GB1155292A - A Method of Making Sinterable and Sintered Shaped Structures and Structures Made Thereby. - Google Patents

Abstract

1,155,292. Filamentary structures. FMC CORP. 20 Sept., 1966, No. 41833/66. Addition to 1,064,271. Heading B5B. [Also in Division C1] A method of making an ablative sinterable structure from a cellulosic spinning solution comprises adding to, and incorporating in, the cellulosic spinning solution at least one glassforming compound, a zirconium compound being the or a glass-forming compound, shaping the spinning solution into the structure through. out which the cellulosic material and glassforming compound or compounds are uniformly distributed, converting the glass-forming compound or compounds to a glass-forming oxide or oxides if the glass-forming compound or compounds does or do not comprise an oxide or oxides, and removing the solvents from the structure, the amount of said glass-forming compound or compounds added to the cellulosic spinning solution being sufficient to provide a ratio of glass-forming oxide or oxides to the cellulosio material of between 0À2:1 and 2:1. Preferably there are at least two glass-forming compounds, both or two of which are sodium silicate and zirconium hydroxide. The cellulosic spinning solution preferably comprises viscose, the spinning solution being shaped in an acid bath, suitably by extrusion through orifices to form filaments. Where viscose is used, the glass-forming compounds, e.g. silica, alumina, zirconia, and boron oxide in finely divided form may be mixed as aqueous dispersions with the viscose solution. Preferably, however the corresponding sodium salts are dissolved in water or sodium hydroxide solution and the solution mixed directly with the viscose. Where a cellulose ether, e.g. an alkali-soluble, waterinsoluble hydroxyethyl cellulose solution is used as a spinning solution, the glass-forming compounds may be introduced in the same manner. Where organic solvent solutions of cellulose ethers or esters are used as the spinning solution, the aforementioned oxides may be introduced in colloidal-sized particles but are preferably added in the form of compounds, e.g. as alkyl silicates, aluminates or borates soluble in the specific solvent. The spinning solutions containing the dispersed or dissolved inorganic or organic silicates, aluminates or borates are then extruded through conventional spinnerets or orifices in the usual manner to form the cellulosic material filaments or other desired structural forms containing the added substances, which are converted or precipitated within the filament by suitable treatment. Thus, where the added substance is sodium silicate and the spinning solution viscose, the spinning solution is extruded into a conventional acid spinning bath to coagulate the viscose and regenerate the cellulose and simultaneously the acid reacts with the contained sodium silicate to precipitate silicic acid in an extremely finely divided and uniformly distributed form. Where an organic solvent solution of e.g. cellulose acetate is used as a base, an alkyl silicate, e.g. tetraethyl silicate may be dissolved in the cellulose ester solution and the filaments formed by extruding the solution through conventional spinnerets in a conventional dry spinning procedure. Subsequently, the filaments are subjected to a suitable treatment with an aqueous bath containing an organic or inorganic acid which will react with the contained silicate to precipitate silicic acid in extremely finely divided form dispersed throughout the filament. A more finely divided form of particle and more uniform distribution of the particles is obtained by incorporating the glass-forming compounds in the form of soluble compounds than is obtained by adding such compounds in powder form. Thus, a product containing both silicon dioxide and zirconium oxide may be obtained with the oxides extremely finely divided and uniformly distributed throughout the product by introducing into the viscose sodium silicate and an aqueous solution of a zirconium salt, e.g. the sulphate. Subsequently, the filaments are dried and where the silicic acid has been precipitated, it may be converted to silica or a hydrate of silica by heat treatment. The structure so obtained may be subjected to elevated temperatures to decompose and carbonize the cellulosic material, and to sinter the shaped structure. Thus the filament or other structure may be subjected to pyrolysis e.g. from 250‹ to about 500‹ C., and the temperature may then be raised to from about 550‹ to about 750‹ C. in the presence of oxygen or air to effect an oxidation and burning-off of the carbon or other remaining organic material. The temperature may then be raised from about 800‹ to about 1500‹ C. or higher and maintained within such range from 10 minutes to about 60 minutes during which period the glass-forming compounds become sintered. The properties of the sintered fibres may be altered by treating the fibres during their production or before the pyrolytic treatment with modifying salts. Thus in forming the fibres from viscose solutions, the wet gel fibres after the final after-treatment or washing may be passed through an aqueous solution of a salt of a modifying agent, e.g. a water-soluble salt of magnesium, aluminium, potassium or boron. When the filaments are to be used to form textile material, the water-soluble salt may be incorporated in a conventional yarn finish applied to the wet gel filaments, the yarn finish serving the conventional purposes in weaving and knitting operations, and being volatilized or burned-off during the subsequent pyrolysis and oxidation treatments. Specified cellulose derivatives which may be used in the process of the invention include cellulose nitrate, cellulose acetate, cellulose butyrate, acetate-butyrate and organic solvent-soluble cellulose ethers. Cellulose acetate spinning solutions containing from about 20% to about 30% or 35% cellulose acetate dissolved in a solvent consisting of e.g. 94% acetone and 6% water may be used. An organic solvent-soluble alkyl silicate, e.g. ethyl silicate, may be dissolved in the cellulose ester solution or finely divided silica may be dispersed therein. The spinning dope is then extruded through the spinneret orifices in a conventional manner either into a warm air chamber or into a liquid bath. The filaments, after collection or after being converted into a fabric, are then treated with an aqueous acidic solution, e.g. 2¢% to 5% by weight aqueous hydrochloric acid whereby the ethyl silicate is decomposed to form silicic acid within the fibre in an extremely finely divided form distributed throughout the individual filaments. Where finely divided silica has been dispersed in the spinning dope, such after treatment is not necessary. The filaments or fabrics may then be subjected to pyrolysis, oxidation and sintering operations, or to other uses. Cellulose nitrate may be similarly utilized, using as solvent a mixture of methyl alcohol and ether. Organic solvent-soluble cellulose ethers may be used as the cellulose base material in like manner. Thus methyl cellulose, ethyl cellulose or benzyl cellulose having a degree of substitution of about 0À5 to 2À5 may be dissolved in dioxane, tetrahydrofurane or dimethylsulphoxide to form the spinning dope in which the glass former is dissolved or dispersed as described above. After spinning and removal of the solvent, the filaments are subjected to a suitable acid treatment to precipitate the silicic acid. In an example, viscose is prepared in a conventional manner, a technical grade sodium silicate being added to the viscose to provide a silicate-cellulose ratio of 0À5:1. The viscose is allowed to ripen for about 24 hours at 18‹ C. A zirconium hydroxide dispersion is made by dissolving zirconium sulphate, mixing the solution with a caustic soda solution containing carboxymethyl cellulose as a dispersing agent and subjecting the mixture to ball milling for 72 hours. The resulting dispersion which was formed of 505 parts zirconium sulphate tetrahydrate, 25 parts of carboxymethyl cellulose and 1700 parts of 7 % caustic soda solution by weight, was introduced into the viscose by injection just before the viscose reached the spinneret. The amount of the dispersion was sufficient to provide a ratio of zirconium oxide to cellulose of 0À5: 1. The spinning solution contained, by weight; 5% cellulose, 2À5% silica, 2À5% zirconia, 7À0% sodium hydroxide, 40À0% carbon disulphide (based on cellulose), 2.6%dimethylamine, and 1À3% of a polyoxyethylene glycol ether of phenol containing an average of 15 ethylene oxide units per mol of phenol. The viscose is spun at a salt test of about 9 to form a 1650 denier, 1500 filament yarn at the rate of about 40 meters per minute. The coagulating and regenerating bath contained 7% by weight sulphuric acid, 20% anhydrous sodium sulphate and 4% anhydrous zinc sulphate and was maintained at about 60‹ C. After withdrawal from the spinning bath the filaments are stretched 75% while being treated with a hot dilute acid solution containing about 3% by weight sulphuric acid and then washed with water at about 70‹ C. The wet gel filaments are then treated with a yarn finishing bath consisting of an emulsion of about 1À5% mineral oil, 0À8% of a silicone oil in water containing a surface active agent, 0À5% gelatin, 10% anhydrous magnesium sulphate, 2% anhydrous aluminium sulphate and 0À5% anhydrous potassium sulphate. The filaments are then passed over drying rolls and collected on a tube. A mass of loose filaments so prepared are placed in an oven and heated for three hours at 375‹ to 380‹C., the filaments then transferred to a muffle furnace at about 925‹ C. and the samples maintained at this temperature for 3 hours. Upon removal of the products from the furnace and cooling to room temperature, the filaments exhibit a whiteness, lustre, resilience and flexibility similar to that of the filaments of the example of Specification 1,064,271. The filaments retained their indivi