US3310611A - Method of producing an artificial graphite body - Google Patents

Description

United States Patent 3,310,611 METHOD OF PRODUCING AN ARTIFICIAL GRAPHITE BODY Roy W. Zocher, Lawrencehurg, Tenn., assignor, by mesne assignments, to Aerojet-General Corporation, El Monte, Califl, a corporation of Ohio No Drawing. Filed July 15, 1964, Ser. No. 382,958 4 Claims. (Cl. 264-.5)

This invention relates to fabrication processes generally, and more particularly to a process for producing a high quality, fine-bodied graphitized body.

The conventional process of producing high quality, fine-bodied artificial graphite begins with an extremely fine coke flour. This coke flour is intimately mixed with a suitable binder material such as coal-tar pitch, furfuryl alcohol, or other related organic compounds. Green or wet strength is thus imparted to the flour and permits the flour to be molded, pressed, or extruded into a desired shape. Once shaped, the body is heated initially to carbonize the coke flour. During this heating, the binder is polymerized, condensed, and partially volatilized. An amorphous carbon residue remains which holds the coke particles together. A subsequent high temperature heating of the body volatilizes the remaining binder and transforms the carbon body into artificial graphite. If further densification is required, the body can be im pregnated with additional coal-tar pitch and then refired as before. A similar process is used to produce high density metal carbide bodies, i.e., tungsten carbide.

This process has a great number of variables which can and do affect the finished product. Even after extensive mixing, the coke flour, binder, and other constituents may not be uniformly mixed. This results in a non-uniform product. A more serious problem, however, is the evolution of gases from the binder during heating of the body. Extreme limitations are placed upon the process by the critical rate of heating required to control the evolution of gases. These gases, if they escape from the green body after it has set, will leave a porous structure. If the gases are retained in the body they will build up pressure that can cause localized voids or hinder migration. In either event, there results a difference in material density which is then magnified by the further heat treatment. The heating must be controlled so that the gases will be evolved and escape before the body sets.

In addition, the gases can also have a marked effect on the dimensions of the final shape. Thermoplastic organic binders soften when they are initially heated thus permitting the gases to swell or pull out the body. The binder itself however shrinks as it carbonizes. This results in both a change in shape of the body and structural weaknesses.

Therefore, an object of the present invention is to provide a novel method of and improved means for producing a fine-bodied graphitized body.

Another object of the present invention is to provide a process for producing a fine-bodied graphitized body having a uniform high density and fine dimensional control.

Still another object of the present invention is to provide a process for producing a fine-bodied graphitized body having a neutron fissionable material dispersed therein.

3,310,611 Patented Mar. 21, 1967 In one aspect, the present invention comprises a process for producing fine-bodied graphite having the following steps:

(1) Uniformly mixing a diluted pre-polymerized furfuryl alcohol binder with fine coke flours;

(2) Vacuum distilling the formed body to remove the diluent;

(3) Forming the green mixture into the desired shape; and

(4) Heating the formed body to first carbonize, then graphitize the body.

In addition, granules of a uranium, or other neutron fissionable compound can be mixed with the fine coke flour to produce a uranium bearing end product. Also powdered metals such as tungsten can be added to produce a metal carbide end product.

Using a diluted pre-polymerized binder and vacuum distilling the mixture before extrusion achieves a more uniform mixture and eliminates a major source of gas evolution during the subsequent heat treating. This alleviates the severe heat treating restrictions and results in a significantly improved carbon yield, a higher graphite density and greater dimensional control.

These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from the following description.

High purity, extremely fine graphite flours are produced by calcining petroleum coke at 1100 C. to 1300 C. to drive off all volatile matter and then pulverizing the calcined coke to the desired particle size and shape. These flours, which are commercially available, have particle distributions 90% of which will pass through a .248 mm. opening screen. Speer Carbon Company, Nuclear Grade II (901-S Medium) was found to possess optimum qualities as far as particle size and shape and chemical purity. Great Lakes Carbon Company, Grade 1008 flour also essentially meets all requirements. The

Y graphite flour is blended with from 13 to 25 weight percent carbon filler, such as #15 C-olumbian lampblack or preferably Thermatomic Carbon Company Thermax.

To permit the forming of this coke flour, it is necessary to impart to it wet or green strength. Thus a suitable binder must be provided. Pre-polymerized furfuryl alcohol has been found to have provided exceptional results. Since the polymerization of furfuryl alcohol produces chain molecules of succeedingly higher molecular weight together with water molecules, pre-polymerization affords an opportunity to remove the water by vacuum distillation before the heat treating of the formed bodies. Thus a major source of gas evolution is removed. Pre-polymerized furfuryl alcohol, having less than 1% free monomer, is commercially available.

To achieve maximum benefits, the viscosity of the monomer free pre-polymerized furfuryl alcohol should be between 250 and 25,000 centipoise at 245 C. In this viscosity range, however, it is extremely difficult to obtain a uniform mixture with the coke flour. It is therefore necessary to dilute the binder with a suitable low viscosity solvent such as acetone, benzene, or other similar hydrocarbons having a boiling point less than 46 C. at 28" of mercury.

Taking a pre-polymerized furfuryl alcohol with less than 1% free monomer, and a viscosity of 250 centipoise at 24.5 C., the quantity of diluent should not exceed 50 weight percent. When using reagent grade acetone as 3 the diluent, its acidity should not exceed 0.003 weight percent as acetic acid as determined by the American Chemical Society Specifications.

A small amount, up to 12 weight percent, of an anhydrous acid binder catalyst can also be added to the binder. p-Toluene sulfonic acid or preferably maleic anhydride can be used effectively to catalyze furfuryl alcohol base binders.

The diluted binder is then added to the graphite and carbon flour mixture and thoroughly blended to a uniform consistency. Mechanical mixing or agitating means would normally be employed to achieve a plastic mixture.

Before shaping the mixture, the binder diluent necessary to effect the dispersion of the binder must be removed. This is accomplished by vacuum distilling the mixture at a temperature sufficient to vaporize the diluent. With an acetone diluent, a temperature of 44 C. is sufiicient. To prevent further polymerization of the pre-polymerized furfuryl alcohol binder, the temperature during this distillation should not exceed 46 C. at 28" of mercury.

The mixture can then be formed into the desired shape, preferably by extrusion. Depending upon the composition of the mixture, extrusion pressures from 1700 to 8000 p.s.i. have been utilized with extrusion rates up to 96 inches per minute.

Once formed, the final shaped article is subjected to thermal processing to (a) complete polymerization and cross linking of the thermal setting binder, (b) decompose the completely polymerized binder to a structured carbon binder phase, and (c) graphitize the carbon binder and filler material.

The binder can be completely polymerized and decomposed in a single firing. Temperatures in the order of 800 to 1100 C. in an inert atmosphere such as helium or argon are required for this carbonization of the body. Heating rates as high as 35 C./hr. are attainable since the binder initially is almost completely pre-polymerized. A suflicient quantity of gases generated over the temperature range during polymerization have been excluded to prevent the disruption of or weakening of the high density, low porosity extrusion.

To graphitize the body, temperatures on the order of 2300 to 2700 C. are required. Although desirable, it is not absolutely necessary to cool the body before the graphitization which also must be performed in an inert atmosphere. 5'50 C./hr. heating rates are possible.

Example An example of the complete process used to consistently produce high quality, fine-bodied graphite having a graphitized density exceeding 1.860 gm./ cc. follows. 2125 gms. of graphite flour are first dry mixed with 375 gms. of carbon filler. Concurrently, 660 gms. of pre-polymerized furfuryl alcohol is wet mixed with 300 gms. of reagent grade acetone and 21.8 gms. of maleic anhydride. The two mixtures are combined and mechanically blended into a plastic mass. After a vacuum distillation at 44 C., the mass is extruded at 40 inches/minute under 3300 p.s.i. to 2500 p.s.i. depending on die configuration.

The extruded shape is then heated to 800 C. over a 24 hour period and allowed to cool. After this carbonization, the shape is quickly heated to 23502700 C. and held at that temperature for approximately two hours to graphitize it.

By introducing granules of a uranium compound such as the oxide or carbide into the dry mixture of graphite and carbon flour and then proceeding with the normal processing, it is possible to obtain uranium in a graphite matrix. A wide selection of particle sizes and good control of the uranium distribution is possible using this method. This dispersion is accomplished with little or no sacrifice in the high strength and density of the artificial graphite produced by the invention. Other suitable neutron fissionable isotopes can likewise be dispersed in the graphite matrix.

In this manner, high density, fine-bodied graphitized bodies can be produced without severe thermal cycling limitations and with good dimensional control.

While specific embodiments of the invention have been illustrated and described, it is to be understood that these embodiments are by way of example only, and that the invention is not to be construed as being limited thereto, but only by the proper scope of the appended claims.

What is claimed is:

1. A method of producing an artificial graphite body comprising the steps of:

(l) diluting a pre-polymerized furfuryl alcohol, having less than 1% free monomer, with a quantity less than 50 weight percent of hydrocarbon solvent having a boiling point less than 46 C. at -28 of mercury;

(2) mixing the solvent diluted, pre-polymerized furfuryl alcohol with fine graphite and carbon flours, at least of which pass a .248 mm. opening screen;

(3) vacuum distilling the solvent from the solvent diluted mixture at a temperature less than 46 C. at 28" of mercury;

(4) extruding the mixture through a die to impart the desired shape;

(5) heating the extruded shape to carbonize and graphitize the mixture into an artificial graphite body.

2. A method of producing an artificial graphite body comprising the steps of:

(1) diluting a pre-polymerized furfuryl alcohol, having less than 1% free monomer and a minimum viscosity of 250 centipoise at 2415 C., with a quantity less than 50 Weight percent of acetone having an acidity less than 0.003

(2) mixing the acetone diluted pre-polymerized furfuryl alcohol binder with fine graphite and carbon flours, 90% of which passes a .248 mm. opening screen;

(3) vacuum distilling the mixture at 44 C. and 28" of mercury to remove the acetone;

(4) extruding the mixture through a die to impart the desired configuration;

(5) heating the extruded body to 1100 C. to carbonize the body; and

(6) heating the carbonized body to 2700 C. to graphitize the body.

3. A method of producing a graphite matrix containing neutron fissionable material comprising the steps of:

(1) dry mixing a compound containing a neutron fissionable isotope with fine graphite and carbon flours, at least 90% of said compound and said flours passing a .248 mm. opening screen;

(2) diluting a pre-polymerized furfuryl alcohol, having less than 1% free monomer, with a quantity less than 50 weight percent of hydrocarbon solvent having a boiling point less than 46 C. at -28" of mercury;

(3) mixing the solvent diluted, pre-polymerized furfuryl alcohol with the flour-neutron fissionable isotope containing compound mixture;

(4) vacuum distilling the solvent from the solvent diluted mixture at a temperature less than 46 C. at 28" of mercury;

(5) extruding the mixture through a die to impart the desired shape;

(6) heating the extruded shape to 'carbonize and graphitize the mixture into a graphite matrix containing neutron fissionable material.

4. A method of producing an artificial graphite body comprising the steps of (1) diluting a pre-polymerized furfuryl alcohol, having less than 1% free monomer, with a quantity less 5 6 than 50 weight percent of hydrocarbon solvent hav- References Cited by the Examiner $r:ur|;?111I1g point less than 46 C. at -28 of UNITED STATES PATENTS (2) adding up to 12 weight percent of anhydrous acid 1,375,879 4/ 1921 Wikle 264-29 binder catalyst to the solvent diluted, pre-polymer- 6 3,124,625 3/1964 Sheinberg 264-.5 ized furf-uryl alcohol; 3,135,665 6/ 1964 Koutz et a1 176-9l (3) mixing the catalyst-alcohol solution with fine 3,137,742 6/1964 SOWden 264-.5 graphite and carbon flours, at least 90% of which 3,156,747 11/ 1964 Burke 264-.5 1) a m g h 1 3,158,547 11/1964 Smith 264.5

vacuum 1st1 mg t e so vent rom t e so vent 1- 10 luted mixture at a temperature less than 46 C. at FOREIGN PATENTS -28" of mercury; 656,694 1/ 1963 Canada. (5) extruding the mixture through a die to impart the desired shape; CARL D. QUARFORTH, Primary Examiner.

(6) heating the extruded shape to carbonize and graph- 15 L. DEWAYNE RUTLEDGE, Examin r.

itize the mixture into an artificial graphite body.

Claims (1)

1. A METHOD OF PRODUCING AN ARTIFICIAL GRAPHITE BODY COMPRISING THE STEPS OF: (1) DILUTING A PRE-POLYMERIZED FURFURYL ALCOHOL, HAVING LESS THAN 1% FREE MONOMER, WITH A QUANTITY LESS THAN 50 WEIGHT PERCENT OF HYDROCARBON SOLVENT HAVING A BOILING POINT LESS THAN 46*C. AT -28" OF MERCURY; (2) MIXING THE SOLVENT DILUTED, PRE-POLYMERIZED FURFURYL ALCOHOL WITH FINE GRAPHITE AND CARBON FLOURS, AT LEAST 90% OF WHICH PASS A .248 MM. OPENING SCREEN; (3) VACUUM DISTILLING THE SOLVENT FROM THE SOLVENT DILUTED MIXTURE AT A TEMPERATURE LESS THAN 46*C. AT -28" OF MERCURY; (4) EXTRUDING THE MIXTURE THROUGH A DIE TO IMPART THE DESIRED SHAPE; (5) HEATING THE EXRUDED SHAPE TO CARBONIZE AND GRAPHITIZE THE MIXTURE INTO AN ARTIFICIAL GRAPHITE BODY.