Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS3903321 A
Publication typeGrant
Publication dateSep 2, 1975
Filing dateJan 22, 1973
Priority dateJan 25, 1972
Also published asDE2203351B1
Publication numberUS 3903321 A, US 3903321A, US-A-3903321, US3903321 A, US3903321A
InventorsHans J Schaad
Original AssigneeHans J Schaad
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method for charging plastic powder electrostatically by friction only
US 3903321 A
Abstract
A method of coating particles with plastic material powder, such as epoxide resin powder without the use of an external electrical field comprises the steps of mixing the powder in a stream of pressurized air, accelerating the air and powder stream to supersonic velocity through a venturi device, electrostatically charging the plastic material powder by friction against a polytetrafluoro ethylene surface. The front portion of the nozzle discharging the material is grounded by a metallic sleeve.
Images(3)
Previous page
Next page
Description  (OCR text may contain errors)

United States Patent [191 Schaad [451 Sept. 2, 1975 METHOD FOR CHARGING PLASTIC POWDER ELECTROSTATICALLY BY FRICTION ONLY [76] Inventor: Hans J. Schaad, Erlenbachstrasse 37, 46 Dortmund-Schuren, Germany [22] Filed: Jan 22, 1973 21 Appl, No.: 325,231

[30] Foreign Application Priority Data Jan. 25, 1972 Germany 2203351 [52] US. Cl. 427/25; 427/27; 239/15; 239/602; 239/DIG. 19; 317/3 [51] Int. Cl. BOSB 5/02; B44D l/O94 [58] Field of Search 117/17; 317/3; 239/15, 239/602, DIG.19; 427/25, 27

[56] References Cited UNITED STATES PATENTS 2,477,947 8/1949 Yadoff 317/3 2,574,284 11/1951 Prochazka.

2,761,997 9/1956 Forest ct al 117/17 3,339,841 9/1967 Beach 1 i 239/15 3,434,859 3/1969 Benjamin.... 117/17 3,507,686 4/1970 Hagenbach..... l17/17.5 3,521,815 7/1970 Szasz 239/15 3,533,835 10/1970 Hagenbach ct a1. ll7/17.5 3,597,668 8/1971 Yoshimine 317/3 3,630,442 12/1971 Probst 239/15 Primary Examiner-Michael Sofocleous Attorney, Agent, or" FirmFitch, Even, Tabin & Luedeka [5 7] ABSTRACT A method of coating particles with plastic material powder, such as epoxide resin powder without the use of an external electrical field comprises the steps of mixing the powder in a stream of pressurized air, accelerating the air and powder stream to supersonic velocity through a venturi device, electrostatically charging the plastic material powder by friction against a polytetrafluoro ethylene surface. The front portion of the nozzle discharging the material is grounded by a metallic sleeve.

3 Claims, 3 Drawing Figures PATENTED SEP 2 975 sum 1 o 3 Z ll |lllallililllvlllillallrltf METHOD FOR CHARGING PLASTIC POWDER ELECTROSTATICALLY BY FRICTION ONLY The present invention relates to a method of coating articles with plastic material powder. expecially epoxide resin powder, in an electrostatic field, and an apparatus for carrying out such method.

It is well known to spray plastic material powder onto articles by means of accelerated air streams, whereby the particles of the plastic material powder are subjected to an electrostatic charge by an electrical field, so that they settle on the grounded or oppositely charged article because of the electrical attraction. Upon baking, the layer of plastic material powder provides a surface coating being superior to conventional lacquer coatings.

However, it is of disadvantage in the known powder coating methods and apparatuses that they require an electrostatic voltage source which, because of the high voltage used (between 30 and 150 kilovolts), necessitates expensive safety measures.

It is the object of the invention to provide a method and an apparatus for the powder coating of articles which do no longer show the abovementioned disadvantages and which, in particular, are more safe in operation, which render possible an ideal distribution of polarization and ionization which, otherwise, is disturbed by foreign charge, which are capable of operating without any additional voltage source for an electrostatic field, and wherein the apparatus is simple, easy and economical to manufacture and operate, and wherein it is possible to adjust the thickness of the coating by adjustment of the flow velocity or of the quantity of air, respectively.

According to the invention, this object is solved in that the plastic material powder is electrostatically charged by friction against polytetrafluoro ethylene (PTFE), and then sprayed onto the article to be coated.

Furthermore, the object according to the invention is solved in that the plastic material powder, after the selfcharging by mutual friction between the plastic material particles and/or with the nozzle walls, is further passed through highly insulating materials only which, preferably, have a very high resistivity of about from to 10 ohms per centimeter.

In a manner being particularly surprising to the expert, it has shown that the good coating according to the invention can be obtained not only by using polytetrafluoro ethylene, but that, further, polytrifluoro monochloroethylene and other fluoro compounds and solid, highly insulating materials, such as amber, mica and quartz glass, may be employed.

Prior condition for the usability is, on the one hand, a high insulating effect which substantially exceeds the insulating effect of e.g. standard glass and the like, and, on the other hand, a surface consistency which safely prevents the plastic material powder from adhering thereto. Additionally, the material must be chemically inert towards the plastic material powder, too.

In the following. an exemplary embodiment of the invention is explained in greater detail by referred to the accompanying three drawings.

FIG. 1 schematically shows in sectional view an apparatus for discharging electrostatically charged plastic material powder, whereby the powder reservoir is attached directly to the jet pipe.

FIG. 2a pistol for the powder coating of articles according to FIG. 1 and FIG. 3a pistol according to FIG. 2 but without having the powder supply attached to the pistol.

The powder spraying apparatus according to the invention comprises primarily a jet pipe I and a mixing section 2 positioned upstream thereof, both components being formed of polytetrafluoro ethylene. The jet pipe 1 per se may consist of a nozzle portion Ic having a subsequent expansion portion lb which is followed by the actual spray pipe la. The spray pipe Ia may merges with a diffusor 7 wherein the velocity of the outflowing powder/air mixture is lowered, and includes a perforated plate 6 which is likewise formed of polytetrafluoro ethylene and which provides a uniform flow distribution.

The powder enters the mixing portion 2 which preferably is a cylindrical chamber, via a passage 8. The passage 8 may be either directly surrounded by a powder reservoir 9, (FIGv 2), or the powder may be fed thereto over a certain distance (FIG. 3).

The rear wall of the mixing or distributor section 2 includes an opening into which a nozzle housing 4 opens. The nozzle housing which likewise consists of polytetrafluoro ethylene, has a nozzle needle passing coaxially therethrough, whereby the quantity of the entering air may be controlled by the position of this needle. Of course, it is also possible to introduce the air via the nozzle housing 4, whereby the quantity of air may be adjusted at some distance, e.g. in the pressurized air feed conduit. In such case, the nozzle needle may be omitted.

The apparatus according to the invention may be formed either as a hand spray gun by being provided with a handle 11 or it may be positioned with some distance from the powder reservoir either stationarily or so as to be movable (FIG. 3).

It is importaant for the generation ofa very high electrostatic field of e.g. up to 200 kilovolts that the whole quantity of powder within the accelerated air flow may contact the polytetrafluoro ethylene only, so that, on the one hand, there are produced ionized and polarized powder particles, and, on the other hand, the resulting charges are not dissipated afterwards.

In a manner being surprising to the expert, the selfcharging of the powder particles renders possible both a polarization and an ionization of the powder particles in ideal combination, so that, when selecting high flow velocities, high thicknesses of the coating in comparison with conventional coating methods may be obtained.

By adjustment of the flow velocity, the deposition of the powder particles may be controlled, so that smaller layer thicknesses may be adjusted, too.

By forming the nozzle with a minimum crosssectional area and a subsequent expansion zone within the extension of a continuous curvature, it is possible to produce supersonic flow velocities.

Surprisingly, the use of polytetrafluoro ethylene in combination with the flow velocity does not only result in that a voltage source becomes superfluous, but also, as mentioned above, in that a superior coating is obtained.

The charging and, thus, the coating depend on the temperature and on the humidity of the pressurized air, Therefore, it is advisable to provide for a pre-drying of the air when operating at high humidity. However, it has been found that an adequate coating by means of the method according to the invention can be obtained even at very high humidities (of above 90 "/1 Epoxide resin is particularly useful for the processing in accordance with the present method. It has been housing. In case of the pistol according to FIG. 3, where coating powder is supplied via tube 8, the control of the powder supply is effected by switch 16 as well.

found. however. that polyester. polyamide and other Referring now to the following table. a number of explastic material powders can be processed with good oriment were performed with apparatus constructed re l o by ing h me hod according to the inin accordance with the drawings using several different vcntion. materials or a combination of materials for the Venturi Surprisingly, the electrostatic properties of polytetranozzle 1 Also, several types of powder including epfluoro ethylene are superior to any other plastic materil0 oxide powders were used with the apparatus of the varials. Enclosed is a Table showing the surprising effect ous materials in attemping to determine the best comobtained in the processing of various types of plastic bination of materials and powders for use with the illusmaterials when using jet pipes and mixing ham r trated apparatus. It was discovered that the use of polymade Of polytetrafluo o hyle e Evidcmlyt A o g tetrafluoro ethylene, particularly when using cpoxide can still be obtained when using po yethylene; ho powders produced the best results. The coating formaever, the use of polytetrafluoro ethylene results in a imtion on the article sprayed was also analyzed. proved effect which is absolutely unexpected by the ex- M ifi ll an electrostatic measuring instrw pert and whi h. in i Of the p i lmOWIedgC ment was used to measure the charge of the powder HbOuI the effect of plastic parts in Powder p y guns, particles in micron amperes. When teflon is used for could not be foreseen, which fact is also demonstrated th atu the average charge was at least one miby the submitted compilation. cron amp and for the epoxide powders, the charge was In order to obtain the effect according to the inveneven 1.5 to 2.5 micron amps as shown in the Table. tion, it is essential that all portions of the mixed flow of R f i m articularl to the Table, it will be driving air and powder are in contact with polytetraeen that first Column is labeled Teflon has a first fluoro ethylene surfaces only, and that a sufficiently subcolumn with numerals showing the micron amperes high flow velocity within the jet pipe is produced by (uA) measured by the electrostatic measuring instruusing driving air of high pressure. ment for the various powders. Thus, the first five epox- Apparently, sonic flow velocity and supersonic flow ide powders measured at least one micron amp charge. velocity may be adjusted with the apparatus according In contrast thereto, when using such materials as PVC to the invention formed in the manner of a Laval nozor polycarbonate or polypropylene for the Venturi tube zle. already by using driving air having a pressure of the average charge of the powder was less than 1 and more than 2 atmospheres. in many cases zero.

Advahmgeously at least the from Portion of the j The coating formations were also analyzed and rated P P l 15 Surrounded by a grounded mctalhc Sleeve 3 during these experiments. A very good coating forma- Order to Prcvcht annoyance of the Operators by Spark tion is indicated in the Table by a numeral 1 in the secgchcmhoh the Surface of the J P P Furthermore, 0nd sub-column under the letter S which refers to coatthis metallic sleeve 3 prov1des a ccrtam improvement i f ti Th the fir t fi e e oxide wder had of the effect according to the inventio a very good coating formation only when Teflon was If spreading device I2 is m unted ithin th nOZZle used for the Venturi nozzle. Several other powders had 14 of jet pipe 1 e.g. by means Of ri s 13, this spr i g a good coating formation as indicated by the numeral device as well as the ribs and the nozzle 14 should con- 2 when u i Teflon. in contrast, the PVC, polycarbonsist of polytetrafluoro ethylene. According to FIGS. 2 ate and polypropylene coatings generally were labeled and 3jet tube 1 may be covered by an outer protective 3s, 4s, and 5s, with 3 being only usable, 4 being sleeve 15 made of PVC or the like. According to FIGS. 5 not usable and 5 indicating no adhesion at all. 2 and 3 switch 16 within the handle ii via a cable 17 by Thus, it is readily apparent that surprising and superior means not shown controls supply of pressurized air results were obtained by using Teflon and epoxide powflowing to the pistol through flexible tube 19 and via ders as set forth in the first upper left hand portion of removable nozzle 18, being screwed into the pistol the Table.

TABLE Coating Formation (S) Current (,u A) and Coating formation (S) l very good with Venturi nonles made of various 2 good materials and with the use of different 3 still usable powders 4 not more usable 5 no adhesion Teflon Glass- (iraph- PVC Hard Poly- Poly Poly- Poly- Alu- TYPE OF Teflon ite- Rubber ethcarproureminum POWDER Teflon ylene hon-ate pylene thane Metal #A S uA S p.A 5 [.LA S ;1.A 5 [LA S tA S [-LA A S p.A S

Epoxide +1.5 1 +1 1 +0.5 2 4 3 (1.5 2 0 4 0 4 (1 4 0 5 Standard Epoxide +2 1 +1.5 2 0 3 o 4 0 3 o 3 0 3 0 3 o 3 0 5 Graphite Epoxide +1 1 +1 2 +1 2 o 4 o 4 0 3 o 4 0 5 0 3 0 5 Black Epoxide +1 1 +1 2 +0.5 3 0 3 o 4 0.5 2 o 4 o 5 0 o 5 Metal Epoxide +2.5 1 +0.5 2 +1.5 1 +0.5 2 +0.5 2 +1 1 0 3 0 3 o 3 (1 5 White Polyester Thermoplast +0.5 4 +l. 2 +l.5 2 5 0 S 0 U 5 O 5 t) S O 5 TABLE Continued Coating Formation (8) Current A) and Coating formation (S) l very good with Venturi nonles made of various 2 good materials and with the use of different 3 still usable powders 4 not more usable 5 no adhesion Teflon Glass- (iraph- PVC Hard Poly- Poly- Poly- Poly Alu- TYPE OF Teflon ite- Rubber eth ear proureminum POWDER Teflon vlene honate pylene thane Metal ;.LA S ;LA S ,uA S #A S 1A S ,uA S ,uA S 4A S #A S #A S Polyesten curable U 5 5 l .5 2 5 5 (l 5 5 5 5 (l 5 Polvester 3 4 0 470 3 +0.53 0 4 O 3 0 4 +0.52 Polyester +5 2 +4 2 +05 3 U 5 O 5 U 4 l) 5 (l 5 (l 5 l) 5 Polvamidc +1 2 0.5 3 +0.5 3 +1 2 +0.5 2 2 +1 2 +l 2 Z (l 5 (327.05 PVC +0.5 3 +05 4 +0.5 4 0 5 (l 5 "()5 2 5 (l 5 U 5 (l 5 Polyurethane +0.5 3 -H),5 3 +l 2 (l 5 (l 5 t) 5 5 t) 5 (l 5 (l 5 positive;

negative: 0 no indication 1 Claim: tion of the charges on said particles, and discharging l. A method of coating articles with an epoxide pow der within an electrostatic field without applying an external charge to the article comprising the steps of: mixing the epoxide powder in a stream of pressurized air within a mixing section of polytetrafluoro ethylene, accelerating the air and powder stream to supersonic velocity through a venturi device of polytetrafluoro ethylene, electrostatically charging the material powder solely by friction of the material flowing against the polytetrafluoro ethylene in said venturi device, passing the charged powder through a spray device having a surface of polytetrafluoro ethylene to prevent dissipaand spraying the clectrostatically charged epoxide powder from the spray device onto the article to be coated.

2. A method in accordance with claim 1 including the step of: grounding a nozzle from which the spray is discharged.

steam of pressurized air within the mixing section.

* l f =l

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2477947 *Aug 24, 1945Aug 2, 1949Electronic Dusting CorpMethod and device for projecting powders carrying electrostatic charges
US2574284 *Mar 8, 1949Nov 6, 1951Rudolf ProchazkaProcess for the production of high electrostatic charges by self-excitation
US2590534 *Jul 22, 1949Mar 25, 1952Georges Truffaut EtsApparatus for producing electrified powders by friction
US2657339 *Jul 23, 1947Oct 27, 1953 Method and apparatus for producing
US2761997 *Jun 15, 1950Sep 4, 1956Magnaflux CorpApparatus for detecting surface discontinuities
US3339841 *Feb 12, 1965Sep 5, 1967Beach Jr Howard WElectrostatic paint spray gun
US3434859 *Dec 29, 1964Mar 25, 1969Harshaw Chem LtdMethod for depositing a coating on the internal walls of capillary or small-bore tubes
US3507686 *Jun 23, 1967Apr 21, 1970Xerox CorpMethod of coating carrier beads
US3521815 *May 7, 1969Jul 28, 1970Szasz ImreGuns for the electrostatic spray coating of objects with a powder
US3533835 *Oct 11, 1966Oct 13, 1970Xerox CorpElectrostatographic developer mixture
US3597668 *Oct 16, 1969Aug 3, 1971Goro FujiiElectrostatic charger for liquid fuel by friction
US3630442 *Jun 16, 1969Dec 28, 1971Randburg Electro Coating CorpElectrostatic coating method and apparatus
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4020393 *Jul 16, 1975Apr 26, 1977Estey Dynamics CorporationElectrogasdynamic coating device having composite non-conductive flow channel, and hollow ionization electrode for an air jet
US4071192 *Mar 29, 1976Jan 31, 1978Coors Container CompanyTribo-electro-gas-dynamic powder charging apparatus
US4090666 *May 19, 1976May 23, 1978Coors Container CompanyGun for tribo charging powder
US4109861 *Sep 16, 1976Aug 29, 1978Solar Suede CorporationElectrostatic flocking system
US4183974 *Mar 24, 1977Jan 15, 1980W. R. Grace & Co.Thin film of resin applied by pulses of powder spraying followed by softening and curing
US4225090 *Sep 7, 1979Sep 30, 1980Toyota Jidosha Kogyo Kabushiki KaishaDevice for painting by electrostatic powder spraying
US4233335 *Mar 6, 1979Nov 11, 1980Etlin Vladimir NCirculating powder through a conduit loop
US4359192 *Feb 12, 1981Nov 16, 1982Toyota Jidosha Kogyo Kabushiki KaishaTriboelectric powder spraying gun
US4630777 *Dec 20, 1985Dec 23, 1986Nordson CorporationPowder spray gun
US4706890 *Apr 15, 1986Nov 17, 1987Ransburg-Gema AgMethod and apparatus for electrostatic coating of articles with powdered coating material
US4765544 *Jan 15, 1987Aug 23, 1988Kopperschmidt-Mueller Gmbh & Co. KgElectrostatic spray gun
US4798338 *Dec 5, 1986Jan 17, 1989Veb Infrarot-Anlagen OranienburgApparatus for the electro-kinetic charging of powdered materials
US4798340 *Jan 9, 1987Jan 17, 1989Esb Elektrostatische Spruh- Und Beschichtungsanlagen G.F. Vohringer GmbhElectrostatic device for powder spraying with triboelectric powder charging
US4886215 *Mar 14, 1989Dec 12, 1989Nordson CorporationHand operated powder spray pistol
US5344082 *Oct 5, 1992Sep 6, 1994Nordson CorporationTribo-electric powder spray gun
US5402940 *Jun 21, 1994Apr 4, 1995Nordson CorporationTribo-electric powder spray gun
US5552191 *Dec 20, 1993Sep 3, 1996Morton International, Inc.Triboelectric coating powder and process
US5637136 *May 8, 1996Jun 10, 1997Morton International, Inc.Triboelectric coating powder and process
US5731043 *May 10, 1996Mar 24, 1998Morton International, Inc.Particle sizes
US5756164 *Jan 22, 1997May 26, 1998Morton International, Inc.Spray depositing thermosetting resin powder of preferential particle size on to a substrate
US5811158 *Apr 22, 1997Sep 22, 1998Kao CorporationMixing paints and fluidity improver which posseses charge control function, blowing paint through outlet using pressurized air
US5851607 *Aug 27, 1997Dec 22, 1998Morton International, Inc.Triboelectric coating powder and process for coating wood substrates
US5885351 *Feb 18, 1997Mar 23, 1999Pfs Thermoplastic Powder Coatings, Inc.Tribocharge applicator device
US5997959 *Oct 4, 1995Dec 7, 1999Societe Lcs InternationalMethod and apparatus for electrostatically painting parts made of dielectric or low-conductivity material
US6156392 *Jul 13, 1999Dec 5, 2000Nylok Fastener CorporationProcess for triboelectric application of a fluoropolymer coating to a threaded fastener
US6202945Aug 14, 1998Mar 20, 2001Kao CorporationMethod and apparatus for electrostatic powder coating
US6645300Feb 28, 2002Nov 11, 2003Nordson CorporationUnipolarity powder coating systems including improved tribocharging and corona guns
DE2816850A1 *Apr 18, 1978Oct 31, 1979Mueller Ernst & CoElektrostatische pulver-spruehpistole
DE2905223A1 *Feb 12, 1979Aug 16, 1979Nordson CorpVorrichtung und verfahren zur elektrostatischen spruehbeschichtung
DE4312262A1 *Apr 15, 1993Oct 20, 1994Gema Volstatic AgElektrostatische Sprühvorrichtung
Classifications
U.S. Classification427/474, 239/602, 361/227, 427/475, 239/DIG.190, 427/459, 239/698
International ClassificationB05B5/047, B05D1/06, B05B5/16, B05B5/025, B05B5/057
Cooperative ClassificationB05B5/1691, B05B5/047, Y10S239/19
European ClassificationB05B5/047