GB1064271A - A method of making ablative, sinterable and sintered shaped structures and structures made thereby - Google Patents

Abstract

1,064,271. Filamentary structures. FMC CORPORATION. July 30, 1963 [July 30, 1962], No. 20217/63. 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, shaping the spinning solution into the structure throughout which the cellulosic material and glass-forming compound or compounds are uniformly distributed, converting the glass-forming compound or compounds to a glass-forming oxide or oxides when the glass-forming compound or compounds does or do not comprise an oxide or oxides, and removing the solvent from the structure, the amount of the 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 cellulosic material of between 0À2 : 1 and 2 : 1. Preferred glass-forming compounds are sodium silicate, sodium aluminate and sodium borate, and 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 compound, e.g. aluminium oxide, boron oxide or silicon dioxide, may be introduced as colloidal sized particles, but preferably is introduced as a sodium salt or solution thereof in water or sodium hydroxide solution. Where a cellulose ether, e.g. an alkali-soluble, water-insoluble hydroxyethyl cellulose solution is used as 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 the base, an alkyl silicate, e.g. tetra ethyl 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 powdered form. Subsequently, the filaments are dried and where the silicic acid has been precipitated, it may be converted to silica or a form of a hydrate of silica by the heat treatment. A high-temperature resistant resin may be incorporated in the said cellulosic spinning solution and is uniformly distributed throughout the structure. Alternatively the structure may be impregnated with a high-temperature resin so that the latter is uniformly distributed therethrough. (Details below.) The structure so obtained may then be subjected to elevated temperatures to decompose and carbonise the cellulosic material, and to sinter the shaped structure. Thus the filament or other structure may be subjected to pyrolysis at from 250‹ C. to about 500‹ C., and the temperature may then be raised to from about 550‹ C. 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‹ C. to about 1,500‹ 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. Where 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 about 94% acetone and about 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 the 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. aqueous hydrochloric acid containing from about 2¢% to 5% by weight 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 ethyl alcohol and ether. Organic solvent-soluble cellulose ethers may be used as the cellulose base material in a 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 dimethyl sulphoxide 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 conventional manner containing 7À5% cellulose, 6À5% caustic soda and 38% carbon disulphide; based on the weight of the cellulose. During mixing, 3À3% dimethylamine and 1À7% of a polyoxyethylene glycol ether of phenol containing an average of 15 ethylene oxide units per mole of phenol is incorporated in the viscose. A technical grade sodium silicate having a Na 2 O: SiO 2 ratio of 1 to 3À25 is added to the viscose to provide equal weights of silica and cellulose. The viscose is then ripened for 24 hours at 18‹ C. and is spun at a salt test of about 9 to form a 1,650 denier, 1,500 filament yarn at the rate of about 40 metres per minute. The coagulating and regenerating bath contained 6À5% by weight sulphuric acid, 14% 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% gelatine, 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. Flat and tubular fabrics are woven from filaments so made. Sintered structures are obtained therefrom by heat treatment in a muffle furnace at about 900‹ C. for a period of about 30 minutes. Filaments prepared by a similar process but omitting the zinc sulphate from the spinning bath are somewhat weaker in strength and can only be satisfactorily heat treated if free from tension. Sintered filaments having diameters of about 3À5 microns are formed by the method described. For the production of ablative structures, a further increase in resistance to high temperature conditions is obtained by incorporating in the cellulosic matrix a high-temperature resistant synthetic resin, e.g. epoxy resins, phenolic resins and urea-formaldehyde resins, which may be incorporated in the spinning solutions by conventional injection spinning methods or by impregnating the shaped article during processing with a solution or dispersion of the resin. The resin is preferably in the precondensate form and polymerizes and becomes a thermoset resinous material during processing of the filaments.