PROCESS FOR MAKING INORGANIC OXIDE FIBERS

Abstract: Alumina or alumina-silica fibers useful for heat insulation purposes are made by dry spinning a spinning dope incorporating an aluminium compound, with or without a silicon compound, an organic polymer such as polyvinyl alcohol to aid fiber spinning and a zinc, cooper or tin salt to reduce dope viscosity and thus allow a higher concentration of aluminium in the dope and to produce a more stable alumina structure in the final fibers. After collection of the spun fibers, for example as a yarn or tow or as a non-woven web, the fibers are heated, usually in air to a temperature of 1,300<o>C to 1,600<o>C, to convert the aluminium and additional metal compounds and if necessary the silicon compound into their oxides and to burn off the organic polymer.

Description: Process for making inorganic oxide fibres Technical Field This invention relates to inorganic oxide fibres and to a process for making them. The refractory proper¬ ties of such fibres provide for them a major use as heat 5 insulation for furnaces. Background Art The principal inorganic oxide fibres hitherto used for this purpose are alumina or alumino-silicate fibres made by melt spinning. These are useful up to tempera- 10 tures in the region of 1400°C but above that they tend to be degraded. Newer techniques have been devised for making fibres of improved properties, and these involve forming a spinn¬ ing dope comprising a solvent incorporating a precursor 15 of the desired metal oxide, namely an aluminium compound, in solution or dispersion and dry spinning the dope into an evaporative atmosphere to form fibres. The spun fibres, which are in the so-called "green" state, are heated to convert the aluminium compound to an oxide. 20 Spinning can be improved by incorporating in the dope a dissolved organic polymer which is burnt out of the fibres during the heating step. A process of this type i ' s described in British Patent No. 1,287,288. Disclosure of the Invention 2.5 It is an object of the present invention to provide a dry-spinning process for making inorganic oxide fibres which produces fibres of improved quality. It is a further and alternative object of the invention to provide a dry spinning process which shows substantial benefits 0 at the spinning stage or the heating stage or both, as compared to the prior art. According to this invention, a process for making OM?I inorganic oxide fibres, which comprises preparing a spinn¬ ing dope comprising a solvent incorporating an aluminium compound in solution or dispersion, and a dissolved organic polymer capable of aiding fibre spinning; spinning the dope into an evaporative atmosphere to form fibres; and heating the fibres to convert the aluminium compound into an oxide, the process being characterised in that an additional metal compound selected from zinc, copper and tin salts is incorporated in the spinning solvent and. is also converted into an oxide by the fibre heating step. The term "fibres" is used herein to include staple fibres , continuous filaments and fibres of intermediate length. The inorganic oxide fibres made by the process of the invention contain alumina in the γ-form which is a defect spinel structure and which, in the absence of contrary factors, tends to be converted at high tempera¬ tures to the more stable but weaker α-form having larger crystallite grain size. However, the presence of the additional metal salt in the dope produces another metal oxide, for example zinc oxide, which improves the stabil¬ ity of the alumina structure. In addition to providing a more stable structure, the presence of the additional metal salt makes it poss¬ ible to achieve the desired average grain size of the alumina crystallites more easily and more quickly during calcination. Crystallite nucleation and grain growth both increase with time during calcination, and it is believed that the additional metal salt may act as a flux which assists nucleation _of the alumina crystallites. This means that a large number of crystallites is formed much more rapidly so that the crystallite grains are not allowed sufficient time to grow beyond the optimum grain size. Thus, the calcination process is shorter and more economical, and it produces a more stable fibre of controlled grain size and therefore better strength. The additional metal salt also gives benefits in spinning the fibres' because it has the effect of reducing dope viscosity, which allows the incorporation in the dope of a higher concentration of the aluminium compound for a given, desired spinning viscosity. The higher concentration aids fibre formation and reduces the amount of evaporation required to remove spinning solvent from the spun fibres. The preferred solvent is aqueous. The aluminium compound is suitably an acid salt such as aluminium chlor¬ ide, aluminium sulphate, aluminium nitrate, aluminium acetate or aluminium chlorhydrate. In an aqueous solvent, it may be present in true solution or as a colloidal solution or sol. The preferred compound is aluminium chlorhydrate. The concentration of the aluminium compound is desir- ably as high as possible commensurate with acceptable spinning viscosity. The latter is preferably in the range 1 to 100 poises (0.1 to 10 Pas) at 18°C, and, bear¬ ing in mind that the other dope ingredients have a signifi¬ cant influence on viscosity, we have found that a conven- ient concentration range ,of the aluminium compound is 50 to 70 per cent by weight. The organic polymer is preferably water-soluble. Suitable polymers include polyethylene oxides, polyethyl¬ ene glycols, polyvinyl alcohols, polyvinyl acetates, polyacrylamides, cellulose ethers, algiπic acids and dextrans, gums and starches, with polyvinyl alcohols being preferred. The concentration of organic polymer in the dope is conveniently from 0.1 to 10 per cent by weight. Higher concentrations can be used but give no additional benefit. The additional metal compound is a salt of zinc or coppper or tin, with zinc salts being preferred. The salt may for example be a halide, nitrate, sulphate or carboxylate. Zinc chloride is particularly effective. The concentration of the additional metal compound in the spinning dope is conveniently from 0.01 to 10 per cent by weight. Improved fibre properties are obtained if the fibre also incorporates silica and for this purpose a silicon compound may be included in the dope. Silica itself may be used as an aqueous colloidal dispersion or sol, or an organic silica compound such as a polysiloxane may be used. A suitable dope concentration of silicon compound is in the range 1 to 10 per cent by weight of silicon. Higher concentrations can be used but fibre properties deteriorate as the concentration is increased much above 10 per cent by weight. The fibres are dry spun by spinning the dope into an evaporative atmosphere which preferably consists essen¬ tially of air. Conventional dry spinning through a multi- hole jet may be used, as may the modified process known as ' blow-spinning in which the extruded filaments are attenuated by impinging jets of air. However, centrifugal spinning is the favoured process because of its much greater production rate compared with the other processes. The centrifugal spinning head may be of the spinning -disc type for shorter, less uniform fibres, or may com- prise a rotating multi-hole spinning jet or jets for producing better quality, longer fibres or continuous filaments. The spun fibres are preferably attenuated as the solvent is being evaporated off, and this may be achieved ' by the action of impinging air currents or by the draw¬ down of a collection device, or by the action of a centri¬ fugal spinning head itself. The fibres may be collected as a continuous filament yarn or tow by a rotating package-forming device, or as a non-woven web on a per¬ vious collecting conveyor or drum. In the case of centri¬ fugal spinning, the outflung fibres may be re-directed by a ring of air currents to form an annular curtain of filaments. This may be collected as a tow by converg¬ ing it through a guide, or as a non-woven web by laying it on a moving screen conveyor. The spun, collected fibres are heated, preferably in an oxygen-containing atmosphere, such as air, to con- vert the aluminium compound and the additional metal compound(s) to their respective oxides. During this operation, the organic polymer is burnt off. The heating step may be carried out continuously by passing the fibre as tow or non-woven web through a furnace, during which passage it is suitably supported on for example a mesh conveyor capable of withstanding the higher temperature in the furnace. The heating step may be effected by heating the fibres up to a temperature in the range 1300°C to 1600°C, and may be carried out as a single stage operation with a furnace .having _a suitable temperature profile. For example, the furnace entry may be at about 600°C, with the temperature rising gradually as the fibres pass through, until the fibres leave the furnace at 1400°C to 1500°C, the total residence time being of the order of 5 to 10 minutes. If the fibres are to be used for high temperature insulation purposes, then it is preferable to minimise any residual shrinkage in the fibres by exposing them O PI during the heating step to temperatures approaching those they will meet in service. Otherwise- the insulation can shrink and leave gaps which expose the furnace walls to the full heat of the furnace. Modes for carrying out the invention The invention is illustrated by the following Examples: Example 1 5280 g of a 50 per cent by weight solution of alu- minium chlorhydrate was mixed with 264 g of polyvinyl alcohol (PVA) ("Denka Poval" K05) of molecular weight 26,000. The mixture was heated to 100°C and held at that temperature for 2 hours to dissolve the PVA, and then allowed to cool before being filtered to remove particles larger than 50 microns. The resulting solution was mixed with another solu¬ tion comprising 220 ml of silica sol ("Nalfloc" N1034A) mixed with 405 ml of aluminium chlorhydrate. To this mixture was added 118 g of anhydrous zinc chloride. The whole mixture was then heated to 65°C, whereupon 1000 g of solid aluminium chlorhydrate was added gradually over a one hour period. The viscosity of the resulting dope was measured at 18°C and was found to be 3 Pas, which was suitable for spinning. The dope was pumped to a centrifugal spinning head comprising two multi-hole jets mounted at opposite ends of a horizontally-rotating shaft and facing oppositely and outwardly. Each jet had 60 holes of 66 microns diameter. The dope was .pumped at a rate of 4.67 ml/min and the shaft was rotated at 5900 r.p.m. to produce spun fibres having a mean fibre diameter of 5 microns. The fibres were spun into air at a temperature of 32°C and a relative humidity of 20 per cent. An annular air slot positioned just above the spinning head directed air under pressure down onto the outflung fibres and 5 redirected them downwards as an annular curtain. The fibres were deposited on the mesh circumference of a rotating drum as a non-woven web. The drum was of 1.5 m circumference, rotated at 20 r.p.m., and was traversed across the path of the spun fibres to facilitate web 10 formation. The web was built up to a basis weight of 100 g/m before being removed from the roller as a l-_ m long strip and placed lengthways on a moving conveyor belt passing through a slot furnace. The inlet and outlet 15 temperatures of the furnace were 640°C and 1000°C respec¬ tively and the residence time of the web in the furnace was 4 minutes. After this heating operation, the web was given a second heat treatment at 1400°C for 2 minutes to substantially remove residual shrinkage. 20 Web strength was measured on an "Instron" tensile tester using 5 x cms square specimens and a 1 cm test length. A mean of ten determinations was taken in both directions. The web produced according to the above procedure had a mean parting strength of 20,000 Newtons/kg 25 (N/kg) in the stronger direction and 12,000 N/kg in the weaker direction, which compared with a figure of only 3000 K/kg in the stronger direction for material made without the inclusion of zinc chloride. When the web made without zinc chloride in the fibre 30 was heated for 1 hour rather than 4 minutes, its strength was improved to 14,000 N/kg in the stronger direction but was still below that of the web made according to the invention. The fibre material made without the inclusion _ zinc chloride was made by the " procedure described above apart from the omission of zinc chloride, and the addition of only 800 g of solid aluminium chlorhydrate which was as much as could be added without raising the viscosity of the dope to a level at which spinning efficiency was reduced. Examples 2 to 6 These Examples show the effect on dope viscosity at 18°C of various zinc salts compared with a control for which no such salt was incorporated. The concentra¬ tion of the zinc salt in the dope is shown in each case and otherwise the conditions are as described in Example 1. Examples 7 and 8 The procedure of Example " 1 was repeated with the difference that the zinc chloride was replaced by stannous chloride (Example 7) and cuprous chloride (Example 8) respectively. Dope viscosity at 18°C and web strength are shown below in comparison with those for Example 1. OMPI

Claims: - 10 - CLAIMS 1. A process for making inorganic oxide fibres, which comprises preparing a spinning dope comprising a solvent incorporating an aluminium compound in solution or dispersion, and a dissolved organic polymer capable 5 of aiding fibre spinning; spinning the dope into an evaporative atmosphere to form fibres; and heating the fibres to convert the aluminium compound into an oxide, the process being characterised in that an additional metal compound selected from zinc, copper and tin salts 10 is incorporated in the spinning solvent and is also con¬ verted into an oxide by the fibre heating step. 2. A process as claimed in claim 1, characterised in that the solvent is aqueous. 3. A process as claimed in claim 1, characterised 15 in that the concentration of the additional metal compound in the spinning dope is in the range 0.1 to 10 per cent by weight. 4. A process as claimed in claim 1, characterised in that the additional metal compound is zinc chloride. 20 5. A process as claimed in claim 1, characterised in that the concentration of the aluminium compound in the spinning dope is in the range 50 to 70 per cent by " weight. 6. A process as claimed in claim 1, characterised 25 in that the aluminium compound is aluminium clorhydrate. 7. A process as claimed in claim 1, characterised in that the organic polymer is a polyvinyl alcohol. 8. A process for making inorganic oxide fibres, which comprises preparing a spinning dope comprising 30 an aqueous solvent incorporating an aluminium compound in dissolved or suspended form, a silicon compound and a dissolved organic polymer capable of aiding fibre spinn¬ ing; dry spinning the dope into an evaporative atmosphere consisting essentially of air to form fibres and heating the fibres in air to convert the aluminium compound into an oxide, the process being characterised in that an additional metal compound selected from zinc, copper and tin salts is incorporated in the spinning solvent and is also ' converted into an oxide by the fibre heating step. 9. A process as claimed in claim 8, characterised in that the concentration of the silicon compound in the spinning dope is in the range 1.0 to 10 per cent by weight of silicon. 10. A process as claimed in claim 1, characterised in that the spinning dope is spun through a multi-hole jet to form fibres. 11. A process as claimed in claim 1, characterised in that the spinning dope is spun into fibres by a blow- spinning process. 12. A process as claimed in claim 1, characterised in that the spinning dope is spun into fibres by a centri¬ fugal spinning process. 13. A process as claimed in claim 1, characterised in that the spun fibres are heated up to a temperature in the range 1,300°C to 1,600°C to convert the aluminium compound and additional metal compound into their respec¬ tive oxides.

Inventors: BRAY ROGER, BRAY, ROGER
Applicants: COURTAULDS PLC
Application year: 1982
Main classification: D01F9
IPC codes: C04B35