder layer, the point where the charge on the layer exceeds its dielectric strength is reached. and a counter-ionization occurs. At this point, any dust is rejected and the dust that adheres loosely on the surface falls off. Also, the charged particles can not easily penetrate the corners, recesses and other hard-to-reach areas of a complicated substrate, for example, a two-sided can body because the electric field lines can not penetrate them due to to the protection or effect of the surrounding conductive body. This is called the Faraday cage effect which states that any empty space (free of charge) enclosed within a conductor is free from any field. As a general rule, an electric field will not penetrate this cavity beyond a depth equal to the radius of the opening of the cavity. The electric field lines will be concentrated in the shoulders (or ridges) of the cavity typically from the front edge facing the spray gun. Some imperfections in the final coating may be the direct result of defects in the powder layer (e.g., of the counterionization and Faraday cage effects) before melting to the powder layer.

The self-limiting effect of ESP has significant implications. For example, non-skilled operators are capable of spraying at least flat substrates with only a brief instruction and training since it is virtually impossible to create slips, drips or warps that are characteristic of liquid finishes applied by spraying. It also makes the relatively easy and practical design of automatic spraying installations possible. For example, multiple electrostatic spray guns can be mounted in reciprocators and placed in staggered opposition to each other in a closed spray booth. The parts to be coated then move between two groups of spray guns where a uniform powder coating is applied. And since the applied plate is self-limiting, a sufficient amount of powder can be loaded and applied to ensure that there are no unduly thin or uncoated areas. The overrun powder is captured in the recovery system and reused. Compared to flame spraying and fluidized bed coating, some main advantages of ESP are that generally thinner films in the order of 75 microns or less can be applied consistently, that it is used in smaller quantities of powder and that they can be coated more evenly on substrates in a more complicated manner. Consequently, ESP has become a previously established technology in which coatings, both thick and thin, can be consistently and uniformly applied to substrates, both in flat surface items and in certain of the complicated or intricate configurations. ESP has proven to be suitable for applying a wide variety of these insulating and dielectric coatings such as plasticized PVC, nylon, cellulose, epoxy, polyurethane, polyester, acrylic resin and hybrid powders on a wide variety of conductive substrates especially metal articles such such as can bodies, wire articles, pipe, tool housings, fire extinguisher bodies, household appliances, floor polishing machinery, sewing machine parts, hospital beds, trailer hooks, auto parts and accessories, motorcycles and biciclestas, furniture for lawns, gardens, office and home, and structural sections and façade elements. The coating of wood dust has been discussed considerably in commerce and literature, but little has been said about how it can be achieved. As Douglas S. Richart stated in his article published in April 1996 on the number of POWDER COATINGS, the coating of wood with low temperature curing powder is almost impossible because the coating must be cured at a temperature less than 83 ° C and the resin should have a flow temperature of approximately 10 ° to 20 ° lower than that. This resin tends to block during storage at normal temperatures. Richard goes on to say that the curing agent must be sufficiently reactive so that the powder will cure in a reasonable amount of time at these low temperatures. But that leads to a possible thermosetting of the resin in the extrusion apparatus. It talks about electrostatically spraying a powder that has an isocyanate blocked to the wood, heating the coating in infrared ovens and other types of furnaces to form a smooth coating and cure the smooth coating with ultraviolet radiation. In its technical bulletins, Boise Cascade shows the use of manual electrostatic spray guns on the lining of its compressed electrically conductive wood board. Resin powders are typically produced by melt mixing techniques for example by homogeneously mixing basic ingredients such as resin, a curing agent, a flow agent, pigments, fillers or fillers and other additives, feeding the mixture to the high shear mixer, for example, an extrusion apparatus wherein the melt-able ingredients melt at a temperature of about 90 ° C at 150 ° C, depending on the type of resin, and the non-meltable ingredients are intermixed with the molten material, for example, by the augers of the extrusion apparatus, feeding the material leaving the mixer usually in the form of a continuous ribbon to a system for example of distribution cylinders and a conveyor where the extruded material is cooled with water to room temperature; crush the extruded material completely cooled in small chips; and then granulate and grind the chips into small particles which are then usually sieved, for example, through a 140 micron sieve to obtain a homogeneous powder. Larger particles are typically recovered and recycled through the mill. A considerable disadvantage, however, of ESP using a corona discharge gun is that a high voltage field is established between the gun and the piece to be coated. The substrates or complicated pieces that have deep angles, recesses, etc. they are very difficult to coat mainly due to the aforementioned Faraday cage effect. The indentations, internal edges, re-entrant corners, beams and overlapping surfaces, for example by linked pieces are typical of the problem areas that need coating. Due to this general inability to effectively coat these areas, the overrun powder captured in the recovery system has become a major problem in the industry which has led to the development of several solutions including new and new corona discharge gun designs. improved as well as more efficient recovery systems. The Faraday cage or the electrostatic effects alone can not explain all the difficulties encountered in problem areas of coating. For example, it has been found that aerodynamic effects are the least partially responsible in some situations especially when there are restricted areas. Air or gas driven dust often poorly deposited in these areas initially, is repeatedly blown in this manner by aerodynamic effects (eg, venturi effects) because satisfactory coating of these areas is often almost impossible.

It has been found that the mentioned difficulties can only be overcome by taking into account the electrostatic and aerodynamic laws. Only a very precise equalization of all the electrostatic and aerodynamic parameters can result in any fundamental improvement in the ESP corona system as well as the equipment, mainly due to the intrinsic electrostatic field between the corona effect and the article to be coated. Another approach has been the use of a triboelectric gun instead of the corona discharge gun in the aforementioned ESP system. With this type of triboelectric gun, as exemplified in US Patents Nos. 3,903,321 and 4,071,192, the loading of the powder occurs by frictional contact of the dust particles carried in the air with the inner tubular surfaces of the gun, depending on the phenomenon of the electrical charge that occurs when two dielectric or insulation materials (ie, powder and gun surfaces) cause them to rub against each other. In theory, the effect that can be achieved depends on the dielectric constant of the materials used. As disclosed by R.P. Lehman in a 1988 article called "Optimized Powder Coating of Critical Objects", an exemplary list of these materials in descending order of their relative dielectric constants from the positive end to the negative end is as follows: Dielectric Series Positive end 24 Polyethylene oxide 23 Polyurethane 22 Plexiglass 21 Epoxy resin 20 Polyvinyl acetate 19 Glass 18 Urea / formaldehyde 17 Wood 16 Polyamide (Nylon) 15 Polyvinyl alcohol 14 Cellulose 13 Metals 12 Rubber 11 Cellulose acetate 10 Polyester resins 9 Polystyrene 8 Anthracene 7 Silicone 6 Nitrocellulose 5 Polyacrylonitrile 4 PVC 3 Polypropylene 2 Polyethylene 1 Polytetrafluoroethylene (PTFE) Negative end To achieve the maximum load capacity, it is believed that the two materials must differ considerably in their electronegativity. It is easy to see from the above list why pure PTFE has generally become the material selected in the industry for the gun rubbing material. It can also be seen, for example, that the epoxy powder is much more suitable than the polyester powder for loading into a gun using PTFE. However, other materials such as nylon have been used successfully as the flotation material when the selected coating powder, such as polypropylene, is too close to the PTFE in the dialectic series. The exemplary list of 24 materials mentioned above is pure resins or pure materials. However, the coating powders do not normally contain only resins, but, for example, up to 50 weight percent filler or filler materials, pigments and other additives that may influence the carrying capacity. All powder coating systems do not lend themselves easily to tribocharging. Due to the low polarity, several resin systems suffer from insufficient load capacity, for example, carboxy-functional polyesters. Attempts have been made to solve this problem through the addition of special additives, eg, amines, in order to obtain the required loading capacity by stabilizing the positive charges. See, for example, European Patent Number 371,528. However, these additives have disadvantages, for example, they can be separated from the powder in the recovery recycling process by having a negative influence on the powder coating properties, etc. Amines, for example, are also catalysts for undesired chemical reactions (e.g., in the pre-mix extrusion apparatus) and can detrimentally affect the curing behavior of the powder paint. An explanation of the triboelectric coating system suggests that the electrons are separated from the dust particles that are positively charged when leaving the PTFE gun and, therefore, they are attracted to the substrate that is connected to the ground. The tribo-gun also usually has an electro (grounded) to remove the equal and opposite charge that accumulates in the tubular body of the gun. See for example, the Journal of Electrostatics, 16, 1985, pages 277 to 286, and the SME Technical Document (FC-89-626) by J. Dailidas from October 16 to 19, 1989, 12 pages, for additional details related with the principles of triboelectronics. Using a tribo-gun, the high voltage and the intense electrostatic field between the spray gun and the article (characteristic of the corona gun) can be dispensed with and the Faraday cage and the effects of counterionization are reduced, therefore , the penetration of the powder into the problem areas of complicated configurations as mentioned above, are greatly facilitated. This replenishment of the corona gun with a tribo-gun reduces the penetration problem generally since one has to control the aerodynamic conditions. Some of the remaining variables that influence the efficiency of triboelectric coating systems are contact time, conductivity, temperature, humidity and particle size. It is known that variations in average particle size (Mv) and particle size distribution (Mx) have important implications in the operation of electrostatic powder coating systems. In general, most electrostatic powders have an Mx of approximately 150 to 200 microns to sub-micron levels, through decreases in Mv within the range of approximately 20 to 50 microns. It is now common practice to remove the coarser particles, for example, by being screened since they are known to have a detrimental effect on the final coating eg reduced smoothness. It is also generally known that ends of less than about 10 microns are detrimental to an efficient coating and can be removed, for example, by air sorting. For more specific details on the various aspects of the prior art, see Polym.-Plast. Technol. Eng., 7 (2), 1976, pages 119 to 220; Kirk-Othmer: Encyclopedia of Chemical Technology, vol. 19, third edition, 1982, pages 1 to 7; Paint and Resin, October 1989, pages 8 and 10; Products Finishing Magazine, January 1990, pages 1 to 8; "The Particle Size Distribution of Powder Coating", by K. Swamborn presented May 15 and 16, 1990, seminar, 24 pages, Hosokawa MicroPul; Proceedings of the Eighteenth Water-borne, Higher-Solids, and Powder Coatings Symposium, February 6 to 8, 1991, "Powder Coating-A Brief Review of the Technology" by D. Richart, pages 191 to 211; Particle News, Publication # 2, 2 pages, British Rema Manuf. Co. Ltd. and U.S. Patent Nos. 3,822,240, 4,027,066, 4,056,653, 4,109,027, 4,113,681, 4,154,871 and 4,172,776. A system and a two-component powder coating process for triboelectric wood siding is described in a co-pending application from the concessionaire called "Two Component Powder Coating System and Method for Coating Wood Therewith" (Correll et al.); serial number (Attorney's Note Number 3209-05-00), filed on May 6, 1996. Additional prior art examples dealing with electrostatic thermosetting coating powders are illustrated by samples J, K and N in the Table 7 that will be presented next.

SUMMARY OF THE INVENTION The main object of the present invention is to electrostatically spray a plastic coating powder through triboelectric coating processes onto wood substrates in an efficient manner in order to minimize over-tress and recovery efforts. A related object of this invention is to provide a low temperature method for producing a protective or decorative continuous coating on wood having a minimum number of pitting. Briefly, these and other objects of this invention are achieved by limiting the amount of coarse and fine particles in the dielectric coating powder so that a mass thereof has a total average particle size (Mv) between about 30 and 45 microns and , critically, have a particle size distribution (Mx) of: from 0 percent to 5 percent greater than 88 microns (= 95 - 100 percent <88 microns), from 85 percent to 95 percent greater than 15.56 microns (= 5 - 15 percent <15.56 microns) and 94 percent to 100 percent greater than 11 microns (= 0 - 6 percent <11 microns). Preferably the mass of the coating powder has an Mx as defined above with 0 percent greater than 88 microns (= 100 percent <88 microns) and an Mv equal to about 30 to 40, preferably about 35 to 40 microns. Especially preferably, the powder mass has an Mx of: O percent greater than 88 microns (= 100 percent <88 microns) and from 85 percent to 90 percent greater than 15.56 microns (= 10 to 15 microns) percent <15.56 microns) and 94 percent to 96 percent greater than 11 microns (= 4 to 6 percent <11 microns) and an Mv equal to approximately 35 - 36 microns. Especially preferably, the powder mass has an Mx of: 0 percent greater than 88 microns (= 100 percent <88 microns). of approximately 88.5 percent greater than 15.56 microns (= 11.5 percent <15.56 microns) and approximately 95.7 percent greater than 11 microns (= 4.3 percent <11 microns) and an Mv of approximately 35.9. Preferably, the powder mass is composed of a formulation based on thermosetting resin. In accordance with a further aspect of the invention, the improvement is also extended to an ESP process for coating substrates, wherein the previously defined mass of the dielectric insulation powder is supplied or charged in a triboelectric spraying gun.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 graphically illustrates the transfer efficiency by plotting the percentage of transfer efficiency versus the percentage by weight of particles smaller than 15.56 microns supplied to the gun. Figure 2 illustrates in graphic form the current of the gun by plotting the current of the gun in microamperes versus percentage by weight of the particles smaller than 15.56 microns supplied to the gun. Figure 3 graphically illustrates the load to the mass by plotting the dust load on the mass of the sprayed powder (C / g) versus the percentage by weight of particles smaller than 15.56 microns supplied to the gun. Figure 4 illustrates in graphic form a commercial execution of multiple powder samples 10-7272 plotting the average value versus the number of coated pieces.

DETAILED DESCRIPTION OF THE INVENTION A more complete understanding of the invention will become apparent from the detailed description given below of the preferred embodiments of the triboelectric coating process and electrostatic coating powder using this powder to coat wood substrates. The critical role of the particle size variable in the tribocharging system has been investigated deeply in an effort to reduce the load and coat variabilities. The coating powders of the invention, therefore, have been designed and formulated specifically for tribocharging, which is a known modification of conventional ESP coating systems and procedures as indicated in the background information. The coating powders applicable under this invention can be any of the aforementioned materials with respect to the corona discharge technique, preferably the thermosettable base powders, including epoxies, acrylics, polyesters, polyurethanes and hybrids thereof. The coating powder of this invention can be used in metal substrates but its specific utility for coating wood makes it highly attractive as an alternately commercially viable for processes such as lamination films and coating liquids that have been used almost universally in the past. For the purposes of this invention, wood is defined as any lignocellulosic material whether it comes from trees or other plants, whether it is in its natural forms or its fibers have been separated, flocked and compressed to form a hardwood , table of fibers of medium density or similar. The particle board, whether normal or treated to improve its electrical conductivity and a oriented board are also within the definition of wood for this invention. Wood having a moisture content of 3 percent to 10 percent by weight is appropriate for the purposes of this invention. As mentioned above, tribocharging offers well-documented advantages in relation to corona discharge since tribo-pistols do not require a high voltage supply, obtain better penetration of Faraday cages, create less back-ionization, achieve a wider scale of minimum and especially maximum coating thicknesses, produce smoother and more continuous films and achieve a more compatible overall performance, especially in complicated configuration facing articles. The improvement focuses on a modification of the particle size that has led to unexpected results in total triboelectric coating efficiency. The Mv and Mx are measured through this text using a MICROTRAC analyzer <; R) from Leeds & Northrop, in dry mode. In one embodiment of the invention, a thermosetting starting powder, CORVE 10-7199 was prepared by the aforementioned conventional technique of stirring the melt and grinding a cooled extruded material. (CORVEL is a registered trademark of the dealership). The resulting particle size distribution (Mx) of an E Sample of 10-7199 as noted in Table 1 below is typical of powders that can be obtained commercially for electrostatic spraying application through, for example, of a Corona discharge ESP equipment. Based on the greater knowledge to date of tribocharge applications and the trade lie, it was first believed that a thicker distribution would improve 10-7199. See Sample F in Table 2 presented below. Even though Sample F provided some improvement, it is generally unacceptable because the thicker distribution caused large swings in performance, since the recovered powder was recycled and mixed with the virgin powder. Also, the unacceptable amounts of the overcoating powder tended to be increased or accumulated in the coating system. Sample E was then further modified by air classification (A / C) resulting in a more critical total distribution. See Samples A, B, C and D of the Table. The resulting dusts from Samples A - D showed a marked improvement in transfer efficiency (see Table 3 below), the Gun Current (see Table 4 below) and the relationship Load to Mass (see Table 5 below), which are three of the measures that are used to more effectively evolve the tribocharging capacity. Quite surprisingly, an optimal point was found within the Mx scale beyond which additional classification did not yield additional benefits. See Figures 1, 2 and 3. The optimum is significant not only from a dust operation point of view but also reduces the potential economic penalties that result from fines removed by re-working. That is, the fines removed by air classification to produce the desired Mx, are typically extruded and re-crushed to reduce waste. Even though this improves the efficiency of the classification, it is an additional step with an associated cost. Therefore, it is essential to minimize the amount of fines that are removed to reach the desired performance characteristics. Figure 1 graphically illustrates the Transfer Efficiency by plotting the percentage of transfer efficiency, E-j- (in the ordinate) versus the percentage by weight of the particles smaller than 15.56 microns supplied to the tribo-gun (in the abscissa). The Et value for a given powder sample or lot is derived by obtaining the weight (D) of the sprayed powder that adheres to a substrate, dividing D by the total weight (W) of the powder supplied by the gun between 100.

Therefore, the formula to calculate the percentage is E- | - = D / W x 100. The values plotted for E-i- typically are an average of three readings per sample. Figure 2 graphically illustrates the Gun Current by plotting the gun current (on the ordinate) versus the percentage by weight of the particles smaller than 15.56 microns that are supplied to the tribo-gun. The value of the gun current for a given powder sample is the current produced in the tribo-gun that is typically measured by a Light Emitting Diode microammeter in the ground circuit for the gun. Typically after starting the operation of the gun charged with powder, the amperage readings are repeated three times at intervals of about one minute. Figure 3 graphically illustrates the Load to Mass by plotting the ratio of the electrical charge in the mass of the powder that is tribo-charged (in the ordinate) versus the percentage by weight of the particles smaller than 15.56 microns supplied by the gun. The ratio of the electric charge in the powder mass (Mc) is defined as a unit of the electric caraga expressed in Coulombs (C) divided by the weight (g) in grams of the powder supplied to the gun; therefore Mc = C / g. The charge value to the mass for a given powder sample is typically obtained by meter readings using a Wolfson electrostatic powder coating test kit. The plotted values are typically an average of three readings for a given sample. In addition, dust classified with air produced in this manner (CORVEL 10-7272) rendered an improved performance compatible in a commercial triboelectric coating system as shown in Figure 4, which graphically illustrates the mean value (in the ordinate) versus the number of coated pieces (in thousands). The Mean Value represents the particle size by means of microns of the powder 10-7272 supplied to the gun through a conventional fluidized bed system including any recovery of recycled. Various different preparations (of the same composition) of the samples or batches of 10-7272 were tested and plotted in Figure 4. The initial samples were all virgins 10-7272 within the distribution scale: from 0 percent to 5 percent greater than 88 microns, from 88 percent to 95 percent greater than 15.56 microns, and from 94 percent to 100 percent greater than 11 microns and an Mv of 30 - 40 microns.

TABLE 1 PARTICLE SIZE SAMPLE A * B O * (material PERCENTAGE CORVEL RETAINED 10--7199 124. 45u 0.0 0.0 0.0 0.0 0.0 88. 00u 0.0 0.0 0.0 0.0 5.4 62. 23u 11.4 8.0 9.3 5.1 11.6 44. 00U 37.1 29.5 30.1 22.9 22.6 31. 11u 65.8 56.5 54.3 45.0 41.4 22. 00u 82.9 75.7 72.6 64.4 55.0 . 56u 91.9 88.5 85.7 78.7 66.7 ll.OOu 97.0 95.7 94.3 89.9 76.8 7. 78u 98.8 98.4 98.1 95.8 84.1 . 50u 100.0 100.0 100.0 96.7 89.3 3. 89u 100.0 100.0 100.0 100.0 94.3 2. 75u 100.0 100.0 100.0 100.0 97.9 1. 94u 100.0 100.0 100.0 100.0 100.0 Average size, Mv 39.6u 35.9u 35.5u 31.2u 31.4u WIDTH OF DISTRIBUTION (Analyzer Channel Reports) 7 7 7 8 11 % < 15.56; 8.07 11.50 14.30 21.30 33.30 SAMPLE G H ** L M *** (material (PERCENTAGE CORVEL CORVEL RETAINED 10-7223) 10-7068) 124. 45u 0.0 0.0 0.0 0.0 88.00u 2.59 2.37 0.0 0.58 62.23u 17.62 22.22 19.7 18.78 44.00u 34.38 44.68 40.07 39.30 31.11u 49.47 63.75 55.44 58.57 22. 00u 61.79 80.17 71.90 76.27 . 56u 72.59 91.57 81.12 89.01 ll.OOu 82.21 98.11 90.58 97.18 7. 78u 88.23 100.00 100.00 100.00 . 50u 92.87 100.00 100.00 100.00 3. 89u 96.64 100.00 100.00 100.00 2. 75u 98.70 100.00 100.00 100.00 1. 94u 100.00 100.00 100.00 100.00 Average size, Mv 35.80u 43.41U 38.95U 40.24U WIDTH OF DISTRIBUTION (Analyzer Channel Reports 12 7 8 8 % < 15.56? U 27.41 8.43 18.88 10.99 * CORVEL 10-7272 (A / C 10-7199) ** A / C CORVEL 10-7223 *** A / C CORVED 10-7068 TABLE 2 PARTICLE SIZE PERCENTAGE RETAINED TO SAMPLE F * 248.90u 1.2 176. OOu 4.5 124.45u 13.1 88. OOu 25.1 62.23u 42.9 44. OOu 59.5 31.11u 69.4 22. OOu 78.9 15.56u 87.2 11. OOu 91.1 7.78u 94.2 5.50 u 96.7 3.89u 98.5 2.75u 100.0 1.9u 100.00 AVERAGE SIZE, Mv 66.4 DISTRIBUTION WIDTH (Analyzer Channel Report) 13% < 15.56u 12.8 * CORVEL thickness 10-7199 TABLE 3 EFFICIENCY OF TRANSFER SAMPLE A B C D E (Percentage) 32.0 32.0 28.3 23.7 18.9 G H I L M 24.71 31.55 38.39 24.46 27.55 TABLE 4 PISTOL CURRENT SAMPLE A B C D E (Microamperes) 1.0 3.25 2.25 2.0 0.5 G H I L M 0.5 2.00 2.00 1.00 2.25 TABLE 5 LOAD TO MASS SAMPLE A B C D E (x 10-7 10.2 11.2 12.0 8.3 2.5 Coulombs / GHILM gram) 8.30 10.79 7.79 6.3 12.2 In another embodiment of the invention, the Mx of another thermosettable starting material or powder commercially available, CORVEL 10-7223 (which was prepared in a similar manner as material 10-7199) points to Sample G in Table 1. And similarly, to Sample F coarse in Table 2, a coarse Sample I of 10-7223 was prepared; see Table 6 below. Even though it shows some improvement, Sample I was generally unacceptable due to similar reasons that are manifested with respect to Sample F. Sample G of material was then further modified to form Sample H, Table 1, by air classification resulting in a more critical total Mx. The powder of Sample H resulting in a similar manner to Samples A, B and C of 10-7272, showed a marked improvement in Transfer Efficiency, Gun Current and Mass Load. See Tables 3, 4 and 5, respectively.

TABLE 6 PARTICLE SIZE PERCENTAGE RETAINED TO SAMPLE I * 248. 90u 0.0 176. OOu 0.0 124.45u 1.67 88. OOu 13.22 62.23u 36.72 44. OOu 63.21 31.Hu 83.15 22. OOu 93.90 15.56u 98.40 11. OOu 100.00 7. 78U 100.00 . 50u 100.00 3. 89u 100.00 2. 75u 100.00 1. 94u 100.00 AVERAGE SIZE, Mv 57.04 DISTRIBUTION WIDTH (Analyzer Channel Reports) 8% > 15.56 98.40 % > 11.00 100.00 % > 88.00 13.22 * CORVEL coarse 10-7223 In still another embodiment of the invention, the Mx of another thermosettable starting powder or commercially available material, CORVEL 10-7068, which was prepared in a similar manner to material 10-7199 and 10-7223, is pointed out as Sample L in Table 1. Sample L of material was then further modified to form Sample M Table 1, by air classification resulting in a more critical overall Mx. The resulting Sample M powder, similar to those of Samples A, B, C and H, showed a marked improvement in Transfer Efficiency, Gun Current and Mass Load. See, respectively, Tables 3, 4 and 5. Accordingly, based on the specific embodiments of the invention; namely Samples A, B, C, H and M, the novel mass of electrically chargeable plastic powder that is tribocharged widely has a particle size distribution (Mx) of (all percentages defined are in percent by weight): 0 percent to 5 percent greater than 88 microns (= 95 - 100 percent <88 microns), from 85 percent to 95 percent greater than 15.56 microns (= 5 - 15 percent <15.56 microns) and from 94 percent to 100 percent greater than 11 microns (= 0 - 6 percent <11 microns), and an average particle size (Mv) of between about 30 and about 45 microns. Preferably, the mass of the dielectric coating powder according to the invention (Samples A, B and C) has an Mx as defined above with 0 percent greater than 88 microns (= 100 percent < 88 microns) and an Mv of about 30 to about 40 microns preferably about 35 to 40 microns. More preferably, the mass of the dielectric powder of the invention (Samples B and C) has an Mx of: 0 percent greater than 88 microns, 85 percent to 90 percent greater than 15.56 microns (= 10 - 15 per cent <; 15.56 microns) and 94 percent to 96 percent greater than 11 microns (= 4 - 6 percent <11 microns), and an Mv of approximately 35 to approximately 36 microns. Especially preferably, the mass of dielectric powder has an Mx as shown in Sample B of: 0 percent greater than 88 microns, approximately 88.5 percent greater than 15.56 microns (= approximately 11.5 percent <15.56 microns ) and approximately 95.7 percent greater than 11 microns (= approximately 4.3 percent <11 microns), and an Mv of approximately 35.9. Preferably, the powder mass according to the invention is a formulation based on a thermosetting resin. For comparison purposes, Samples J, K and N of the thermosettable paint powders of the prior art are indicated in Table 7, which is presented below. The particle size of Samples J and K was too thick, that is, the percentage retained was greater than 88 microns which is too high and was expected to function in almost the same unacceptable way as that of Samples F and I coarse previous The particle size of Sample N is too low in the retained percentage of both 15.56 and 11 microns; therefore, Sample N was expected to function in an unacceptable manner like the previous similar D Sample.

TABLE 7 (PREVIOUS TECHNIQUE) PARTICLE SIZE SAMPLE J * K ** M * * * PERCENT ^ JE RETAINED TO 124.45u 0.0 4.64 0.0 88. OOu 8.44 21.12 0.0 62.23u 27.48 46.28 6.42 44. OOu 51.27 68.74 26.80 31. Hu 70.21 83.27 46.05 22. OOu 82.05 91.53 64.19 . 56u 90.87 96.46 77.83 11. OOu 95.97 99.02 87.47 7. 78u 98.45 100.00 92.19 . 50u 100.00 100.00 94.60 3. 89u 100.00 100.00 100.00 2. 75u 100.00 100.00 100.00 1. 94u 100.00 100.00 100.00 AVERAGE SIZE Mv 48.Hu 63.36u 31.91u DISTRIBUTION WIDTH (Analyzer Channel Report) 9 9 % < 15.56u 9.13 3.54 22.17 * Becker ER-7493 SG ** Tiger White Base *** Lilly 907 B-S The triboelectric coating process of the present invention is particularly effective in situations such as when shaping the wood substrate. The grooves and ridges present a specific problem for electrostatic coating process is due to the Faraday effect. This is because the electric charge generated by friction as the powder flows along the TEFLON plastic surface inside the gun are relatively small compared to the load collected as the powder flows through the gun. of a corona discharge cloud. Wood cabinet doors are examples of a profiled wood substrate and are the door linings represented by the drawings in U.S. Patent No. 5,489,460, which further illustrates the type of wood panels that are particularly susceptible to powder coating by the method of this invention. The grooves and sharp edges of these panels are very well covered with a flat line coating apparatus with the nozzles formed to direct a portion of the powder against them. These panels, as well as the flat surface panels, such as those used to be ping-pong tables are particularly well coated by triboelectric guns on a flat line conveyor having electrically conductive bands around the circumference of the conveyor belt. The apparatus for this coating is disclosed in a series of patents assigned to Nordson Corporation. These are the North American Patents Numbers 4,498,913, 4,590,884, 4,723,505, 4,871,380, 4,910,047 and 5,018,909. A suitable flat line powder coating apparatus comprises a conveyor extending through a powder coating booth wherein a wooden article held and moved by the conveyor belt is triboelectrically coated by a plurality of guns located adjacent to each other. the other one and in more than one row. The powder is forced into the guns under a pressure of about 2.81 kilograms per square centimeter and the air at a pressure of about 1.41 kilograms per square centimeter is passed into the powder conduits just before the powder passes into the nozzles. The article that carries the powder is then transported through a curing oven that has several heating zones, some of which are heated by infrared lamps and others by heat convection and still others by a combination of the two. The coating and curing line speeds may be the same or different depending on the length of the curing furnace. The line speed through the powder application booth can be from about 1.52 to about 45.72 meters per minute, but preferably it is from about 6.10 to about 30.48 meters per minute. The line speed through the curing furnace on the other hand can be from about 1.52 to about 6.15 meters per minute, depending on the temperature of the furnace, the length of the furnace and the specific coating powder used. The curing temperature may vary from about 180 ° to, but not including the decomposition temperature of the powder. It is preferred to maintain the curing temperature within a range of about 88 ° C to about 150 ° C and still especially preferred, maintain the curing temperature of approximately 121 ° to 150 ° C. When a crystalline epoxy resin is used, a curing temperature of about 82 ° C is particularly appropriate. It is preferred that the line, coating and curing speeds be adjusted to an oven length so that they are balanced. Preheating the panel before the coating step is preferred in some cases, e.g., to help the powder reach its flow temperature in the first zone of the kiln and also to minimize degassing during curing. The oven can have several heating zones of both the infrared type and the convection type and also a combination of the two. The TRIAB Speedoven sold by Thermal Innovations Corporation is appropriate for the purposes of this invention. The film thickness of the cured coating is at least about .0254 millimeter and can be much greater, such as about .203 millimeter or even greater if there is practical need for this. The film thicknesses of about .102 millimeters to about 1152 millimeters are regularly achieved by the method of this invention even at coating speeds of approximately 30.48 meters per minute. In the laboratory tests of the additional mode that were carried out to determine the characteristics of the wood coating of the powders prepared on a common basis having a particle size distribution within the invention and which are outside the scope of the invention. invention, that is, the only variables remain in the particle size distribution. The identical size pieces of the pressed particle board of 13.34 centimeters by 6.99 centimeters were triboelectrically coated with a hand gun using 10 grams of powders sprayed out completely. The thickness of the coating was then measured after a curing cycle of 10 minutes at a temperature of 150 ° C. The results of the five points measured in each piece in millimeters were as follows: TABLE 8 Upper Lower Right Right M Sample 0 (CORVEL 265-148-2 * Red Brandy 1.4 Sample P (CORVEL 23-9296 * Red Brandy 4.1 4.8 6.0 TABLE 8 (Continued) Lower Upper Left Left Average Sample 0 (CORVEL 265-148-2 * Red Brandy 2.25 2.33 Sample P (CORVEL 23-9296 * Red Brandy 6.2 3.9 5.00 * Brandy Red - 0.4 part / 100 parts Red Cromaftal 3B resin (Ciba-Geigy) and Red Violet 13-7011 (Hoescht).

The powder of Sample 0 had a particle size distribution outside the invention and Sample P had a particle size distribution within the invention. It can be seen from Table 8 that the powder of Sample P (invention) provided a thicker coating on the test panels of the pressed particle board. With this description of the invention in detail, those skilled in the art will appreciate that various modifications can be made to the invention without deviating from the spirit of the invention. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described. Instead, it follows from the fact that the scope of the invention is determined by the appended claims and their equivalents.

Claims (20)

R E I V I N D I C A C I O N E S:

1. A mass of electrostatically chargeable plastic powder for a triboelectric coating application having a particle size distribution (Mx) comprising (all percentages are defined as a percentage by weight): from 95 percent to 100 percent less than 88 microns, from 5 percent to 15 percent less than

15. 56 microns, and from 0 percent to 6 percent less than 11 microns, and having an average particle size (Mv) of between about 30 and about 45 microns. 2. A coating powder mass according to claim 1, wherein the Mx comprises: 100 percent less than about 88 microns, and having an Mv of between about 30 to about 40 microns.

3. A coating powder mass according to claim 2, wherein the Mx comprises: from 10 percent to 15 percent less than 15.56 microns, and from 4 percent to 6 percent less than 11 microns, and It has an Mv of between about 35 and 36 microns.

4. A coating powder mass according to claim 3, wherein the Mx comprises: 100 percent less than 88 microns, about 11.

5 percent less than 15.56 microns, and about 4.3 percent less than 11 microns , and an Mv of approximately 35.9. A mass of the coating powder according to claim 1, wherein the powder is a thermosetting resin composition.

6. In a triboelectric process for coating a substrate with powder fed to a triboelectric spray gun and whose powder is sprayed onto the substrate, the improvement wherein the feed powder is as defined in claim 1.

7. A process of according to claim 6, wherein the powder is a thermosetting resin composition.

8. In a triboelectric process for coating a substrate with powder fed to a triboelectric spray gun and whose powder is sprayed onto the substrate, the improvement wherein the feed powder is as defined in claim 4.

9. A process of according to claim 8, wherein the powder is a thermosetting resin composition.

10. In a triboelectric process for coating a substrate with a plastic powder loaded to a triboelectric spraying gun and the powder of which is sprayed onto the substrate, the improvement wherein the powder loading is a mass of powder having a size distribution of particle comprising: 100 percent less than 88 microns, 5 percent to 15 percent less than 15.56 microns, and 0 percent to 6 percent less than 11 microns, and which has an Mv of between about 30 and approximately 40 microns.

11. A process according to claim 10, wherein the powder is a thermosetting resin composition.

12. In a triboelectric process to coat a substrate with plastic powder loaded to a triboelectric spray gun and whose powder is sprayed onto the substrate, the improvement where the load is a mass of powder having a particle size distribution- comprising: 100 percent less than 88 microns, 10 percent to 15 percent less than 15.56 microns, and 4 percent to 6 percent less than 11 microns, and which has an Mv of between about 35 and 36 microns.

13. A process according to claim 12, wherein the powder is a thermosetting resin composition.

14. In a coating process for coating a wood substrate where the plastic powder is fed to a triboelectric spray gun and where the powder is then sprayed onto the wood substrate, the powder has an average particle size Mv of between about 30 and 45 microns, the improvement where the powder has a particle size distribution Mx comprising, all in porcnetajes by weight: from 95 percent to 100 percent less than 88 microns, 5 percent one hundred to 15 percent less than 15.56 microns, and from 0 percent to 6 percent less than 11 microns.

15. A process according to claim 14, wherein the powder has an Mv of between about 30 and 40 microns and an Mx comprising: 100 percent less than 88 microns, from 5 percent to 15 percent less than of 15.56 microns, and from 0 percent to 6 percent less than 11 microns.

16. A process according to claim 14, wherein the powder has an Mv of between about 35 and 36 and an Mx comprising all percentages by weight: 100 percent less than 88 microns, from 10 percent to 15 percent less than 15.56 microns, and 4 percent to 6 percent less than 11 microns.

17. A process according to claim 16, wherein the powder has an average particle size Mv of about 35.9 and an Mx of: 100 percent less than 88 microns, approximately 11.5 percent less than

15. 56 microns, and approximately 4.3 percent less than 11 microns.

18. A process according to claim 14, wherein the powder is a thermosetting resin composition.

19. A process according to claim 16, wherein the powder is a thermosetting resin composition.

20. A plastic coating for wood substrate produced by the process according to claim 14, having a continuous decorative protective plastic coating on the same. SUMMARY OF THE INVENTION An electrically chargeable dielectric coating powder and a triboelectric coating process for applying the powder to wood substrates are described. The coating powder is a mass of a finely-divided thermally fusible dielectric plastic material having an average particle size (Mv) of between 30 and 45 microns, and a particle size distribution (all percentages defined are in percent by weight). weight) of: from 95 percent to 100 percent less than 88 microns, from 5 percent to 15 percent less than 15.56 microns, and from 0 percent to 6 percent less than 11 microns. Preferably the coating powder has 0 percent greater than 88 microns and an Mv = of about 30-40, preferably 35-40 microns. Preferably, the particle distribution further includes: from 10 percent to 15 percent less than 15.56 microns, and from 4 percent to 6 percent less than 11 microns, and an Mv of between about 35 and 36 microns . Particularly preferably, the particle distribution further includes: about 11.5 percent less than 15.56 microns, and about 4.3 percent less than 11 microns, and an Mv of about 35.9. Preferably a powder in a thermosetting resin composition.