US2355639A

US2355639A - Pencil lead

Info

Publication number: US2355639A
Application number: US487654A
Authority: US
Inventors: Monie A Ferst; Charles F Wysong
Original assignee: M A FERST Ltd
Current assignee: M A FERST Ltd
Priority date: 1943-05-19
Filing date: 1943-05-19
Publication date: 1944-08-15
Anticipated expiration: 1961-08-15
Also published as: BE460908A

Description

Aug 15, 1944. YM. A. FERST ET AL PENCIL LEAD Filed May 19, 1943 $3 5 WW V m Patented Aug. 1944 I canes srArss PATENT Wis raucm LEAD Monie l rel-st and came; a Wysong, Atlanta, 63., assignors to M. A Fen-st, Ltd" Atlanta, (5a., a corporation of Georgia Application May 19,- 1943, serial No. 487354 (or. mo -s3 1 7 Claims. The invention relates to pencil leads or mark an object the provision of an element of increased strength against breaking, as compared with existing elements of a comparable grade of softness or hardness.

It is a further object of the invention 'toprovidea marking element which will have improved uniformity of character in all parts of each individual element, as well as among the individual. elements in each grade of softness or hardness. f v

It is a further object'of the invention to provide a marking element which will carry a larger proportion of the impregnant material than comparable elements now in use.

In the drawing accompanying and forming a part hereof Figure 1 is a side elevation of a pencil'lead somewhat enlarged.

particles at H and the bond material at it. For

purpose ofillustration the pores or lacunae it,

which are formed as described below; are shown very much exaggerated in size as compared with the graphite particles lland the presumably plate-like bond portions l2. 7 The dimensions of the particles measurable. in microns. and the particles of material used to form thepores it are also exceedingly minute, therefore the showing of the drawing is idealized and a correct showing of relative sizes or proportions is impossible, particularly where a wide variation is possible with the invention.

By "porosity is meant the percentage of pores by volume of the specimen ready for impregnation which can be filled by gas or liquid. The range of porosity of 'graphitic, lead pencil leads now in use is between sixteen per cent for hard leads and forty per cent at the outside for'soft leads. In accordance with the present invention the percentage of porosity is increased by an of graphite are.

ing elements of ceramic character and has as I Graphitic marking elements such as are in 7 common use in lead pencils are ceramic in char acter'and are preponlierantly composed of powdered graphite as a marking material and iinely divided clay as a bond which bond is glassy in characikr-as a result of the firing step of manufacturef'lhe finest particles of the clay are as small as substantially one-tenth micron and may range to substantially two microns while the graphite particles usually range between about onemicrofi and a possible eighty microns in well made elements. o

The 'naturallyoccurring interstices between particles of such sizes are exceedingly small.

Furthermore it is considered) desirable in the art, to so choose-the different sizes of particles of the material as to, provide as compact a body as possible for resultant strength and uniformity, but with resultant reduction of capacity for impregna'nt. r

In accordance with the present invention, an

increase in porosity is brought about by inclusion' of material'in themix, while preparing the same for molding, whichadded material is afiected or removed during the process of manu-' facture to leave an additional volume of pores in the body of the completed marking'element.

The material so included may be removed by volatilization, by extraction with a solvent, or

the added material may be reduced in volum as by charring. v

The ideal result contemplated by the present invention maybe obtained by a choice of graphite andiboncl particle sizes such as to result in no porosity, that is maximum compactness, and a provision of removable material, or material whose volume could be reduced by treatment, in

quantity and in fineness of particle size to pro-- vide the exact size and number of pores desired fora particular result.

- The porosity would then be-under full control and if the mixing is properly done. the resu twould be maximum uniformity; It is desirable to approach as closely as possible to this ideal.

Among the materials that may be charred or otherwise reduced in volume in the firing of the lead are soaps, vegetable dusts or fibers and synthetic fibersf Such substances while not completely removed are sufliciently reduced in volamount sufiicient to have an appreciable efiect ume to provide increased porosity in predetermihable amount.

Examples of materials that may be removedprior to or, during the firing step hre naphtha- ;onles, waxes, sulphur ahd other organic mate A separatemlass of substances which may be used forthe purpose of the invention is finely divided metal or metallic salts reducible to free metal under firing conditions. The resulting metal in the fired lead can then be removed by pregnant filled pores or la unae of substantially increased size whereby e marking material yields more readily to abrasion and is consequently more friable for a given fiexural or point in writing in any given degree of softness or hardness because porosity will be more accurately controlled. The writing qualities of a lead are drastically altered by any change in porosity, other factors being equal. The pores normally developedin formation are subject to much varia-- tion due to variations in water content, extrustrength. If an oil and/or a waxy impregnant is used the increased proportion of oil and/or wax provides greater lubrication resulting in a smoother mark. The function of the oil and/or wax impregnant being to cause the graphite to 4 adhere to the paper, the mark will appear more intense. Further, the porosity being controlled, the leads of the invention, will have a more uniform body and, also, be more uniform in marking and in writing in every degree of softness and hardness.

A number of difierent impregnants are contemplated in the impregnation of the porous body of the invention. The impregnant may contain water soluble coloring matter in which case the lead will produce a graphite mark or a colored graphite mark and if the color is water soluble to sufilcient extent a copying-pencil leadwill be provided of the so-called indelible type. The impregnant may be a water soluble chromate in which case the lead will produce a mark that can be used for transfer purposes. The impregnant, which is water soluble can contain an ionogen in which case the lead will produce a mark which will conduct electricity. The conductivity can be controlled by the amount of ionogen used.

It is important that the material of the lead aside from the porosity producing material shall be as compact as possible to provide a strong skeleton interspersed with the pores produced by the removed material.

A pencil lead made in accordance with the invention will bestronger for a comparable sof ness and softer for a comparable strength than the usual pencils sincethe impregnant adds strength to the material.

The increase in breaking strength is not inconsistent with-an increase of yield to abrasion noted above. The element may be regarded as a skeleton of graphite and bond surrounding the impregnant filled pores. As wear exposes a-pore the surrounding skeletal substance will present edges thereof backed up-as resisting abrasiononly by the impregnant which; of itself has little resistance to displacement. Therefore, the abrasion pressure, method of tempering and so forth. The type of pores developed by this invention will be unaflected by most of these factors, hence they would bemore uniform than the normal ,pores. Since the two types of porosity would be additive, the overall uniformity would be improved. 4 v

The expression "reduced in volume as used in the appended claims includes the reduction to zero or complete removal of the distending material.

Minor changes may be made in the steps of the process or in the characteristics of the article produced thereby within the scope of the appended claims, without departing from the spirit of the invention. I a

W claim: X

1. The process of producing graphitic marking e1ements.which comprises: preparing a mixture including graphite, bonding material, and a discrete distending material; forming a marking element thereof; firing the thus formed element; subjecting the element, at some stage subsequent to the forming operation, to treatment acting to reduce the volume of the distending particles, and filling the thus formed pores with an impregnant.

2. The process of producing graphitic mark-' ing elements which comprises: preparing a mixture comprising graphite, clay, and. a discrete distending material; forming a marking element thereof; firing the thus prepared element; subjecting the material,- at some stage subsequent to the forming operation. to treatment to remove the particles of distending material so as to leave vacant the spaces occupied thereby during formation of the element; and filling said vacant spaces with an impregnant.

3. The process of producinggraphitic marking elements which comprises: preparing a mixture comprising graphite, bonding material and a discrete vaporizable distendingJniaterial; said last named material comprising from five to forty per cent by volume of the mass; forming a marking element from the thus prepared mix, firing the formed element with consequent vaporizasion can break off some of the skeletal material and rub off some of the impregnant into'adherence to the paper. In resistance to fiexure, however, the impregnantfilled pores are interspersed fiowof the material since it is substantially intion of" the distending material; and filling the resulting pores with an impregnant.

4. The process of claim 3 wherein the distending material is powdered naphthalene.

5. A graphitic marking element comprising a body formed of graphiteand bonding material and having pores throughout the mass, the total volume of the pores being abnormally large-said pores filled with an impregnant.

6. A graphitic marking. element comprising a skeletal body of graphite particles bonded together by fired clay, enclosing pores which are of a larger volume than the volume of the interstices normally occurring between particles of graphite and bond of marking elements, said pores filled with impregnant.

'l. The structure of claim 5wherein the impregnant is an oily and/or waxy material.

\ with the skeletal material and can only yield by vention will be more uniform'in marking and Id 8. The structure of claim 5 wherein the impregnant is an oily and/or waxy material containing oil or, wax soluble materials.

9. The-structure of claim 5 wherein the impregnant is an oily and/or waxy material containing a wax soluble coloring material.

10. The structure of claim 5 wherein the impregnant is a water soluble waxy material.

" 11. The structure of 'claim'5 wherein the im-' the im- 1'1. The process of producing graphitic marking elements which comprises: preparing a mixture including finely divided graphite and bonding material, the sizes and quantities of each size of particles of the material chosen to pro,- vide a minimum of voids, adding at least five per. cent by volume of discrete material to said mixture; forming a marking element of said mixture;., firing the thus formed element; subjecting the element, at some stage subsequent to the forming operation, to treatment acting to 4 reduce the volume of the particles of the discrete material; and filling the thus formed pores with an impregnant.

MONTE A. FERST. CHARLES F. WYSONG.