WO1997032118A1

WO1997032118A1 - Composite fibre products and processes for their production

Info

Publication number: WO1997032118A1
Application number: PCT/GB1997/000466
Authority: WO
Inventors: Stuart Samuel Boffey; Paul Martin Lawford Asher
Original assignee: Imperial Chemical Industries Plc
Priority date: 1996-02-27
Filing date: 1997-02-20
Publication date: 1997-09-04
Also published as: CN1216595A; CN1082610C; ES2146461T3; AU1885097A; EP0883736B1; DE69701796T2; DE69701796D1; EP0883736A1

Abstract

A composite fibre product is described which comprises a plurality of inorganic fibres and a binder which is substantially uniformly distributed throughout the fibre product. The composite fibre product has a laminar shear strength of at least 0.1 MPa. Also described is a process for the production of a composite fibre product which comprises impregnating a fibre mass comprising a plurality of inorganic fibres with a liquid binder system comprising a binder material and a carrier liquid and then subjecting the impregnated fibre mass to a heating step which involves the use of dielectric heating.

Description

COMPOSITE FIBRE PRODUCTS AND PROCESSES FOR THEIR PRODUCTION

The present invention relates to composite fibre products comprising a plurality of inorganic fibres which are bound together with a binder and to processes for their production. More particularly, the present invention relates to composite fibre mats which can be used for resiliently mounting the fragile ceramic or metal monoliths which are found in catalytic converters and diesel particulate filters in a surrounding metal casing.

Catalytic converters and diesel particulate filters are routinely fitted to automobiles and other road going vehicles in order to purify the exhaust gases which are generated by the engine. These devices usually comprise a ceramic honeycomb monolith which is housed within a metal casing and provides a support for the catalyst. The ceramic monolith comprises a plurality of tiny flow channels and is a fragile structure which is susceptible to damage when subjected to the kind of vibrational forces which prevail when any road going vehicle is in use. Moreover, the monolith and the surrounding metal casing are subjected to extremely high temperatures in use which causes them to expand, but not to the same extent. In consequence, the mounting system which is used to mount the ceramic monolith in its metal casing must insulate the monolith from the attendant vibrational forces and compensate for any difference between the expansion of the monolith and the casing. In this way, the stresses to which the monolith is subjected during use as a result of differential expansion or vibrational forces can be maintained at an acceptable level.

It is known to use a composite fibre mat to resiliently mount the fragile ceramic monolith which is found in catalytic converters and diesel particulate filters in a surrounding metal casing, see for example US-4,011,651 and WO-94/24425. The fibre mat is arranged in the annular space between the monolith and the surrounding metal casing and is maintained under compression in the annular space so exerting a radial pressure on the monolith and the casing which retains the monolith in place.

The present invention provides a composite fibre product comprising a plurality of inorganic fibres and a binder and a process for its production. The composite fibre product can take the form of a flexible mat which can be used to mount ceramic or metal monoliths found in catalytic converters and diesel particulate filters in their metal casings.

According to a first aspect of the present invention there is provided a composite fibre product, particularly a mat, which comprises a plurality of inorganic fibres and a binder which is substantially uniformly distributed throughout the fibre product, said composite fibre product having a laminar shear strength of at least 0.1 MPa.

The inorganic fibres may be any of the inorganic fibres known in the art. However, when the composite fibre product is a mat which is to be used for resiliently mounting the ceramic or metal monoliths contained in catalytic converters and diesel particulate filters, the fibres will need to be thermally stable (i.e. will not degrade) at the high operating temperatures prevailing in such devices. Typically, the fibres contained in composite fibre mats which are to be used in such mounting applications will be thermally stable at temperatures in excess of 700°C, preferably in excess of 800°C and more preferably in excess of 900°C.

Thermally stable inorganic fibres include ceramic fibres such as alumina, mullite, aluminosilicate, aluminoborosi1icate, zirconia and titania fibres as well as vitreous glass fibres. The preferred thermally stable inorganic fibres are polycrystalline inorganic fibres, particularly polycrystalline inorganic oxide fibres, such as alumina, mullite, aluminosilicate, aluminoborosi1icate, zirconia and titania fibres. Of these, alumina fibres, by which term we are also intending to include alumina fibres comprising a few weight % of silica added as a phase stabiliser, are particularly preferred. The fibres are preferably short staple fibres having a length in the range of from 1 to 10 cms and a mean diameter in the range of from 1 to 10 microns. Especially preferred alumina fibres are those sold in the form of a loosely bound, low density mat by Imperial Chemical Industries PLC under the trade name Saffil which are thermally stable at temperatures in excess of 1000°C.

The composite fibre products of the invention may comprise two or more different types of inorganic fibre. In this embodiment, the different fibre types may be intimately mixed or they may be segregated and arranged in definite patterns, e.g. in discrete layers.

The binder may be an inorganic material, but is preferably organic. Suitable organic binders are more particularly described in US-4,011,651 and WO-94/24425, the disclosures in which are incorporated herein by way of reference. Preferably the binder is an organic polymer.

One suitable binder is a copolymer based on n-butyl acrylate and acrylonitrile.

Preferred binders are those obtained on curing a curable polymer composition. Preferred examples of curable polymer compositions are those comprising a combination of an acrylic polymer and a cross-linking agent, particular an epoxy group containing cross-linking agent such as an epoxy resin. Curable polymer compositions of this type will typically comprise from 90.0 to 99.0 % by weight, preferably from 95.0 to 99.0 % by weight of the acrylic polymer and from 1.0 to 10.0 % by weight, preferably from 1.0 to 5.0 % by weight of the cross-linking agent. The acrylic polymer is suitably a homopolymer or copolymer comprising monomer units derived from at least one acrylic monomer selected from the C.__~ alkyl (C, . alkyl)acrylates, and in a preferred embodiment is a homopolymer or copolymer comprising monomer units derived from at least one acrylic monomer selected from the C alkyl (meth)acrylates, for example methyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. An especially preferred binder is that obtained on curing a composition comprising an acrylic polymer based on butyl acrylate and an epoxy resin cross-linking agent.

When a composite fibre mat of the invention is to be used for mounting a ceramic or metal monolith in a catalytic converter or in a diesel particulate filter, it is preferred to use an organic binder which will be substantially pyrolysed/burned out by the high temperatures to which the mat will be subjected in use. In addition, the organic binder is preferably one which will not lead to the generation of toxic emissions when it is pyrolysed/burned out and for this reason is preferably free of chlorine and nitrogen.

The binder contained in the composite fibre product of the present invention is substantially uniformly distributed throughout the fibre product. Preferably the distribution of the binder in the composite fibre product is such that the percentage by weight of binder in each 1 mm region of the product based on the total weight of the product in that region is within 40 % , more preferably within

30 % and particularly preferably within 20 % of the overall percentage by weight of binder in the product based on the total weight of the product. In an especially preferred embodiment, the distribution of the binder in the composite fibre product is such

3 that the percentage by weight of binder in each 1 mm region of the product based on the total weight of the product in that region is within 10 % of the overall percentage by weight of binder in the product based on the total weight of the product.

The thickness of the composite fibre product will depend on the intended end use for the product. However, when the product is a composite fibre mat for mounting a ceramic or metal monolith in a catalytic converter or in a diesel particulate filter, it will typically have a thickness in the range of from 3 to 15 mm, preferably in the range of from 5 to 12 mm and more preferably in the range of from 5 to 9 mm.

The loading of the binder in the composite fibre product will typically be in the range of from 2 to 15 % by weight and preferably in the range of from 5 to 15 % by weight based on the total weight of the product .

The composite fibre product of the invention typically has a

3 density in the range of from 30 to 700 kg/m , preferably in the range

3 of from 100 to 500 kg/m and more preferably in the range of from 100 to 350 kg/m³.

The composite fibre product of the invention has a laminar shear strength, by which is meant the force which has to be applied in order to bring about delamination of the product, of at least 0.1 MPa, preferably of at least 0.2 MPa and more preferably of at least 0.3 MPa. The laminar shear strength can be conveniently measured on an Instron or similar machine using a three point bend test. Preferably the composite fibre product is also capable of exerting a pressure of at least 1.0 kgf/cm² , more preferably in the range of from 1.5 to 4.0 kgf/cm² when a sample of the product having a thickness in the range of from 5 to 10 mm is compressed to a thickness of 3 mm between two plates and the binder removed.

According to a second aspect of the present invention there is provided a process for the production of a composite fibre product, particularly a mat, which comprises impregnating a fibre mass comprising a plurality of inorganic fibres with a liquid binder system comprising a binder material and a carrier liquid and subjecting the impregnated fibre mass to a drying step which involves heating the impregnated fibre mass so as to at least substantially remove the carrier liquid, characterised in that the impregnated fibre mass is held under compression during at least a part of the drying step.

According to a third aspect of the present invention there is provided a process for the production of a composite fibre product, particularly a mat, which comprises impregnating a fibre mass comprising a plurality of inorganic fibres with a liquid binder system comprising a binder material and a carrier liquid and subjecting the impregnated fibre mass to a heating step, characterised in that the impregnated fibre mass is held under compression during at least a part of the heating step.

According to a fourth aspect of the present invention there is provided a process for the production of a composite fibre product, particularly a mat, which comprises impregnating a fibre mass comprising a plurality of inorganic fibres with a liquid binder system comprising a binder material and a carrier liquid and subjecting the impregnated fibre mass to a heating step, characterised in that the heating step involves the use of dielectric heating, such as microwave or radio frequency heating. In this fourth aspect of the present invention, the impregnated fibre mass is preferably held under compression during at least a part of the heating step.

The fibre mass which is impregnated in the processes of the present invention may comprise a plurality of discrete fibres or it may take the form of a multi-fibre product in which the individual fibres are assembled into a low density mat or blanket which is loosely held together by fibre intertwining or perhaps more robustly consolidated by some other means such as weaving, knitting, stitching, needle-punching or vacuum packing. Preferably the fibre mass which is impregnated in the processes of the present invention is a multi-fibre product having a thickness in the range of from 10 to 60 mm, more preferably in the range of from 30 to 50 mm, and an area density in the range of from 0.2 to 2.0 kg/m², more preferably in the range of from 1.0 to 2.0 kg/m². The inorganic fibres and preferred inorganic fibres for use in the processes of the present invention are as described previously in connection with the composite fibre product.

The processes of the present invention may be used to prepare composite fibre products from two or more different types of inorganic fibre. In this embodiment, the different fibre types may be intimately mixed or they may be segregated and arranged in definite patterns, e.g. in discrete layers. The liquid binder system may comprise an inorganic binder material, but preferably comprises an organic binder material, such as a polymer, and an organic or aqueous carrier liquid which is able to dissolve or disperse the organic binder material. Suitable organic binder materials are more particularly described in US-4,011,651 and WO-94/24425, the disclosures in which are incorporated herein by way of reference, and include polymers as well as curable polymers or prepolymers which can be cured in situ on the impregnated fibre mass as part of the drying step or in a subsequent processing step. The polymer may be a material which allows for the recovery of fibres from waste product generated in a process of the present invention or in subsequent processing/finishing operations.

One suitable binder system comprises an aqueous dispersion of a copolymer based on n-butyi acrylate and acrylonitrile.

Preferred binder systems are those comprising a dispersion, preferably an aqueous dispersion, of a curable polymer composition, sometimes termed a resin or latex. Examples of preferred curable polymer compositions are those comprising a combination of an acrylic polymer and a cross-linking agent, particular an epoxy group containing cross-linking agent such as an epoxy resin. Curable polymer compositions of this type will typically comprise from 90.0 to 99.0 % by weight, preferably from 95.0 to 99.0 % by weight of the acrylic polymer and from 1.0 to 10.0 % by weight, preferably from 1.0 to 5.0 % by weight of the cross-linking agent. The acrylic polymer is suitably a homopolymer or copolymer comprising monomer units derived from at least one acrylic monomer selected from the C alkyl (C. . alkyl)acrylates, and in a preferred embodiment is a homopolymer or copolymer comprising monomer units derived from at least one acrylic monomer selected from the C alkyl (meth)acrylates, for example methyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. An especially preferred binder system is one comprising an aqueous dispersion of an acrylic polymer based on butyl acrylate and an epoxy resin cross-linking agent. When the liquid binder system is one comprising a curable polymer composition, it may also comprise a catalyst to accelerate the curing process.

It will be appreciated from the above, that by the term liquid binder system we are also intending to include binder systems which comprise dispersions or suspensions of finely divided solids in liquid vehicles.

The liquid binder system will typically comprise from 0.5 to 50.0 % by weight of the binder material and from 50.0 to 99.5 % by weight of the carrier liquid. Preferably the liquid binder system will comprise from 0.5 to 10.0 % by weight, more preferably from 1.0 to 5.0 % by weight of the binder material and from 90.0 to 99.5 % by weight, more preferably from 95.0 to 99.0 % by weight of the carrier liquid.

Various techniques may be used to impregnate the mass of inorganic fibres. For example, the individual fibres may be thoroughly dispersed in the liquid binder system and the resulting dispersion cast into sheets using a paper making process which involves removing excess carrier liquid, e.g. by vacuum. Alternatively, where the mass of inorganic fibres takes the form of a multi-fibre product in which the individual fibres are assembled into a low density mat or blanket, the mat or blanket may be simply immersed or soaked in the liquid binder system. Alternatively, the low density fibre mat may be sprayed with the liquid binder system.

Before the impregnated fibre mass is subjected to the drying/heating step, it will often be convenient to remove any excess carrier liquid. This can be achieved by pressing the impregnated fibre mass between rollers or plates, by placing it under vacuum or by centrifuging.

After the fibre mass has been impregnated with the liquid binder system, and generally following removal of any excess carrier liquid, the impregnated fibre mass is subjected to a drying/heating step. In the processes of the second and third aspects of the present invention and preferably also in the process of the fourth aspect of the present invention, at least a part of the drying/heating step is conducted while the impregnated fibre mass is held under compression.

The impregnated fibre mass should be held under compression until such time as the binder material is able to hold the fibres together and significantly limit the expansion of the composite fibre product once the compressive forces are released. Generally, the whole of the drying/heating step will be performed while the impregnated fibre mass is held under compression, but it may be possible to perform just the final stages of the drying/heating step in this manner and still obtain satisfactory results. During the drying/heating step, substantially all and preferably all of any residual carrier liquid will be removed.

The pressure which is applied during the drying/heating step to compress the impregnated fibre mass will generally be in the range of from 5 to 500 KPa, preferably in the range of from 5 to 200 KPa. In general, the pressure applied is such as to produce a composite fibre

3 product having a density in the range of from 30 to 700 kg/m , preferably in the range of from 100 to 500 kg/m , more preferably in

3 the range of from 100 to 350 kg/m . When the fibre mass which is impregnated is a multi-fibre product having a thickness in the range of from 10 to 60 mm, e.g. in the range of from 30 to 50 mm, and an area density in the range of from 0.2 to 2.0 kg/m², e.g. in the range of from 1.0 to 2.0 kg/m², the resulting impregnated fibre mass will generally be compressed to a thickness in the range of from 2 to 5 mm during the drying/heating step. This pressure is conveniently applied in a batch process by sandwiching the impregnated fibre mass between plates and then squeezing the plates together, e.g. by means of clamps, spring loaded clips or hydraulic presses. Alternatively, in a continuous process, it may be convenient to generate the necessary compressive forces on the impregnated fibre mass by feeding it through an arrangement of rollers or belts.

In the processes of the second and third aspects of the present invention, a conventional oven may be employed to carry out the drying/heating step, but in a preferred embodiment dielectric heating such as microwave or radio frequency heating is employed since it tends to result in an appreciably more uniform distribution of the binder in the final composite fibre product. In the processes of each of the second, third and fourth aspects of the present invention, it is preferred to use a combination of dielectric heating and a conventional heating means, such as a flow of hot air. Typically, the drying/heating step will involve heating the impregnated fibre mass to a temperature in the range of from 80 to 200°C, preferably in the range of from 100 to 170°C. Temperatures in the range of from 140 to 160°C are especially preferred.

When the liquid binder system comprises a curable polymer composition, as is preferred, the drying/heating step may be followed by a further processing step in which the curable polymer composition is cured. This curing process preferably involves the polymer composition undergoing some form of cross-linking reaction. However, the temperatures which are employed in the drying/heating step are usually sufficient to remove any residual carrier liquid and cure the curable polymer composition so that a separate curing step is generally unnecessary. When a separate curing step is employed, however, the impregnated fibre mass will generally be held under compression for the duration of the curing step.

The composite fibre product of the present invention may also contain one or more other materials. Suitable materials for inclusion in the composite fibre product include the layer minerals, particularly the expandable layer silicate minerals such as vermicul ite. The incorporation of another material into the composite fibre product may be achieved by adding the material to a liquid binder system used in the preparation of such a product. Alternatively, a composite fibre product prepared in accordance with the present processes may be post-treated with a solution or dispersion of the material to be incorporated.

Composite fibre products of the present invention may be used as mounting mats to mount ceramic and metal monoliths in catalytic converters and diesel particulate filters or to support the ceramic monoliths found in hot gas filtration units and coal gasification plants. Composite fibre products of the invention may also be usefully employed in gasket applications and as a high temperature insulation material.

The present invention is now illustrated but not limited with reference to the following examples.

Example 1

Samples of "Saffil" low density mat having a size of about 500 mm by 200 mm were cut from a bulk product having a known area density of from 1.2 to 1.6 kg/m² and a thickness of from 30 to 50 mm. These samples were weighed and then transferred to a tray where they were soaked in a latex (Acronal 35D, a 50 % aqueous dispersion of a copolymer based on n-butyl acrylate and acrylonitrile available from BASF) which had been diluted to a solids content of around 3 % w/w. The impregnated samples were then sandwiched between two sheets of PTFE-coated glass fibre mesh and these sandwiches were then placed between two sheets of glass fibre filled silicone resin board of size 500 mm by 200 mm by 12 mm. The resin boards were then pressed together using G-clamps until the impregnated "Saffil" layers were reduced to a thickness of about 5 mm (equal to an applied pressure of about 0.5 bar (50 KPa)), and held in this position with clips. During this assembly excess latex drained from the samples.

The completed mould assemblies were then placed on the belt of an air/radio frequency (RF) assisted oven and the belt speed was adjusted to give a residence time of between 15 and 45 minutes. The RF power to the oven was set at about 5.5 KW and the temperature of the air in the oven was adjusted to about 150°C. The samples were removed from the oven when the latex had been fully dried and cured (cross-linked). The clips and the boards were then carefully removed from the samples and the PTFE mesh peeled off to reveal the final composite fibre mats which had a thickness in the range of from 7 to 8.5 mm. Example 2

Samples of "Saffil" low density mat having a size of about 500 mm by 200 mm were cut from a bulk product having a known area density of from 1.0 to 2.0 kg/m² and a thickness of from 30 to 50 mm. These samples were weighed and then transferred to a tray where they were soaked in a latex (60 % aqueous dispersion of a butyl acrylate based polymer containing 3 % w/w of Epikote (TM) 828 epoxy resin cross-linking agent) which had been diluted to a solids content of around 5 % w/w. The impregnated samples were then sandwiched between two sheets of PTFE-coated glass fibre mesh and these sandwiches were then placed between two sheets of glass fibre filled silicone resin board of size 500 mm by 200 mm by 12 mm. The resin boards were then pressed together using G-clamps until the impregnated "Saffil" layers were reduced to a thickness of about 5 mm (equal to an applied pressure of about 0.5 bar (50 KPa)), and held in this position with clips. During this assembly excess latex drained from the samples.

The completed mould assemblies were then placed on the belt of an air/radio frequency (RF) assisted oven and the belt speed was adjusted to give a residence time of between 15 and 45 minutes. The RF power to the oven was set at about 5.5 KW and the temperature of the air in the oven was adjusted to about 150^*C. The samples were removed from the oven when the latex had been fully dried and cured (cross-linked). The clips and the boards were then carefully removed from the samples and the PTFE mesh peeled off to reveal the final composite fibre mats which had a thickness in the range of from 7 to 8.5 mm.

Claims

Claims :

1. A composite fibre product which comprises a plurality of inorganic fibres and a binder which is substantially uniformly distributed throughout the fibre product, said composite fibre product having a laminar shear strength of at least 0.1 MPa.

2. A composite fibre product as claimed in claim 1 wherein the inorganic fibres are thermally stable at temperatures in excess of 700"C.

3. A composite fibre product as claimed in claim 1 or claim 2 wherein the inorganic fibres are ceramic fibres.

4. A composite fibre product as claimed in claim 2 or claim 3 wherein the inorganic fibres are polycrystalline inorganic oxide fibres selected from the group consisting of alumina fibres, mullite fibres, aluminosilicate fibres, aluminoborosi 1icate fibres, zirconia fibres and titania fibres.

5. A composite fibre product as claimed in claim 4 wherein the inorganic fibres are alumina fibres.

6. A composite fibre product as claimed in any one of claims 1 to 5 wherein the inorganic fibres are short staple fibres having a length in the range of from 1 to 10 cms and a mean diameter in the range of from 1 to 10 microns.

7. A composite fibre product as claimed in any one of claims 1 to 6 wherein the binder is an organic material.

8. A composite fibre product as claimed in claim 7 wherein the binder is an organic polymer.

9. A composite fibre product as claimed in claim 7 or claim 8 wherein the binder is a material derived from curing a curable polymer composition comprising an acrylic polymer and an epoxy group containing cross-linking agent.

10. A composite fibre product as claimed in claim 9 wherein the acrylic polymer is a homopolymer or copolymer comprising monomer units derived from at least one acrylic monomer selected from the C, alkyl (meth)acrylates.

11. A composite fibre product as claimed in claim 9 wherein the binder is a material derived from curing a curable polymer composition comprising an acrylic polymer based on butyl acrylate and an epoxy resin cross-linking agent.

12. A composite fibre product as claimed in any one of the preceding claims wherein the distribution of the binder in the composite fibre product is such that the percentage by weight of binder in each 1 mm region of the product based on the total weight of the product in that region is within 40 % of the overall percentage by weight of binder in the product based on the total weight of the product.

13. A composite fibre product as claimed in claim 12 wherein the distribution of the binder in the composite fibre product is such

3 that the percentage by weight of binder in each 1 mm region of the product based on the total weight of the product in that region is within 30 % of the overall percentage by weight of binder in the product based on the total weight of the product.

14. A composite fibre product as claimed in any one of the preceding claims which has a thickness in the range of from 3 to 15 mm.

15. A composite fibre product as claimed in any one of the preceding claims wherein the loading of the binder in the product is in the range of from 2 to 15 % by weight based on the total weight of the product .

16. A composite fibre product as claimed in any one of the preceding

3 claims which has a density in the range of from 30 to 700 kg/m .

17. A composite fibre product as claimed in claim 16 which has a

3 density in the range of from 100 to 500 kg/m .

18. A composite fibre product as claimed in any one of the preceding claims which has a laminar shear strength of at least 0.2 MPa.

19. A composite fibre product as claimed in any one of the preceding claims which is capable of exerting a pressure of at least 1.0 kgf/cm² when a sample of the product having a thickness in the range of from 5 to 10 mm is compressed to a thickness of 3 mm between two plates and the binder removed. - 15 -

20. A composite fibre product as claimed in claim 19 which is capable of exerting a pressure in the range of from 1.5 to 4.0 kgf/cm² when a sample of the product having a thickness in the range of from 5 to 10 mm is compressed to a thickness of 3 mm between two plates and the binder removed.

21. A process for the production of a composite fibre product which comprises impregnating a fibre mass comprising a plurality of inorganic fibres with a liquid binder system comprising a binder material and a carrier liquid and subjecting the impregnated fibre mass to a heating step, characterised in that the heating step involves the use of dielectric heating.

22. A process as claimed in claim 21 wherein the impregnated fibre mass is held under compression during at least a part of the heating step.

23. A process for the production of a composite fibre product which comprises impregnating a fibre mass comprising a plurality of inorganic fibres with a liquid binder system comprising a binder material and a carrier liquid and subjecting the impregnated fibre mass to a drying step which involves heating the impregnated fibre mass so as to at least substantially remove the carrier liquid, characterised in that the impregnated fibre mass is held under compression during at least a part of the drying step.

24. A process for the production of a composite fibre product which comprises impregnating a fibre mass comprising a plurality of inorganic fibres with a liquid binder system comprising a binder material and a carrier liquid and subjecting the impregnated fibre mass to a heating step, characterised in that the impregnated fibre mass is held under compression during at least a part of the heating step.

25. A process as claimed in any one of claims 21 to 24 wherein the fibre mass which is impregnated is a multi-fibre product in which the individual fibres are assembled into a low density mat or blanket.

26. A process as claimed in claim 25 wherein the multi-fibre product has a thickness in the range of from 10 to 60 mm and an area density in the range of from 0.2 to 2.0 kg/m².

27. A process as claimed in any one of claims 21 to 26 wherein the inorganic fibres are thermally stable at temperatures in excess of 700°C.

28. A process as claimed in claim 27 wherein the inorganic fibres are ceramic fibres.

29. A process as claimed in claim 27 or claim 28 wherein the inorganic fibres are polycrystalline inorganic oxide fibres selected from the group consisting of alumina fibres, mullite fibres, aluminosilicate fibres, aluminoborosi1icate fibres, zirconia fibres and titania fibres.

30. A process as claimed in claim 29 wherein the inorganic fibres are alumina fibres.

31. A process as claimed in any one of claims 21 to 30 wherein the inorganic fibres are short staple fibres having a length in the range of from 1 to 10 cms and a mean diameter in the range of from 1 to 10 microns.

32. A process as claimed in any one of claims 21 to 31 wherein the liquid binder system comprises an organic binder material and an organic or aqueous carrier liquid which is able to dissolve or disperse the organic binder material.

33. A process as claimed in claim 32 wherein the organic binder material is a polymer.

34. A process as claimed in claim 32 or claim 33 wherein the liquid binder system comprises a dispersion of a curable polymer composition comprising an acrylic polymer and an epoxy group containing cross-linking agent.

35. A process as claimed in claim 34 wherein the acrylic polymer is a homopolymer or copolymer comprising monomer units derived from at least one acrylic monomer selected from the C. alkyl (meth)acrylates. - 17 -

36. A process as claimed in claim 34 wherein the liquid binder system comprises a dispersion of a curable polymer composition comprising an acrylic polymer based on butyl acrylate and an epoxy resin cross-linking agent.

37. A process as claimed in any one of claims 32 to 36 wherein the dispersion is an aqueous dispersion.

38. A process as claimed in any one of claims 21 to 37 wherein the liquid binder system comprises from 0.5 to 50.0 % by weight of the binder material and from 50.0 to 99.5 % by weight of the carrier liquid.

39. A process as claimed in claim 38 wherein the liquid binder system comprises from 1.0 to 5.0 % by weight of the binder material and from 95.0 to 99.0 % by weight of the carrier liquid.

40. A process as claimed in any one of claims 22 to 24 wherein the whole of the drying/heating step is performed while the impregnated fibre mass is held under compression.

41. A process as claimed in any one of claims 22 to 24 wherein the pressure which is applied during the drying/heating step to compress the impregnated fibre mass is in the range of from 5 to 500 KPa.

42. A process as claimed in claim 41 wherein the pressure which is applied during the drying/heating step to compress the impregnated fibre mass is in the range of from 5 to 200 KPa.

43. A process as claimed in claim 23 or claim 24 wherein dielectric heating is employed in the drying/heating step.

44. A process as claimed in any one of claims 21, 22 or 43 wherein the dielectric heating means is microwave or radio frequency heating.

45. A process as claimed in any one of claims 21, 22, 43 or 44 wherein a combination of dielectric heating and conventional heating is employed in the drying/heating step.

46. A process as claimed in any one of claims 21 to 45 wherein the impregnated fibre mass is heated to a temperature in the range of from 80 to 200^*C in the drying/heating step.

47. A process as claimed in claim 46 wherein the impregnated fibre mass is heated to a temperature in the range of from 100 to 170°C in the drying/heating step.