THERMALLY BONDED FIBROUS PRODUCTS

Abstract: A thermally bonded fibrous product is composed of a sintered blend of aluminosilicate fibres, silica powder and boron nitride powder.

Description: This invention relates to thermally bonded fibrous products. Ceramic fibres are often used to provide thermal insulation from very high temperatures. For many applications, it is convenient and often necessary to form the fibres into rigid shapes such as boards or special configurations. Two approaches are used to accomplish bonding of ceramic fibres to obtain these shapes. Chemical bonding is the most common. A heat or air setting binder, usually in liquid form, is added to the fibres and, through drying, the application of low to moderate heat, or chemical reaction, the binder becomes rigid and literally glues the fibres into a rigid structure. Such binders include colloidal oxide suspensions, silicate solutions and thermoplastic suspensions. The strenghts of these systems are relatively low, typically having flexural strength properties of 138 to 345 kPa (20 to 50 lbf/in²) or "psi". More binder can be added to increase the strength. However, the additional binder adversely affects thermal insulating properties. A second family of rigid ceramic shapes has been developed that utilises more expensive firing schedules to sinter or fuse the fibres into a rigid structure. Flexural strengths are 10 to 20 times that of chemical bonding at the same density with no sacrifice in insulating properties. Difficulties in matching thermal expansion and preventing destructive crystal growth in the fibres have been experienced. Some success has been reportedly achieved through the use of a blend of silica fibres and aluminoborosilicate fibres. This technology still depends on the use of very expensive, high purity fibres and teaches against the use of lower grade fibres or lower grade particulates. According to one aspect of the invention, a thermally bonded fibrous product is provided from compositions comprising a sintered blend of aluminosilicate fibres, silica powder, rather than silica fibre, and boron nitride powder. Thermally bonded fibrous products in accordance with the invention advantageously provide inexpensive constructions of high thermal insulating value with improved strength at relatively low densities. Preferably, the components of the product, on a weight basis, consist essentially of 75% aluminosilicate fibre, 20% silica powder (-325 mesh: US standard mesh size) and 5% boron nitride powder (-325 mesh: US standard mesh size) sintered by firing at a temperature of at least 1288°C (2350°F). The use of silica powder, rather than silica fibres, as a bond former, is unique in that conventional thermal bonding theory teaches the use of silica fibre as a bonding agent. The invention will now be further described by way of illustrative and non-limiting example. Examples of the invention disclosed hereinbelow describe the construction or formation of thermally bonded fibrous products, comprising a sintered blend of primarily aluminosilicate fibres, silica powder and boron nitride powder, that are low in cost, and have high strength, low density and high thermal insulating value with improved machineability. The aluminosilicate thermally bonded fibrous products are particularly suited for use as diesel soot filters, kiln furniture, combustor liners and burner tubes, and in other high temperature applications. Suitable fibres for the practice of this invention may consist essentially of approximately equal parts of alumina and silica, such as KAOWOOL (trade mark) brand ceramic fibre marketed by The Babcock & Wilcox Company. The aluminosilicate fibres have average diameters of about 3 micrometres (microns). Particulates that can be employed in the formation of aluminosilica products embodying the invention include -325 mesh (US standard mesh size) silica powder and smaller amounts of -325 mesh (US standard mesh size) boron nitride powder. Suitable silica powders include high purity quartz silica powder (-325 mesh: US standard mesh size), as is available from Atlantic Equipment Engineers, and high purity amorphous fumed silica (12 nm or millimicrons), marketed under the trade mark AEROSIL 200 by Degussa. Boron nitride powder marketed as UCAR HCV by Union Carbide has been found to provide a suitabe boron nitride. The following examples are illustrative and explanatory of the invention and not restrictive thereof. A number of billets measuring 216 mm (8.5 inches) square and 38 mm (1.5 inches) thick were prepared from compositions of aluminosilicate fibre, silica powder and boron nitride powder which were first added to deionised water. Starting materials included a first system having, by weight, 76.2% aluminosilicate fibre, 19% silica powder and 4.8% boron nitride powder, and a second system having, by weight, 89.5% aluminosilicate fibre, 7.2% amphorous fumed silica powder and 3.3% boron nitride powder. The solution was mixed in a high shear Arde Barinco CJ-4 laboratory mixer for a sufficient time to disperse the solids within the solution. The aluminosilicate fibre was then added and the modified solution was further mixed for a sufficient time to disperse the fibre component in the solution. After dispersion, a small amount of Betz 1260 polymer was added to flocculate the solution. Billets were formed from the flocculated fibre/powder slurry in a casting/pressing tower. The tower had a top and bottom platen, each platen being constructed such that a vaccum could be applied and liquid removed through the platen. After the slurry was added, it was allowed to free-drain. The resultant pad was then pressed by movement of the bottom platen to a desired thickness. During the pressing operation, a vacuum was applied to the top and bottom platen. The pad was oven dried, at approximately 121°C (250°F), until thoroughly dry. The dried parts were then fired in an electric furnace, at a heat-up rate of about 220°C/h (400°F/h) and held at the sintering temperature for 90 minutes. The aluminosilicate fibre-silica powder-boron nitride composition was then fired at 1288°C (2350°F), and the aluminosilicate-fumed silica-boron nitride was fired at 1371°C (2500°F). Room temperature modulus of rupture (MOR) and density were determined in the as-fired condition, and after reheats to 982°C (1800°F), 1149°C (2100°F) and 1316°C (2400°F). Table I (below) reflects the values of these properties for sample billets formed from the first system composition while Table II (below) reflects the values of these properties for sample billets formed from the second system composition. The billets displayed no shrinkage at the reheat temperatures of 982°C (1800°F) and 1149°C (2100°F). At the reheat temperature of 1316°C (2400°F), sample billets of the first system composition experienced less than 7% volumetric shrinkage and sample billets of the second system composition experienced less than 2% volumetric shrinkage. Analysis has indicated that boron nitride is released upon firing above the temperature of 649°C (1200°F) and fluxes the silica powder to form a fused bond at the fibre intersections, thereby producing an extremely strong shape. In addition, the boron nitride aids in stabilisation of the fused bond against crystallisation. When blends in the desired portions, by weight, of approximately 75% aluminosilicate fibre are formed and fired, a very strong, lightweight insulation results.

Claims: 1. A thermally bonded fibrous product having high thermal insulating value, high modulus of rupture and low density, the product comprising a blend of aluminosilicate fibres, silica powder and boron nitride powder formed by sintering at a temperature of at least 1288°C (2350°F). 2. A thermally bonded fibrous product according to claim 1, wherein said blend, by weight, consists essentially of 75% to 90% aluminosilicate fibre, 7% to 20% silica powder and 3% to 5% boron nitride powder. 3. A thermally bonded fibrous product according to claim 2, wherein said blend, by weight, consists essentially of 76.2% aluminosilicate fibre, 19% silica powder and 4.8% boron nitride powder. 4. A thermally bonded fibrous product according to claim 2, wherein said blend, by weight, consists essentially of 89.5% aluminosilicate fibre, 7.2% amorphous fumed silica, and 3.3% boron nitride powder. 5. A thermally bonded fibrous product according to claim 4, wherein the sintering temperature is 1371°C (2500°F). 6. A thermally bonded fibrous product according to any one of the preceding claims, wherein the silica powder consists essentially of -325 mesh (US standard mesh size) silica powder and the boron nitride powder consists essentially of -325 mesh (US standard mesh size) boron nitride powder. 7. A thermally bonded fibrous product according to any one of the preceding claims, wherein the silica powder consists essentially of 12 nm amorphous fumed silica.

Inventors: HILL, CHARLES ALEXANDER
Applicants: THERMAL CERAMICS, INC.
Application year: 1987
Main classification: C04B30
IPC codes: C04B30, C04B38