US3419840A - Composition resistor - Google Patents

Description

Dec. 31, 1968 c. LrwRlGHT 3,

' COMPOSITION RESISTOR Filed Nov. 18, 1965 FIG. I

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[I II II 1/ II 11 II 1/ 1/ 1 7 {gm w l 'ggwarg uvvzuron CARL L. WRIGHT ATTORNEY United States Patent 3,419,840 COMPOSITION RESISTOR Carl L. Wright, Youngstown, N.Y., assignor to Air Reduction Company, Incorporated, New York, N .Y., a corporation of New York Filed Nov. 18, 1965, Ser. No. 508,498 3 Claims. (Cl. 338-426) ABSTRACT OF THE DISCLOSURE Carbon composition resistor having a pyrolytic silica dispersed in the composition.

The present invention relates to electrical resistors and more particularly relates to volumetric carbon composition resistors and methods of making the same.

Carbon composition resistors are widely used in a number of industries in great quantities. Consequently, continuous efforts are being made to facilitate mass production of large numbers of volumetric resistors.

Resistors of the type described above generally comprise a conductive body or core portion formed of a non-conductive inert filler material such as silica, clay,

ashes, and the like, and a conductive carbonaceous material such as carbon black, lamp black, graphite, etc., dispersed in an organic binder such as pitch, phenolformaldehyde resin, silicone resin, and other similar materials. The proportions of the ingredients are varied to obtain the desired resistance values and other electric properties.

Generally these resistors are provided with an insulating shell formed of a material comparable with the conductive portion and often formed of the same materials omitting the conductive constituent.

It has been the practice to form the insulator shell in partly compacted form having a tubular shape and the outside diameter corresponding to that of the desired finished product. The insulator shell is positioned in a die cavity and filled with the powdered material which is to form the conductive core. The entire assembly is then compacted into a resistor of the composition, density, and outer dimensions desired. Outwardly extended leads contact the conductive core to provide means for coupling the resistor to the operative circuit.

In instances where an insulating shell is not necessary or desired the molding of the conductive core is commonly done by pouring the powdered conductive material into single or multicavity dies and compacting the material from either end with suitable punches.

Conventional examples of methods and apparatus for molding carbon composition resistors are set forth in United States Patents Nos. 2,454,508 and 3,013,240.

A long continuing problem in the volumetric carbon composition resistor field has been the excessive caking or bridging of the powdered shell or core mix which obstructs the smooth flow of the powder into the die cavity. The problem is particularly severe in the case of small size resistors which are molded using die sizes in the order of 0.045 in. diameter.

An additional problem of the prior art in manufacturing carbon composition resistors is the low yield of resistors heretofore obtainable from a particular mix. One factor which creates low yield is the inability of the individual particles in the resistor mix to flow freely which prevents uniform dispersion of the particles. By yield is meant the percentage of resistors produced from a given mix which have a resistance value which does not deviate more than a predetermined amount from the desired resistance value. In general, resistors which fall 3,419,840 Patented Dec. 31, 1968 ICC outside the allowed deviation in resistance are scrapped with the obvious resulting economic loss.

It is therefore of utmost necessity to achieve the best possible flow characteristics for the mix which reduces or eliminates die filling problems and allows for an even and uniform dispersion of the individual powdered particles in the mix.

I have discovered that by blending small quantities by weight of pyrolytic silica with the powdered mix the flow characteristics of the mix are greatly increased, with improved die filling properties and improved uniformity in resistors formed from the mix. In addition, because of the improved flow characteristics and more uniform dispersion of the particles throughout the mix, other electrical characteristics of such resistors are generally superior to prior art resistors.

It is an object of this invention to provide resistors having improved electrical characteristics.

It is an additional object of this invention to provide a conductive composition having improved die filling properties.

A further object of this invention is to increase the yield of carbon composition resistors from a given conductive composition.

Yet another object of the invention is to provide a method of manufacturing carbon composition resistors which results in a high yield.

These and other objects of the invention will be readily apparent as the description proceeds with reference to annexed drawings in which:

FIG. 1 is a block diagram outlining the method of manufacturing the resistor of this invention; and

FIG. 2 is a longitudinal cross-sectional view of the resistor of this invention.

The pyrolytic silica utilized in carrying out the invention is a submicroscopic particulate silica. A suitable silica is available commercially from the Cabot Company under the trade name Cab-o-sil. Cab-o-sil is a submicroscopic particulate silica prepared in a hot gaseous environment (1100 C.) by the vapor phase hydrolysis of a silicon compound. Cab-o-sil has an extremely fine particle size, one gram containing over 11 million billion particles, and covers an area of about 200 sq. meters. Cab-o-sil has the following chemical and physical properties:

Silica content (moisture free) percent min-.. 99.9 Particle size range 0015-0012; Surface area (nit. adsorption) sq. meters/g 200:25 Specific gravity 2.2 Refgractive index 1.46 pH (4% aqueous dispersion) 3.6-4.2 Loose bulk density (approx) lb./cu. ft 2.0-3.5

Other blends of pyrolytic silica having the same or similar characteristics as Cab-o-sil may obviously be used in carrying out my invention.

The amount of pyrolytic silica blended with the core or shell mix is dependent upon the number of fines (fines being described as material which will pass through a 200 mesh screen) present in the granulated mix. I have found that for the most common core mixes the optimum amount of Cab-o-sil by weight which should be blended with the mix is approx. 0.5% to 1.0%. However, should the number of fines be extremely high, i.e. greater than 50%, the quantity of pyrolytic silica should be somewhat greater. Granular mixes having an extremely small amount of fines should accordingly have a smaller percentage by weight or p-yrolytic silica blended therewith. In no known commercial resistor mixes should the required amount of pyrolytic silica lie outside the range of 0.4% to 2.0% by weight of the mix.

Although the problem of yield is not present with respect to the shell, the same relative amount of Cab-o-sil may be advantageously used to improve the flow of the mix during processing.

Referring to FIG. 2 of the drawing, a resistor is shown generally at 1 and is formed of a conductive core portion 1, insulator shell 2, and terminals 3.

FIG. 1 of the drawing is a block diagram which illustrates the steps in the prior art processes in which pyrolytic silica is blended into the resistor composition mix to form the resistor of FIG. 2. In the manufacture of the resistor the entire process consists of selecting and mixing the basic resistor ingredients, subjecting the mix to dry granulation, screening the dry granulated particles into the proper size, blending pyrolytic silica with the mix, molding the mix to form a solid resistor body according to well known molding processes, such as for example illustrated in US. Patents Nos. 2,454,508 and 3,013,240, and baking, impregnating, sorting, and finishing the molded resistor body. Each of the above process steps is well known and well documented in the prior art with the exception of the blending of pyrolytic silica into the resistor mix.

In addition to blending the pyrolytic silica into the mix following the screening of the dry granulated mix, the pyrolytic silica may be blended into the mix just prior to the screening step.

As an example of a resistor core formulation which contains pyrolytic silica, a conventional formula as fo lows was prepared.

Silica flour lb 265 Phenolic resin (60% solids) lb 115 Silane g 250 Carbon black lb Acetone lb 20 One hundred pounds of the above mix was selected and /2 by weight pyrolytic silica was blended with the mix for 9 min. in a Patterson-Kelley V Blender. The resulting mixture was then processed into a resistor in accordance with well known processes as generally outlined in FIG. 1.

The yield of resistors made from the above formulations With a permissible i10% deviation from the desired resistance value jumped from a prior 10% yield to a 40% to yield. In addition, remarkably improved flow was observed in the die filling operation.

The improvement of the die filling characteristics and the increase in yield of resistors from a particular mix are believed to result from the effect of the pyrolytic silica segregating the fine particles of the mix and keeping them segregated so that the particles cannot bridge or cake throughout the mixing and resistor processing steps.

Obviously many modifications and variations of the invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically disclosed.

I claim:

1. A carbon composition resistor comprising a conductive core member having pyrolytic silica dispersed therein in an amount equal to .4 to 2% of the total weight of said core, and electrical terminal means attached to said core member.

2. A carbon composition resistor as in claim 1 including an insulating jacket surounding said core member.

3. A carbon composition resistor comprising a conductive core member, electrical terminal means connected to said core member, and an insulating jacket surrounding said core member, the composition of said conductive core member comprising a nonconductive filler material, a carbonaceous conducting material, an organic binder, and 0.5% to 1% by weight of said core being pyrolytic silica.

References Cited UNITED STATES PATENTS 3,074,817 1/ 1963 Gentner 252-506 3,353,135 11/1967 Chadwick 338-275 X LEWIS H. MYERS, Primary Examiner.

ELLIOT GOLDBERG, Assistant Exan'ziner.

US. Cl. X.R.

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