Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2003-03-20
2004-06-08
Edwards, N. (Department: 1774)
Compositions: ceramic
Ceramic compositions
Refractory
C428S364000
Reexamination Certificate
active
06746979
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an alumina fiber aggregate and its production method.
BACKGROUND ART
Alumina fiber aggregates, by making use of their excellent heat resistance, are worked into alumina fiber blankets, etc., and used as heat insulator, etc. Such alumina fiber blankets can be produced, for instance, in the following way.
Spinning is carried out with a spinning solution containing basic aluminum chloride, a silicon compound, an organic polymer and water by the blowing method and the obtained alumina short fiber precursor aggregate (laminated sheet) is subjected to calcining, if necessary after needling. In the calcining step, the volatiles are removed and at the same time crystallization of alumina and silica is advanced to convert them into alumina fibers. Consequently, an alumina fiber aggregate comprising alumina short fibers is obtained. This production method is generally called precursor fiberizing method.
It is to be noted that when the diameter of alumina short fibers is small, these fibers tend to scatter and are bad in handling characteristics and also undesirable for the working environmental hygiene. It is pointed out that the fibers with a diameter of less than several &mgr;m have the problems such as easy entrance into pulmonary alveoli of the human body through respiration.
The present invention has been made in view of the above circumstances, and its object is to provide an alumina fiber aggregate comprising alumina short fibers having their diameter enlarged to suppress scattering of the fibers, and a method of producing such an alumina fiber aggregate.
DISCLOSURE OF THE INVENTION
As a result of earnest studies, the present inventors have found that the diameter of alumina short fibers varies depending on the properties of the spinning solution used, and it is possible to obtain alumina short fibers with a large diameter and a narrow distribution of fiber diameter by using a spinning solution of specific properties and optimizing the spinning conditions. This finding has led to the attainment of the present invention.
The present invention have been completed on the basis of the above finding, and in the first aspect of the present invention, there is provided an alumina fiber aggregate comprising alumina short fibers having an average diameter of 4.0 to 10.0 &mgr;m, with the smallest diameter being not less than 3.0 &mgr;m.
In the second aspect of the present invention, there is provided a method of producing an alumina fiber aggregate comprising alumina short fibers having an average diameter of 4.0 to 10.0 &mgr;m, with the smallest diameter being not less than 3.0 &mgr;m, which method comprises carrying out spinning with a spinning solution containing basic aluminum chloride, a silicon compound, an organic polymer and water by the blowing method, and calcining the obtained alumina short fiber precursor aggregate, the said spinning solution being one in which the aluminum/silicon ratio is from 99/1 to 65/35 calculated as Al
2
O
3
/SiO
2
ratio by weight, the concentration of basic aluminum chloride is 180 to 200 g/L, and the concentration of organic polymer is 20 to 40 g/L.
Hereinafter, the present invention is explained in detail. Firstly, for the convenience of explanation, the method of producing an alumina fiber aggregate according to the present invention is explained.
The production method of the present invention comprises carrying out spinning with a spinning solution containing basic aluminum chloride, a silicon compound, an organic polymer and water by the blowing method, and calcining the obtained alumina short fiber precursor aggregate (precursor fiberizing method). The method comprises essentially a spinning solution preparation step, a spinning step and a calcining step, with a needling step incorporated optionally between the spinning step and the calcining step.
Spinning Solution Preparation Step
Basic aluminum chloride Al(OH)
3−X
Cl
X
can be prepared, for example, by dissolving metallic aluminum in hydrochloric acid or an aluminum chloride solution. The value of X in the above chemical formula is usually 0.45 to 0.54, preferably 0.50 to 0.53. As the silicon compound, silica sol is preferably used, but water-soluble silicon compounds such as tetraethyl silicate and water-soluble siloxane derivatives are also usable. As the organic polymer, water-soluble high-molecular compounds such as polyvinyl alcohol, polyethylene glycol and polyacrylamide are preferably used. The polymerization degree of these polymers is usually 1,000 to 3,000.
In the present invention, as the said spinning solution, it is important to use a solution in which the aluminum/silicon ratio is 99/1 to 65/35 calculated as Al
2
O
2
/SiO
2
ratio by weight, the concentration of basic aluminum chloride is 180 to 200 g/L, and the concentration of organic polymer is 20 to 40 g/L.
When the amount of the silicon compound is below the above-defined range, alumina composing the short fibers tends to convert into &agr;-alumina and also the short fibers tends to be weak due to coarsening of the alumina grains. On the other hand, when the amount of the silicon compound exceeds the above-defined range, the amount of silica produced with mullite (3Al
2
O
3
.2SiO
2
) is increased to reduce heat resistance of the product. The aluminum/silicon ratio, calculated as Al
2
O
3
/SiO
2
ratio by weight, is usually 99/1 to 65/35, preferably 99/1 to 70/30, more preferably 98/2 to 72/28.
When the concentration of basic aluminum chloride is less than 180 g/L or when the concentration of the organic polymer is less than 20 g/L, appropriate viscosity can not be obtained, resulting in a reduced fiber diameter. That is, because of too much free water in the spinning solution, the drying speed in spinning by the blowing method is low, causing excessive advance of drawing and variation of diameter of the spun out precursor fibers, making it impossible to obtain the short fibers enlarged in diameter and narrow in distribution of diameter. Further, productivity lowers when the concentration of basic aluminum chloride is less than 180 g/L. On the other hand, when the concentration of basic aluminum chloride exceeds 200 g/L or when the concentration of the organic polymer exceeds 40 g/L, the desired spinning solution can not be obtained because of too high viscosity. The preferred concentration of basic aluminum chloride is 185 to 195 g/L and the preferred concentration of the organic polymer is 30 to 40 g/L.
The said spinning solution is prepared by adding a silicon compound and an organic polymer in a basic aluminum chloride solution so that the concentrations of the basic aluminum chloride and the organic polymer will fall in the above-defined ranges. The viscosity of the spinning solution at normal temperature is usually 1 to 1,000 poises, preferably 10 to 100 poises.
Spinning
Spinning (fiberization of the spinning solution) is carried out by the blowing method in which the spinning solution is supplied into a high-speed spinning air stream, whereby alumina short fiber precursor with a length of several ten to several hundred mm can be obtained.
The spinning nozzle used for the said spinning operation is not specifically restricted structurally, but it is preferably of a structure in which, as for instance described in European Patent No. 495,466 (Japanese Patent No. 2,602,460), the air stream blown out from the air nozzle and the flow of spinning solution forced out from the spinning solution supply nozzles are parallelized to each other, and the parallel flow of air, after having been well rectified, is brought into contact with the spinning solution. In this case, the diameter of the spinning nozzle is usually 0.1 to 0.5 mm, the amount of the solution supplied from the spinning solution supply nozzles is usually 1 to 120 ml/h, preferably 3 to 50 ml per nozzle, and the gas flow rate from the air nozzle is usually 40 to 200 m/s per slit.
According to the spinning nozzle described above, the spinning solution forced out from the spinning solution supply nozzles won'
Ikeda Norio
Sasaki Toshiaki
Shoji Mamoru
Edwards N.
Mitsubishi Chemical Functional Products, Inc.
Roos Richard J.
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