US1881444A - Manufacture of resistance units - Google Patents

Description

Oct. 11, 1932. J. A. FLANZER "ANUFACTURE OF RESISTANCE UNITS Filed July 5, 1928 INVENTOR Joseph lanzeY ATTORNEYS Patented Oct. 11, 1932 UNITED STATES PATENT OFFICE JOSEPH A. FLA NZER, OF BROOKLYN, NEW YORK, ASSIGNOR '10 TECHNIDYNE CORPORATION, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK *HANUFACTURE or Application filed July 5,

This invention relates to the art of making resistances and has special reference to improvements in the manufacture of resistances of the type set forth and claimed in the copending application of Lester L. Jones et al., Serial No. 167,583 filed February 11, 1927, which issued as Patent 1,773,105, Aug. 19, 1930, and has more particular reference to improvements in resistance units and the art of making the same such as described and claimed in another copending application of Lester L. Jones et al., Serial No. 260,- 987 -filed March 12, 1928, which issued as Patent 1,847,653, March 1, 1932.

In the said copending application Serial No. 167,583, there is disclosed the art of making a resistance paint or composition capable of being applied, painted or coated on mica, glass, and other bases to form thin non-hygroscopic resistance films, the said resistance paint embodying a finely divided conducting material, preferably colloidal graphite, held in a binder of a metallic phosphate and phosphoric acid. This paint composition, after being applied to a. suitable base of an insulating material, is baked to relatively hign temperatures, this treatment producing a per manent or set resistance coating which is tough and durable, which adheres perfectly to glass, porcelain, lavite, mica, etc., which is practically insoluble in plating solutions so that it becomes practical to electro-plate co per nickel or silver thereon, and which ba ed coating is substantially non-hygroscopic so that the resistance unit suffers no deterioration due to exposure to moisture and suffers no variation in resistance which would be due to moisture absorption.

I have empirically determined that resist ance coatings or films having physical properties substantially similar to those which characterize these resistance unit may be made, provided certain precautions are observed, Without the emplo ment of a metallic phosphate and may there ore be made by appropriate compositions of finely divided conductive particles, such as colloidal graphite and phosphoric acid. I have discovered that the metallicphosphate, when added either in a liquid or solid form to the paint composi- RESISTANCE UNITS 1928. Serial No. 290,514.

tion, serves a a keeper or stabilizer for the paint coatings in such a manner as to render less critical the making and'treatment of these resistance units. I have found moreover that, by suitably regulating the com- 85 position of the paint and the method of baking the painted coatings, stabilizing or keeper agencies may be dispensed with without substantially altering the desirable physical characteristics of the resulting resistances. One of the prime objects of my present invention relates to the art of manufacturing resistance units embodying these discoveries.

In the aforesaid copending application 55 Serial No. 260,987 of Lester L. Jones and myself, there is disclosed the art of making resistance units embodying these painted or coated resistance films or strips combined with electro-plated resistance coatings for producing electrical resistance elements having predetermined temperature co-efiicients. In making resistance elements of determinable temperature coefficients, a metallic resistance film of relatively low specific resistivity is electro-plated over painted films or coatings of the nature above referred to, the painted film forming a base which adheres very firmly to an insulating base and to which very firmly adheres the electro-plated resista0 ance film. I have found that this principle of combining an electroplated resistance film With and over a paint film may be utilized with great efiicacy in the making or production of relatively low resistance units employable, for example, for heating purposes, the electro-plated film comprising the resistance element and the painted film comprising the base upon which the electro-plated film is deposited. A. further prime object of my present invention therefore centers about the provision of an electrical resistance unit embodying this principle and this combination of resistance films and coatings.

To the accomplishment of the foregoing and such other objects as may hereinafter appear, my invention consists in the method, the steps of the method, and the resistance elements hereinafter particularly described ma and sought to be defined in the claims, in which:

Fig. 1 is a view of a resistance element embodying the new painted coating of my present invention; 1

Fig. 2 is a view of a resistance element embodying the principles of my present invention and showing the same adapted for use or employable as a heating unit; and

Fig. 3 is a view of a modified form of resistance element embodying the invention.

As described in the aforementioned a lication of Lester L. Jones et al., Seria 0. 167,583, the paint composition with which the resistance films are made is obtained by mixing a finely divided such as a colloidal conducting material, more particularly colloidal graphite, in a solution of aluminum in a slight excess of phosphoric acid, these ingredients being compounded to form a freely flowing paint, the aluminum in the phosphoric acid producing an aluminum phosphate. This paint composition may be applied to glass, porcelain rods, to glazed or enameled surfaces or to mica to form resistance coatings of very thin films of the order of magnitude of 0.1 to 0.25 thousandths of an inch, such coatings being formed of continuous and unbroken layers. After coating, the painted unit is then heated as by baking to relatively high temperatures, for example to about 500 for a period of ap-' proximately one hour, this baking causing the final setting of the paint to coatings which are tough, durable and of constant resistance values.

I have found that in these paints the binding action is due to the phosphoric acid and that the metallic phosphate, to wit, the aluminum phosphate in the aforementioned example, acts as a keeper or stabilizer, that is, the metallic phosphate acts to retain a small amount of phosphoric acid in the film, and when the resistance film is baked, the metallic phosphate prevents rapid solution of the phosphoric acid in water. I' have found that this stabilizer also makes the paint somewhat more fluid and hence easier to apply in the liquid state. Finished resistance units made with the metallic phosphate when properly baked contain a certain proportion of residual phosphoric acid to aluminum phosphate, this proportion being of the order of 1 c. c. of phosphoric acid to 1 gram of aluminum phosphate. If a paint is prepared with less acid in proportion to aluminum phosphate, the resulting resistance film is mechanically weak; on the other hand, the upper limit of acid to aluminum phosphate is 6 c. c. of acid to 1 gram of aluminum phosphate in a paint having a total volume of 35 c. c. (including water as the other ingredient). If a greater proportion of acid is used, the amount of aluminum phosphate introduced becomes too small to retain enough of the acid to form the best type of binder. This is probably due to the fact that with an excess of acid the resistance unit loses the-excess on baking and a point is reached where the pro ortion of acid to hosphate is about 1 to 1. f, however, the lm of paint originally contains a ver small amount of aluminum )hos hate and if the unit loses most of the p os oric acid on bakin the total amount of acid left and retained by the aluminum phosphate is too small to furnish enou h bond between the graphitic particles, resu ting in a mechanically weak paint. And thus it is important not only to have more than a certain minimum amount of acid in proportion to aluminum phosphate but to have a certain amount of aluminum phosphate present in a given volume of paint.

I have found by experimental determination that the lower limit of aluminum phosphate desirable is 1 grams in 30 c. c. of paint. The upper limit of aluminum phosphate is not critical but the addition of larger quantities of aluminum phosphate in 30 c. c.

of paint will give a higher resistance film, since here the proportion of graphitic particles to aluminum phosphate drops.

With the use of these paints employing a metallic phosphate, I find that there is little or no criticalness in the baking process. When the excess of phosphoric acid has been removed'by baking, the baking should he stopped, although the stopping point is not very critical as there appears to be a resistance to the further removal of acid when the optimum ratio is reached. The opitmum ratio may be the point at which the supersaturated solution of aluminum phosphate in phosphoric acid reaches a limit and the rate of removal of phosphoric acid becomes vex y small as the saturation approaches this limit; but in spite of this it is possible to remove more of the phosphoric acid by baking at a higher temperature for a longer time. The temperature, however, may be controlled within relatively wide limits, and the baking time may similarly be controlled within relatively wide limits, so that there is practically no criticalness in the baking of these units.

I have discovered that suitable resistance paint-s may be made by dispensing with the metallic phosphate ingredient, the compounding of the paint and the baking of the coatings being attended with a greater criticalness. More specifically I have found that by properly regulating the temperature and time of baking and the consistency of the paint, desirable resistance units can be made which are substantially constant in resistance value, are non-microphonic and can be electroplated with copper or nickel without decomposition of the resistance paint. The baking temperature and time are critical, however, and should be determined for every concentration of acid in the paint, there being no one definite set of conditions which apply to all kinds of paints. The limits, however, of paint consistency, time and temperature are determinable as will now be described.

I have found that the minimum acid concentration for a good paint is 3 c. c. of 85% syrupy phosphoric acid in a total volume of 33 c. c. containing 20 c. c. of water and 10 c. c. of colloidal graphite. While this may be set as the minimum acid concentration, I have found no upper limit of acid concentration since units may be made from paints all the way up to 10 c. c. of acid in 40 c. c. of total volume of paint. The only upper limit that may be placed as far as acid concentration is concerned for proper manufacturing conditions is the same as the lower limit of 3 c. c. of acid in 33 c. c. total volume, because units made with a higher concentration require careful treatment in the drying process, etc. Otherwise, the higher concentrations are satisfactory and resistance units may be made therefrom.

The baking of these units is more critically a function of the time, temperature being held constant. If the unit is heated or baked too long a time, all of the phosphoric acid may be driven off and there is left merely a film of graphite particles which is mechanically weak and may be removed by rubbing. This, however, does not occur rapidly unless the temperature is raised to higher points. The probable reason for this is that when the residual acid reaches a certain minimum value, the rate of evolution of acid falls off to a small value due to mutual protecting action between the colloidal graphite particles and the phosphoric acid. As to suitable baking temperatures, I have found that on an 'isolantite base 650 C. is a good temperature. At this temperature free phosphoric acid is removed from the paint fairly rapidly and for the 3 c. c. paint composition thirty minutes will suflice to properly bake the unit.

- Where higher acid concentrations are used the time of baking must be' increased proportionately, the upper limit being about two hours.

Referring now more in detail to the drawing and having reference first to Fig. 1 thereof, I show one form of resistance unit made with the paint of my present invention, said resistance unit comprising an insulating base 10 of any suitable shape shown as tubular or cylindrical in the drawing, which may be glass, enamel, isolantite, etc. coated with a thin film 11 of the resistance paint which may be applied with a brush or which may be obtaincd by dipping the base 10 in a bath of the paint. After coating or dipping, the unit is baked to the temperatures and for the period of time mentioned to produce the satisfactory resistance unit.

In Fig. 3 of the drawing I show a modified form of the invention in which a predetermined resistance vahie is obtained for the coated unit. In this form of the invention the base 10' is coated with a paint film 11' of the composition of my present invention, and after baking, portions of the resistance film 11 are excised or cut as at 12l2 to produce a zigzag strip of resistance having a predetermined length and hence a predetermined resistance magnitude. The resistance unit may be connected in any desired circuit by attaching the leads or conductors 13 and 14 to the ends 15 and 16 respectively of the resistance strip.

In the resistance paint of the present invention, the phosphoric acid and the colloidal graphite comprise the only two active ingredients. To these ingredients, however, may be added materials which are inert. I have found, for example, that where materials such as carborundum which are inert are added, that the introduction of such materials produces no other efi'ect than an increase in resistance value which is proportional to the amount of inert material added. However, the addition of materials inert 1n the cold state may become active during the process treatment of the resistance units. For example, materials may be introduced to an acid paint which are not necessarily soluble in cold, dilute acid but may become soluble in hot, concentrated acid. Such materials would give rise to the formation of metallic phosphates in phosphoric acid such as are described and claimed in the aforementioned application, Serial No. 167,583. Thus, in lieu of starting with a solution of aluminum in phosphoric acid, the paints may be made by adding to the acid a desired proportion of bauxite'or powdered aluminum oxide which does not dissolve in the cold, dilute acid but becomes soluble in the hot, concentrated acid Therefore, such materials which appear as more or less inert in their behavior in the cold paint may form metallic phosphates at high temperatures resulting in the production of stabilixing agents for the phosphoric acid.

As heretofore stated, it is the further object of my present invention to combine with these resistance paints electro-plated resistance films producing resistance units of relatively low specific resistivities adaptable particularly as heating units. Referring now, particularly, to Figure 2 of the drawing, I show such a heating unit as comprising a cylindrical ortubular base 17 which may bemade of sheet metal coated with a vitreous film 18 such as enamel to supply an insulating base, the said enamel insulating base being first coated with a thin film of the resistance paint 19 upon which is electroplated a second resistance film 20 of nickel, copper or the like. The electro-plated reduring the baking step. i

sistance film 20 is of comparatively low resistivity while the resistance paint 19 is of comparativel high resistivity so that the electro-plate film forms the efi'ectlve resistance coating. To produce a predetermined magnitude or value of resistance, the coating 20 and ,if desired, the sub-coating 19 are spiralled after the manner set forth in the patent to Lester L. Jones, No. 1,635,184 of July 12, 1927, the said spiralling being indicated by the reference character 21. As shown, the spiralling at the'ends of the resistance unit is of small pitch, while that at the intermediate region thereof is of a higher pitch, this for the purpose of developing a more intense heating at the ends of the unit and therefore a more uniform heating (due to a difference in radiation and conduction) for the length of the unit. Contact may be made with the resistance coating by means of the bands 22 and 23 clamped on the ends of the thus formed resistance element.

I have found that a tube such as shown in Figure 2 of the drawing the nickel film of which is spiralled to a resistance value of 100 ohms may be used on ordinary home lighting power lines with an applied voltage of 110 to 120 volts. The average nickel film of 0.2 thousandthsof an inch thickness may be spiralled to 100 ohms on a five inch length by using about five threads of spiralling per inch.

One of the important advantages of the paint composition is evident from this combination of the same with a plated resistance coating. The nickel plating solution used in the eleetro-plating step is not critical as to the plating period nor as to the voltage or temperature of the plating bath, the binder of the resistance paint or film 19 or 11 being insoluble in the plating solution. The paint film adheres very firmly to the insulating base and forms a very firm basis or support for the deposit or plated metal film. A unit such as that shown in Figure 2 of the drawing possesses the further advantage of having a positive temperature co-eflicient. As a result of this, the heating unit heats up very rapidly when first connected to the electric current, the current automatically being cut down as the temperature increases. For example, the power consumed at the moment of heating may be about 140 watts per tube as against 7 0 watts when the tube becomes hot. Should the tube shown in Figure 2 then be left operating without heat being withdrawn therefrom, the current through the unit drops automatically as the temperature rises, thereby preventing burning out of the resistance element and also resulting in increased economy.

The manner of making resistance devices embodying my present invention and the many advantages thereof will, in the main, be fully apparent from the above detailed description thereof. It will be further a parent that while I have described the pre erred forms of the invention and the preferred steps in the process of making resistance units, etc. many modifications and changes may be made therein without departing from the spirit of the invention defined in the following claims.

I claim:

1. A resistance element for heating purposes comprising a cylindrical insulation base provided with a painted resistance film having a relatively high specific resistivity and provided with a second resistance film having a relatively low specific resistivity and consisting of a metallic deposit electro-plated over the full resistance portion of said painted resistance film, the said second resistance film being spirally cut to produce a predetermined value of resistance for the element, the pitch of the spiral being smaller near the ends than at the middle of said resistance element.

2. A resistance element for heating pur poses comprising a cylindrical insulation base provided with a resistance film, the said film being spirally cut to produce a predetermined value of resistance for the element, the pitch of the spiral being relatively small near the ends of the resistor and relatively large at the middle of the resistor so as to promote uniform heating over the length of the resistor.

3. The method of making a resistance unit, which includes mixing colloidal graphite, water, and phosphoric acid in proportions equivalent to 10 cc of the graphite, 20 cc of water, and from 3 to 10 cc of syrupy phosphoric acid to make a resistance paint, coating an insulation base with a thin film of the paint, and baking the painted unit at a temperature of about 650 C. for a period of from approximately one-half to two hours according as 3 to 10 cc of the phosphoric acid has been used.

Signed at New York in the county of New York and State of New York this 2nd day of July A. D. 1928.

JOSEPH A. FLANZER.

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