Hardened shaped article

Abstract

A shaped article including inorganic fibers including SiO2 and an alkaline earth metal, wherein the shaped article is treated with a hardening agent.

Description

TECHNICAL FIELD

The invention relates to a hardened shaped article formed of inorganic fibers.

BACKGROUND ART

Inorganic fibers are lightweight, easy to handle and excellent in heat resistance, and hence, they are used as a heat resisting sealing material, for example. On the other hand, in recent years, a problem has been pointed out that inorganic fibers are inhaled into a human body and the inhaled fibers invade the lung to cause disorders. Under such circumstances, bio-soluble inorganic fibers which do not cause or hardly cause disorders even if inhaled into a human body have been developed.

However, a shaped article produced with such bio-soluble fibers has a drawback of low absolute strength.

Patent Document 1 discloses a mastics containing bio-soluble alkaline earth metal silicate fibers and colloidal silica. Patent Document 1 describes that the mastics could exhibit excellent storage stability by using the colloidal silica as a binder and the storage stability could be improved by further containing a chelating agent such as EDTA (ethylene diamine tetraacetate).

Patent Document 2 discloses a Xonotlite-type calcium silicate board is impregnated with an inorganic binder in order to suppress generation of dust.

RELATED ART DOCUMENTS

Patent Documents

• Patent Document 1: JP-A-2002-524384
• Patent Document 2: JP-A-2008-13402

SUMMARY OF THE INVENTION

Subject to be Solved by the Invention

The invention has been made in view of the above-mentioned problem, and one object thereof is to provide a high-strength shaped article comprising inorganic fibers.

Means for Solving the Problems

As a result of intensive studies, the inventors have found that a shaped article can be improved in the strength by a treatment with a hardening agent, and have completed the invention.

According to the invention, the following hardened shaped article and the method of hardening a shaped article are provided.

1. A shaped article comprising inorganic fibers comprising SiO₂ and an alkaline earth metal, wherein the shaped article is treated with a hardening agent.
2. The shaped article according to 1, wherein the hardening agent is colloidal silica.
3. The shaped article according to 1 or 2, wherein the shaped article is treated with a metal ion free-chelating agent and the hardening agent.
4. The shaped article according to 3, wherein the chelating agent is EDTA.H.3(NH₄) or EDTA.H.2(NH₄).
5. The shaped article according to 1 or 2, wherein the shaped article is treated with an acid and the hardening agent.
6. The shaped article according to 5, wherein the acid is a hydroxy acid.
7. The shaped article according to 5, wherein the acid is at least one selected from the group consisting of citric acid, malic acid, tartaric acid and salicylic acid.
8. The shaped article according to 1 or 2, wherein the hardening agent is an acidic colloidal silica or a cationic colloidal silica.
9. The shaped article according to 8, wherein a pH value of the acidic colloidal silica is about 2 to about 5.
10. The shaped article according to any one of 3 to 9, wherein elution of an alkali metal or alkaline earth metal is suppressed.
11. The shaped article according to 10, wherein the alkali metal or alkaline earth metal is potassium, sodium, calcium or magnesium.
12. The shaped article according to any one of 1 to 11, wherein the inorganic fibers have the following composition.

Total of SiO₂, ZrO₂, Al₂O₃ and TiO₂: about 50 wt % to about 82 wt %

Total of alkali metal oxides and alkaline earth metal oxides: about 18 wt % to about 50 wt %

13. The shaped article according to any one of 1 to 11, wherein the inorganic fibers have the following composition.

SiO₂: about 50 wt % to about 82 wt %

Total of CaO and MgO: about 10 wt % to about 43 wt %

14. The shaped article according to any one of 1 to 13, wherein the inorganic fibers comprise SiO₂, CaO and MgO as main components.
15. The shaped article according to any one of 1 to 14, wherein the inorganic fibers are bio-soluble.
16. A method of hardening a shaped article comprising treating a shaped article comprising inorganic fibers comprising SiO₂ and an alkaline earth metal with a hardening agent.

Advantageous Effects of the Invention

According to the invention, a high-strength shaped article comprising inorganic fibers can be provided.

MODE FOR CARRYING OUT THE INVENTION

A hardened shaped article of the invention can be obtained by treating a shaped article formed using inorganic fibers comprising SiO₂ and an alkaline earth metal with a hardening agent. As the hardening agent, colloidal silica can be used. The colloidal silica includes alkaline colloidal silica (pH8 to 10), acidic colloidal silica (pH2 to 6) and cationic colloidal silica.

The hardening treatment is carried out by immersing a shaped article in a hardening treatment solution, or applying or blowing a hardening treatment solution to a shaped article, followed by drying, for example. As a solvent, water, monovalent alcohols such as ethanol and propanol, and divalent alcohols such as ethyleneglycol can be used. Water content in the shaped article after hardening treatment is usually about 5 wt % or less. The water content is confirmed based on the weights before and after drying.

By the above-mentioned hardening treatment, hardness increases and strength and handling properties (problems such as hand marks after grasping, nipping out during processing, no sharp corner caused by cutting, and falling of dust) are improved.

When a shaped article is treated with alkaline colloidal silica as the hardening agent, alkaline earth metal ions, in particular Ca ion and Mg ion are eluted to the hardening treatment solution. The ions may react with the hardening agent at the surface of the shaped article, causing generation of cracks when heated to a temperature of 600° C. or higher. The reason therefor may be that the reaction of silica and Ca ions, etc. causes change in volume.

To suppress the generation of cracks, a shaped article is treated with a metal ion-free chelating agent and a hardening agent. The inventors consider that the chelating agent traps the eluted alkaline earth metal and forms a protective layer at the same time to suppress the reaction of the alkaline earth metal and the hardening agent and to prevent cracks from generating. The chelating agent has two or more electron-donating groups, and therefore it can form a metal chelate compound. The electron-donating group includes a carboxy group and a hydroxy group. The chelating agent used in the invention does not contain alkaline earth metals and alkali metals. Examples of the chelating agent include one having a composition of EDTA (ethylene diamine tetraacetate).H.3(NH₄) and one having a composition of EDTA.H.2(NH₄).

As the hardening agent used together with the chelating agent, colloidal silica can be used. The colloidal silica includes alkaline colloidal silica, acidic colloidal silica and cationic colloidal silica. The concentration of the chelating agent can be arbitrarily set. When using the alkaline colloidal silica as the hardening agent, it is preferably used in a concentration of 0.5 wt % or more, relative to the hardening treatment solution having a solid content of 10 wt %.

Alternatively, to suppress the generation of cracks, a shaped article may be treated with a hardening treatment solution having a pH value of 6 or lower (preferably a pH value of 1 to 6, more preferably 2 to 5, and particularly preferably 3 to 5). By making the hardening treatment solution to be acidic, elution of an alkaline earth metal is suppressed. As a result, the inventors concider that the reaction of the alkaline earth metal and the hardening agent is suppressed so that the generation of cracks is prevented.

Specifically, a shaped article is treated with an acid and the hardening agent. The acid may be a week acid or a strong acid. The week acid includes citric acid, malic acid, tartaric acid, salicylic acid, glycolic acid, lactic acid, mandelic acid, benzilic acid, coumaric acid and acetic acid. The strong acid includes sulfuric acid, hydrochloric acid and nitric acid. In view of no trouble with generation of gas, the week acid is preferable.

Also, acidic colloidal silica (for example, one having a pH value of 1 to 6 or 2 to 5) may be used. As the acidic colloidal silica, commercial products such as SILICADOL 20A (manufactured by Nippon Chemical Industrial Co., Ltd.), CATALOID SN (manufactured by JGC Catalysts and Chemicals Ltd.), etc. may be used.

As substances having both of the chelating effect and the nature of making the pH to be acidic, hydroxy acids such as citric acid, malic acid, tartaric acid and salicylic acid may be given.

When cationic colloidal silica (for example, one having a pH value of 4 to 6) is used as the hardening agent, elution of an alkaline earth metal can be suppressed. In this case, the inventors consider that the cationic colloidal silica forms a protective film on the surface of a shaped article to suppress the reaction of the alkaline earth metal and the hardening agent. The cationic colloidal silica is colloidal silica having a positive charge at the surface. For example, one may be given which contains a multivalent metal ion such as aluminum ion or an organic cationic compound on the surface or inside of the colloidal silica particles, and thus has the cation-charged surface. As the cationic colloidal silica, commercial products such as SNOWTEX AK (manufactured by Nissan Chemical Industries Ltd.) may be used.

The shaped article of the invention is constituted by containing inorganic fibers. For instance, it contains 20 to 99 wt % of the inorganic fibers. The inorganic fibers are formed of SiO₂, CaO and MgO as main components. The “main components” means that total of these substances occupy 90 wt % or more, or 95 wt % or more.

The inorganic fibers may be bio-soluble fibers having the following composition.

Total of SiO₂, ZrO₂, Al₂O₃ and TiO₂: 50 wt % to 82 wt %

Total of alkali metal oxides and alkaline earth metal oxides: 18 wt % to 50 wt %

Also, the inorganic fibers may be bio-soluble fibers having the following composition.

SiO₂: 50 to 82 wt %

Total of CaO and MgO: 10 to 43 wt %

The bio-soluble fibers are divided roughly to Mg silicate fibers containing more MgO and Ca silicate fibers containing more CaO. As the Mg silicate fibers, one having the following composition can be exemplified.

SiO₂: 66 to 82 wt %

CaO: 1 to 9 wt %

MgO: 10 to 30 wt %

Al₂O₃: 3 wt % or less

Other oxides: less than 2 wt %

As the Ca silicate fibers, one having the following composition can be exemplified.

SiO₂: 66 to 82 wt % (for example, it can be 68 to 80 wt %, 70 to 80 wt %, 71 to 80 wt % or 71 to 76 wt %)

CaO: 10 to 34 wt % (for example, it can be 20 to 30 wt % or 21 to 26 wt %)

MgO: 3 wt % or less (for example, it can be 1 wt % or less)

Al₂O₃: 5 wt % or less (for example, it can be 3.5 wt % or less, or 3 wt % or less, and it can be 1 wt % or more, or 2 wt % or more)

Other oxides: less than 2 wt %

The above-mentioned inorganic fibers may or may not contain, as the other oxides, at least one selected from alkali metal oxides such as K₂O and Na₂O, Fe₂O₃, ZrO₂, TiO₂, P₂O₄, B₂O₃ and R₂O₃ (where R is selected from Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, and mixtures thereof). Each of the other oxides can be contained in an amount of 0.2 wt % or less or 0.1 wt % or less.

Further, total of SiO₂, CaO, MgO and Al₂O₃ may be more than 98 wt % or more than 99 wt %.

The bio-soluble fibers may contain other components in addition to SiO₂ and the alkaline earth metal oxides (for example, at least one of MgO and CaO). The bio-soluble fibers may further contain, for example, one, or two or more selected from the group consisting of alumina (Al₂O₃), titania (TiO₂), zirconia (ZrO₂), iron oxide (Fe₂O₃), manganese oxide (MnO) and potassium oxide (K₂O).

Bio-soluble inorganic fibers are inorganic fibers having a physiological saline dissolution ratio at 40° C. of 1% or more, for example.

The physiological saline dissolution ratio can be measured by the following method, for example. Specifically, at first, 1 g of a sample obtained by pulverizing inorganic fibers to 200 meshes or less and 150 mL of a physiological saline solution are charged in a conical flask (volume: 300 mL) and the flask is installed in an incubator at 40° C. Next, the conical flask is continuously subjected to horizontal vibration of 120 rotations per minute for 50 hours. Subsequently, the concentrations of elements (mg/L) contained in a filtrate obtained by filtration are each measured by means of an ICP emission spectroscopic analyzer. Then, the physiologically saline dissolution ratio (%) is calculated based on the measured concentrations of the elements and the contents (mass %) of the elements in inorganic fibers before dissolution. Specifically, if the elements subjected to the measurement are silicon (Si), magnesium (Mg), calcium (Ca) and aluminum (Al), the physiologically saline dissolution ratio C (%) is calculated according to the following formula: C(%)=[Amount of filtrate (L)×(a1+a2+a3+a4)×100]/[Mass of inorganic fibers before dissolution (mg)×(b1+b2+b3+b4)/100]. In this formula, a1, a2, a3 and a4 are the measured concentrations (mg/L) of silicon, magnesium, calcium and aluminum, respectively, and b1, b2, b3 and b4 are the contents (mass %) of silicon, magnesium, calcium and aluminum contained in inorganic fibers before dissolution, respectively.

The shaped article includes articles formed of inorganic fibers such as molds, blankets, blocks, boards, papers and felts, other than shapeless articles such as mastics (which may contain an inorganic and/or an organic binder). However, the shaped article of the invention comprising inorganic fibers does not include Xonotlite-type calcium silicate boards as disclosed in Patent Document 2.

The shaped article can contain an organic binder, an inorganic binder, an inorganic filler, etc., in addition to the inorganic fibers. As these components, ones usually used can be employed, as long as they do not impair the effect of the invention. The organic binder includes starches, acrylic resins, polyacrylamides, pulps and acrylic emulsions. The inorganic binder includes colloidal silica such as anionic colloidal silica and cationic colloidal silica, alumina sol, bentonite and clay minerals.

The hardened shaped article of the invention is obtained by the above-mentioned treatment so that the hardening agent is impregnated from the surface thereof.

EXAMPLES

Production Example 1

<Production of Organic and Inorganic Boards (Shaped Article)>

An organic board having a size of 300 mm in length, 300 mm in width and 50 mm in thickness (density: 250 kg/m³) was produced from bio-soluble fibers A (content of SiO₂: 74 mass %, content of CaO: 25 mass %, content of MgO: 0.3 mass %, and content of Al₂O₃: 2 mass %), starch and polyacrylamide (organic binder), and colloidal silica (inorganic binder).

Also, An inorganic board having a size of 300 mm in length, 300 mm in width and 50 mm in thickness (density: 250 kg/m³) was produced from the bio-soluble fibers A, colloidal silica, an alumina sol and bentonite (inorganic binder), inorganic fine particles (silica, zircon, calcium carbonate, zirconia, cordierite, wollastonite, etc.), and acrylamide (organic binder).

Production Example 2

<Production of Organic and Inorganic Boards (Shaped Article)>

An organic board and an inorganic board were produced in the same manner as in Production Example 2 except that bio-soluble fibers B (content of SiO₂: 76 mass %, content of CaO: 4 mass %, content of MgO: 18 mass %, and content of Al₂O₃: 2 mass %) were used in place of the bio-soluble fibers A.

Example 1

<Hardening Treatment>

The organic board and inorganic board produced in Production Example 1 were treated with a hardening solution containing alkaline colloidal silica (pH: approximately 9, solvent: water) to harden and dry the surfaces of the boards. The strength of the boards increased after hardening. Further, hardness of the hardened organic and inorganic boards were measured by a hardness tester (ASKER Rubber Hardness Tester C type manufactured by ASKER). Table 1 shows the results.

Comparative Example 1

The organic and inorganic boards produced in Production Example 1 were evaluated without conducting the hardening treatment. Both the organic and inorganic boards were brittle. The hardnesses thereof were measured in the same manner an in Example 1. Table 1 shows the results.

TABLE 1
Fiber (A)
	Organic Board	Inorganic Board
	Before	After	Before	After
	hardening	hardening	hardening	hardening
Hardness (°)	50	85	40	75

Example 2

<Hardening Treatment>

The hardening treatment was carried out in the same manner as in Example 1 except that the organic board and inorganic board produced in Production Example 2 were used. Hardness was measured in the same manner as in Example 1. Table 2 shows the results.

Comparative Example 2

The organic and inorganic boards produced in Production Example 2 were evaluated without conducting the hardening treatment. Both the organic and inorganic boards were brittle. The hardnesses thereof were measured in the same manner an in Example 1. Table 2 shows the results.

TABLE 2
Fiber (B)
	Organic Board	Inorganic Board
	Before	After	Before	After
	hardening	hardening	hardening	hardening
Hardness (°)	50	70	45	65

Example 3

[Chelating Agent and Hardening Agent]

<Hardening Treatment>

The organic and inorganic boards produced in Production Example 1 were treated with a hardening treatment solution (pH: approximately 9, solvent: water) containing EDTA diammonium salt and alkaline colloidal silica to harden and dry the surface thereof. The chelating agent was used in an amount of 1 wt %, relative to the hardening treatment solution having a solid content of 10 wt %.

<Heating Test>

The hardened organic and inorganic boards were heated at a temperature of 800° C., 900° C., 1000° C. and 1100° C. for 24 hours, respectively, and the outer appearances (with or without a crack) thereof were observed visually. For comparison, the hardened organic and inorganic boards produced in Example 1 were also heated in the same manner and observed visually. One with no cracks is represented by “⊚”, one wherein small cracks are generated on the surface by “◯”, one wherein large cracks are generated on the surface by “▴”, and broken one by “x”. Table 3 shows the results.

	TABLE 3
	800° C.	900° C.	1000° C.	1100° C.
Alkaline colloidal	Organic	⊚	◯	▴	X
silica	Board
Alkaline colloidal	Inorganic	⊚	⊚	⊚	▴
silica	Board
Alkaline colloidal	Organic	⊚	⊚	⊚	⊚
silica + Chelating	Board
agent
Alkaline colloidal	Inorganic	⊚	⊚	⊚	⊚
silica + Chelating	Board
agent

Example 4

[Acid and Hardening Agent]

<Hardening Treatment>

The organic and inorganic boards produced in Production Example 1 were treated with a hardening treatment solution containing an acid and alkaline colloidal silica (hardening agent) to harden and dry the surface thereof. As the acid, citric acid, malic acid, tartaric acid and salicylic acid were used in an amount of about 1 wt %, relative to the hardening treatment solution having a solid content of 10 wt %, to adjust the treatment solutions to pH 3 to 4.

<Heating Test>

The tests are conducted in the same manner as in Example 3. Tables 4 and 5 show the results.

TABLE 4
Organic Board (Fiber A)
Hardening treatment solution	800° C.	900° C.	1000° C.	1100° C.
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
citric acid
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
malic acid
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
tartaric acid
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
salicylic acid

TABLE 5
Inorganic Board (Fiber A)
Hardening treatment solution	800° C.	900° C.	1000° C.	1100° C.
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
citric acid
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
malic acid
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
tartaric acid
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
salicylic acid

Example 5

[Acid and Hardening Agent]

<Hardening Treatment>

The organic and inorganic boards produced in Production Example 1 were treated with the hardening treatment solution containing citric acid and alkaline colloidal silica (hardening agent) to harden and dry the surface thereof. Citric acid was used in amounts of 0.4 wt %, 0.6 wt %, 1.5 wt % and 8 wt % to adjust the treatment solution to pH 2 to 5.

<Heating Test>

The test was conducted in the same manner as in Example 3. Table 6 shows the results.

	TABLE 6
	pH
	5	4	3	2
800° C. × 24 h
	Organic Board	⊚	⊚	⊚	⊚
	Inorganic Board	⊚	⊚	⊚	⊚
900° C. × 24 h
	Organic Board	⊚	⊚	⊚	⊚
	Inorganic Board	⊚	⊚	⊚	⊚
1000° C. × 24 h
	Organic Board	⊚	⊚	⊚	⊚
	Inorganic board	⊚	⊚	⊚	⊚
1100° C. × 24 h
	Organic Board	⊚	⊚	⊚	⊚
	Inorganic Board	⊚	⊚	⊚	⊚

Example 6

[Acid and Hardening Agent]

<Hardening Treatment>

The treatment was conducted in the same manner as in Example 4 except that the organic board and inorganic board produced in Production Example 2 were used.

<Heating Test>

The test was conducted in the same manner as in Example 4. For comparison, the hardened organic and inorganic boards obtained in Example 2 were heated and observed in the same manner as in Example 4. Tables 7 and 8 show the results.

TABLE 7
Organic Board (Fiber B)
Hardening treatment solution	800° C.	900° C.	1000° C.	1100° C.
Alkaline colloidal silica	◯	◯	▴	X
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
citric acid
Alkaline colloidal silica +	⊚	⊚	◯	▴
malic acid
Alkaline colloidal silica +	⊚	⊚	◯	▴
tartaric acid
Alkaline colloidal silica +	⊚	⊚	⊚	◯
salicylic acid

TABLE 8
Inorganic Board (Fiber B)
Hardening treatment solution	800° C.	900° C.	1000° C.	1100° C.
Alkaline colloidal silica	⊚	◯	▴	X
Alkaline colloidal silica +	⊚	⊚	⊚	⊚
citric acid
Alkaline colloidal silica +	⊚	⊚	⊚	◯
malic acid
Alkaline colloidal silica +	⊚	⊚	⊚	◯
tartaric acid
Alkaline colloidal silica +	⊚	⊚	◯	▴
salicylic acid

Example 7

[Acidic Colloidal Silica and Cationic Colloidal Silica]

<Hardening Treatment>

The organic board and inorganic board produced in Production Example 1 were treated with acidic colloidal silica (SILICADOL 20A (manufactured by Nippon Chemical Industrial Co., Ltd.), pH 2 to 4) or cationic colloidal silica (SNOWTEX AK (manufactured by Nissan Chemical Industries Ltd.)) (hardening agent) to harden and dry the surface thereof.

<Heating Test>

The test was conducted in the same manner as in Example 3. Tables 9 and 10 show the results.

TABLE 9
Organic Board (Fiber A)
Hardening treatment solution	800° C.	900° C.	1000° C.	1100° C.
Acidic colloidal silica	⊚	⊚	⊚	⊚
Cationic colloidal silica	⊚	⊚	⊚	⊚

TABLE 10
Inorganic Board (Fiber A)
Hardening treatment solution	800° C.	900° C.	1000° C.	1100° C.
Acidic colloidal silica	⊚	⊚	⊚	⊚
Cationic colloidal silica	⊚	⊚	⊚	⊚

Example 8

[Acidic Colloidal Silica and Cationic Colloidal Silica]

<Hardening Treatment>

The treatment was conducted in the same manner as in Example 7 except that the organic board and inorganic board produced in Production Example 2 were used.

<Heating Test>

The test was conducted in the same manner as in Example 7. Tables 11 and 12 show the results.

TABLE 11
Organic Board (Fiber B)
Hardening treatment solution	800° C.	900° C.	1000° C.	1100° C.
Acidic colloidal silica	⊚	⊚	⊚	⊚
Cationic colloidal silica	⊚	⊚	⊚	⊚

TABLE 12
Inorganic Board (Fiber B)
Hardening treatment solution	800° C.	900° C.	1000° C.	1100° C.
Acidic colloidal silica	⊚	⊚	⊚	⊚
Cationic colloidal silica	⊚	⊚	⊚	⊚

INDUSTRIAL APPLICABILITY

The hardened shaped article of the invention can be used for various uses as general heat insulating materials for high temperature, heat insulating lining materials for a ceiling and wall of furnace, heat insulating materials and backup materials.

Although only some exemplary embodiments and/or examples of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments and/or examples without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.

The documents described in the specification are incorporated herein by reference in its entirety.

Claims

1. A shaped article comprising 20 wt % to 99 wt % of bio-soluble inorganic fibers comprising SiO2 and an alkaline earth metal, wherein the shaped article is impregnated from the surface thereof with a hardening agent which is colloidal silica and the surface is hardened.

2. (canceled)

3. The shaped article according to claim 1, wherein the shaped article is impregnated with a metal ion free-chelating agent in addition to the hardening agent.

4. The shaped article according to claim 3, wherein the chelating agent is EDTA.H.3(NH4) or EDTA.H.2(NH4).

5. The shaped article according to claim 1, wherein the shaped article is impregnated with an acid together with the hardening agent.

6. The shaped article according to claim 5, wherein the acid is a hydroxyl acid.

7. The shaped article according to claim 5, wherein the acid is at least one selected from the group consisting of citric acid, malic acid, tartaric acid and salicylic acid.

8. The shaped article according to claim 1, wherein the hardening agent is an acidic colloidal silica or a cationic colloidal silica.

9. The shaped article according to claim 8, wherein a pH value of the acidic colloidal silica is about 2 to about 5.

10. (canceled)

11. (canceled)

12. The shaped article according to claim 1, wherein the inorganic fibers have the following composition.

Total of SiO2, ZrO2, AI2O3 and TiO2: about 50 wt % to about 82 wt %

Total of alkali metal oxides and alkaline earth metal oxides: about 18 wt % to about 50 wt %

13. The shaped article according to claim 1, wherein the inorganic fibers have the following composition.

SiO2: about 50 wt % to about 82 wt %

Total of CaO and MgO: about 10 wt % to about 43 wt %

14. (canceled)

15. (canceled)

16. A method of hardening a shaped article comprising impregnating a shaped article comprising 20 wt % to 99 wt % of bio-soluble inorganic fibers comprising SiO2 and an alkaline earth metal with a hardening treatment solution comprising colloidal silica, and drying the shaped article.

17. The shaped article according to claim 1, wherein the inorganic fibers are Mg filicate fibers having the following composition or Ca silicate fibers having the following composition.

[Mg silicate fiber] SiO2: 66 wt % to 82 wt % CaO: 1 wt % to 9 wt % MgO: 10 wt % to 30 wt % AI2O3: 3 wt % or less Other oxides: less than 2 wt %

[Ca silicate fiber] SiO2: 66 wt % to 82 wt % CaO: 10 wt % to 34 wt % MgO: 3 wt % or less AI2O3: 5 wt % or less Other oxides: less than 2 wt %

18. The method according to claim 16, wherein the hardening treatment solution further comprises a metal ion free-chelating agent or an acid.

19. A shaped article hardened by the method according to claim 16 or 18.