GB1575144A - Method for covering articles with powdered materials and articles produced thereby - Google Patents

Description

(54) METHOD FOR COVERING ARTICLES WITH POWDERED MATERIALS, & BR< PRODUCED THEREBY (71) We, SUMITOMO DUREZ COMPANY LIMITED, a corporation organized under the laws of Japan, of 2-2, Uchisaiwaicho-lchome, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to a method for covering articles with powdered materials and articles produced thereby.

For many years articles have been coated with various liquid compositions at room temperature or elevated temperatures for hereby exemplified purposes of electrical insulation, corrosion prevention, protection from chemical attack, and decoration, etc.

In recent years, however, in order to meet the requirements for natural resources saving, pollution abatement, and improvement of working environment in handling solvent-containing liquid coatings, as well as the requirements for mechanization and labor saving in coating workshops, instead of said coatings various methods and equipments for powder covering with powdered materials have been tried in a number of industrial fields and some of the methods have actually been realized. Thus in the coating industries, powder covering with various application methods has already been put into actual use, replacing conventional coating in limited cases, and these methods are expected to enlarge their application fields further in the future.

Also, in such industries as adhesives, ceramics, the powder coating process is expected to be more widely used with the development in coating methods and equipments.

Following are some of these generally known coating methods: (1) Fluidized bed coating process which comprises dipping a preheated article into a bed where powdered material is fluidized by blowing compressed air, whereby the temperature of said article is held higher than the melting point of said powdered material, the covered powder thus being melted to form a film; (2) Electrostatic spraying process which comprises ejection of a powdered material with compressed air from a nozzle while electrostatically charging the powdered material with a high voltage by an electrode provided at the nozzle, and thus depositing the powdered material on an earthed article;; (3) Electrostatic fluidized bed coating which comprises applying a high voltage by way of an electrode to a powdered material initially fluidized by compressed air, and dipping an earthed article into the upper dilute phase of the fluidized powder, the powdered material thus being electrostatically deposited on the article; and (4) Other processes such as melt spraying, rolling over, or cascade spreading.

The above methods from (1) to (4), however, have respective disadvantages as listed below: (1) As for the fluidized bed coating process: (a) This process cannot be applied to an article which should not be preheated to a temperature higher than the melting point of a powdered material; (b) This process- requires much man power when a thicker film formation is intended, because the thickness of a single coating is at most 01 mm, and therefore, repeated operation is necessary for a thicker film; (c) Although this process can be applied to an article which has undergone wetting treatment with a liquid in place of preheating, it is difficult to obtain an even film through baking, because the powdered material usu ally is heterogeneous in particle size and only forms a thin film by said process; and (d) This process does not give an even deposit of a powdered material onto a composite article because indivi dual components of the article can not evenly be preheated.

(2) As for the electrostatic spraying process: this process tends to result in an uneven deposit of powdered material especially on a composite article.

(3) As for the electrostatic fluidized bed coating process: (a) Coating by this process is liable to result in partial growth or partial detachment of the deposited pow dered material at the surface of an article during or after deposit when a thick covering is intended and is also liable to result markedly in an uneven deposit of powdered ma terial on a composite article, because some components of said article may have less affinity to powdered ma terial.Attempts were made to ob tain an even deposit and to eliminate said partial growth after deposit of powdered material by intermittent tapping of the article, but these have been frequently unsuccessful be cause partial detachment of the de posited powdered material occurs; (b) When a thicker covering is necessary it takes too much time because the powdered material must be gradu ally deposited onto the article in the upper dilute phase of the fluidized bed; (c) This process requires a high voltage (generally DC 60-100 kV) to charge the powdered material, which is con sequently hazardous;; (d) This process requires recovery of the powdered material at the top of a fluidizing chamber because a large volume of compressed air must be blown into the fluidized bed, and this makes it difficult directly to vent the top of fluidizing chamber to open air; this is one of the reasons for low efficiency (low productivity) of this process; and (e) This process requires a narrowly con trolled particle size distribution of the powdered material controlled so as to ensure uniform fluidization and even deposit.

The present invention provides a method for powder coating an article wherein the article is vibrated at a frequency within the range from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns while coating the article with powder which has been charged with a DC voltage, the article optionally having a periodic motion superimposed on its vibration and optionally being coated in a fluidized bed, the apparent volume of the powder in the fluidized bed being 1-05 to 1-5 times as large as that in the stationary state.

More particularly, the invention provides a method for covering an article with a powdered material, which comprises setting an article in a fluidized powdered material, which powdered material has been charged with a DC voltage, the apparent volume of which powdered material is 105 to 15 times as large as that in the stationary state, and keeping said article vibrating continuously with a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns until said powdered material is even deposited onto the surfaces of the article.

The invention further provides a method for covering an article with a powdered material, which comprises setting an article in a spray of powdered material which powdered material has been charged with a DC voltage, said powdered material having been ejected from a nozzle with compressed air, and keeping said article vibrating continuously with a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns until said powdered material is evenly deposited onto the surfaces of the article.

The invention also provides a method for covering an article with a powdered material, which comprises setting an article in a fluidized powdered material, which powdered material has been charged with a DC voltage, the apparent volume of which powdered material is 105 to 15 times as large as that in the stationary state, and keeping said article vibrating continuously with a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns and simultaneously causing the article to move periodically like a swing until said powdered material is evenly deposited onto the surfaces of the article.

The present invention makes it possible to provide a method for covering an article with a deposit of powdered material wherein a more even deposit may be obtained in a shorter time with greater safety in operation, and less necessity for narrow control of the particle size of the powdered materials.

In the method of the invention, an article is covered with a powdered material while keeping said article vibrating continuously or keeping said article vibrating continuously with periodic motion in a fluidized bed or in a spray of powdered material within a high voltage electrostatic field (relative to the article) generated by an electrode which applies a charge to said powdered material.

The uninterrupted vibration of the article begins from the moment when the article enters the fluidized bed and continues until an even deposit of powdered material is made. Further, in many cases, it is generally sufficient to keep the article continuously vibrating or to keep the article continuously vibrating with periodic motion while said article is in the fluidized bed.

Said vibration in this invention is a continuous one having a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns depending on sorts and characteristics or properties of said powdered material, and also on the conditions for deposit. A vibration with a frequency from 2,000 to 8,000 per minute and an amplitude from 10 to 100 microns is preferred for restraining the deposited powdered material from both partial growth and partial detachment. Said continuous vibration in the present invention is entirely different from any intermittent pulsation in spite of its effectiveness for deposit of powdered material. When any intermittent pulsation is applied, neither partial growth nor partial detachment can effectively be restrained and this leads to an uneven deposit.

Where a periodic motion is used, this may be given simultaneously with the vibration to an article during such a period that the article is set in a fluidized powdered material and an even deposit of powder can be made onto the surfaces of the article.

Further, in many cases, it is sufficient to move the article periodically only when it is set in the fluidized powdered material. The periodic motion, in this case, need not be continuous, and may be intermittent. The mode of said periodic motion differs with sorts, nature and forms of the article or sorts characteristics or properties and deposit conditions of the fluidized powdered material, etc. For example, when a powdered material such as an epoxy resin powdered compound for electrical insulation is deposited onto such a small article, as electronic parts, in general, it is mostly sufficient to make the article swing as a pendulum. In these cases the preferable periodic motion is from 0'3 to 3 seconds in cycle and from 10 to 300 mm in amplitude.

A cycle from 0 5 to 2 seconds and an amplitude 30 to 200 mm is particularly preferred for restraining the deposit of powder from both partial growth and partial detach ment.

One of the advantageous features of the present invention is that the deposit can be made in the high density phase of fluidization and another advantageous feature is that the process can be carried out at a relatively low voltage, which naturally leads to increased safety. More particularly, during the deposit of powdered material, a low voltage is applied intentionally to avoid the danger from electric leakage, because deposit of said powdered material is performed in the high density phase fluidization.

Said voltage naturally varies depending on sorts and characteristics or properties of powdered material to be deposited; however, in the present invention it is preferable to use a relatively low DC voltage from 5 to 60 kV to charge the powdered material.

The covering process according to this invention involves positioning an article in a mildly fluidized bed of powdered material so as to allow said powdered material sufficiently to deposit onto the entire surface of said article whereby a thick covering may be obtained by a single-run process. Said mild fluidization causes substantially no dust emission, so that dust traps may be dispensed with.

Said mild fluidization is a fluidization stage wherein the "apparent volume ratio" of the powdered material becomes generally from 105 to 15, preferably from 1 1 to 12, depending on sorts and characteristics or properties of the powdered material being fluidized. The "apparent volume ratio" is hereby defined as the ratio of the apparent volume of the powdered material in the stationary state before fluidization.

As described above, the present inventors have found that a thick and even covering onto an article may be obtained by keeping the article continuously vibrating or keeping the article continuously vibrating with periodic motion through a single shorttime deposit in the high density phase of fluidization, the powdered material charged with a relatively low voltage.

It was also found that an even deposit of powdered material, without partial growth and partial detachment, could be obtained by keeping said article continuously vibrating when applied to the powder spraying process, where the powdered material is ejected from a nozzzle by compressed air, and at the same time applying a high voltage between the nozzle and the article.

The advantages of the invention are demonstrated well when the article to be coated is made of a composite material, or when its surface is rugged or markedly irregular.

Advantageously, the article to be covered with powdered material in an electrostatic field is pretreated. Pretreatments include heating, impregnation or wetting with a liquid substance, or a combination of heating and impregnation or wetting.

In the present invention said heating of the article does not necessarily require a temperature higher than the melting point of the powdered material, as is in the case with the conventional fluidized bed coating process without using electrostatic charge.

Whatever the extent of preheating may be, even at a lower temperature such as for example from room temperature to ó0 C., it is still effective.

The liquid substances which may be used in the present invention to wet an article include, for example, such common liquid substances as thermoplastic resins, thermosetting resins, plasticizers, solvents, and inorganic substances. More precisely described, the liquid substances include, for example; starches, epoxy resins, polybutenes; polymer homologous ethylene glycols, phthalic esters and water glass. It is frequently desirable to coat or impregnate the article previously with these liquids so as to improve the performance of the finished film, for example with regard to adhesion, moisture resistance, and the absence of pinholes.

The powdered materials which can be applied by the present invention include all of the powdered materials which can cover an article, such as for example oragnic powdered materials including synthetic resins and elastomers such as, for example, epoxy resins, acrylic resins, polyesters, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyurethanes, polyamides, ethylene-vinyl acetate copolymers, polyvinyl butyral, synthetic rubbers, phenolic resins, melamine resins, urea resins, and fluorocarbon resins; and natural products such, for example, as starch and rosin; compounds (mixtures) therefrom or a formation of two or more of above-mentioned substances as major components and if necessary with colorants, stabilizers, curing agents; inorganic powdered fillers such, for example, as calcium carbonate, silica and alumina frit.

The range of particle size of the powdered material is distributed preferably from 5 to 200 microns.

The method of the present invention is applicable to any article which requires covering, for example electronic parts, electric parts, mechanical parts and products obtained therefrom, construction or building materials, industrial products, notions and every day articles, and ceramic products.

To summarize, in the case of the electrostatic fluidized bed process, this invention is characterized by (1) keeping the article vibrating continuously or keeping the article continuously vibrating with periodic motion throughout said setting so as to have an even deposit in a shorter time, (2) setting said article in the high' density phase of fluidized powdered material, the apparent volume of the material in the high density phase being 1 05 to 1 5 times as large as that in the stationary phase, (3) fluidizing the powdered material more mildly, and (4) applying a relatively low voltage charge applied to the powdered material.The main advantages of this method of the present invention (that is, of effecting vibration with or without periodic motion) over the conventional methods are (a) a more even covering, (b) a shorter time required for deposit, (c) a thicker deposit in a single run, (d) an easier control of deposit thickness by effecting vibration or vibration with periodic motion, and (e) a greater tolerance for particle size control of said powdered material.

On the other hand, the conventional electrostatic spraying process which is carried out by ejecting a powdered material from a nozzle with compressed air and depositing the powdered material on an article with an electrostatic force is improved by this invention which keeps said article vibrating continuously throughout its presence in the atmosphere of ejected powdered material so that its surface receives an even deposit of the powdered material. Thus this invention offers the further advantages of (a) a more uniform deposit, and (b) an easier control of its thickness by regulating the frequency and amplitude of the vibration.

The method of the present invention preferably includes also baking, namely a commonly required aftertreatment, to form a film from the deposited powdered material.

The following Examples illustrate the invention: Example 1 Covering of a cast iron pipe fitting with an inner diameter of 25 mm, for water supply: A heavy-duty anticorrosive epoxy resin powdered compound (SUMILITERESIN PR-52000K, a product of Sumitomo Durez Co., Ltd.) was mildly fluidized with compressed air in a fluidizing chamber to an apparent volume ratio of 1.15. The earthed pipe fitting was set in the fluidized bed with continuous vibration at a frequency of 3,600/minute and an amplitude of 50 microns and periodic motion at a cycle of I second and an amplitude of 100 mm. In this process a voltage of DC 30 kV was applied to the powdered compound and it was allowed to deposit electrostatically deposit for 2 seconds on the surfaces of said pipe fitting. The fitting was taken out from the fluidized bed in one second under said vibration with periodic motion, after which this vibration with periodic motion was further continued until excess deposit had been removed. The surface of the fit- ting thus treated was entirely covered with an even deposit of the compound without revealing any partial growth or partial detachment of the deposited compound. The thus-processed fitting was then baked in an oven at 1800C. for one hour to form an extremely even and smooth film with a thickness of 0 4 mm, which completely covered all its surfaces.

Example 2 Covering of cast iron pipe fitting with an inner diameter of 25 mm, for water supply: The surfaces of the fitting were pretreated by wetting with a liquid mixture comprising 100 parts by weight of a liquid epoxy resin (Epikote 828, a product of Shell Chemical Co., Ltd.) and 90 parts by weight of a liquid anhydride curing agent (Methyl Nadic Anhydride, a product of Hitachi Chemical (two., Ltd.). "Epikote" and "Nadic" are trade marks.

A heavy-duty anticorrosive epoxy resin powered compound (SUMILITERESIN PR-52000K, a product of Sumitomo Durez Co., Ltd.) was mildly fluidized with compressed air in a fluidizing chamber with an apparent volume ratio of fluidization of 1 15.

Said pretreated fitting was set in said fluidized bed, while keeping the fitting continuously vibrating at a frequency of 3,600/ minute and an amplitude of 50 microns with periodic motion at a cycle of 1 second and an amplitude of 100 mm. The powdered compound was charged by applying a voltage of DC 30 kV and was allowed to deposit for 1 second onto the surfaces of the fitting by electrostratic force. The fitting was taken out from the fluidized bed in one second under said vibration with periodic motion, after which this vibration with periodic motion was further continued until excess compound was removed. The surfaces of the fitting thus processed were completely covered with an even deposit of the compound without revealing any partial growth or partial detachment of the deposited compound.This fitting covered in such a short time was then baked in an oven at 1800C. for one hour. The film formed was extremely even and smooth having a thickness of about 0 4 mm.

As compared with the process described in Example 1, the deposit time in this process could be shortened and the film formed was entirely free from pinholes and had a more even and smooth surface.

Example 3 Covering: of a cast iron pipe fitting with an inner diameter of 25 mm, for watersupply: The fitting was preheated to 60"C. and subjected to the same process from deposit to baking with the same compound as in Example 1. The resulting film, about 05 mm in thickness, covering the surfaces of.

the fitting was very even and smooth. As compared with the process clarified in Example 1, in this process, a thicker film is formed within the same deposit time.

Example 4 Covering of a tab-inserted type polyethylene terephthalate film capacitor (0 01 micro-Farad): Said element was first vacuum impregnated with a liquid mixture comprising 100 parts by weight of liquid epoxy resin (Epikote 828, a product of Shell Chemical Co., Ltd.) and 90 parts by weight of liquid acid anhydride curing agent (Methyl Nadic Anhydride, a product of Hitachi Chemical Co., Ltd.). The pretreated element was then kept continuously vibrating at a frequency of 6,000/minute and an amplitude of 20 microns, and set in an epoxy resin powdered compound (electric insulation type SUMILITERESIN PR-52121, a product of Sumitomo Durez Co., Ltd.) fluidized with compressed air to an apparent volume ratio of 1.1 in a fluidizing chamber.The said compound was charged by applying a voltage of DC 40 kV and was allowed to deposit electrostatically for one second onto said element. The element was taken out from the fluidized bed while vibrating, after which said vibration was continued until excess compound had been removed. The surfaces of the element was found to be completely covered with the powdered compound including even the edges of the polyethylene terephthalate films where no metallic foils were provided. The deposit of said compound was so even that neither partial growth nor partial detachment of the compound could be found.The resulting compound-deposited element was baked for 2 hours in an oven at 1200C. The film thus formed was even and smooth and its thickness was 0 4 mm all over the surfaces thereof, free from pinholes, and excellent in electrical insulation.

As compared with the process, wherein said element is not pretreated by vacuum impregnation or surface wetting with a liquid substance, this process of Example 4 permits better deposition of the powdered compound in a shorter time and results in an even and smooth film formation free from pinholes by baking.

Example 5 Covering of hybrid integrated circuits with transistor and ceramic capacitor elements on alumina-substrate: The surfaces of said hybrid integrated circuits were wetted with the same liquid mixture as used in Example 4 and then electrostatically covered with an epoxy resin powdered compound for electrical insulation (SUMILITERESIN PR-52114, a product of Sumitomo Durez Co., Ltd.) mildly fluidized with compressed air, keeping said article vibrating continuously at a frequency of 5,000/minute and an amplitude of 80 microns, to an apparent volume ratio of l 1 in a fluidizing chamber. The fluidized compound was charged by applying a voltage of DC 30 kV and was allowed to deposit electrostatically for 2 seconds on the circuit substrate and element.The article was then taken out from the fluidized bed while still vibrating, and further kept vibrating until excess compound had been removed.

Thus, in a short time, a sufficient deposit of said compound was obtained on all the surfaces of said substrate where circuits were running or not, as well as the surfaces of said elements therewith and the surfaces of intricate portions where the substrate circuits were connected to the transistors or ceramic capacitors. The deposit of said compound was even throughout the surfaces and neither partial growth nor partial detachment occurred. The powder-covered article was then baked for one hour in an oven at 1500C. The resulting film was even and smooth with a thickness of about 03 mm throughout said surfaces and it was excellent in electrical insulation properties.

Example 6 Covering of a copper wire with a diameter of 1 0 mm diameter, which wire was kept continually vibrating at a frequency of 2,000/minute and an amplitude of 50 microns: Said copper wire was passed for one second through a powdered polyvinyl chloride compound (Aron Compound SF, a product of Toa Gosei Chemical Co., Ltd.) had been mildly fluidized to an apparent volume ratio of 1 1 with compressed air in a fluidizing chamber and electrostatically charged a voltage of DC 20 kV, so as to allow the fluidized powder to deposit electrostatically onto said copper wire. After having been taken out from the fluidizing chamber, said copper wire was kept continuously vibrating until excess compound had been removed.

Thus, there was obtained an even deposit of said compound on the copper wire without partial growth or partial detachment of the compound. The copper wire thus processed was then baked by passing through an oven at 2000 C. with an effective duration time of 5 minutes to obtain an even and smooth film of about 0 3 mm in thickness surrounding the wire.

Example 7 Sheet-steel enamelling: A surface-degreased steel sheet specimen was earthed and set in a powdered frit (black undercoat for porcelain enamel, major components: feldspar and borax) mildly fluidized to an apparent volume ratio of 13 with compressed air in a fluidizing chamber with said sheet-steel specimen kept continuously vibrating at a frequency of 3,000/minute and an amplitude of 50 microns. The powdered frit was charged with a voltage of DC 40 kV and was allowed to deposit electrostatically for 10 seconds onto the specimen. The vibration was terminated simultaneously when the specimen was taken out from the fluidizing chamber.

An even deposit of said frit onto all sides of the surface of the specimen was obtained.

The deposited frit revealed neither partial growth nor partial detachment. This processed specimen was fired in an oven at 850 C. so as to fuse the frit completely.

The film thus formed was extremely even and smooth, and had a thickness of about 0 15 mm. In the same way as mentioned above, a powdered frit for over-coat (major components: feldspar and silica) was allowed to deposit electrostatically onto the said film-formed specimen and fired. It was confirmed that the enamel film thus formed was about 025 mm in thickness and uniform throughout, and was excellent in evenness and smoothness, and was entirely free from pinholes.

Example 8 Covering a steel cover for belt drive: The said cover was covered by electrostatic spraying process in the following way: an epoxy resin powdered compound (SUMI LITERESIN PR-52000, a product of Sumitomo Durez Co., Ltd.), charged by applying a voltage of DC 80 kV, was sprayed from a nozzle with compressed air onto said cover which was kept continuously vibrating at a frequency of 3,500/minute and an amplitude of 80 microns, so as to allow the compound to deposit evenly onto the cover by electrostatic force. The vibration was terminated immediately after completion of the spraying. An even deposit of said compound was obtained throughout the surfaces of the cover. The cover thus processed was baked for 05 hour in an oven at 200 C. It was found that the cover thus processed had an even and smooth film of about 0 15 mm in thickness.

Comparative Example 1 Covering of a cast iron pipe fitting with an inner diameter of 25 mm for water supply: (1) By an ordinary electrostatic fluidized bed dipping technique: a heavy-duty anticorrosive epoxy resin powdered compound (SUMILITERESIN PR-52000K, a product of Sumitomo Durez Co., Ltd.) was charged by applying a high voltage of DC 70 kV and was allowed to deposit electrostatically for 15 seconds onto the fitting. The fitting thus processed was baked for one hour in an oven at 180"C. to form a film, about 03 mm in average thickness.However, the deposit of said compound in the deposition step was unsatisfactory, because the maximum deviation in thickness of the formed film finally from the average of 0 3 mm was found to be + 0 2 mm; the thickest part was the side which had been faced upward during deposition of the compound. Moreover, during the baking step, the deposit layer showed partial detachment caused by a slight tap. After baking, 6 out of the 20 processed pieces were found defective in appearance on account of uneven surface covering.

(2) By adopting an ordinary fluidized bed dipping technique, as an alternative to (1), a fitting was preheated to 200"C. and was dipped into the fluidized bed of the said powdered compound. As to the resulting deposit of said compound onto the surfaces of the fitting, partial growth on the top side was removed by intermittent taps applied to the covered fitting. After the preheating and dipping had been repeated three times, the fitting thus processed was baked for one hour in an oven at 18(4"C. The film formed onto the surface of the fitting was about 0 3 mm in average thickness.

But the fitting thus processed was not satisfactory because of the irregularity in thickness of the film which showed deviation +0 15 mm as maximum from the average value of 03 mm, and moreover, this process required too many man-hours, because four repeated deposits were necessary since the deposit for a single-run was too thin to form a successful film thereon.

Comparative Example 2 Covering of a tab-inserted type polyethylene terephthalate film capacitor (0 01 micro-Farad): The said capacitor element was vacuumimpregnated and in the same manner with the liquid mixture as in Example 4. In accordance with a conventional electrostatic fluidized bed dipping method, an epoxy resin powdered compound for electrical insulation (SUMILITERESIN PR-52121, a product of Sumitomo Durez Co., Ltd.) was charged by applying a high voltage of DC 70 kV and was allowed to deposit electrostatically deposit for 12 seconds onto said pretreated element. The compound apparently deposited onto all surfaces thereof including even the surface of the polyethylene terephthalate film edge where no metallic foils were provided.Then, the element was baked for two hours in an oven at 1200C. The resulting film had a thickness of about 03 mm in the middle part of said element where the metallic foils were embedded, and the film formed by baking had nearly the same thickness at the metallic-foil-free edge. However, on the whole, evenness of thickness was somewhat poor.

The deposit time of said compound had to be undesirably lengthened. As compared with the process described in Example 4, this process was found to be poor in evenness of the film, as well as in productivity.

Comparative Example 3 Covering of hybrid integrated circuits with transistor and ceramic capacitor elements on alumina substrate: The surfaces of said article were wetted with the same liquid mixture as that in Example 4. An epoxy resin powdered compound for electrical insulation (SUMILI TERESIN PR-52114, a product of Sumitomo Durez Co., Ltd.) was charged by applying a high voltage of DC 70 kV and was electrostatically deposited for 20 seconds onto said pretreated article. Partial growth was observed, although the compound was deposited onto the surfaces including the edges and back side of the alumina substrate where no circuits had been provided, and intricate parts where the circuits on the alumina substrate connects to the transistors or ceramic capacitors.A slight tap inflicted on said hybrid integrated circuits, was found helpful to remove the said partial growth, however, this causes partial detachment of the deposited compound.

resulting in decreased uneven thickness.

Said integrated circuits and elements therewith were then baked for one hour in an oven at 1500C. The film, about 0 3 mm in thickness, formed onto various parts of the article was poor in evenness of thickness and smoothness of the surfaces thereof even in the flat area of said article.

Comparative Example 4 Covering of copper wire 1 0 mm in diameter: By applying an ordinary fluidized bed dipping method, said copper wire was preheated at 200"C. and was continuously passed through a powdered polyvinyl chloride compound (Aron Compound SF, a product of Toa Gosei Chemical Co., Ltd.) fluidized bed with an apparent volume ratio of 1 1, so as to allow said powdered compound to deposit around the wire. Partial growth of said compound on the wire was removed by applying intermittent taps to the wire after it passed through the fluidized bed. Said preheating and dipping process was repeated three times more and thereafter, thus processed copper wire was baked to obtain a film of said compound surrounding it having a thickness of 0 3 mm.

The film formed by the process in the present Comparative Example 4 was not desirable because of partial unevenness of the thickness, the maximum deviation of it from the average value 0 3 mm being +0 1 mm. Moreover, this process required many man-hours, because the quantity of a single deposit was so small that adequate film thickness could only be obtained by four repeated process.

Comparative Example 5 Covering a steel cover for belt drive: In accordance with a common electrostatic spraying process, an epoxy resin powdered compound (SUMILITERESIN PR52000, a product of Sumitomo Durez Co., Ltd.) was charged by applying a high voltage of DC 80 kV and was ejected with compressed air allowing it to deposit on the said belt cover with an electrostatic force. After baking for 0.5 hour in an oven at 2000C., a film, 0 15 mm in thickness, on the surface of the cover was obtained. The film thus formed was not satisfactory because of partial unevenness of the thickness, the maximum deviation from the average thickness of 0 15 mm being + 0 06 mm.The ejected compound deposited onto said cover by the spraying process was apt to partially detach if a slight accidental tap was given to the cover before and during the baking, and this gave rise to an uneven film after completion of the baking.

WHAT WE CLAIM IS:- 1. A method for covering an article with a powdered material, which comprises setting an article in a fluidized powdered material, which powdered material has been charged with a DC voltage, the apparent volume of which powdered material is 1 05 to 1 5 times as large as that in the stationary state, and keeping said article vibrating continuously with a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns until' said powdered material is evenly deposited onto the surfaces of the article.

2. A method for covering an article with a powdered material, which comprises setting an article in a spray of powdered material, which powdered material has been charged with a DC voltage, said powdered material having been ejected from a nozzle with .compressed air, and keeping said article vibrating continuously with a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns until said powdered material is evenly deposited onto the surfaces of the article.

3. A method for covering an article with a powdered material, which comprises setting an article in a fluidized powdered material, which powdered material has been charged with a DC voltage, the apparent volume of which powdered material is 1 05 to 1-5 times as large as that in the stationary state, and keeping said article vibrating continuously with a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns and simultaneously causing the article to move periodically like a swing until said powdered material is evenly deposited onto the surfaces of the article.

4. A method according to Claim 1, wherein the article is pretreated prior to setting it in the fluidized and electrostatically charged powdered material.

5. A method according to Claim 2, wherein the article is pretreated prior to setting it in the sprayed and electrostatically charged powdered material.

6. A method according to Claim 3, wherein the article is pretreated prior to setting it in the fluidized and electrostatically charged powdered material.

7. A method according to Claim 4, wherein the pretreatment comprises heating, impregnation, and wetting alone or in combination of two or more in any order.

8. A method according to Claim 5, wherein the pretreatment comprises heating, impregnation and wetting alone or in combination of two or more in any order.

9. A method according to Claim 6, wherein the pretreatment comprises heating, impregnation and wetting alone or in combination of two or more in any order.

10. A method according to any one of Claims 1, 4 and 7, wherein the article is selected from electronic parts, electric parts, mechanical parts, or products obtained therefrom, construction or building materials, industrial products, notions and everyday articles, and ceramic products.

11. A method according to any one of Claims 1, 4, 7 and 10, wherein the powdered material is selected from epoxy resins, acrylic resins, polyesters, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyurethanes, polyamides, ethylene-vinyl acetate copolymers, polyvinyl butyral, synthetic rubbers, phenolic resins, melamine resins, urea resins, fluorocarbon resins, starch, rosin, or compounds therefrom or a formulation of two or more of the abovementioned substances, and inorganic frit.

12. A method according to any one of Claims 2, 5 and 8, wherein the article is selected from electronic parts, electric parts, mechanical parts, or products obtained therefrom, construction or building materials, industrial products, notions and everyday articles, and ceramic products.

13. A method according to any one of Claims 2, 5, 8 and '12, wherein the pow

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (21)

**WARNING** start of CLMS field may overlap end of DESC **. surrounding it having a thickness of 0 3 mm. The film formed by the process in the present Comparative Example 4 was not desirable because of partial unevenness of the thickness, the maximum deviation of it from the average value 0 3 mm being +0 1 mm. Moreover, this process required many man-hours, because the quantity of a single deposit was so small that adequate film thickness could only be obtained by four repeated process. Comparative Example 5 Covering a steel cover for belt drive: In accordance with a common electrostatic spraying process, an epoxy resin powdered compound (SUMILITERESIN PR52000, a product of Sumitomo Durez Co., Ltd.) was charged by applying a high voltage of DC 80 kV and was ejected with compressed air allowing it to deposit on the said belt cover with an electrostatic force. After baking for 0.5 hour in an oven at 2000C., a film, 0 15 mm in thickness, on the surface of the cover was obtained. The film thus formed was not satisfactory because of partial unevenness of the thickness, the maximum deviation from the average thickness of 0 15 mm being + 0 06 mm.The ejected compound deposited onto said cover by the spraying process was apt to partially detach if a slight accidental tap was given to the cover before and during the baking, and this gave rise to an uneven film after completion of the baking. WHAT WE CLAIM IS:-

1. A method for covering an article with a powdered material, which comprises setting an article in a fluidized powdered material, which powdered material has been charged with a DC voltage, the apparent volume of which powdered material is 1 05 to 1 5 times as large as that in the stationary state, and keeping said article vibrating continuously with a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns until' said powdered material is evenly deposited onto the surfaces of the article.

2. A method for covering an article with a powdered material, which comprises setting an article in a spray of powdered material, which powdered material has been charged with a DC voltage, said powdered material having been ejected from a nozzle with .compressed air, and keeping said article vibrating continuously with a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns until said powdered material is evenly deposited onto the surfaces of the article.

3. A method for covering an article with a powdered material, which comprises setting an article in a fluidized powdered material, which powdered material has been charged with a DC voltage, the apparent volume of which powdered material is 1 05 to 1-5 times as large as that in the stationary state, and keeping said article vibrating continuously with a frequency from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns and simultaneously causing the article to move periodically like a swing until said powdered material is evenly deposited onto the surfaces of the article.

4. A method according to Claim 1, wherein the article is pretreated prior to setting it in the fluidized and electrostatically charged powdered material.

5. A method according to Claim 2, wherein the article is pretreated prior to setting it in the sprayed and electrostatically charged powdered material.

6. A method according to Claim 3, wherein the article is pretreated prior to setting it in the fluidized and electrostatically charged powdered material.

7. A method according to Claim 4, wherein the pretreatment comprises heating, impregnation, and wetting alone or in combination of two or more in any order.

8. A method according to Claim 5, wherein the pretreatment comprises heating, impregnation and wetting alone or in combination of two or more in any order.

9. A method according to Claim 6, wherein the pretreatment comprises heating, impregnation and wetting alone or in combination of two or more in any order.

10. A method according to any one of Claims 1, 4 and 7, wherein the article is selected from electronic parts, electric parts, mechanical parts, or products obtained therefrom, construction or building materials, industrial products, notions and everyday articles, and ceramic products.

11. A method according to any one of Claims 1, 4, 7 and 10, wherein the powdered material is selected from epoxy resins, acrylic resins, polyesters, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyurethanes, polyamides, ethylene-vinyl acetate copolymers, polyvinyl butyral, synthetic rubbers, phenolic resins, melamine resins, urea resins, fluorocarbon resins, starch, rosin, or compounds therefrom or a formulation of two or more of the abovementioned substances, and inorganic frit.

12. A method according to any one of Claims 2, 5 and 8, wherein the article is selected from electronic parts, electric parts, mechanical parts, or products obtained therefrom, construction or building materials, industrial products, notions and everyday articles, and ceramic products.

13. A method according to any one of Claims 2, 5, 8 and '12, wherein the pow

dered material is selected from epoxy resins, acrylic resins, polyesters, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyurethanes, polyamides, ethylene-vinyl acetate copolymers, polyvinyl butyral, synthetc rubbers, phenolic resins, melamine resins, urea resins, fluorocarbon resins, starch, rosin, or compounds therefrom or a formulation of two or more of the abovementioned substances, and inorganic frit.

14. A method according to any one of Claims 3, 6 and 9, wherein the article is selected from electronic parts, electric parts, mechanical parts, or products obtained therefrom, construction or building materials, industrial products, notions and everyday articles, and ceramic products.

15. A method according to any one of Claims 3, 6, 9 and 14, wherein the powdered material is selected from epoxy resins, acrylic resins, polyesters, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyurethanes, polyamides, ethylene-vinyl acetate copolymers, polyvinyl butyral, synthetic rubbers, phenolic resins, melamine resins, urea resins, fluorocarbon resins, starch, rosin, or compounds therefrom or a formulation of two or more of the abovementioned substances, and inorganic frit.

16. A method as claimed in any one of Claims 1 to 15, wherein the article coated with the powdered material is baked to form a film from the material.

17. A method as claimed in Claim 3, carried out substantially as described in any one of Examples 1 to 3, herein.

18. A method as claimed in Claim 1, carried out substantially as described in any one of Examples 4 to 7 herein.

19. A method as claimed in Claim 2, carried out substantially as described in Example 8 herein.

20. A coated article, whenever prepared by a method as claimed in any one of Claims 1 to 19.

21. A method for powder coating an article wherein the article is vibrated at a frequency within the range of from 1,000 to 10,000 per minute and an amplitude from 5 to 200 microns while coating the article with powder which has been charged with a DC voltage, the article optionally having a periodic motion superimposed on its vibration and optionally being coated in a fluidized bed, the apparent volume of the powder in the fluidized bed being 1-05 to 1 5 times as large as that in the stationary state.