IL33891A - A process for the production of fibers,textiles and shaped articles composed of an inorganic material - Google Patents

Description

/30/85 33891/2 O»-IXIBI »DopB »iain ,o»a»o ns^'V i» nn »a.niK »R noma mas-nan o*asij?a A process for the production of fibers, textiles and shaped articles composed of an inorganic material The invention, relates to a process for reducing the content of carbon from a composition that comprises a mixture of carbon and at least one metal-containing compound. The process comprises contacting said composition with carbon dioxide at a temperature sufficiently high to effect reaction between said carbon and said carbon dioxide.

The art is aware of processes for producing inorganic articles by decomposing and removing the organic portion of mixtures of an organic composition and a metal-containing compound. Among such known processes is the "relic process" disclosed in United States Patents os. 3,385,915; 3,399,979; 3,1*03,008 and 3,1*06,025, to Bernard Η· Hamling. These patents disclose a relic process for the production of inorganic articles (i.e., fibers, textiles, and shaped articles) composed of metal oxides, metal nitrides, metal carbides, and elemental metals. In the relic process, a preformed organic polymeric material (referred to herein as a "precursor") is loaded with one or more metal compounds, and the loaded precursor is then subjected to conditions to decompose and at least partially volatilize the precursor without destroying the structural integrity of the precursor, to produce the inorganic article as a relic product. Whether or not a metal oxide, a metal nitride, a metal carbide, or an elemental metal article is produced depends upon the specific treatment given to the loaded precursor during and ater the decomposition and volatilization of said precursor.

There is also known from British Patent Ho.858,061 , a process of forming a fuel element for a nuclear reactor by preparing compacts composed of an oxide powder of a fissionable material, a resinous binder and a lubricant, removing the binder and the lubricant by heating the compact© to a temperature between 500° and 900°C. in a free-flowing atmosphere of carbon dioxide, and thereupon sintering the residue at a higher temperature.

During the initial heating, the hydrocarbons of the binder and lubricant break down into carbon and volatile tars and resins* The volatile compounds are driven off by the steady stream of carbon dioxide gas, whereas the carbon reacts with th carbon dioxide and passes off as carbon monoxide. The carbon content of the pressed compacts is thereby reduced from an initial value of 5000 to 7000 parts per million to 20 to US parts per million.

The sintering is carried out only subsequent to the carbon removal, and is effected by subjecting the oxide residue to a temperature of about 1700°C. for a period of approximately 10 hours in a non-oxidizing atmosphere such as argon or hydrogen. The product thus obtained has a density closely approaching the theoretical density, which is indication of a substantially complete merging or "gross sintering" of the individual particles.

Other processes for producing inorganic articles by decomposing, and volatilizing the organic portion of a mixture - 1a - of an organic material and a metal-containing compound are known in the art. For instance, one such method is to spin viscose rayon from a spinning dope in which a metal-containing compound is dispersed. The rayon thereby produced contains the metal-containing compound, and inorganic articles can be produced by decomposing and volatilizing the rayon.

An important feature of the relic process disclosed by Hamling is that the decomposition and volatilization of the precursor must be carried out under conditions such that the structural integrity of the precursor is maintained.

Therefore, the said decomposition and volatilization of the precursor is carried out so as to avoid ignition of the precursor. (As explained in the above-mentioned four patents to Hamling, the term "ignition" refers to an uncontrolled temperature increase, and not solely to combustion accompanied by flame.) The decomposition and volatilization is carried out by heating the precursor under controlled conditions. In the usual case , the loaded precursor is heated in air , and the precursor decomposes to form carbon and volatile decomposition products. Subsequently (and in some cases, concurrently with the decomposition) the carbon is removed by oxidation. In order to avoid ignition of the precursor when air or other strong oxidizing agent is used as the oxidizing agent for the decomposition and volatilization, the temperature of the loaded precursor is increased very slowly. As a result, heating times of the order of from about 12 to about 24 hours are not uncommon.

Hamling also indicates that shorter heating times However, there are certain disadvantages attendant with the use of water vapor for this purpose. Special equipment must be used for handling the water vapor. Water vapor is corrosive especially at the temperatures needed to effect reaction between water and carbon so as to volatilize the carbon in the form of carbon monoxide. (The reaction between carbon and water does not proceed with an appreciable rate until temperatures of the order of about 740°C. are reached.) Because of the corrosive nature of water vapor at these temperatures , expensive corrosion-resistant equipment must be used.

The present invention is based upon the discovery that several useful and valuable advantages can be obtained when carbon dioxide is used as the oxidizing agent for reduction of the content of carbon from compositions comprising mixtures of carbon and at least one metal-containing compound. Such compositions are encountered when practicing the relic process and other processes for producing inorganic articles by decomposing and at least partially volatilizing the organic portion of a mixture of an organic composition and at least one metal-containing compound. Said advantages include the fact that the carbon oxidation can be carried out much more rapidly than when air or other strong oxidizing gas is used as the oxidizing agent. Also, expensive corrosion resistant equipment is not needed , as is the case when water vapor is used as the oxidizing gas. In many cases, the inorganic articles that are produced by the process of the invention are stronger and more uniform than those that were heretofore In its broadest aspect, the process of the invention comprises contacting a composition that comprises a mixture of carbon and at least one metal-containing compound with carbon dioxide at a temperature sufficiently high to effect reaction between said carbon and said carbon dioxide, and to thereby reduce the content of carbon in said composition.

The composition that comprises a mixture of carbon and at least metal-containing compound can be obtained by methods that are known to the art. For instance, a precursor can be loaded with at least one metal compound in accordance with the relic process , and the precursor is decomposed to form a carbonaceous char containing one or more metal compounds intimately dispersed therein. The carbon content of this material is then reduced by contacting it with carbon dioxide in accordance with the present invention. The invention will be more fully described immediately below by reference to one of its preferred aspects , which resides in the production of metal oxide articles (i.e., fibers, textiles and shaped articles) by the relic process.

In this aspect, a first composition comprising an intimate mixture of an organic material and at least one metal-containing compound is produced by loading a preformed organic polymeric material (i.e., "precursor") with at least one metal compound. The loading can be effected by the method taught in the Hamling Patent No. 3,385,915, that was referred to above. One desirable way to form said first composition is to first immerse a precursor such as a rayon or other Such metal compounds which can be used include zirconyl chloride, thorium nitrate, yttrium trichloride, ammonium meta-tungstate, and the compounds of other metals including beryllium, aluminum, tantalum, niobium, magnesium, calcium, silicon, boron, strontium, lanthanum, cerium and other rare earths, hafnium and the like. In many cases, the metal compound that is loaded in the precursor is in the form of a chloride, oxychloride or a nitrate. Because chlorides, oxy-chlorides , and nitrates can be rather corrosive and as a result they can have a somewhat deleterious effect on the properties of the ultimate metal oxide product, it is desirable in many cases to wash out the anion from the loaded precursor by washing the loaded precursor with aqueous ammonium hydroxide. This procedure has the beneficial result of washing out the corrosive anion, and also of forming the hydroxide/oxide form of the metal. In the case of many of the metals that are useful in this aspect of the invention, such as zirconium, the hydroxide form behaves in certain circumstances as though it were the metal oxide combined with water of crystallization. After the ammonia treatment, the loaded precursor is preferably water-washed to remove the excess ammonia. After the precursor is loaded with the desired metal compound or compounds, it is then subjected to conditions to decompose the precursor. The loaded precursor is heated either in an inert gas such as nitrogen, argon, or the like, or in an atmosphere of carbon dioxide (the carbon dioxide can be diluted with an inert gas) to decompose the precursor to a present at the time.) At this point, the material comprises an intimate mixture of the carbonaceous char which is mostly carbon and at least one metal compound. In the next step, the mixture of carbon and metal compound is contacted with carbon dioxide or mixture thereof with an inert gas at a temperature sufficient to effect reaction between the carbon dioxide and the carbon. The atmosphere during this step will usually contain at least about 1 and preferably at least about 5 volume per cent of carbon dioxide, any remainder being inert gas. In general, a temperature of at least about 740°C, and preferably at least about 800°C, is employed in order to cause the reaction between carbon and carbon dioxide to proceed at a substantial rate. Much higher temperatures can also be used, if desired. For instance, temperatures up to 1200°C. , and higher, can be used in some cases. The upper temperature limit at which the process is carried out is preferably just below that temperature at which gross sintering of the desired metal oxide relic product occurs. By "gross sintering" of the metal oxide product, is meant, for example, sintering between the individual fibers of a textile material. This results in undesirable stiffening and embrittlement of the product. However, when producing a metal oxide article in accordance with the relic process , the structural integrity of the final relic product depends in part on a consolidation and sintering of the individual metal oxide particles. However, this desired sintering occurs on what could be termed a molecular scale rather than on a gross while at the same time molecular scale sintering is desired.

As a guide, the approximate temperatures at which undesired gross sintering occurs with several metal oxides are set forth in the following table : TABLE I Metal Oxide Melting Point. °C. Sintering Temperature. °C.

Zr02 2677 1190 MgO 2800 1250 La203 2305 1000 Y203 2410 1055 BeO 2550 1125 CaO 2600 1150 SrO 2415 1060 HfO 2777 1240 * and other rare earth metal oxides.

The preferred metal compounds for use in the invention include the compounds of metals which form the oxides listed in Table I.

The decomposition of the precursor to form various decomposition products and carbon and the volatilization of the carbon by reaction with carbon dioxide can take place concurrently. The decomposition of the precursor can be carried out very rapidly without adverse effects on the quality of the final relic products because the atmosphere of either carbon dioxide, an inert gas such as nitrogen or argon, decomposition products will occur. The decomposition of the precursor is believed to yield a char which contains a network of small diameter pores. The carbon dioxide then reacts with the carbon constituting the walls of these pores. The time within which the loaded precursor is heated up to reaction temperature (the "reaction temperature" referred to is that temperature at which the reaction between carbon dioxide and carbon becomes significant, i.e., in most cases at about 740°C.) can be very short, for instance, from as little as a few minutes (e.g., one to two minutes, or less) up to about 1 hour or more. The specific time that is selected within which to heat the loaded precursor up to reaction temperature will depend , in part , upon factors such as the specific nature of the precursor, the nature of the metal compound (s) and the oxide product to be formed, and upon the geometry of the article. For instance, a thin article such as yarn, tow, a thin woven cloth, or the like, can often be heated up to reaction temperature quite rapidly. However, a thick article such as a relatively thick felt, a shaped article such as a foam, may require a longer period of time within which to be heated up to reaction temperature.

Upon attaining reaction temperature, the carbon begins to react with the carbon dioxide in accordance with the following equations : (1) C + C02 ^ > CO + C(0) (2) C(0) ^ CO The carbon-removal in accordance with the foregoing reactions ten hours, or more, and preferably from about one-half to about eight hours, at temperatures of about 740°C. to 800°C. At higher temperatures, the reaction time can be even shorter.

Thus , the specific reaction time selected depends , in part , upon the particular temperature employed for the carbon-removal reaction. When substantially all of the carbon has been removed, any further heating of the metal oxide article can be done in air or other atmosphere, as desired. Further heating will be desirable in some cases to effect a greater degree of consolidation and strengthening of the article via the molecular scale sintering that was discussed above. Such further heating will be known to those skilled in the art, for example, from the disclosure of the Hamling patent that was referred to above.

In this aspect of the invention, the process can be used to produce improved metal oxide fibrous materials such as yarns, tows, rovings , woven, knitted, or felted fabrics, tapes, as well as many other types of metal oxide articles such as those that are described in the said Hamling patent. The preferred preformed organic polymeric material is cellulosic in nature. Thus, cotton, regenerated cellulose such as rayon, linen, regenerated cellulose films such as cellophane, cellulose sponges, wood, paper, wood pulp, and the like, are preferred preformed polymeric organic materials. Rayon or other regenerated cellulose fibrous materials are especially preferred. (By "fibrous materials" is meant any material that is made of fibers, such as individual fibers and textiles.) is useful for removing carbon from mixtures of carbon and at least one metal-containing compound in other processes , such as the relic process for producing metal nitrides that is disclosed in U.S. Patent 3,399,979, to B. H. Hamling, in the relic process for producing elemental metal articles that is disclosed in U.S. Patent 3,406,025 to B. H. Hamling, and in the relic process for producing metal carbide articles that is disclosed in U.S. Patent 3,403,008, to B. H. Hamling. In the relic processes for producing metal nitride articles and elemental metal articles, carbon-removal is an essential step that is carried out. In the relic process for producing metal carbide articles, reduction of the carbon content is desirable in some cases, for instance, where the amount of carbon left in the carbonaceous char is in excess of the amount of carbon that is desired for reaction with the metal or metal compound in the production of the metal carbide in the carburization step.

It is preferred in the practice of the invention that the temperatures employed in the process do not exceed the temperature at which gross sintering of the metal or metal compound (s) that is present occurs. In most cases during the reaction with carbon dioxide, the metal compound or compounds present will be in the form of the oxide. This can occur in many cases, even though the oxide will subsequently be processed to form, for instance, a carbide or elemental metal article. Thus, in practice, it is preferred that the metals employed in the invention have oxides whose gross employed. In some cases, however, the invention can be carried out at temperatures that are higher than the gross sintering temperature of the metal compound or compounds present. For instance, in the production of monofilaments, gross sintering between individual fibers is not a factor in some cases.

The equipment that is employed for carrying out the process of the invention can be a forced draft oven, a tube reactor, or the like, that is equipped with heating means, means for handling the loaded precursor, means for controlling the atmosphere, means for introducing carbon dioxide, and the like. The invention has made it possible to carry out the relic process for the production of metal oxide fibers , textiles, and shaped articles on a continuous basis, because of the short heating times that can be used and because of the improvement in strength of the products which makes handling easier.

The process of the invention is customarily carried out at about atmospheric pressure. The reaction times, and the temperature at which the reaction between carbon and carbon dioxide occurs, may change to a small extent at different pressures. However, there is no apparent advantage to be obtained by practicing the process at anything other than about atmospheric pressure.

The inorganic articles that can be produced by the process of this invention have known utility. They are useful, for instance, in the preparation of insulation, catalyst supports, structural members especially where extreme conditions of either temperature or corrosive environment or both are The invention is further illustrated by the examples which follow: EXAMPLE 1 A 20.0 meter length of Villwyte rayon yarn (1650 denier/720 filament/1 ply) weighing 3.49g was pre-swelled in ethylenediamine (EDA) in water having a concentration of 80 to 100 weight per cent, for 1/2 hour at 25eC. The pre-swelled rayon was thoroughly washed in water for approximately 15 minutes to remove excess EDA. After centrifuging out the excess water, the rayon contained 0.84g water/g rayon. The pre-swelled yarn was immersed in a 2.2 molar ZrOCl2 solution (containing YCI3 equivalent to 6 weight per cent Y2O3 based on weight of Zr02) for 1/2 hour at 25°C. The excess ZrOC^/ Cl^ solution was removed from the yarn by centri-fugation followed by drying under 175g tension to untwist and straighten the yarn bundle. The ZrOCl^ loaded yarn was very flexible and had a luster equivalent to the starting rayon. The loaded yarn was converted to the hydroxide/oxide form by immersion in concentrated ammonium hydroxide for 10 minutes , followed by a water wash for removal of excess base and NH^Cl, and by redrying at 175g tension. A 2.00 meter section of the loaded converted yarn heated in air showed that the yarn contained zirconia equivalent to 0.32g Zr02/g rayon.

A 78 cm section of the yarn was pyrolyzed in CO2 at a flow rate of 130cc/min at 600°C./hr to 600°C. , followed by a heating rate of 42°C./hr from 800° to 1115°C. with a 1-hour hold at the maximum temperature. The yarn was tensioned with i h The resulting Zr02 fibers after removal of all carbon, had a denier of 913 and an average fiber diameter of 6.9 microns.

Shrinkage amounted to 44% of the original loaded , converted length. Single fiber measurements using an Instron tensile testing apparatus gave strengths of 181,000 psi and a modulus of elasticity of 29 x 106 psi.

EXAMPLE 2 A rayon bias tape was immersed in a solution containing 660g Th(N03>4/liter and 4g CeCl3/liter for 40 hours. Excess impregnating solution was removed by centrifugation, and the tape was dried in air at room temperature. The tape was next immersed in 57% aqueous ammonium hydroxide for 1/2 hour, and then rinsed thoroughly with water and redried. At this point the tape contained the equivalent of 0.22g The dried, neutralized tape was heated to 400°C. in a stream of dry nitrogen, then to 900°C. in a stream of carbon dioxide, and was held at 900°C. in carbon dioxide for one hour. 400eC. was reached in 65 minutes, and 900°C. in 150 minutes. The resultant Th02-Ce203 tape, which was white to off-white in color, was further heated at 1100°C. for 1 hour in air. Little, if any, change in color or loss in weight occurred with this latter treatment. The final product was flexible and had a breaking strength of approximately 6 pounds per inch of width. X-ray diffraction showed strong, well defined peaks characteristic of thorium dioxide.

Claims (1)

1. 3389l/l/¾/3 · Claims: 1 · A process for the production of fibers, textiles and shaped articles composed of an inorganic material which includes inpregnatlng a preformed organic material with at least one metal compound, heating the impregnated material to decompose the organic component, thereby forming a composition comprising uncombined carbon having an elemental metal or a metal compound dispersed therein, at least partially removing the uncombined carbon by contacting said composition with carbon dioxide while at εζ. elevated temperature and sintering the residue, characterized in that the removal of at least part of the uncombined carbon is effected by heating said composition in the presence of the carbon dioxide, or a mixture of carbon dioxide and inert gas containing at least M of carbon dioxide by volume, to a sintering temperature above 71+Q°C. but below the gross sintering temperature of the residue. 2· A process according to claim 1 , characterized in that the preformed organic material is cellulosic. 3· A process according to claim 1 or 2, characterized in that the heating in the presence of carbon dioxide is carried out for a sufficient time to remove substantially all of the carbon from said composition and to produce thereby a metal oxide article*