US2538562A - Electrostatic coating method and apparatus - Google Patents

Description

Jan. 16, 1951 D. s. GUSTIN ET AL 5 5 ELECTROSTATIC comm METHOD AND APPARATUS Filed May 50, 1945 4 Sheets-Sheet l INVENTOR5 Q BY mum ATTORNEY n- 16,1951 D. s. GUSTIN Em 2 8, 2

ELECTROSTATIC COATING METHOD AND APPARATUS Filed May 50, 1945 4 Sheets-Sheet 2 INVENTORS 0.5. 5057/ 5. 14 h/fl/lV/0 BY m ATTORNEY Jan. 16, 1951 D. s. GUSTIN ET AL 2,533,562

ELECTROSTATIC COATING METHOD AND APPARATUS Filed May 30, 1945 4 Sheets-Sheet 5 INVENTORS 9.5. 60577 /7. n4 h/fivv/o ATTORN EY Jan. 16, 1951 D. s. GUSTIN ET AL I 2,538,562

ELECTROSTATIC comm; METHOD AND APPARATUS 4 Sheets-Sheet 4 Filed May 30, 1945 arsesare:

Y "The temperature "to which the container is subjected hasbeen found to be important. No't only does *heating of the container =cha'nge' it from --an electrically insulated. body to an electri- Cally" conducting -'container serving as an elecwas as hereinaften'desoribed,but the temperature also determines the pa-ttern assumed by the coating particles. It hasbeen Ffound that the temperature limits f'or good -c'oa'tings are not abrupt but rang'e from about175-. to aboiit 400 C. ilfowever, heating the "container -to a tem-a perature "near the :upper 'limit of "the rang'e at 7 fRferring again to Fig. 1, 'it wi-ll be' noted that the smoke "outlet -38 from the smoke generator aligns coaxia'lly with the container 33 when in position upon the rollers 3'4 and 35 '-so that :ithe fluorescent 'unaterial smoke is passed "into the container. In-order to accommodatecontainers off varying 1 length, '-'small -tubular ==pieces of vitre- '6usmateri'al-orthe like 49 are shown 'p'ositioned a-t each end-of the-container to thus act "as temporary extensions of the latter, and a suitable gasket or the like 50 is position'ed betw'een the "tubular eXtens-iorr fli -and thebracktS-Z'to prevent theescape *of' smoke, thereby causing it te -pass through the tubular extension into the conmine-res. "The enew divided fluorescent powder thus suspended in the air in the -form of srnolceis then iorii'lzeid and electrostatioally precipitated orito the inner surface "of the -container. 'A 'high voltage direct 'cu'rreritgenerator 52 an h generates a ciu'rentranging 'fromS'O to 50-mi'croamperes 'at'a voltage 6f' f1'0m"-50U0' t0 1 5, 000 Volts is -ca'rr ie'd by the base d 'This generator may' be of-any suitable type, it' ha's been found c0nvenient" -to ernploy -a power --pack'-'compr'ising a transformer together with suitable rectifier tubes wl'iich =will give-the desired 13. C. output energy, and sinoe the gen'erator per se forms no par't of F the present invention; it is 'be'lieved unnecessary that it be "further herein described-in detail. -It'- should' sliiifice-to say that one side "of the genera terns gr musc essuewnatse to the"metall io=-partssdf the apparatus, while the 'other side le -connected "toan' insulated tenn'inal fi4 supported byanuphardlyextending bracket 55 from "the base 5.

This bracket 55 also serves as-"oneinsulated hearing for an elongated electrode 55 provided w'ith a *sharopo'inted end 51 and: which-is' furt-her supporte'd' byan -additional insulated bearin'g" 5'8 likewise supported by a bracket 59 eXte-ndingupward'ly "from the base Electrical connection is made "from t'he insulated terminal 54" to the electrode 5 t by means of a brush contamt*"5i 'so that only the electrode "56 is atone'polarityywhile other em {parts or -fthe -apparatus "are "at :fiahiisitefpdlarity due to the 'ground onnectiori 53.

' The'lectrdde -56 isuuovd'so' thatitsshar ened enti e! p'asses longitudinally andccoax'iailyrofithe container m by a m'echa'nism-.comprising alrackbar BE having an angular-projection 63 :to which one end of the electrode 53 is secured. angular projecti'on is supported by small rollers or casters 64 engaging a track 65 secured to' the base *5, and the' rack-bar B2 meshes with r a small gear 66 afixed to a shaft 61 journalled to an upstanding bracketL'EB. .;Al'l'arige pulley 623,also-con nested 1 to i'the waiver -is rotated by a reversible electric motor ll. and-belt-connection 2. "Suitable limit switches 13 are provided which are adapted to be engaged by adjustable projections "-14 Anarried by the angular 1'aok-bar projection 63-=so as to stop the-electrical motor -13 when the 'sharpenedend S'i er v the electrodefifihas reached its ln practioing ithe method of the present inven tion, the apparatus-above described "is oper ed in"'-the 'follow'ingmanner. The container to -"b'e coated is first-placed upon the rollers "34 and 3}: indirect communicationwith the outlet 'tfi'of the smoke generator l, "either with or without the tubular extension M as required, and motor W started by the operatorclosing "a control switch -15, thus causing/rotation of the rollers 3 and 35-=as'-well as the coritainer3-3 in the direction in dicate'd by the arrows in Fig. "2. The gas jets 41 are then "lighted to 'heat the container maths operator closes a'revers'ing switch' 'lfi to startthe motor -l-fi so as to cause movementof therackbar 62 and electrode 56 to ther'ight as showniin Fig. '1. This accordingly causes the electrode 55 to slide through the insulated -'supported bearirigs carriedby"thebrackets' 55 and EE until the sharpened-electrode end'E-i "reaches the farthest end of the container 33, as shown in Fig. lfia't which time the projection 2 1 will engage the limit-switch 13"to stop the electric motor 10.

The electric motor 19 is then 's'tarted-by'closure of 'aswitchT'! which thus rotates the worm feed T0, "forcing 'the'finely dividedfluorescent powder within thereservoir through'the outlet Se. "The -va1ve'24'is then opened, allowing the air origas' under pressure to pass through the "chamberfifl where 'it sucks' up the fiuorescent powder, thus -suspending'itin theair in the form ofsmolge whichis exhausted through conduit '25 intothe reservoir '1. "This 'smokethen passes out of the reservoir 7 through'the out1et3llfand since the latter isin direct communication with thewitre- 'ous'container 3-3 either with or without the.tubu lar extensions AQythesmokeppasses into the container with leakage thereof being prevented'by 'thega-ketBO.

"The Operator next closes a'switch' 18' to connect thehigh'voltage D. '0, generator 52 to a -'souroe"of*electrica1 energy of the customary domestic potentia'l' of about volts, which accordinglyimpress'es a highD. C. voltage between the electrodes-fitandall metal partsof the-appa- "ratus. "A-lso, *since'the'gas flames are ionized; they "form "suitable -lectrodes for contacting' "the "Vitreous container 33 which, being'hot, isrendered"-'seIrii-coriducting, not only by the "gas flames, but by "virtue of its contact 'with the metal rollers 35 and 35, thus impressing the high "voltage across the vitreous container as one electrode and the axially disposed elongated electrode 56.

"IhemQtorH1 is againstarted'by closure ofthe reversing switch ls'so' as to withdraw theelectrode -56 from within the container :33. the

sharpened end- 51 *is movedrelatively-slowly,the

"higlr'ymtagoimpressed betweenthe-electrode 56 and the vitreous container 33 ionizes the fluores- 63 of the rack-bar 62 will engage the other limit switch 13 to again stop the electric motor M.

In describing the operation of the apparatus in the manner above noted, it should be understood that the coatingigjcan be just as readily applied by the high electrostatic field generated and during movement of the electrode 56 into the container 33 in the same manner as described.

relative to movement of the electrode 56 out of the container. However, it is preferable to coat the container only during outward movement of the electrode 56 since obviously the electrode must be outside of the container in order to remove the latter from the rollers. In order to save time so as to coat envelopes during either direction of movement of the electrode 56, the smoke generator can be made so that it detachably connects to the container and pivots about an axis so as to. allow withdrawal of the container from the electrode if desired.

It sometimes happens that in a fluorescent lamp coated by the foregoing electrostatic method, the liquid mercury often dislodges part of the coating, either by the free mercury rolling about and producing streaks in the coating or by sudden impact and splattering of the mercury, causing the coating tobe torn ofi in a characteristic ring shape. 7 c

To obviate this disadvantage, it was found that, by passing a suitable moistening material or agent through the electrostatically-coated fluorescent envelope and condensing or volatilizingj out, upon subsequent drying either in air or by application of heat to hasten drying, the coating why moistening the electrostaticallyapplied coating imparts toughness and greater tenacity thereto has yet been made, it is believed due to the fact that after application of the particles to the container wall, such as shown particularly in Fig. 8, the passing of the moistening material or agent into the container wets the particles to a different surface which is apparent, not only from thestandpoint of compactness and smooth surface, as shown by the comparison between the untreated cross-sectional view of Fig. 8 with the treated cross-sectional view of Fig. 9, but also from Figs. 10 andll. For example, in Fig. 10 it 'will be noted that the light reflected by the surface of the unmoistened electrostatically applied coating is far from uniform. Moreover, the coating has no definite pattern but a few isolated aggregates or agglomerates with the majority of the granules apparently being flattened or endwise and reflect very little light.

A comparison, however, with the moistened and dried electrostatically applied coating of Fig. 11 clearly shows that it is orientated and forms a continuous systematic network or interlocking of ranule aggregates or agglomeratesextending over substantially theentire surface, thus reflecting light from substantiallythe entire surface. Since a fluorescent lamp for best efficiency must reflect as well as transmit light, it can be seen from Fig. 11 that the efliciency is greatly enhanced by the increase of reflected light by the treated coating.

The treatment of the electrostatically .applied coating in accordance with the present invention also greatly increases the amount of transmitted light which is naturally a furthe requisite of an efficient fluorescent lamp. This can be seen from a comparison between Figs. 12 and 13 wherein very little light is transmitted through the untreated coating of Fig. 12 as can be observed from the extremely few white spots therein shown. Since the untreated coating is less compact, this may be due to scattering of the radiation among the coating granules or particles. In Fig. 13, however, the transmitted light is distributed like the reflected light, over substantially the entire surface much in thesame form or orientated netattendant increase in efficiency of the fluorescent lamp and at'the same time produces a tougher and more tenacious coating as above noted.

Moreover, by comparing the amount of both reflected and transmitted light from a fluorescent lamp coated by the method of the present invention with that of a lamp coated by the well known prior art flush method,as can be seen from Figs.

slight degree. Upon volatilization or condensing out of the moistening material or agent, and subsequent drying as above mentioned, there apparently results an expansion and contraction of the particles which increases their surface tension, causing more tenacious adherence.

Whatever the explanation, the fact remains that the electrostatically coated fluorescent lamp can withstand heavy impacts after this latter treatment without the particles in any way being quite readily loosened the particles.

Moreover the treated coating has an entirely 14 and 715, it is obvious notionly that the coating agglomerates are of an entirely different formation, but that the reflected and transmitted light resulting from the coating of the present invention is greatly increased over that of theflush coated fluorescent lamp. c

Accordingly, upon complete coating of the container or envelope 33, it is removed from the apparatus of Fig! 1 and inserted in the treating apparatus as shown in Fig. 4, which may form a part of the same machine orbe separate therefrom, as desired. This apparatus as shown comprises a base member 80 having a spring-clip fastener 82 projecting therefrom, 'as well 'as a metallic fitting 83. Although a plurality of such devices may be provided so that numerous lamps may betreated simultaneously, for the sake of \ivhile at the top a threaded bushing 86 and gasket a1 is provided to-forma fluid-tight connection withthe container 33 whenthe latter is inserted zi t s i -cl 82...

A Y-connection 88 having branches 89 and 913 communicates with the passageway 84 and with a source of low pressure air and a steam source,

through manipulation of respective valves 92 and 93. By operation of these valves, steam, together with low pressure air in the proper proportion,.is introduced through the passageway 84 and upwardly through the envelope 33. By proper mixture of the steam and air, the distribution of moisture will be substantially uniform from one end of the envelope 33 to the other without the formation of water droplets which might mar the coating.

There is, however, for every different percentage mixture of air and steam (water vapor) a certain dew-point temperature above which liquid water will not condense from the mixture, but below which it will condense. Also, the lower the percentage of steam in the air, the lower the dewpoint temperature will be. Therefore, if the envelopes are essentially at the same temperature throughout their length, on passing the airsteam mixture through them, each section of each envelope will condense moisture and thus be heated by the latent heat of condensation of water. Just as soon as enough water has condensed to raise the temperatureof the lower section of the envelope to the dew-point, no more moisture will condense on this section and the air-steam mixture will pass on by, condensing on the upper section of the envelope until the same dew-point is reached and the same quantity of water has condensed per unit area of the envelope. The envelopes are then allowed todry by passing air therethrough by closing of the steam valve 93, or they may be removed from the steaming apparatus and heated to more quickly dry them, if desired.

This steaming step fixes the coating so that it is more tenacious and resistant to mechanical injury from the mercury as before mentioned, thus conditioning the envelope for fabrication into a completed lamp 94 as shown in Fig. 5. The filamentary electrodes 95 and 96 are sealed into the ends of the envelope in the manner well known in the art, and after evacuation the lamp is filled with a gaseous medium, such as argon, neon or a mixture thereof, together with a small quantity of mercury. By reference to Fig. 5 and to the .microphotographic views of Figs. 9, 11 and 13, it will be seen that the fluorescent particles form an adhesive bond with the interior envelope wall and there is an entire absence of any intervening layer of foreign material which would otherwise impair the visible light given off by the fluorescent material under excitation by the ultra-violet radiations.

It will thus become obvious to those skilled in the art that a method and apparatus is provided by the present invention which enables the electrostatic precipitation of dry fluorescent powders directly onto the glass bulb or container for a fluorescent lamp from a cloud of smoke. Moreessence over, since-thercoating is applied; while the con- :tainer is'hot, it' iS'DOSSiblGT to combine the coating operationwithglass: tube makingmachinesin such mannerthat; the glass: never becomes cold from the time itis: drawn until; after it is completely coated- It: is also significant that since the coat:- ing particles are ionized and precipitatedfronira cloud of smoke, a uniform coating both in thickness. andparti'cledistribution and size is obtained as no velocity component is imparted to the particles by fluid pressure during precipitation. In-

i asn uch asndbinder required bat the material is fixed by a moistening treatment, a-closer tenacious coating results with the inherent fiuorescent properties of the material being in no way impaired, thus not only increasing the efiiciency of the light output from the completed fluorescent lamp, but also producing an entirely different color of the light'from that of a lamp in which a binder is used. A further economy resulting from the method of the present invention resides in the fact that substantially all of the fluorescent material is used and none is lost.

Although one specific embodiment of the present invention has been shown and described, it is to be understood that stil further modifications thereof may be made without departing from the,

spirit and scope of the appended claims.

We claim:

1. The method of forming a uniform coating of a stabile fiuorescent material on the inner surface of the vitreous envelope for a fluorescent lamp which comprises generating a smoke by suspending finely divided fluorescent powder in a gaseous medium, applying a slight pressure to the smoke to cause it to drift into the envelope in substantially a definite direction, subjecting the smoke to a highelectrostatic field to cause ionization of the suspended particles and deviation thereof from their normal direction of drift with attendant coating of the envelope walls solely by precipitation of such particles, and passing a readily condensable vapor which does not decompose the coating material into the envelope over the precoated surface thereof, and subsequently drying the same to make the particles adhere more tenaciously to the envelope.

2. The method of forming a uniform coating of a stabile fluorescent material on the inner surface of the vitreous envelope for a fluorescent lamp which comprises generating the smoke by suspending finely divided fluorescent powder in a the smoke to cause it to drift into the envelope in substantially a definite direction, subjecting the smoke to a high electrostatic field to cause ionization of the suspended particles, and deviation thereof from their normal direction of drift with attendant coating of the envelope wall solely by precipitation of such particles, and passing steam and air into the envelope over the precoated surface thereof, and subsequently drying the same to make the particles adhere more tenaciously to the envelope.

, 3. The method of forming a uniform coating of a stabile fluorescent material on the inner surface of. .thezvitreous envelope for a fluorescent lamp which comprises generating a smoke by suspending =finely divided fluorescent powder in a gaseous medium, applying a slight pressure totion thereof from their normal direction of drift with attendnt'coating ofthe'lenvelope wa'll solely by precipitation of such particles; and passing 'a vo1ati1e:fl1iijc'1 which does notdec ompose the coat- :ing into his envelope over'tlie pfecoated surface 'thereofy nd subsequently drying the same to make the jaarticles adhere more tenaciously (to the envelope.

DANIEL s. GUSTIN. ALBERT W. WAINIO.

REFERENCES CITED The following references are of record in the 'file of this patent:

Number Number Australia m 19,1939

Claims (1)

1. THE METHOD OF FORMING A UNIFORM COATING OF A STABILE FLUORESCENT MATERIAL ON THE INNER SURFACE OF THE VITREOUS ENVELOPE FOR A FLUORESCENT LAMP WHICH COMPRISES GENERATING A SMOKE BY SUSPENDING FINELY DIVIDED FLUORESCENT POWDER IN A GASEOUS MEDIUM, APPLYING A SLIGHT PRESSURE TO THE SMOKE TO CAUSE IT TO DRIFT INTO THE ENVELOPE IN SUBSTANTIALLY A DEFINITE DIRECTION, SUBJECTING THE SMOKE TO A HIGH ELECTROSTATIC FIELD TO CAUSE IONIZATION OF THE SUSPENDED PARTICLES AND DEVIATION THEREOF FROM THEIR NORMAL DIRECTION OF DRIFT WITH ATTENDANT COATING OF THE ENVELOPE WALLS SOLELY BY PRECIPITATION OF SUCH PARTICLES, AND PASSING A READILY CONDENSABLE VAPOR WHICH DOES NOT DECOMPOSE THE COATING MATERIAL INTO THE ENVELOPE OVER THE PRECOATED SURFACE THEREOF, AND SUBSEQUENTLY DRYING THE SAME TO MAKE THE PARTICLES ADHERE MORE TENACIOUSLY TO THE ENVELOPE.

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