Alumina-based fibrous mass, process for producing same, and use

Abstract

To provide an alumina-based fibrous mass having a high areal pressure and is usable as a holding material for an exhaust gas cleaners and a production process thereof; the alumina-based fibrous mass has a chemical composition containing an Al.sub.2O.sub.3 in an amount of 70% or more and less than 90% and having a total pore volume of 0.0055 mL/g or less.

Claims

1. A process for producing the alumina-based fibrous mass having a chemical composition comprising an Al.sub.2O.sub.3 in an amount of 70 weight % or more and less than 90 weight % and having a total pore volume of 0.0055 mL/g or less, comprising: (I) a step in which an inorganic fiber source and a spinning aid are mixed together and concentrated under a reduced pressure to obtain a viscous spinning dope, (II) a step in which the spinning dope is extruded through fine holes into the air and dried to obtain a precursor, and (III) a step in which the precursor is burned, in this order, wherein, the step of burning the precursor in said step (III) comprises a degreasing step in which the burning is performed to 800° C. at a burning rate of 3° C./min or lower while performing evacuation at a rate of 0.1 to 3 Nm.sup.3/h per kg of the precursor, and a crystallization step in which after the degreasing step, the inorganic fiber source is burned and crystallized.

Description

EXAMPLES

Example 1-1

(1) A 3,650-g portion of an aqueous aluminum oxychloride solution having an Al.sub.2O.sub.3 concentration of 20.0% by mass was mixed with 1,286 g of a silica sol having an SiO.sub.2 concentration of 21% by mass and 800 g of an aqueous polyvinyl alcohol (polymerization degree: 1,700) solution (concentration: 10% by mass), followed by concentrating under a reduced pressure to prepare a spinning dope having a viscosity of 3,500 mPa.Math.s.

(2) From a diameter of 0.2 mm (hole-to-hole distance: 3.5 mm) formed in the sidewall of a hollow disk which had a diameter of 350 mm and was being rotated at a rotational speed of 2,600 rpm, the spinning dope was ejected to form liquid fibers, which were then conveyed, while being suspended in and dried with 200° C. hot air, to a collection chamber of the type in which suction was conducted from below. Thus, precursor fibers were accumulated.

(3) They were burned by using a roller-hearth furnace in the air. The burning consisted of the degreasing step up to an ambient temperature of 800° C. in which the temperature was continuously raised at a rate of 3° C./min while performing evacuation at a rate of 1.5 Nm.sup.3/h per kg of the precursor, and the crystallization step from above 800° C. to 1,200° C. in which the temperature was raised at a rate of 20° C./min and held at 1,200° C. for 30 minutes.

(A) Example 1-2 and Comparative Examples 1-1 and 1-2

(4) Alumina-based fibrous masses were produced in the same manner as in Example 1-1, except that the chemical composition was changed. The properties of the alumina-based fibrous masses obtained are shown in Table 1.

(5) TABLE-US-00001 TABLE 1 Example Example Comp. Ex. Comp. Ex. 1-1 1-2 1-1 1-2 Chemical Al.sub.2O.sub.3 (%) 73 70 95 60 composition Burning Rate of burning to 800° C. (° C./min) 3 3 3 3 conditions Maximum burning temperature (° C.) 1200 1200 1200 1200 Period of holding at maximum burning 30 30 30 30 temperature (min) Evacuation condition per kg of precursor 1.5 1.5 1.5 1.5 (Nm.sup.3/h) Properties Total pore volume (mL/g) 0.0015 0.0018 0.0023 0.0012 Specific surface area (m.sup.2/g) 0.8 0.7 2.6 0.6 Mullite ratio (%) 2 2 1 3 Areal pressure (N/cm.sup.2) in compression 110 106 65 92 at 0.5 g/cm.sup.2 Areal pressure (N/cm.sup.2) in compression 45 41 29 35 at 0.4 g/cm.sup.2 Areal pressure (N/cm.sup.2) in compression 20 19 11 14 at 0.3 g/cm.sup.2 Remarks Heat resistance decreased

(B) Example 2-1 and Comparative Example 2-1

(6) Alumina-based fibrous masses were produced in the same manner as in Example 1-1, except that the burning rate in the degreasing step was changed. The properties of the alumina-based fibrous masses obtained are shown in Table 2.

(7) TABLE-US-00002 TABLE 2 Exam- Exam- Comp. ple ple Ex. 1-1 2-1 2-1 Chemical Al.sub.2O.sub.3 (%) 73 73 73 composi- tion Burning Rate of burning to 800° 3 2 10 condi- C. (° C./min) tions Maximum burning tem- 1200 1200 1200 perature (° C.) Period of holding at max- 30 30 30 imum burning tempera- ture (min) Evacuation condition per 1.5 1.5 1.5 kg of precursor (Nm.sup.3/h) Proper- Total pore volume 0.0015 0.0012 0.0058 ties (mL/g) Specific surface area 0.8 0.7 6.4 (m.sup.2/g) Mullite Ratio (%) 2 2 2 Areal pressure (N/cm.sup.2) 110 112 87 in compression at 0.5 g/cm.sup.2 Areal pressure (N/cm.sup.2) 45 50 29 in compression at 0.4 g/cm.sup.2 Areal pressure (N/cm.sup.2) 20 25 12 in compression at 0.3 g/cm.sup.2

(C) Examples 3-1 and 3-2 and Comparative Examples 3-1 and 3-2

(8) Alumina-based fibrous masses were produced in the same manner as in Example 1-1, except that the rate of evacuation per kg of the precursor was changed. The properties of the alumina-based fibrous masses obtained are shown in Table 3.

(9) TABLE-US-00003 TABLE 3 Example Example Example Comp. Ex. Comp. Ex. 1-1 3-1 3-2 3-1 3-2 Chemical Al.sub.2O.sub.3 (%) 73 73 73 73 73 composition Burning Rate of burning to 800° C. (° C./min) 3 3 3 3 3 conditions Maximum burning temperature (° C.) 1200 1200 1200 1200 1200 Period of holding at maximum burning 30 30 30 30 30 temperature (min) Evacuation condition per kg of precursor 1.5 0.1 3 0.01 10 (Nm.sup.3/h) Properties Total pore volume (mL/g) 0.0015 0.0037 0.0014 0.0072 0.0061 Specific surface area (m.sup.2/g) 0.8 0.8 0.6 5.6 0.8 Mullite ratio (%) 2 2 2 2 2 Areal pressure (N/cm.sup.2) in compression 110 103 98 87 82 at 0.5 g/cm.sup.2 Areal pressure (N/cm.sup.2) in compression 45 42 44 37 33 at 0.4 g/cm.sup.2 Areal pressure (N/cm.sup.2) in compression 20 18 19 14 12 at 0.3 g/cm.sup.2

(D) Examples 4-1 to 4-4 and Comparative Examples 4-1 and 4-2

(10) Alumina-based fibrous masses were produced in the same manner as in Example 1-1, except that the maximum burning temperature and holding period in the crystallization step were changed. The properties of the alumina-based fibrous masses obtained are shown in Table 4.

(11) TABLE-US-00004 TABLE 4 Ex. Ex. Ex. Ex. Ex. Comp. Ex. Comp. Ex. 1-1 4-1 4-2 4-3 4-4 4-1 4-2 Chemical Al.sub.2O.sub.3 (%) 73 73 73 73 73 73 73 composition Burning Rate of burning to 800° C. (° C./min) 3 3 3 3 3 3 3 conditions Maximum burning temperature (° C.) 1200 1230 1000 1200 1200 1250 1500 Period of holding at maximum 30 30 30 5 60 30 30 burning temperature (min) Evacuation condition per kg of 1.5 1.5 1.5 1.5 1.5 1.5 1.5 precursor (Nm.sup.3/h) Properties Total pore volume (mL/g) 0.0015 0.0011 0.0038 0.0032 0.0012 0.0041 0.0001 Specific surface area (m.sup.2/g) 0.8 0.3 3.2 3.4 0.2 0.3 0.2 Mullite ratio (%) 2 5 1 2 2 10 50 Areal pressure (N/cm.sup.2) in 110 103 98 92 94 85 78 compression at 0.5 g/cm.sup.2 Areal pressure (N/cm.sup.2) in 45 42 41 40 42 28 16 compression at 0.4 g/cm.sup.2 Areal pressure (N/cm.sup.2) in 20 17 18 16 15 11 12 compression at 0.3 g/cm.sup.2 Remarks Areal pressure Areal pressure decreased due decreased due to too high to too high mullite ratio mullite ratio

(E) Reference Example

(12) For reference, commercial products of alumina-based fibers (commercial product A and commercial product B) were measured for total pore volume, specific surface area, mullite ratio, and areal pressure, and the results thereof are shown in Table 5.

(13) TABLE-US-00005 TABLE 5 Com- Com- Exam- mercial mercial ple prod- prod- 1-1 uct A uct B Chemical Al.sub.2O.sub.3 (%) 73 72 80 composi- tion Proper- Total pore volume (mL/g) 0.0015 0.0061 0.0059 ties Specific surface area (m.sup.2/g) 0.8 0.6 0.2 Mullite ratio (%) 2 2 1 Areal pressure (N/cm.sup.2) in 110 85 65 compression at 0.5 g/cm.sup.2 Areal pressure (N/cm.sup.2) in 45 42 32 compression at 0.4 g/cm.sup.2 Areal pressure (N/cm.sup.2) in 20 18 12 compression at 0.3 g/cm.sup.2

(14) From the results shown in Table 1 to Table 5, it can be seen that the alumina-based fibrous mass of the present invention can be produced only when a specific Al.sub.2O.sub.3 incorporation amount and specific production conditions are used in combination. According to this production process of the present invention, an alumina-based fibrous mass which exhibits a high areal pressure can be easily produced by using conventional devices or the like. Meanwhile, since the areal pressure decreases considerably when the mullite ratio exceeds 5%, it is desirable to inhibit the mullite ratio from exceeding 5%, by regulating the temperature and holding period during the crystallization step.

(15) While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

(16) This application is based on a Japanese patent application filed on Oct. 31, 2012 (Application No. 2012-240883), the entire contents thereof being incorporated herein by reference. Furthermore, all the references cited herein are incorporated herein as a whole.

INDUSTRIAL APPLICABILITY

(17) Since the alumina-based fibrous mass of the present invention has a dense structure and a high areal pressure as compared with conventional alumina-based fibers, it can be used, for example, as a holding material of the exhaust gas cleaner of a motor vehicle or the like.