GB2147925A

GB2147925A - Coating of mineral fibres

Info

Publication number: GB2147925A
Application number: GB08425506A
Authority: GB
Inventors: Dr Alan Briggs; Dr David Leslie Segal; David Vivian Clee Jones; Roy Frederick Preston; Graham Raw
Original assignee: ENERGY SECRETARY OF STATE FOR; UK Secretary of State for Energy
Current assignee: ENERGY SECRETARY OF STATE FOR; UK Secretary of State for Energy
Priority date: 1983-10-10
Filing date: 1984-10-09
Publication date: 1985-05-22
Also published as: GB8327058D0; GB8425506D0; GB2147925B

Abstract

Aluminosilicate fibres are coated with chromium(III) oxide by contacting with a chromium- containing dispersion and by calcining. The chromium-containing dispersion comprises a dispersion of chromium(III) oxide powder in a sol of chromium(III) oxide. Fibres so-coated, e.g. by spraying the dispersion, have been found to be resistant to sintering at elevated temperatures, for example up to 1500 DEG C, and not to suffer excessive shrinkage when formed into usable artefacts such as blankets for high temperature thermal insulation.

Description

SPECIFICATION Coating of fibres This invention relates to a method of coating aluminosilicate fibres with chromium(lll) oxide wherein the fibres are contacted with a chromium-containing dispersion and calcined.

Fibrous aluminosilicate is a known and useful high temperature insulating material, and it is known to improve its performance by providing it with a chromium(lll) oxide coating. For example, US Patent No 4312 911 describes producing a chromium(lll) oxide coating on aluminosilicate fibres by spraying an aqueous chromium(lll) ion containing solution onto the fibre under conditions to precipitate a chromium(lll) oxide hydrate thereupon, followed by calcining to convert the hydrate to the oxide.

Fibres coated as above may be in the form of a usable artefact such as a blanket or mat, or in the form of individual fibres for subsequent formation into a usable artefact. The latter is preferred because of difficulties of penetrating an artefact with a dispersion. However, the above method has been found to give coated fibres which suffer from considerable shrinkage when formed into a mat or other artefact.

The present invention provides a method of endowing aluminosilicate fibres with the ability to withstand high temperatures in severe environments whilst not suffering from excessive shrinkage if and when formed into a mat or other artefact.

The invention provides a method of coating aluminosilicate fibres with chromium(lll) oxide wherein the fibres are contacted with a dispersion of chromium(lll) oxide powder in a -sol of chromium(lll) oxide followed by calcining.

Fibres so-coated have been found to be resistant to sintering at elevated temperatures, for example up to 1 500 C, and no to suffer from excessive shrinkage when formed into usable artefacts.

It is believed that the chromium(lll) oxide acts by providing a more refractory barrier between the surfaces of adjacent fibres thereby reducing their tendency to sinter. It is also possible that chromium(lil) oxide diffuses into the fibre and increases the viscosity of the aluminosilicate and reduces its ability to devitrify and shrink. It is believed that the use of chromium(lll) oxide partly in powder form in the present invention, i.e. of larger particle size than in a solution or a sol, is significant. Thus, the effect of the chromium(lll) oxide may be enhanced because the larger particles thereof would require longer times and/or higher temperatures to complete their dispersion by diffusion into the fibres.

Preferably, the invention is carried out at elevated temperatures; for example, it may be carried out on fibres immediately after their formation from a molten mixture by methods known in the art. The coated fibres may then be formed into an artefact usable, e.g. for heat insulation purposes, by methods known in the art. For example, they may be compressed and needled to form an insulating mat.

It may be possible to carry out the invention on an artefact composed of aluminosilicate fibres. It may, however, be difficult in practice to obtain adequate penetration of the dispersion into the artefact and hence satisfactory coating of the fibres. The invention may, though, satisfactorily be carried out on a loose assemblage of fibres which may then be formed into a usable artefact.

The contacting of the fibres with the dispersion may be carried out by methods known in the art such as dipping or spraying, of which the latter is preferred. Where the spraying is carried out on hot fibres, the contacting and calcining steps of the invention occur simultaneously.

The concentration and composition of the dispersion may be varied in accordance with specific requirements, provided, of course, that it is capable of giving rise to a satisfactory coating on the fibres. Generally, it is preferable that the dispersion contains a greater proportion by weight of non-colloidal material (i.e. powder) than of colloidal material, though the very much smaller particle size of the latter will, in most cases, ensure that there are more colloidal particles than non-colloidal particles in the dispersion. An example of the ratio of non-colloidal to colloidal matter in the dispersion expressed as Cr2O3 by weight may be about 8:1. Where the dispersion is to be applied by spraying, it should be noted that it has to be sufficiently dilute for the spraying operation to be carried out.

The aluminosilicate fibres may be of any convenient form, for example as supplied commercially by Morganite Ceramic Fibres Limited under the Trade Mark "KAOWOOL". The fibres may, for example, have a diameter in the range from 5 to 1 5 ,um.

The chromium(lll) oxide coating may conveniently be less than 1 ym in thickness, for example about 0.3 pm.

Several ways of carrying out the invention wili now be described below by way of example only.

Example 1 Preparation of Dispersion An aqueous dispersion was prepared by dispersing chromium(lll) oxide power (particle diameter-0.3 ym) in a chromium(lll) oxide aquasol prepared as described in UK Patent Specification No 2 059 933A. The dispersion had a concentration of 447 gI-1 of Cr203, of which 400 gI - 1 was in the form of the powder and 47 gI - was in the form of colloidal particles. The dispersion was then diluted in the ratio of 1 5:1 with water to give a dispersion for use as described below.

Coating of Fibres A Triton Kaowool ceramic fibre blanket (128 kg m - 3) supplied by Morganite Ceramic Fibres Limited containing 45% alumina and made by fusion/blowing a mixture of silica and alumina was the starting material. A sample of the blanket (3m2; 25 mm thick) was cut into 600 mm X 600 mm squares and each square separated carefully into 1 2 layers. Each layer was placed in a coating furnace, heated to 400"C and sprayed five times with the above diluted dispersion. A De Vilbiss Type CGA-50231 spray gun was used; the spraying-air pressure was 189 to 196 kPa and the working distance was 1 50 mm.

The first ten layers of blanket were weighed before and after coating and the total weight increase, representing the weight of the coating, was 4%.

Fabrication of Blanket and Tests The above coated layers were reconstituted as blanket and fabricated into insulation modules.

Example 2 Coating of Fibres A Bulk E Coarse Triton Kaowool ceramic fibre supplied by Morganite Cermaic Fibres Limited containing 45% alumina and made by fusion/blowing a mixture of alumina and silica was the starting material. 1 kg thereof was hand-shredded into small pieces and placed, in 709 batches, in a cylindrical mesh tumbler. The tumbler was placed in a coating furnace, heated to 400to and the fibre sprayed whilst being tumbled as described in Example 1.

Shrinkage Tests For comparison purposes, standard shrinkage tests were carried out on the above coated material and on the uncoated starting material. The results, shown below, indicate the improved resistance to shrinkage of the coated material.

MATERIAL DENSITY SHRINKAGE (kgm-3) (%) Uncoated 128 12 Coated 114 2.5 Example 3 Single aluminosilicate fibres were dipped into the undiluted dispersion described in Example 1, dried and calcined at 1000"C. The chromium(lll) oxide coating thereby produced was coherent and was approximately 0.3 ,um thick.

Claims

1. A method of coating aluminosilicate fibres with chromium(lll) oxide wherein the fibres are contacted with a dispersion of chromium(lll) oxide powder in a sol of chromium(lll) oxide, followed by calcining.

2. A method as claimed in claim 1 wherein the contacting step is carried out on fibres at an elevated temperature immediately after their formation.

3. A method as claimed in claim 1 or claim 2 wherein the coated fibres are formed into an artefact.

4. A method as claimed in claim 1 wherein the contacting step is carried out on fibres in the form of an artefact.

5. A method as claimed in any of the preceding claims wherein the dispersion contains a greater proportion by weight of non-colloidal chromium(lll) oxide than of colloidal chromium(lll) oxide.

6. A method as claimed in claim 5 wherein the ratio of colloidal to non-colloidal chromium (III) oxide is 8:1 by weight.

7. A method as claimed in any of the preceeding claims wherein the fibres have a diameter in the range of 5 to 1 5 ,um.

8. A method of coating aluminosilicate fibres with chromium(lll) oxide substantially as described herein with reference to any of the examples.

9. Aluminosilicate fibres coated with chromium(lll) oxide by a method as claimed in any of the preceding claims.