US2252277A - Molded porous electrical brush and the like - Google Patents

Description

Aug. 12, 1941. J. R E ETAL 2,252,277

MOLDED POROUS ELECTRICAL BRUSH AND THE LIKE Filed April 20, 1939 MHz/E5 OFCONDIICTII/E Mars/2m 4W INVENTORS 504/0 Pqerlas 0F PURE mew/v6 JU857/I/YCE dimes, Elk Y g,

ATTORNEY.

Patented Aug. 12, 1941 MOLDED POROUS ELECTRICAL BRUSH AND THE LIKE James R. Tate, Toronto, Ontario, Canada, and Ralph L. Henry, Detroit, Mich.

Application Apr-H20, 1939, Serial No. 268,940

10 Claims.

This invention relates to novel electrically conductive bodies produced from molded plastic materials, and to method procedure by which the same are obtained.

It is an object of this invention to provide an electrical motor and generator brushes, resistors,

contacts and for other uses requiring electrically conductive material wherein, from the electrical standpoint, maximum conductivity in a given direction with relatively high resistance in direction transverse thereto is required, as in electrical brushes, or wherein a resistance characteristic of predetermined degree is required, as in electrical resistors, and wherein, from a mechanical standpoint, a comparatively light weight but nevertheless strong and tough material is desired.

This invention has for another object to provide a novel method of producing a body of desired shape having the porous or interstitial physical character, which "includes introducing into and distributing through a selected base molding mixture, from which it is to be formed, a pore forming substance which is subject to physical change under applied heat, whereby voids are formed in the body mass, while at the same time a residue of the substance remains within the body mass in a form and relation calculated to provide an additional bond auxiliary to that of the base binder element.

This invention, from a more specific standpoint, has for a further object to provide a novel commutator brush formed from a molded mixture of electrically conductive material and a nonconductive synthetic resin binder, such e. g. as disclosed in prior United States Letters Patent to Ralph L. Henry No. 1,556,990 dated October 13, 1925, but so treated by an added pore forming ingredient and under the novel procedure according to this invention as to provide, in its finished form, a porous or interstitial mass calculated to reduce its density while nevertheless including means operating to reenforce the primary binding effect of the resin binder element, whereby a very light, and yet strong and long wearing brush is provided, and also one which possesses the desirable longitudinally stratified relation of its conductive and binder elements which is adapted to provide good lengthwise conductivity while ofiering greater crosswise resistance, the crosswise resistance being increased by the porosity of the brush mass; thus providing a brush adapted to attain, in use, better commutation with less commutator wear and less running noise.

To illustrate the principles of procedure according to this invention and the character of the molded body having the desired porous or interstitial form attained thereby, the application thereof to the production of a commutator brush will now be described.

In the accompanying drawing, Fig. 1 shows the initial molding mixture, with its content of solid particles of pore forming substance disprsed therethrough, deposited in a mold for shaping; and Fig. 2 shows, conventionally, a sectional view of the finish body after the pore forming substance has been evacuated so as to form dispersed voids having a residue of said pore forming substance deposited on the walls of said voids in the form of hardened adherent coatings.

In the manufacture of a molded commutator brush, conductive material, such e. g. as graphite, copper powder or the like, is mixed with a binder material comprising a solution ofa condensation product or synthetic resin, such e. g. as Bakelite. These materials or ingredients are thoroughly intermingled and worked together cold into a doughy mass by means of suitable mixing apparatus, and thereafter the mixture thus attained is dried, under the application of comparatively low heat and, if desired, under a reduced atmosphere (as in a vacuum pan), to drive off the solvent of the resin solution.

The conductive material and binder are combined in suitable'relative proportions to provide a mixture thereof wherein the proportion of the binder ingredient is preferably about 15 per cent and not in excess of 25 per cent.

After the mixture is dried, the same is reduced to a powdered state by means of suitable grinding mechanism, whereupon the powdered mixture is spread in suitable receptacles for deposit in an oven and therein subjected to heat approximating 250 degrees F. for a suificiently long period (usually about two hours) to temper the same. After tempering the mixture may again be ground to remove lumps and to assure its reduction to a uniformly fine powder.

Preparatory to moldin the base mixture thus obtained into desired body forms, a. suitable.

quantity of pore forming substance is added thereto. Various substances adaptable for the purpose may be used so long as the same are ca pable of being broken down by heat and evacuated from the molded mass with pore forming effect, while at the same time leaving ahardened residue, preferably having some electrical conductivity, which functions to provide an auxiliary binding effect calculated to enhance the townness and tensile strength of the finished brush body.

By way of example, one pore forming substance qualified to provide the effects above mentioned is any carbohydrate capable of melting or liquefying when subjected to heat so as to be, in the main, exuded or discharged from the molded body so as to leave the desired pores or interstices dispersed throughout the body mass. Granulated sugar is such a carbohydrate, and may be satisfactorily utilized, since the solid discrete crystalline particles thereof are readily intermingled with and thoroughly distributed throughout the mass of molding mixture when intermingled with the constituents of the latter. The proportion of sugar to molding mixture is subject to more or less wide variation, depending upon the degree of porosity desired in the finished product, the range of this variation may run from five to fifty per cent of sugar relative to the quantity of molding mixture. A relative proportion which has been found to be quite satisfactory for electrical brush purposes is one part of sugar to ten parts of molding mixture. The molding mixture with the added sugar is deposited in mold cavities of either a hot or cold mold, and is thereupon compressed therein to form and set the mixture so as to produce plates of brush material or individual brush units as desired.

By way of further example, another pore forming substance which may be utilized is boric acid, which not only breaks down under applied heat with pore forming eifect upon the mass in which it is incorporated, but also, when broken down leaves a residue which produces an additional or auxiliary binding effect to enhance the toughness and tensile strength of the ultimate molded brush body. To prepare the boric acid for introduction into the powdered base mixture, the same is first thoroughly dried under heat which is preferably not in excess of 250 degrees F. The base mixture is deposited in suitable mixing apparatus and thereupon the dried boric acid is screened thereinto and thoroughly worked through and intermingled with said base mixture. The base mixture and boric acid are thus intermingled in suitable relative proportions to provide a final mixture comprising, preferably, 1 part boric acid to 11 parts of the base mixture. These proportions, however, are subject to more or less variation, since the proportion of boric acid in the total mixture may range from about 5 to 20 per cent.

After the boric acid is thoroughly mixed with the base mixture to provide the final molding mixture, the latter is filled into the receivin cavity or cavities of either a hot or cold mold, and is thereupon compressed therein to form and set the mixture so as to produce molded plates or desired body forms.

The molded plates or bodies I, as shaped by the molding operation, are removed from the molds and deposited in a suitable oven for baking under a progressively increasing temperature, ordinarily starting at approximately 250 degrees F. and rising finally to approximately 500 degrees F. Preferably the baking temperatures are raised step by step at desired intervals of time until the maximum temperature is reached, at which latter temperature the baking is continued for several hours. The eifect of the baking operation is twofold, in the first place, the applied heat causes the resin binder to turn over" and become infusible and insoluble, and in the second place, the applied heat acts to break down the pore forming constituent with pore forming and other effects.

Under the baking heat, if sugar is the pore forming substance utilized, the initial temperature being above the melting point of said sugar, the same will melt or liquefy and thereupon exudingly discharge from the molded body, leaving the desired voids 2 dispersed throughout the mass of the latter. As the sugar particles liquefy upon melting, the resultant liquid tends to expand and is thus forced to enter any crevices or small fissures left between the conductive and binder constituents of the molded body mass, while at the same time being caused to film over the wall surfaces of the spaces or voids left by the evacuation of the positions formerly occupied by the initially solid particles of sugar. Subsequent to this, and as the baking temperature progressively rises, a temperature condition is arrived at which is sufficiently high to carbonize or coke the remaining infiltrating and filming residue 3 of the liquefied sugar, and when this carbonization or coking is complete, the resultant conversion of such residue 3 into hard carbon serves to reenforce the primary bonding effect of the synthetic resin binder which has also become solidified and infusible, thus enhancing the mechanical strength and toughness of the finished brush body.

Under the baking heat, as it progresses, if boric acid is the pore substance utilized, the boric acid is broken down into boron oxide and water vapor, the water vapor going off or being evacuated leaving the pore forming voids 2 upon the walls of which the residue 3, provided by the boron oxide, deposits. Under the elevated baking temperatures approaching and attaining the maximum, the boron oxide, which deposits on the pore walls, fuses and becomes hard and closely adherent thereto. In this pore wall coating state, the fused boron oxide functions as an additional binding agent auxiliary to the resin binder content of the body I. In addition to the advantages above pointed out and attendant upon the use of boric acid as a pore forming substance, further advantages are that it is not necessary to subject the same to any preliminary processing other than drying the same as stated; and that the fused boron oxide residue remaining in the finished brush body will neither dust off nor carbonize.

It has been found by test that commutator brush bodies, made according to this invention and as above described, give better wearing life, better commutation, less commutator wear, and less running noise. By reason of its relatively low density and consequently lighter weight, the brush has less tendency to bounce or chatter, and therefore rides the commutator more closely and as a result runs more quietly. This is due to the fact that, because of its porous or interstitial physical character and consequent lower density, its inertia on the bounce is considerably less than that of a solid brush, and therefore it will be off the commutator less, 1. e. for shorter periods.

When the conductive material of the brush is graphite and the binder comprises an insoluble and infusible synthetic resin, a stratified or laminated structure is produced as disclosed in the above referred to prior Henery patent; the strata or laminae being perpendicular to the directlon of molding pressure. Such structure tends to provide a brush of high cross resistance. This feature is not only maintained in the porous brush body made according to the instant invention, but the cross resistance is even increased by the presence of the dispersed voids in the brush body mass.

While, as above pointed out by way of example, boric acidlor sugar may be satisfactorily utilized as the pore forming substance because of the ability of .the same to break down from solid particles under heat, thereupon leaving but a residue of substance coating the walls of the space initially occupied by the mass of the particle, it will be understood that other pore forming substances which will act in substantially corresponding manner are to be considered equivalent to the boric acid or sugar within the scope of the following claims.

It will also be understood that, under the broader aspects of the instant invention, the base molding mixture may comprise any well known conductive material and any suitable binder which is capable of having the bore forming material interspersed therethrough prior to molding and which is capable of being brought to desired shape by a molding operation.

Having described the instant invention, What is claimed is:

1. The process of making an electrical brush or the like having a porous body of low density which consists in forming a molding mixture by intermingling with a base mixture, comprising conductive material and a suitable binder, solid particles of a pore forming substance of a character adapted to break down under heat with resultant mass reduction and conversion into a residue deposit having auxiliary binding elfect upon the molded mixture; compressing the molding mixture to a desired body form; and then subjecting the molded body to increasing heat sufficient to first break down evacuate a substantial portion of the pore forming substance to thereby leave voids dispersed throughout the molded body mass and to deposit the residue of said pore forming substance upon the walls of said voids, and then to convert said residue into a hardened coating adherent to the void wall surfaces.

2. The process of making an electrical brush or the like having a porous body of low density which consists in forming a molding mixture by intermingling with a base mixture, comprising conductive material and a synthetic resin binder capable of becoming insoluble and infusible when solidified by heat, solid particles of a pore forming substance of a character adapted to break down under heat with resultant mass reduction and conversion into a residue deposit having auxiliary binding effect upon the molded mixture; compressing the molding mixture to a desired body form; and then subjecting the molded body to increasing heat suflicient to first consolidate the base mixture thereof while at the same time breaking down and evacuating a substantial part of the pore forming substance to thereby leave voids dispersed throughout the molded body mass, and to deposit the residue of said pore forming substance upon the walls of said voids, and then to convert said residue into a hardened coating adherent to the void wall surfaces. I

3. The process of making an electrical brush or the like having a porous body of low density which consists in forming a molding mixture by intermingling with a base mixture, comprising conductive material and a suitable binder, solid particles of sugar; compressing the molding mixture to a desired body form; and then subjecting the molded body to heat sufilcient to liquefy and evacuate the sugar particles and thus break down the latter to leave voids dispersed throughout the body and to thereupon deposit a carbon residue upon the walls of said voids in a hardened and adherent condition.

4. The process of making an electrical brush or the like having a porous body of low density which consists in forming a molding mixture by intermingling with a base mixture, comprising graphite and a synthetic resin binder capable of becoming insoluble and infusible when solidified by heat, solid particles of boric acid; compressing the molding mixture to a desired body form; and then subjecting the molded body to heat sumcient to evacuate the water content of said boric acid particles and thus break down the latter to boron oxide to leave voids dispersed throughout the body and to thereupon deposit'said boron oxide upon the walls of said voids in a fused, and self hardening and adherent condition.

5. The process of making an electrical brush or the like having a porous body of low density as defined in claim 2, wherein the pore forming substance added to the molding mixture ranges in amount from 5 to 50 per cent of the resultant mixture.

6. The process of making an electrical brush or the like having a porous body of low density as defined in claim 2, wherein the heat applied to the molded body is periodically increased during the baking step up to approximately 500 degrees F. as a maximum.

7. The process of making an electrical brush or the like, having a porous body of low density, which consists in forming a molding mixture by intermingling with a base mixture, comprising graphite and a synthetic resin binder, solid par-- ticles of sugar, compressing the molding mixture to a desired body form; and then subjecting the molded body to heat suiiicient to liquefy and evacuate the sugar particles and thus break down the latter to leave voids dispersed throughout the body, and to thereupon deposit a carbon residue 4 upon the walls of said voids in a hardened and adherent condition.

8. The process of making an electrical brush or the like,- having a porous body of low density which consists in forming a molding mixture by intermingling with a base mixture, comprising conductive material and a suitable binder capable of becoming insoluble and infusibl when solidified by heat, solid particles of boric acid; compressing the molding mixture to a desired body form; and then subjecting the molded'body to heat suflicient to evacuate the water content of said boric acid particles and thus break down the latter to boron oxide to leave voids dispersed throughout the body, and to thereupon deposit said boron oxide upon the walls of said voids in a fused, and self-hardening and adherent con dition.

9. A porous electrically conductive body of low density, comprising a molded and baked mixture consisting of an electrically conductive material and a binder, and provided with interspersed voids, the walls of which are coated with a hard adherent carbon residue left thereupon by evacuatlon during the baking procedure 01' an initially intermingled proportion of sugar.

10. A porous electrically conductive body of low density, comprising a molded and baked mixture consisting of an electrically conductive ma; terial and a binder, and. provided with interspersed voids, the walls or which are coated with a hard adherent deposit of boron oxide left thereupon by evacuation during the baking pro-- cedure of an initially intermingled proportion of boric acid.

JAKE R. TATE. RALPH L'. HENRY.

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