US2327462A - Electrodeposition of insulating material - Google Patents

Description

5. RUBEN ELECTRODEPOSITION OF INSULATING MATERIAL Filed Aug. 12, 1939 5 Sheets-Sheet 1 INVENTOR Samuel fiah'n/ ATTORNEY Aug. 24, 1943. s. RUBEN ELECTRODEPOSITION OF INSULATING MATERIAL Filed Aug. 12, 1939 3 Sheets-Sheet 2 l l 44/. 1 I

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Jamaal flaen/ ATTORNEY Aizg. 24, 1943. s. RUBEN ELECTRODEPOSITION OF INSULATING MATERIAL 5 Sheets-Sheet 5 Filed Aug. 12, 1939 INVENTOR v (Jami tel fink ATTORNEY Patented Aug. 24, 1943 UNITED STATES PATENT OFFICE ELECTRODEPOSITION OF INSULATING MATERIAL 15 Claims.

This invention relates to a method of electrically depositing insulating materials on metals, particularly resistance wire formed of refractory metal compositions. This application is a continuation in part of my co-pending applications bearing Serial Numbers 67,599 filed March 7, 1936, now abandoned; 71,365 filed March 28, 1936, now abandoned; 91,027 filed July 17, 1936, now Patent No. 2,204,623; 123,759 filed February 3, 1937, now abandoned; and 127,- 101 filed February 23, 1937, now Patent No. 2,213,969.

An object of the invention is to provide a method whereby an insulating material may be electrically deposited on metal surfaces.

A further object is the provision of a process whereby metal members of very fine or unusual dimension may be coated with an insulating material.

Another object is the provision of a method for coating refractory wire with an insulating material having a high resistance to heat.

Still another object is the provision of a process for electrophoretically depositing and bonding a thin hard high temperature resistant insulation upon refractory wire elements to be used as electrical resistors.

Other objects of the invention will be apparent from the following description and accompan ing drawings taken in connection with the appended claims.

The invention comprises the features of construction, combination of elements, arrangement of parts and methods of manufacture and operation referred to above or which will be brought out and exemplified in the disclosure hereinafter set forth, including the illustrations'in the drawings.

In the drawings:

Figure 1 is a side elevation, partly in section,

of a multi-layer resistance element made according to the invention;

Figure 2 is a perspective view 'of the'element; Figure 3 is a longitudinal section through modified element;

Figure 4 is a section through another modification; and Figure 5 is a view of a section of coated resistance wire with part of the coating removed;

Figure 6 is a section through another modifi cation of the invention; Figure 7 is a graph showing the effect of oxidation of the coating in respect to increase in resistivity, and

Figure 8 illustrates the process of applying the insulating coating to the refractory wire.

While, a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made in the method of procedure and the construction of parts danger of burn-outs. pied by the insulation seriously limits the without departing from the spirit of the invention. In the following description parts will be identified by specific names for convenience, but the invention 1S not intended to be limited beyohd the scope of the claims.

Heretoiore, in the commercial manufacture of resistance elements, several methods have been employed. in one, the wire was wrapped with a layer of thick insulating fibrous material such as asbestos; in another, the wire was embedded in a mass of plastic cement after which the mass was haired to dry and harden the cement; a third means of obtaining insulation was to wind the bare wire on an insulating form with sumcient spacing between turns to avoid short-wircxntmg contact between them.

Units or the above types could not conveniently or eniciently be made multi-layer coil form. it will be appreciated that the asbestos or cement coverings of the type described are inherently and necessarily relatively thick or massive resulting in excessive heat insulation. This produces a high temperature differential between the inside and outside of the element. The ele-. ment is thus not as quickly responsive to electric current through the wire nor can high current values be safely handled by the element without Likewise the space occuamount of wire which can be contained in an element of a given size. The space factor is also very poor in a bare wire element and added precautions are necessary to avoid danger of contact with the bare wire.

.In my application bearing Serial Number 71,- 365, I describe a flexible, high-temperature, electrical heating element comprising a wire of refractory metal coated with a thin, hard and adherent insulating layer of uniform thickness, the layer being hard, adherent and nonconductive at temperatures at which the heatmg'element is operated. The layer is composed of inorganic insulating material and a much smaller amount of organic resinous hinder, the percentage of latter material present being insufiicient to render the insulating layer conductive at temperatures beyond its carbonization point. The coating is applied by making the wire the anode in a bath tonite, or its equivalent.

Instead of using a resinous solution in which to suspend the insulating materials during their application to the wire, I prefer to use an inorganic suspension me-- dium. An important improvement is the use of a high temperature heat treatment to sinter or fire the coating upon the wire, either before the wire is Wound into the form of a resistor element or after it has been so wound. This firing treatment is preferably carried out at a temperature of from 700 C. to 1100 C.

In applications where maximum insulation resistivity is necessary between the wire and the exterior and where maximum mechanical bonding of the insulation to the wire is desirable, I have found it preferable to avoid the use of any organic binder in the coating bath. In place of such organic binder, I use a hydrated silicate, such as bentonite, which, when mixed with water, will allow suspension of the finely divided insulating materials, for instance kaolinite and talc, to be maintained in the solution and which forms an adhering gel in suspension. The effect of the bentonite, especially when the coated wire has been fired as above described, is to make the coating denser and more adherent to the wire.

I have found it desirable to use only enough bentonite to afiord necessary suspension of the solids, to impart sufficient conductivity to the bath and to bring about the required coating density. The percentage of bentonite is preferably held down for the reason that the insulation resistance of the coating at high temperatures with an excess amount of bentonite is poor. The best percentage seems to be about by weight of the total insulating solid materials used, the useful limits appearing to be from 5% to Preferably, the bentonite used is of 600 mesh size or finer.

Finely divided hydrated materials of the kaolinite and talc type are the most desirable insulating compounds, due to the fact that they are excellent suspension materials in the electrolytic method of coating used and also because they are highly suitable for the firing and sintering operation. While kaolinite alone may be used with the bentonite, the presence of the talc increases the smoothness of the coating and increases its resistivity at high temperatures. Finely divided chromium oxide, beryllium oxide or titanium oxide may be substituted for the tale or kaolinite with less satisfactory results, the latter materials being superior. The most desirable combination appears to be a mixture of kaolinite and talc in bentonite. For coating small size wires, 1 use a mixture of 45% by weight of kaolinite, 45% by weight of talc and 10% by weight of bentonite, the bentonite having previously been mixed with distilled water to form a hydrogel. About 300 grams of solid are used to 1200 grams of water. Before the mixture is applied to the wire, it is ballmilled for about twenty-four hours in order to eliminate any possible large particles and to insure a fine grain deposition. It is important that the materials after being ground should have a grain size not larger than 400 mesh and preferably smaller.

The wire used for the resistance element may be of any suitable-composition, such as nickelchromium, iron-chromium, tantalum-iron-chromiuni, etc., and of any desired size. The invention makes it possible to use very fine wire, as low as 0.001 inch in diameter and the coating may, of course, also be deposited upon larger Wires of all commercial sizes.

eliminates the combined water of hydration, in-

creases the coating density and causes the insulation to be bonded to the base. At this point the coated wire is in the form illustrated in Figure 5, the resistance element In comprising nickel chromium wire I I and coating I2. For some purposes the wire may be used in this form but where considerable handling is required and where the coated wire is to be wound upon mandrels of relatively small diameter it is desirable to apply a flexible waterproof top coating of a material such as methyl methacrylate, cable lacquer, tung oil, indene or other resin, etc. Accordingly, the sintered wire may be directly passed through a solution of 7% methyl methacrylate polymer in'acetone and thereafter dried at a temperature of about C. and spooled'for future use. The wire so produced is sufficiently tough and flexible to be wound or bent into any form ordinarily required in the finished element Without danger of chipping or cracking. It is likewise resistant to mechanical shocks and abrasions so that the coated wire may be handled by a winding machine. In the manufacture of resistance units, the organic top coating may be expelled as a gas or vapor from the finishel unit, either immediately after the wire has been wound into form or where a metal casting is used at the time of or prior to the application of the metal to the unit. Alternatively, the top coat may be allowed to remain, in which case it may gradually volatilize after the resistor has been put in use.

The coated wire having the characteristics above described may be used to form resistance elements of various types and for various purposes such as fixed resistors for radio circuits and the like, heating elements, soldering iron ele-' ments, hot plates, energy dissipating units and the like. It may be wound on forms of metal or ceramic either in ordinary spiral or in non-inductive form. 1

Figures 1 and 2 show a fixed resistor for use in general circuit applications. This comprises a spool or form I3 of insulating material such as porcelain, Levite, Isolantite, sillimanite or the like. The coated wire I 0 is wound without spacing between turns, on form I3 in multi-layer fashion without the interposition of any extra in-- sulating layers and the ends are scraped clean of coating material and electrically connected to terminal rivets I4 and I5, respectively, which pass through the end flanges of form I3, the electrical connection being made by a mechanical riveting operation or by Welding. A pair of terminal lugs I6 and I! are secured to the ends of form l3 by the respective rivets and form the electric terminals of the element.

If greater rigidity and higher abrasive resist ance is desired in the finished element it can be obtained by applying a glazing material prior to any of the above heat treatments. For example, the wound element, before heating, can be sprayed, dipped or painted with a hydrolyzed ethyl silicate solution, preferably with finely divided kaolinite and talc suspended therein. A suitable mixture consists of 50% kaolinite and cate may be applied if high voltages are not encountered. The best glazing effects can be obtained by heating at least to 1000 C.

The element of Figures 1 and 2 may be covered and protected by a metal sleeve [3, of aluminum or other metal which fits overthe flanges on form l3 and has its ends spun down over the ends of the form thereby completely enclosing the Winding ID. A pair of lugs l9 and secured to sleeve I8 serve as mounting means for the element.

Figure 3 shows an element of different shape, the coated wire I!) in this case being wound on a shorter insulator form or spool 2|, the wire being connected to terminal lugs 22 and 23. A metal sleeve 24 fits over spool 2| and is spun down over its ends. This form is adapted to be mounted on a bolt passing through central hole 25 in spool 2|.

Figure 4 shows an element having a cast metal sheath. In this case the coated, wire In is wound without spacing between turns on an insulating form 26 having a large diameter flange 2!v at one end and a small diameter flange 28 at the other. The wire is connected to rivets 29 and 30 whose heads are countersunk in the irmer face of flange 21. The countersunk bores are filled with alundum cement after the wires are attached to insulate the connections. A pair of terminal lugs 3i and 32 are held by rivets 29 and 30 on the end of form 26. A metal sheath 33 is cast over the winding after the element has been heated as described to eliminate organic material. This may be done by placing the wound element into a mold and casting the metal about it. The metal sheath may be made flush with the outer edge of flange 21 as shown. The metal is cast directly against the coated wire winding. Due to the prior elim ination of the volatile organic material the casting will be free of blow holes.

A suitable alloy for the cast sheath where low temperatures only are to be encountered consists of:

Per cent Aluminum 4.1 Magnesium .1 Copper 2.7 Zinc 93.1

an aluminum spool having small diameter flange structed in plug-in form, prongs being substituted for the terminals shown. As-aluminum sheath 4| has been cast directly over the winding to protect the unit and to aiford a rapid heat transference. This heat transference may be further improved and the unit can be made capable of dissipating higher wattage by etching or otherwise roughening the outersurface of the casting and also by blackening the casting.

The coating described so far and its method of application, with the exception of the top coating materials, have dealt solely with the use of inorganic materials. It is possible, however, and for some purposes desirable to use a type of coating in which shellac or other organic binder is substituted for the bentonite. When thus used the shellac may either be removed by a subsequent heating operation in which case the finished coating will-consist solely of inorganic materials, or a relatively small amount of shellac may be left in the coating.

The most practical working limits for shellac are 10% to 30%. The preferred proportions of shellac will vary somewhat, however, with different sizes of wire. Thus with l and 2 mil wire the preferred amount of shellac is 10%, with 4 to 6 mil wire 16% and with 10 mil wire about 22%.

Other organic binders than. shellac can be used such asthe various resins (natural or synthetic) for example, rosin, copal and the like, the various gums. for instance, dammar, arabic, tragacanth, and rubber latex and certain oils such as tung oil and linseed oil. However, none of these has been as satisfactory as shellac.

When shellac is used in place of bentonite, it is desirable -in most cases to employ the high temperature heating only after the wire has been wound on the resistance element. In such cases the high temperature 1000 oven is not required, the resistance element being heated up to 1000 C. after the wire has been wound into final form.

For use where low voltage insulation only is required it may only be necessary to heat the winding above the vo'latilizing ordecomposition temperature of the shellac or other organic binder. For shellac a. temperature of about 420 C. may be sufficient. At this temperature the shellac appears to decompose, the majority of the product going ofi as vapor and only asmall amount of non-volatile residue remaining, probably in the form of carbon. If the unit is to be used in this form it is to be preferred that the original coating shall have a lower percentage of binder so that the amount of carbon left in the coating will be insuflicient to cause any material current leakage. For a shellac binder, the percentage of shellac originally in the coating should preferably be not larger than 30%. The heating may be carried on in an oven or by passing current through the winding and may take place in air or in an inert gas atmosphere.

While the above heating may be sufficient for low voltage operation it will generally b preferred especially where high voltage wil1 be encountered to heat the element to a higher temperature, namely one at which the residue (carbon) will burn out leaving a coating free from organic matter. For this purpose the temperature should be raised to 455 C. or above. The resulting coating is highly resistant to current leakage, the resistance normally amounting to several megohms per linear inch of coated wire. This coating is suitable for high voltage high temperature use. A metal sheath can be cast directly against the coated wire if desired in which case the coating will fully insulate the wire from the cast sheath. In this case it is of advantage, however, to keep the percentage of shellac originally in the coating to a minimum (preferably below to keep down the porosity in the final coating from which the organic material has been volatilized and oxidized out).

When a greater abrasion resistance is desired, as where the coated wire is to be used without any protective sheath, it is preferred that the wire element be sintered and fired at 1000 C. The resulting sintered element is ready for use without further treatment either with or without a protective sleeve or a cast metal sheath. The sintering temperature can be reduced by adding a small percentage of fusible borates or silicates to the coating material before applying to the wire but for minimum leakage at high temperatures the use of such additional agents is not desired.

The preceding description of the various possible heat treatments which may be employed when an organic binder is used may be briefly summarized as follows: The wire is first provided with a coating consisting of a finely divided inorganic insulating material and an organic binder whereby a coated wire is produced of sufiicient flexibility to enable Winding or otherwise forming into a resistance element of desired shape by hand or with a winding machine or other forming apparatus. The wound (or formed) element can be used in this form where low temperatures only are to be encountered. Where hi her temperatures are to be encountered the coated wire element is given a heat treatment which changes the composition and physical properties of the coating. The composition is thus changed so as to reduce or eliminate the organic material which could not stand up under the higher temperatures which the element is expected to encounter during the coating of the metal sheath or in subsequent use. The physical characteristics are changed by the heat treatment to produce a more rigid, less flexible coating which would ordinarily be chipped or cracked if it were attempted to manipulate or rewind the coated wire in this condition. However, since the element has already been formed or wound prior to the heat treatment it is unnecessary to apply any further manipulations after treatment and'hence the coating is preserved intact.

The heat treatment, as described above, may extend to one of the following temperatures:

(1) Volatilizing temperature at which the volatile constituents of the organic binder are driven off. Suitable for low voltage operation at high temperatures preferably with metal sheath, 375 C.

(2) Oxidizing temperature at which any residual carbon left by the organic binder is burned out. Suitable for high voltage 450 C. operation at high temperatures preferably with cast metal sheath.

(3) Sintering temperature at which the particles of inorganic material are sintered together.

Suitable for high voltage operation at high temperatures With or without any protective sleeve or sheath.

The effect of the heat treatments and resultant oxidation of the shellac binder wire coating is i1- lustrated in Figure 7 which is a graph, the curves of which show resistivity of insulation on units formed from resistance wire having a 0.003" insulating coating and in which the total contacting area of the insulation is 4.6 sq. in. Curve 62 shows the resistivity of such a unit, the insulating coating of which has been heated only up to the point at which the coating becomes tanned. Curve 44 shows the result of heating such a unit to a temperature where the coating becomes white due to oxidation and/or elimination of any carbonaceous material, the heating temperature being in excess of 450 C. The insulated wire, the characteristics of which are shown in curve 44, was wound on a transite core. Curve 43 illustrates the resistivity of a coated wire identical with that referred to in curve 44, excepting that in this case the wire was wound on an aluminum core.

Figure 8 illustrates a simple continuous method of applying the insulating coating to the resistance wire. The positively charged uncoated nickel chromium wire 50, passes via pulley 5|, into talc-lzaolinite bentonite bath 52, held in negatively charged metal container 53. Pulleys 54 and 55 serve to direct the course of the wire through the solution and up through electrically heated oven 56. It will be noted that the coated wire is baked after each pass into the bath. The finished wire completely coated, is wound on spool 57. Battery B provides the platin current and heater H supplies the temperature for the baking oven. The solution 52, is continuously agitated by propeller 58.

Where it is desired to add an organic Waterproof top coating to the wire, an additional bath of a suitable material such as indene resin, may be added, the wire passing through the resin bath after the inorganic coating has been completed, and being again heated to drive off volatilizable vapors prior to being spooled.

It will be seen that the present invention provides a method for producing an insulated resistance Wire element in which the insulation, fired in situ upon the wire, is thin, hard, uniform, continuous, adherent, indestructible, of sufficient flexibility to permit of its being Wound into coil form, which may be constructed in compact multi-layers without danger of short-circuiting between turns and which conveniently allows the casting of a protective and heat-radiating hard metal sheath directly against and around the insulated wire without damage to the wire.

What is claimed is:

1. In the method of insulating a flexible con ductor, said conductor constituting the anode in an electrophoretic assembly, the steps comprising electrophoretically depositing upon said conductor from an aqueous dispersion, a mixture of finely divided inorganic insulating material and a water gelling hydrated silicate, the amount of inorganic insulating material being greater than the amount of said hydrated silicate.

2. In the method of insulating a flexible conductor, said conductor constituting the anode in an 'electrophoretic assembly, the steps comprising electrophoretically depositing upon said conductor from an aqueous dispersion, a mixture of finely divided inorganic insulating material and bentonite, the amount of inorganic insulating material being greater than the amount of bentonite.

3. The method described in claim 2 characterized in that the inorganic insulating material comprises kaolinite.

4. In a continuous process for making a flexible insulated resistance wire capable thereafter of being wound in coil form to provide a resistor unit in an electrical circuit, the steps which comprise making a wire of electrical resistance material the anode in an electrophoretic assembly containing an aqueous dispersion of a water gell ing hydrated silicate and finely divided refrac- 5. The method described in claim 4 characterized in that a top coating is'flnally applied to said coated wire. i

6. The method described in claim-4 characterized in that the refractory insulating material comprises talc.

'7. The method described in claim 4 characterized in that the refractory insulating material comprises kaolinite and talc;

8. The method described in claim 4 characterized in that the coated wire is heated in an atterial the anode in an electrophoretic assembly containing an aqueous dispersion of bentonite and finely divided refractory insulating material, the amount of'refractory material exceeding the amount of bentonite, providing a cathode in contact with said dispersion, passing said resistance .wire through said dispersion whereby the hentonite is deposited upon said wire simultaneously 7 with said finely divided'insulating material, said.

coated wire thereafter passing through an oven whereby the coating is baked and further bonded to the wire.

11. The process described in claim 10 characterized in that the bentonite comprises to 20% of the total amount of solids in the aqueous dispersion.

12. In a continuous process for making 'a flexible insulated resistance wire, the steps which comprise making the wire the anode in an electrophoretic assembly containing an aqueous dispersion of bentonite having in suspension finely divided refractory insulating material comprising kaolinite and talc, the amount of refractory insulating material exceeding the amount of bentonite, providing a cathode in contact with said dispersion, passing said resistance wire through said dispersion whereby the bentonite is deposited upon said wire simultaneously with said finely divided insulating material, thereafter baking the coated wire to drive off water and further bond the coating to the wire.

13. In a continuous process for making 9. flex- I ible insulated resistance wire, the steps which comprise making a wire of electrical resistance material the anode in an electrophoretic assembly comprising a dispersion containing water and a hydrated silicate having theproperty of gelling when mixed with water, said dispersion having suspended therein finely divided refractory insulating material, the amount of refractory insulating material exceedin the amount of hydrated silicate, providing a cathode in contact with said dispersion, and applying a potential to said electrodes whereby the silicate and said finely dividedinsulatingmaterial are simultaneously deposited upon the wire, thereafter repeating said steps with the coated wire until the desired thickness of insulation is obtained.

14. The process described in claim 13 characterized in that the wire is baked after the application of each coating.

15. A continuous method of providing a tier.- ible refractory insulating coating for a resistance wire of the nickel-chromium type which comprises making said wire the anode in an electrophoretic assembly containing an aqueous dispersion of a water gelling hydrated silicate having kaolinite suspended therein, the amount'of kaolinite being greater than the amount of water gellin hydrated silicate, providing a cathode in contact with said dispersion, passing said wire through said dispersion. whereby the kaolinite is deposited upon said wire and simultaneously therewith the water gelling silicate is likewise deposited upon said wire, the kaolinite being deposited in a greater amountthan the water gelling silicate, thereafter baking said coated wire to a temperature sufilcient to dehydrateand firmly bond the deposited material to the wire.

SAMUEL RUBEN.

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