US 2781273 A
Description (OCR text may contain errors)
Feb. 12, 1957 w, KOCH 2,781,273
METHOD OE'MAKING BLOCK TALC AND RELATED MATERIALS Filed Aug. 26, 1955 mz/m JAM BY M M ATTOP/Vfy INVENTOR United States Patent METHOD OF MAKING BLOCK TALC AND RELATED MATERIALS William J. Koch, Silver Spring, Md., assignor to the United States of America as represented by the Secretary of Commerce Application August 26, 1953, Serial No. 376,767
4 Claims. (Cl. 106-39) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to me of any royalty thereon in accordance with the provisions of 35 United States Code (1952), section 266.
The present invention relates to a method of making block talc products and in particular to a method of producing artificial block talc having high green strength. Natural block talc has been used for some time for spacers in electron tubes and for similar uses. However, the best source of natural block talc is found in India and the material is therefore relatively difiicult to obtain. Also, in the event of a national emergency the supply of the natural material would be cut off leaving the country without a source of supply of the material. Another difficulty arises when the electrical properties required are such that the natural blocks have to be fired at over 1800 F., since at these temperatures the product will tend to crack and warp, making handling and processing very difiicult. Further, as a result of fractures and impurities which exist in the natural blocks it is often difficult to obtain large blocks which can be used for the intended purposes. M
Numerous attempts have previously been made to produce an artificial block talc. The artificial product must conform to rather rigorous specifications. Since the fired product is very hard and difiicult to work on it is preferable to be able to machine the block to the desired configuration before firing. Therefore the green strength (strength before firing) must be relatively high to allow such operations to be performed. Also the shrinkage during firing must be uniform and small to prevent warpage of the piece and to maintain the dimensions 0f the machined piece accurate.
In the prior art it has been found necessary to use binders in order to obtain the aforementioned characteristics, However, as a result of the poor electrical properties of available hinders the resulting product has not been usable in applications where high dielectric materials are required.
It is therefore an object of the present invention to provide a method for producing block talc from the readily available powdered talc which method does not require the use of binders.
It is another object of the present invention to produce artificial block talc which has sufficient green strength so that it may be machined prior to firing.
It is another object of the present invention to provide a method for making artificial block talc which is almost entirely free of structural defects.
It is another object of the present invention to provide a method for producing artificial block talc having very low shrinkage during firing but which shrinkage is uniform for all dimensions of the material.
It is another object of the present invention to provide a method for producing artificial block talc which produces a product that is relatively absorptive and there- 2,781,273 Patented Feb. 12, 1957 fore relatively easy to degas when used for electron tube structures.
Other uses and advantages of the invention will become apparent upon reference to the specification and drawings in which:
Fig. 1 is a diagrammatic illustration of a fluid pressure system employed in one step of the process accord ing to this invention;
Fig. 2 illustrates the step of evacuating the aggregate containing sack prior to compression and firing;
Fig. 3 shows the sack containing the aggregate, and
Fig. 4 shows various forms of the block talc produced in accordance with the method of this invention.
In accordance with the present invention -100 percent powdered talc is mixed with 0-12 percent clay and 0-10 percent barium carbonate. The mixture must contain less than 5 percent impurities. The mixture is ground until the particles are not more than 10 microns in diameter. It is then sealed in a rubber sack from which all air has been evacuated and subjected to hydrostatic pressure of at least 40,000 pounds per square inch for a minimum of one minute to form a generally spherically shaped briquette. The material thus produced may then be machined or cut to any desired form as illustrated in Fig. 4, after which it is fired at between 1800 to 2315 F. for between 30 minutes to one hour with a total firing time of 2 to 9 hours. The time and/ or temperatures used will depend upon the properties desired in the final product.
In the preferred embodiment of the invention 80 percent powdered talc is mixed with 10 percent barium carbonate and 10 percent clay. To insure that the particles in the mixture will be of the required size-that is, not more than 10 microns and preferably 2-3 microns-each batch is ball milled wet for 24 hours and then dried. The dry mixture is then passed through a pulverizer to break up the agglomerated particles. The barium carbonate is used to chemically bind the silica which always appears as an impurity in talc. If the silica is not bound it goes through a quartz inversion which produces expansion and therefore cracking of the block. If the silica is combined with barium oxide, which is formed during the heating process as a result of the decomposition of barium carbonate into barium oxide and carbon dioxide, this inversion cannot take place and cracking is prevented. However, if the material is to be fired to only 1800 F., the barium carbonate may be eliminated, since the final product will be sufiiciently porous to allow expansion to occur without cracking, since the silica crystals will go into the spaces within the piece.
The clay used is Florida kaolin, since of the clays tried this has been found to contain the least impurities and still give the highest green strength to the unfired material. After the mixture has been ground, it is placed in a thin membrane rubber sack which is tied off and then evacuated to eliminate all air and other gases. The sack is then placed in a tightly sealed vessel which has been filled with a suitable liquid. The sack is suspended in a liquid to insure that uniform pressure will be applied to all areas of the mixture. This was found essential to insure the production of a final product free of cracks and other physical defects and also to insure uniform shrinkage when the product is fired. The mixture is subjected to a pressure of at least 40,000 pounds per square inchthe higher the pressure the more uniform the end product. Pressure is maintained for at least one minute and then released. The pressed specimens are hard enough to be machined but soft enough to allow ease in handling.
In order to prepare the material for briquetting, the ground aggregate is placed in a thin, leak-proof flexible sack 1 made of rubber or other fluid impervious material as shown in Figs. 2 and 3. Evacuation of the sack is per.
formed in a. conventional manner by inserting a needle 2 in the throat of sack 1 connected to a vacuum line 3. A tie-string 4 is laced around the throat portion and the sack is then sealed off by concurrently withdrawing the. needle and tightening the draw string.
The necessary pressure. for briquetting the aggregate is obtained by the use of special high-pressure equipment such as is manufactured by the Superpressure Division of the American Instrument Company and described on pages 30 and 75 of their catalogue, entitled Aminco Superpressure Equipment. The apparatus, as diagrammatically illustrated in Fig. 1 comprises a high-pressure chamber 5 capable of withstanding pressures exceeding 50,000 p. s. i. A closed fluid system is employed in which oil 6 supplied by a pump 8 is applied under pressure to the pressure vessel 5. The aggregate-containing sack 1 is suspended in the chamber 5a of the pressure vessel 5 as shown in Fig. 1 after which the end closure 7 is clamped in place. Fluid pressure is applied to the chamber through the highpressure inlet 7a.
The block may be fired for between 30 minutes to one hour at temperatures ranging from 1800 F. to 2315 F., with a total firing time of 2 to 9 hours, depending upon thev properties desired in the end product. As a result of the expulsion of moisture from the product and chemical reactions within the product, the rate of temperature rise cannot be too rapid or the piece will be destroyed. Therefore, although the pieces are fired for only 30 minutes to one hour, the heating cycle will take anywhere from two to nine hours, depending upon the final temperature to be attained and the time of firing at that temperature. Firing at 1800 F. produces a relatively porous material, which, as previously mentioned, is useful in electron tubes where it is necessary to degas the material. If firing takes place at 2315 F., steatite porcelain is formed which is a very hard, nonporous, material. The rate of increase of temperature in firing is not critical and can be approximately 150 C. (300 F.) per hour. As previously mentioned, the shrinkage during firing is relatively low and almost completely uniform. Of course, the percentage of shrinkage will vary with the firing temperature and the length of time fired.
Referring to Table 1, it will be seen that when the material was initially subjected to 44,000 pounds per square inch pressure and then fired at 1800 F., the percentage of the absorption was 7.67,
and the linear shrinkage along the three dimensions was approximately 2.8 percent, the greatest variation being 0.08 percent. When the material was fired at 2000 F. for one hour, the percentage of absorption was 4.13 and the linear shrinkage in the three dimensions was approximately 5 percent, maximum, with a variation in the three dimensions of 0.17 percent. When the test was made using a hydrostatic pressure of 100,000 pounds per square 4 inch and fired at 2000 F., it was found that the absorption was approximately 5.5 percent and the shrinkage during firing was approximately 2 percent.
It has been found that this material stores very well since its green strength is relatively high. Also, very few structural defects are found, and therefore almost 100 percent of the blocks formed can be used.
Furthermore, it has been found that this process may be used for relatively small, as well as relatively large bodies. This is particularly interesting since firing at 2000 F. is considered rather severe treatment for this product.
It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of my invention as defined in the appended claims.
What is claimed is:
1. The method of preparing block talc which comprises preparing a mixture of -100 percent talc, 0-12 percent clay and 0-10 percent barium carbonate, the materials in the mixture having a maximum particle size of 10 microns and containing a maximum of 5 percent impurities, subjecting the mixture to a minimum of 40,000 p. s. i. hydrostatic pressure and firing the product at a temperature of between 1800-23l5 F. for between 30 minutes to 1 hour, with a total firing time of 2 to 9 hours.
2. The method of preparing block talc which comprises preparing a mixture of 80-100 percent talc, 0-12 percent clay and 0-10 percent barium carbonate, the materials in the mixture having a maximum particle size of 10 microns and containing a maximum of 5 percent impurities, placing the mixture in a pliable sack, evacuatting the air from the sack, subjecting the mixture to a hydrostatic pressure of at least 40,000 p. s. i. and firing the product at between 1800" F. and 2315 F. for between 30 minutes to 1 hour, with a total firing time of 2 to 9 hours.
3. The method of preparing artificial block talc which comprises preparing a mixture containing 80-100 percent powdered talc, 0-12 percent clay, 0-10 percent barium carbonate and a maximum of 5 percent impurities, the particle size of the materials in the mixture preferably being 2-3 microns and not more than 10 microns, placing the mixture in a pliable sack, exacuating the air and other gaseous materials from the sack, subjecting the mixture to a hydrostatic pressure of at least 40,000 p. s. i. and firing the product at between 1800 F. and 2315 F. for between 30 minutes and 1 hour, with a total firing time of 2 to 9 hours.
4. The of preparing block talc which comprises preparing a mixture containing 80 percent talc, 10 percent Florida kaolin and 10 percent barium carbonate, the materials in the mixture having a particle size of from 2 to 3 microns and having less than 5 percent impurities, placing the mixture in a pliable sack, evacuating the air from the sack, subjecting the mixture to between 40,000 and 100,000 p. s. i. hydrostatic pressure, and firing the product at between 1800 F. and 2000 F. for 1 hour, with a total firing time of 2 to 9 hours.
References Cited in the file of this patent UNITED STATES PATENTS 2,332,343 Rigerink Oct. 19, 1943 FOREIGN PATENTS 441,115 Great Britain 1934