US2462125A

US2462125A - Electrophoretic coating of metal articles

Info

Publication number: US2462125A
Application number: US528619A
Authority: US
Inventors: Oakes Lesley Francis
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1943-07-23
Filing date: 1944-03-29
Publication date: 1949-02-22
Anticipated expiration: 1966-02-22

Description

Patented Feb. 22 1949 EIiECTROPHORETIC C ATINGIOEMETAL ARTICLES e Lesley Francis"bakes, Alexandria; near Sydney,

Australia, *assignor to International Standard lct'ric Corporation, New.Y -k,o p'oration of- Delaware 2 N6"Drawing. Application March 29, 144; '"Serial N0. 528,619. In AustraliaJ-uly 23, 1943 11' Claims.

This invention relates tonthe 'electrophoretic application of coating materials (such as aluminium oxide, or silicomdioitide, or a mixture of said -f-oxides,-or-an alkaline earth carbonate suchas "barium carbonate, or strontium carbonatejor a mixturepf such carbonates) to heater elements for thermionic valves or other metal articles required to have a coating thereon for'enhancing thermionic emissivity or for insulation purposes rthe like.

Existing electrophoretic ocating methods are not i'rholly'satisfactory; because the coatingma- 'terial particles in *suspension inthe electro- 'phoreticbath' (that is tosay, in colloidal solution, or non-colloidalsuspension, or partly one and am me other) tendtocoalesce or coagulate in the-bath fluid, and-then rapidly precipitate, or at best, are sparsely deposited on the article being' coatedas' relativelylarge particles which do not readily or tenaciously adhere thereto-as an ii-nbroken uniform coating.

' Themain object of-the present invention is the provision of an electrophoretic bath fluid which avoids; orpractically 'avoids, thisdisability. A,

further object is "therprovision of suchf a fluid -='-'and amethod "of employing same;-"which in -a -gneral way 'enablea 'highly'efiective electro- -phoreticcoating to-beobtained in a more efficient manner than has been the-case heretofore.

*The -inain' characteristic of this invention may best'ated as consistingin an electrophoretic'bath fluid composed of a suspensionofa coating material in an organic liquid, and a dispersive agent comprising a member of 9 the duali'group 'cons'i'sting ofi tannic acid; and a miXt'ure'of an alkylolaniinewsuch as' triethanolamine) with nitrocellulose. lin -the'*case "ivh'ere the dispersive 'agent is a mixture- 0f an :alkylolamine' and nitrocellulose -these two ingredierlts inely be present in almost any proportion of one in relation to the "other. fl'owevenin the-course'of"experiment, I have found that when the dispersive ag'ent" consists eritiie ly5-"or almost entirely, of -tri'eth'anol'am'ine the objects of the invention are insufiiciently realised; also, nitrocellulose by itself was found to gi-ve a- -'deposited coating which by reason of 'gelatifiousnature, lacked mechanicalstrength andadl ies'ive "tenacity. "Consequently it; is "pre- The quantity of coating--ma-terial included in, the bath is such that its weight-amounts to.-from about 3% 'to -eabout+20% of theweig-ht of' the organic liquid, az preferred proportion beingabout 10%. "The dispersive-agent is included insuch quantity that'its Weight amounts to fromxabout 0.3% up to as much-as 10% of -the weight of organic liquid, a preferred proportion being about 1%. I I

.Inthe coating of articles accordinghereto, it is foundthat the electricalresistance ofthe coatings is somewhat impaired bythe-rpresencesof carbonaceous residues therein. This forms. a "basis ofcomparison of the relativemerits of the two kinds of dispersiveagent named herein.

' In the case of tannic acid the carbon residues may only be. removed from a coating with difficulty, but an advantage due to this dispersive agent ..is that thebath voltagemay be very low;

f for example, from 22- to 50 volts D. C. On the "other hand, bath voltage inwthevcase of an alkylol- 'amine fiitrocellulose dispersive. agent, may ref {quireto be as'mli'chzas 300 volts D. 0., but with "thin coatings .the carbon content ..is practically 25 negligible, andvvith thick coatings (or thin coatings, for that 'matterY'the carbon may be readily -removed by burning. A further consideration in -regard to" the 'use' of tannic acid, is that the deposited coating r'ea'dily' diffuses into any drops'of "bath liquid-Which.areallowed' to remain on the article after removalthereof'from the bath, or into the bath'liquid'itself if the article is allowed "'to remain therein after' the current is turned off. "Thesela'st-mentioned' difiiculties may be readily overcome, however, by the precaution of removingthe coated article from the bath before the 'currentis'switched'off; and by giving the article a vigorous shake immediately after it'has been --rem'ovedfromthebath.

40 "The following exam'plesf illustrate preferred modesof" carrying the invention into practical The b'ath materials cons'ist bf the "ingredients set forth in the following Table' Ain (or in about) the proportion's th'ere stated:

Theparticle sizeof the o'xides is preferablybf the order of 40 microns or less. .It is e sential, or at least desirable, that all traces of moisture. be removed from the aluminium oxide. This may be readily effected by heating it at about 1000 C. for about 1 hour; and, if it is not to be used immediately, bottling it as soon as it is cool enough for that operation.

The bath is prepared by thoroughly mixing the nitrocellulose and triethanolamine together (using a mortar and pestle, for example) to form a homogeneous paste. The oxides are then added to this paste together with suflicient of the acetone to give a doughy consistency to the whole. The resulting doughy mass is then thoroughly kneaded to ensure breaking up of oxide agglomerates. Further acetone is then added in small quantities, and with continued kneading or stirring, until a thin creamy consistency is reached. The remainder of the acetone is then added and the bath fluid thus obtained is given a final stirring, and for preference is stirred from time to time until required for use.

The steps of preparing the bath fluid may be performed manually or mechanically. The essentiality in the mixing, is that all the solid particles be thoroughly Wetted. The mixing steps described above achieve this result in a satisfactory manner.

In coating an article, the bath fluid described above may be contained in a beaker or other vessel and the article constitutes the anode. A suitable material for the cathode is nickel. The cathode may be formed as a loop which loosely fits the vessel interior. The bath fluid is preferably stirred continuously, or at least frequently,

during the coating operation. The potential difference across the electrodes may be from 100 to 300 volts D. C. We have found that 200 volts, giving a current density of about 250 ma. per square inch of anode area, gives a satisfactory result. At this voltage, the article is immersed in the bath for a period of from 10 to 30 seconds, depending on the coating thickness required. If the article remains in the bath appreciably longer than 30 seconds it is found that gas forms on the anode which tends to dislodge the deposited oxide film. Consequently if the required coating thickness is greater than that which is obtainable within about 30 seconds immersion, it is necessary to effect the coating in two or more stages, and between these stages, to rid the article of gaseous accumulation, and preferably, at the same time eifect a stabilization or consolidation of the material already coated. These requirements may be effected by frltting the coating film to the wire (between immersions) by ignition in wet hydrogen at a temperature of about 1600 C. Experiment has shown that the fritting operation gives a sound adherent coating which has a long useful life and is substantially proof against relatively rough usage. Because of this, and irrespective of whether the coating is applied in one or several stages, it is desirable that fritting, as described, be effected as a final step in the coating process.

Where the required thickness of coating is such as to necessitate three or more immersions in the electrophoretic bath, the accumulation of carbonaceous residues in the coating becomes deleterious. This accumulation may be readily removed by preceding or replacing the step of fritting, following the second immersion, by the step of burning the carbonaceous matter in air and at low temperature; that is to say, at a temperature I which is not substantially greater than the ignition temperature of carbon.

A cycle of operations which gave a satisfactory result in the coating of themiionic valve heater elements is as follows:

ExlmrLI 2 The bath materials consist of the ingredients set forth in the following Table B in (or in about) the proportions there stated:

Table B Aluminium oxide grams 10 Silicon dioxide 7 do 3 Tannic acid do 0.3 Acetone mls The particle size of the oxides and the dryness of the aluminium oxide are selected or conditioned as set forth in Example 1.

The bath is prepared by thoroughly mixing the oxides and the tannic acid together with some of the acetone, added intermittently and in small quantities, until a creamy consistency is arrived at. The remainder of the acetone is then added and the whole mixed and stirred as set forth under Example 1.

In coating an article, the article is used as anode with a cathode of nickel, and the voltage applied to the electrodes may be about 36 volts D. C. The article is immersed for from 10 to 30 seconds depending on the coating thickness required. Where a thicker coating than that obtainable in 30 seconds is required, the coating may be effected in several stages each of which (or the first and last of which) is (or are) followed by fritting (as, and for the purposes, already described herein).

A cycle of operations which gave a satisfactory result in the coating of thermionic valve heater elements is as follows:

It will be appreciated that although in both examplesset forth herein the article to be coated is used as anode, the article may constitute the cathode, depending on the migration direction of the coating material suspensoids due to the electrical properties thereof and the nature of the organic liquid employed. In short, the dispersive agent is effective independently of the migration direction. A I I.

great'advantage over the'use of a single one of the two '-'dispersive agents, 'inasmuch '-as, while inclusion'of tannic acid with the alkylolaminenitrocellulose mixture-enables operating voltages to fbe gsomewhat lowered, it also introduces the difiiculty'(referred to herein) of removing carbon-fromadeposited coatingyand for this reason the alkylolamine-nitrocellulose mixture (by'itself) is the preferred dispersive agent when it is essential that'the coatings be free of carbon, and tannic acid (by itself) is preferred when freedomfrOin carbon is notabsolutely essential.

It is to be noted further,"in regard to the alkylolamine-nitrocellulose member of the dual'group, that alkylolamines other than triethanolamine (such as-monoethanolamine or diethanolamine, for example) may be employed. In our experiments, however, bestand satisfactory results have been obtained with ,iiriethanolamine and consequentlythatcompound is presently preferred as against other alkylolamines.

What is claimed is:

1. In a process for coating a metal article with an inorganic refractory substance by electrophoresis; the step that comprises making the article the anode of two electrodes in a bath having substantially the following composition expressed in proportions based on weight:

Parts An organic liquid of the class consisting of acetone and methanol 100 Inorganic refractory substance 3 to 20 A dispersive agent 0.3 to 1 said dispersive agent being a mixture consisting of an alkylolamine and nitrocellulose each constituting at least by weight of the said dispersive agent.

2. In a process for coating a metal article with an inorganic refractory substance by electrophoresis; the step that comprises making the article the anode of two electrodes for a period of to 30 seconds in a bath having substantially the following composition expressed in proportions based on weight:

Parts An organic liquid of the class consisting of acetone and methanol 100 Inorganic refractory substance 3 to A dispersive agent 0.3 to 1 said dispersive agent being a mixture consisting of an alkylolamine and nitrocellulose each constituting at least 5% by weight of the said dispersive agent.

3. In a process for coating a metal article with an inorganic refractory substance by electrophoresis; the step that comprises making the article the anode of two electrodes for a period Of 10 to seconds in a bath having substantially the follo'wing' com osition ex ressed in proper-'- tions based on weight: I

Parts An organicliquid of the class consisting of "aceton'e'and methanol Inorganic refractory substance "3 to A dispersive agent .''0.3 to' 1 said dispersive agent being a mixture consisting of an alkylolamine and nitrocellulose each c'onstituting at least 5% by weight 'ofthe s'aid dispersive agent, the potential applied to the electrodes beinglOO to'300 volts direct current.

4. In 'a process for coating a metal article with an inorganic refractory substance by' electrophoresis; the step that comprises making the article the anodeof'two electrodes in a bath having substantially the following composition expressedin--proportions based on weight? Parts An-organicliquid of the-class consisting of acetone and methanol 100 Inorganic refractory substance -10 A dispersive agent -n 1 said dispersive agent being a mixture consisting of an alkylolamine and nitrocellulose each-constituting at least-5% by weight of the 'said dise persive agent.

5.'In a process for coating a metal article with an inorganic refractory substance by electrophoresis; the step that comprises making'the article the anode of two electrodesfor a period of 10 to 30 seconds in a bath having-substantially the following composition expressed in'proportions based on weight:

Parts An organic liquid of the class consisting of acetone and methanol 100 Inorganic refractory substance 10 A dispersive agent 1 said dispersive agent being a mixture consisting of an alkylolamine and nitrocellulose each constituting at least 5% by weight of the said dispersive agent.

6. In a process for coating a metal article with an inorganic refractory substance by electrophoresis; the step that comprises making the article the anode of two electrodes for a period of 10 to 30 seconds in a bath having substantially the following composition expressed in proportions based on weight:

Parts An organic liquid of the class consisting of acetone and methanol 100 Inorganic refractory substance 10 A dispersive agent 1 said dispersive agent being a mixture consisting of an alkylolamine and nitrocellulose each constituting at least 5% by weight of the said dispersive agent, the potential applied to the electrodes being 100 to 300 volts direct current.

'7. A process as defined in claim 1 wherein the alkylolamine is triethanolamine. l

8. A process as defined in claim 4 wherein the alkylolamine is triethanolamine.

9. In a process for coating a metal article with an inorganic refractory substance by electrophoresis; the step that comprises making the article the anode of two electrodes in a bath having substantially the following composition expressed in proportions based on weight:

Parts An organic liquid of the class consisting of acetone and methanol 100 Aluminum oxide 10 Silicon dioxide 0.3 Nitrocellulose 0.7 Triethanolamine 1.0

10. A bath for use in process for coating a metal article with an inorganic refractory substance by electrophoresis using the article to be coated as the anode of two electrodes, having substantially the following composition expressed in proportions based on weight:

Parts An organic liquid of the class consisting of acetone and methanol 100 Inorganic refractory substance 3 to 20 A dispersive agent 0.3 to 1 said dispersive agent being a mixture consisting of an alkylolamine and nitrocellulose each constituting at least 5% by weight of the dispersive agent.

11. A bath for use in process for coating a metal article with an inorganic refractory substance by electrophoresis using the article to be coated as the anode of two electrodes, having substantially the following composition expressed in proportions based on weight:

Parts Acetone 100 Aluminum oxide Silicon dioxide 0.3 Nitrocellulose 0.7 Triethanolamine 1.0

8 said aluminum oxide and silicon dioxide having a particle size of microns or less.

LESLEY FRANCIS CAKES.

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

UNITED STATES PATENTS OTHER REFERENCES Recuei1 des Travaux Chimiques des Pays-Has, vol. 58 (1932) pp. 662 through 665.

Colloidal Phenomena, by Hauser, published in 1939 by McGraw-Hill Book Company; p. 159.