GB2047297A - Mineral-fibre boards - Google Patents

Abstract

A handleable, low-density, foam- like fibrous material comprises inorganic fibres and a binding agent which is a phosphate, a silica sol or an aluminium chlorohydrate binding agent. A wet-lay process for the production of such materials comprises forming an aqueous suspension of inorganic fibres, incorporating the binding agent in the suspension, and removing water from the suspension. Fibrous materials of density below 100 Kg/m<3>, preferably below 50 Kg/m<3> are provided containing less than 10% by weight based on the material of the binding agent.

Description

SPECIFICATION Low-density fibrous materials This invention relates to low-density fibrous materials and in particular to low-density materials comprising inorganic fibres, particularly polycrystalline inorganic oxide fibres.

Inorganic fibres are used extensively in a wide variety of applications and usually they are employed in the form of a non-woven structure with or without binding agents. A common method of forming the fibres into the desired structures is the so-called "wet-lay" process wherein the fibres are suspended in a liquid medium, usually an aqueous medium, the resulting suspension or slurry is cast into the desired shape and the liquid medium is removed by drainage or evaporation or both. Substrates by or against which the slurry is shaped are frequently water-permeable to permit drainage of water from the shaped slurry and a reduced pressure is often applied externally of such substrates to assist in the removal of water through the substrate.

The wet-lay process is particularly suitable for forming structures from short staple fibres. Upon removal of the liquid medium from the slurry, the fibres do not pack closely together and there is produced a low density material wherein the fibres are loosely packed and lie in essentially parallel planes, usually horizontally, although they are randomly oriented in those planes.

The low-density materials so produced lack structural integrity and are little more than a mass of loose fibres. They cannot be picked up and handled in fabrication processes. Consequently it is customary practice to incorporate an additive in the suspension such that upon removal of water the resulting fibrous material has structural integrity and can be picked up and handled. For producing lowdensity felts a polymer latex is usually employed as additive and fibrous materials of density in the range of from 100 to about 1 60 kg/m3 are produced. Strong, rigid materials known as and resembling boards are produced using starch/alumina mixtures as additive to provide high-density materials of density usually greater than 1 60 kg/m3.

We have now found that if a selected binding agent is incorporated in the slurry prior to removal of the liquid medium, then upon removal of the liquid medium there is obtained a fibrous material which exhibits structural integrity and is handleable. As in the case of low-density materials produced in the absence of an additive, the material comprises a relatively open fibrous configuration of greater voidage than conventional fibrous structures, and is of relatively low density, for example below 100 kg/m3.

According to the present invention there is provided a handleable low-density fibrous material comprising inorganic fibres and a binding agent which is a phosphate, a silica sol or an aluminium chlorohydrate binding agent.

the low-density fibrous materials may be made by a wet-lay process and according to a further feature of the invention there is provided a process for the production of a low-density fibrous material which comprises forming an aqueous suspension of inorganic fibres, incorporating a binding agent in the aqueous suspension which is a phosphate, a silica sol or an aluminium chlorohydrate binding agent and removing water from the suspension.

Removal of water from the suspension results in a low-density material wherein the fibres occupy a loosely-packed configuration as they did in the suspension before removal of the water. Although not possessing the true cellular structure associated with foams, the fibrous material bears resemblance to a foam and can rightly be regarded as a foam-like material. It exhibits the lightweight and thermal insulation characteristics of true foams. It may be used in any application where a light-weight fibrous material is desirable; it is especially useful in thermal insulation applications.

Using the process of the present invention, handleable fibrous materials of structural integrity and of density below 100 and even below 50 kg/m3 may readily be produced using a small amount, for example less than 10% by weight based on the fibres of the binding agent. By comparison, as discussed hereinbefore, materials made from the same fibres by the same wet-lay process but using a conventional additive, e.g. a latex not containing the selected binding agent, typically have a density of the order of 100 to 1 60 kg/m3.

Fibrous materials of density as low as 50 kg/m3 are readily obtained by the process, and the incorporation in the suspension of a foaming agent enables materials of even lower density, for example as low as 25 or 30 kg/m3 to be produced.

The fibrous materials comprise inorganic fibres, preferable short staple inorganic fibres, and the fibres may be made of any inorganic material for which the specified binding agents exhibit afl affinity.

Examples of suitable inorganic fibres are glass fibres, metal fibres and especially metal oxide fibres such as alumina fibres, alumino-silicate fibres, zirconia fibres and iron oxide fibres. We especially prefer fibrous materials comprising alumina fibres. Fibres of different materials may be employed if desired, for example mixtures of polycrystalline alumina fibres and aluminosilicate fibres, or layered materials comprising one or more layers of polycrystalline alumina fibres and one or more layers of aluminosilicate fibres. Preferably the inorganic fibres are short staple fibres and especially preferred are fibres of length in the range of from 5 to 10 mm. The diameter of the fibres may vary widely, for example from 2 microns to 20 microns.

The fibrous material may if desired contain materials other than the inorganic fibres and the selected binding agent. Thus, for example, the material may contain organic fibres (which may be natural or man-made) provided that the proportion of inorganic fibres is sufficient to provide the open fibrous structure described and required for the achievement of low-density materials.

Similarly, the fibrous material may contain organic or inorganic fillers and strength-improving agents which may be, for example, in particulate form. By way of example, the material may and preferably does contain powdered inorganic fillers such as boehmite (hydrated alumina) or Kaolin (China clay) to improve its compressive strength. These and other additional materials are readily incorporated in the fibrous materials by incorporating them in the aqueous suspension from which the material is formed.

As described hereinbefore, the fibres in the fibrous material may be all of the same type or they may be mixtures of different types of fibres. It will be appreciated that the nature of the fibres may effect the choice of particulate filler or compressive strength improver used since a filler which provides optimum compressive strength improvement with one type of fibres may not necessarily be the best filler for use with other types of fibres. For example we have observed that powdered Boehmite produces a greater improvement in the compressive strength of materials made from aluminosilicate fibres ("Kaowool") than is achieved in materials made from alumina fibres ("Saffil") whilst powdered Kaolin (China clay) leads to a similar improvement in both aluminosilicate fibre materials and alumina fibre materials.It is believed, though we do not wish to be bound by any particular theory, that this is due to the difference in charge of the fibres in water, aluminosilicate fibres being negatively charged and alumina fibres being positively charged whilst Boehmite is positively charged and Kaolin is negatively charged and aluminium chiorohydrate binding agents are positively charged as described hereinafter.

Irrespective of theory, however, we prefer to use negatively-charged additives for making fibrous materials containing alumina fibres.

The amounts in the aqueous suspension of fibres, binding agent and particulate additives for improving the compressive strength of the resulting fibrous materials may vary within wide limits. The amount of fibres in the suspension can be any amount used in conventional wet-iay processes, though in order to provide high voidage in the resulting fibrous material the amount of fibres will normally be low, for example less than 5% by weight of the suspension and typically about 1.5% to 2% by weight of the suspension. The binding agent may be present in a large excess since any binding agent solution which drains from the fibrous material during its formation can be re-used to prepare a fresh fibre suspension, if required afte.r the addition of further binding agent.Normally the amount of the binding agent in the suspension will be from 5% to 20% by weight based on the weight of fibres in the suspension. The amount of particulate additive is not critical and can be chosen to suit the desired density of the resulting fibrous material; in general increasing the amount of the additive increases the density of the resulting fibrous material. As a guide only, amounts of additive of from 10% to 50% by weight based on the fibre content of the suspension will be sufficient for most practical purposes.

Removal of water from the suspension will normally be effected by drainage of free water followed by evaporation of residual water from the fibrous materials, for example by heating the materials at e.g.

100 to 1 500C to dry them. The wet material after removal of excess water has a degree of structural integrity and may be handleable to the extent of being removable from the vessel or surface in or on which it is formed and transferrable to an oven for drying. It may be advantageous to incorporate in the suspension a material, for example an organic polymer, which increases the wet or green strength of the fibrous material prior to drying it but which can subsequently be removed by heating the dried material at an elevated temperature. The material may be heated to remove such temporary aditives at any temperature up to the decomposition temperature of the inorganic fibres.For example materials made of polycrystalline alumina fibres can be heated at temperatures up to 1 5000C. Most carbonaceous materials may be removed by heating at about 8000 C.

As discussed hereinbefore, one or more foaming agents may be incorporated in the aqueous suspension to facilitate the production of very low density fibrous materials, e.g. below 50 kg/m3. Any of the very large number of conventional foaming agents or surfactants known for aqueous systems may be used providing they are inert to the inorganic fibres and the selected binding agent and to any other components of the aqueous suspension. Hydrocarbon surfactants or fluorochemical surfactants may be employed and they may be selected such as to be removed from or remain in the fibrous material during subsequent heat-treatment operations.Typical of the surfactants which may be used are long-chain alkyl sulphates (e.g. alkyl chains of 1 5 to 40 carbon atoms) and fatty acid esters, and aikylene oxide polymers and copolymers for example copolymers of ethylene oxide and propylene oxide.

The binding agent used in the present invention is aluminium chlorohydrate, Al2(0H)5Cl.2H20.

Aluminium chlorohydrate is commercially available in the form of a powdered solid or as a 50% solution in water in which it is readily soluble at room temperature. It is used extensively as a binding agent for aluminosilicate refractory compositions. When the compound is dissolved in water, aluminium chlorohydrate is present in the form of colloidal polymeric cations and chloride ions so that it behaves like a positively-charged colloid species.

By the term "phosphate binding agent" as used throughout this specification there is meant any substance which in solution in water provides phosphate ions and includes complex phosphate salts as well as simple phosphate salts and phosphoric acid. Thus, for example, the binding agent may be ammonium phosphate or a simple metal phosphate, e.g. aluminium phosphate or a complex metal phosphate containing halogen atoms, for example aluminium chlorophosphate. A particularly preferred phosphate binding agent for alumina or alumino-silicate fibres is a hydrated complex aluminium phosphate containing chlorine atoms available under the trade name "Winnofos" from Imperial Chemical Industries Limited.

The aqueous suspension may be formed into any desired shape prior to removal of the water, to form shaped low-density fibrous materials. Examples of shaped materials which can be produced are pads, blocks and boards although complex shapes are attainable using suitably designed moulds.

Preferably the surfaces of the mould used to shape the suspension are porous to water to allow free water to drain from the suspension. Preferably the mould is made of a material for which the binding agent has no great affinity which might otherwise interfere with removal of the shaped fibrous product from the mould. As in conventional wet-lay processes, reduced pressure may be created externally of the mould surfaces to assist removal of excess water from the suspension by drainage through the mould surfaces.

Shaped fibrous material, e.g. block or pads, comprising layers of different fibres are readily made by the wet-lay process herein described. To form such a composite material, a suspension of one type of fibres can be laid down and the water removed by drainage, and then a suspension of another type of fibres can be laid over the first layer. In this way multi-layer materials can be produced. For example there can be formed a material comprising relatively inexpensive aluminosilicate fibres ("Kaowool") "faced" with more expensive polycrystalline alumina fibres ("Saffil").

By a similar technique, composite materials comprising one or more layers of the fibrous material bonded to a non-fibrous substrate, such as fire-brick, vermiculite board or cementitious material, can be produced by applying the suspension to any porous substrate to which the binding agent will adhere and allowing the water to drain from the suspension into the substrate.

Fixing devices for attaching the fibrous materials to surfaces, for example to the internal walls of furnaces and kilns, can readily be incorporated during the formation of the materials by the technique of adding further layers of fibres to an existing layer whilst the latter is still wet. Thus for example, a low density fibrous block having integral fixing devices in the form of threaded bolts or lugs can be produced by laying down a layer of fibres and allowing the water to drain from the layer, locating the fixing devices with their anchors in or on this layer and whilst the first layer of fibres is still wet laying down another layer of fibres over the first layer so that the anchors of the fixing devices are located in the interior of the resulting fibrous block.

Amongst the major uses of the low-density fibrous materials of the invention is the thermal insulation of furnaces and kilns such as those employed in the ceramics industries. In such uses, the fibrous material may be subjected to chemicals used to apply glaze finishes to the ceramics which cause damage to the fibrous material. For this reason, fibrous materials used as linings for kilns are sometimes coated with a refractory wash. The fibrous materials of the invention may be so treated, but we prefer to incorporate a glaze-resistant substance such as a refractory wash in the fibrous material itself or at least in the working surface layer of the material.Thus, as described hereinbefore, a material laid down by the wet-lay process can be coated whilst still wet with a thin or "facing" layer of a suspension of fibres in which a refractory wash has been incorporated so providing a working face resistant to glazes.

Alternatively the fibrous material may be provided with a working surface of a resistant substance such as fire-brick by the procedure described herein for making composite articles.

The invention is illustrated but in no way iimited by the following Examples: EXAMPLE 1 Loose, staple, polycrystalline "Saffil" alumina fibres (140 g) of average length 8 mm and average diameter 3 microns were added to water (10 I) in a plastic container. Aluminium chlorohydrate solution (150 ml, 23.5% Al203) and hydrated alumina powder (37.5 g "Cerasol" available from the British Aluminium Company) were mixed together in a Kenwood mixer for 2 minutes after which the mixture was added to the fibres and water in the plastic container. The resulting mixture was agitated for 1 minute using a Silverson mixer to form an aqueous suspension of the fibres.

The suspension was poured into a perforated brass mould of dimensions 20 cm x 20 cm square and 10 cm height and as liquid drained from the mould, the fibrous mass was compressed to a depth of 6 cm using a lightweight polystyrene square (20 cm x 20 cm). The fibrous mass was immediately removed from the mould by inverting the mould with the polystyrene square still in position and lifting the mould from the fibrous mass, leaving the mass on the polystyrene square. A square of cloth of open weave made from polytetrafluoroethylene-coated glass fibres was placed over the fibre block, followed by a perforated stainless steel plate. The block assembly was turned over such that it sat on the steel plate and the assembly was placed in an oven and heated at 130 to 1 400C for 12 hours to dry it.

The assembly was removed from the oven and allowed to cool and the fibre block was separated.

The block had a density of 67 kg/m3. The block was heated at 1 4000C for 24 hours after which treatment it was observed that linear shrinkage of the block was 2.35%.

EXAMPLE 2 Using the procedure described in Example 1, a fibrous block comprising a 70:30 mixture of staple polycrystalline "Saffil" alumina fibres (70%) and staple "Kaowool" aluminosilicate fibres (30%) was produced from the following materials: "Saffil" alumina fibres 1159 (av. length 8 mm: av. diam 3 microns) "Kaowool" aluminosilicate fibre 50 g (av. length 20 mm: av diam 3 microns) Hydrated Alumina powder 40g ("Cerasol") Aluminium Chlorohydrate solution 163 ml (23.5% Al203) Water 10.6 litres After drying at 130-1 400C in an oven as described in Example 1, the mixed-fibre block was heated at 1 4000C for 24 hours after which time the linear shrinkage of the block had been 2.5%.

EXAMPLE 3 Using the procedure described in Example 1, a fibre block comprising a 50:50 mixture of staple polycrystalline "Saffil" alumina fibres (5 /0) and staple "Kaowool" aluminosilicate fibres (50%) was produced from the following materials.

"Saffil" alumina fibres 93 g (av. length 8 mm: av diam micron) "Kaowool" aluminosilicate fibres 93 g (av. length 20 mm: av diam 3 microns) Hydrated alumina ("Cerasol") 42.5 g Aluminium chlorohydrate solution 174 ml (65% Al203) Water 11 litres The dry, mixed-fibre block had a density of 91.7% Kg/m3 and its linear shrinkage after heating at 1400"C for 24 hours was 3.35%.

EXAMPLE 4 Using the procedure described in Example 1, a fibre block comprising a 35 :65 mixture of "Saffil" alumina fibres (35%) and "Kaowool" aluminosilicate fibres (65%) was produced from the following materials: "Saffil" alumina fibres 64g "Kaowool" aluminosilicate fibres 150 g Hydrated alumina ("Cerasol") 45.6 g Aluminium chlorohydrate solution 190 ml (23.5% Al203) Water 11.6 litres The dry, mixed-fibre block had a density of 104 Kg/m3 and its linear shrinkage upon heating at 1 4000C for 24 hours was 2.14%.

EXAMPLE 5 Using the procedure described in Example 1 a fibre block was produced from the following materials: "Saffil" alumina fibres 140 g Aluminium chlorohydrate solution 150 ml Kaolin* 40g Water 10 litres *Grade Supreme available from British China Clays.

The dry block had a density of 85 Kg/m3 and the linear shrinkage noted after heating the block at 1 4000C for 24 hours was 2.14%.

Fibre blocks produced as above were heated at temperatures up to 1 4000C and their thermal conductivities were determined. Thermal conductivity was measured by the hot wire method described in ASTM special publication 660 which is based upon the German draft standard DIN 51046.

Temperature (OC) K-value watt/m-k 25 0.046 200 0.070 400 0.095 600 0.130 800 0.170 1000 0.215 1200 0.280 1400 0.290 EXAMPLE 6 Using the mixing procedure described in Example 1, two fibre suspensions were produced to the formulations: Suspension I Suspension II "Saffil" alumina fibres 300 g "Kaowool" aluminosilicate fibres 1060 g Al. chlorohydrate solution 325 ml 1150 ml Kaolin 80 g 280 g Water 20 litre 20 litre Suspension I was poured into a perforated brass mould of dimensions 30 cm x 30 cm x 25 cm height and liquid was allowed to drain from the mould for 1 minute.The surface of the fibrous mass in the mould was levelled (depth 5 cm) and then Suspension II was added to the mould on top of the fibrous mass, Suspension II being poured onto a polystyrene square held at an angle so that the suspension was deposited without greatly disturbing the surface of the fibrous mass in the mould.

The resulting fibrous mass in the mould, (total 10 cm) was removed from the mould and dried as described in Example 1, drying being at 130-1 400C for 12 hours.

EXAMPLE 7 A fibre block comprising a layer (5 cm thick) of "Saffil" alumina fibres and a layer (5 cm thick) of "Kaowool" aluminosilicate fibres and provided with a protroduding bolt for attaching the block to a furnace wall was produced by the procedure described in Example 7, except that prior to adding Suspension II to the mould there was applied over the wet fibre mass in the mould (i.e. the mass deposited from Suspension I) a square (15 cm x 15 cm) piece of stainless steel expamet mesh to the centre of which had been welded a threaded bolt of length 5 cm, the bolt pointing upwards. The layer of aluminosilicate fibres was laid down and the fibre mass was removed from the mould and dried as is described in Example 1.

Eight dry blocks, each with a protruding bolt, were made and these were assembled by means of the bolts on the walls of a Shelley furnace with "Kaowool" cement between adjacent blocks. The furnace was heated at 140DOC for three separate 12 hour periods. At the end of this test the wall was examined and both the blocks and the wall surface they created were found to be completely satisfactory.

EXAMPLE 8 This example describes the production of blocks comprising the basic two-layer fibre block fitted with fixing bolt described in Example 8 to which has been added one or more non-fibrous layers to form graded temperature furnace building blocks.

A The procedure described in Example 8 was repeated, except that prior to drying of the wet fibre block a layer (thickness 2.5 cm and dimensions 30 cm x 30 m) of vermiculite board was applied over the surface of the wet aluminosilicate fibre layer, the vermiculite board having a hole bored at its centre through which protruded the fixing bolt embedded in the wet fibre block. The vermiculite board was made by cementing together heat-exfoliated vermiculite granules and is available under the trade name "Vicuclad". The board was pressed lightly onto the wet fibre block and the assembly was dried at 130--1400C for 12 hours.The dry composite block consisted in the thickness direction of a face layer of "Saffil" alumina fibres, an intermediate layer of "Kaowool" aluminosilicate fibres and a backing layer of "Vicuclad" board, the layers being firmly bonded together and the composite block being provided with a fixing bolt.

B A composite block was made as described in A but using a 1 cm thick block of firebrick instead of the "Vicuclad" board. The layer of firebrick became firmly bonded to the layer of aluminosilicate fibres.

C The procedure described in Example 8 was repeated, except that prior to drying the fibre block a layer (3 mm thick) made of firebrick was pressed lightly onto the layer of "Saffil" alumina fibres.

The block of firebrick became firmly bonded to the layer of "Saffil" alumina fibres to provide a composite block consisting in the thickness direction of a facing layer of firebrick, an intermediate insulation layer of high temeprature "Saffil" alumina fibres and a backing layer of aluminosilicate fibres. The composite block is useful for lining furnaces wherein a glazing treatment is carried out, the firebrick facing layer being resistant to refractory glazes.

D Using a combination of procedures A and C, a composite block was made consisting in the thickness direction of a facing layer of firebrick, a layer of "Saffil" alumina fibres, a layer of aluminosilicate fibres and a backing layer of "Vicuclad" board, the block being provided with a threaded fixing bolt.

E Using procedure A a composite block was produced having as backing layer a "Saffil" alumina fibre board instead of the "Vicuclad" board. The board is a commercially-available material consisting of "Saffil" alumina fibres bonded together in a dense material with an alumina-based refractory binder.

EXAMPLE 9 Suspension: "Safil" alumina fibre 160 g "Winnofos" binding agent 180 g Foaming Agent 20 ml Water 151 A complex aluminium phosphate binding agent (180 g) available under the trade name "Winnofos" from Imperial Chemical Industries Limited was dissolved in water (500 ml) in a Kenwood mixer. The solution was transferred to a 30 litre capacity vessel containing water (14 litres).

A solution of a foaming agent (20 ml) in water (500 ml) was then prepared in the Kenwood mixer and added to the solution of the phosphate binding agent so that the vessel contained 1 5 1 of aqueous solution. The foaming agent was a 50:50 mixture of an ethylene oxide/propylene oxide copolymer ('Mololan' P100) and a long chain alkyl sulphate ('Empicol' TCR).

Loose, staple, polycrystalline "Saffil" alumina fibre of average length 8 mm and average diameter 3 microns (160 g) was added to the solution which was then stirred rapidly with a Silverson mixer for 2 minutes to provide an aqueous suspension of the fibres. "Saffil" is a Registered Trade Mark of Imperial Chemical Industries Limited.

The suspension was transferred to a 12-inch diameter circular paper - or board -- making mould (6 mesh) mould made of wire to allow drainage of water from the suspension. Provision was present for reducing the pressure externally of the mould to assist removal of water from the suspension. After removal of the free water from the mould by drainage, the resulting circular fibrous pad was removed from the mould and dried by heating it at 1 500C in an oven.

The dried fibrous pad was of thickness 7.9 cm and weight 1 82 g. its density was 33.1 kg/m3.

Thermal conductivity tests on a sample of the pad using a Tinsley probe indicated a thermal conductivity of 0.025 W/mK. A 3-inch square block was cut from the dry pad and heated at 1 400OC in an oven for 2 hours. The degree of shrinkage of the block after this treatment was found to be less than 1%.

EXAMPLE 10 Suspension: "Saffil" alumina fibre 1 60 g Phosphoric Acid 150 ml Powdered boehmite 67 g Water 161 Powdered behmite (67 g) available under the trade name 'Cerasoi' and concentrated phosphoric acid (150 ml) were mixed with water (1 litre) in a Kenwood mixer and the mixture was transferred to a 30 litre capacity vessel containing water (15 I). "Saffil" alumina fibre (160 g) as described in Example 9 was added to the vessel and the resulting mixture was stirred rapidly for 30 seconds using a Silverson mixer to form an aqueous suspension of the fibres.

The suspension was transferred to a 12-inch wire mould as described in Example 9. The resulting fibrous pad was dried at 1 500C and then heated at 11 000C for 2 hours.

The pad had a density of 58 kg/m3 and its thermal conductivity measured using a Tinsley probe was found to be 0.037 W/mK. Shrinkage tests (as in Example 9) on two 3-inch square blocks cut from two further pads made by the process and heated at 14000C for 24 hours showed shrinkages of 1.15% and 1.32%.

Thermal conductivity tests at various temperatures up to 1 4000C were performed by the hot wire method on a further pad made by the process The results are shown below. For purposes of comparison, results are also shown below of hot-wire thermal conductivity tests carried out on a 'Saffil' alumina fibre board made from alumina fibres by a wet-lay process (excluding a phosphate binding agent) and containing powdered boehmite. The board had a density of 1 92 kg/m3.

Thermal Conductivity (W/mK) Temperature Low Density Mat Board 25 0.054 0.083 90 0.073 200 0.101 300 0.090 550 0.149 600 0.157 1000 0,289 0.244 1400 0;358 0.379 EXAMPLE 11 Suspension: "Saffil" alumina fibre 1 60 g Phosphoric Acid 150 ml Powdered boehmite ("Cerasol") 60 g Acrylic latex ('Ciago' latex) 20 ml Foaming Agent 30 ml Water 161 Powdered boehmite ('Cerasol'-60 g) and concentrated phosphoric acid (150 ml) were mixed with water (1 litre) in a Kenwood mixer and the mixture was transferred to a 30 1 capacity vessel containing water (15 I).

An acrylic polymer latex (20 ml of 'Ciago' acrylic latex), 'Saffil' alumina fibre as described in Example 9 (160 g) and a foaming agent (30 ml) as described in Example 9 were added to the vessel and the mixture was stirred for 2 minutes using a Silverson mixer to form an aqueous suspension.

The suspension was transferred to a 12-inch mould as described in Example 9 and the resulting wet pad was dried by heating at 1 500 C, then the dry pad was heated at 11 000C for 2 hours.

The thickness of the dry pad was 4.3 cm and its density (measured after the drying at 1 500 C) was 93.7 kg/m3.

EXAMPLE 1 2 Suspension: "Saffil" alumina fibre 80 g 'Kaowool' aluminosilicate fibre 80 g Phosphoric Acid 150 ml Powdered boehmite ('Cerasol') 67 g Foaming Agent 20 ml Water 161 The phosphoric acid and powdered boehmite were mixed with 1 litre of the water in a Kenwood I mixer and the mixture was transferred to a 30 1 capacity vessel containing 14.5 1 of water.

The foaming agent (as in Example 9) was stirred with 500 ml of water in the Kenwood mixer and transferred to the 30 1 capacity vessel.

The inorganic fibres were added to the vessel and the mixture was stirred for 2 minutes using a Silverson mixer to form an aqueous suspension of the fibres.

A 12-inch diameter pad formed from the suspension as described in Example 9 and dried at 1 500C then heated at 11 003C had a thickness of 5.7 cm, a weight of 221 g and a density of 58.7 kg/m3.

EXAMPLE 13 Loose, staple polycrystalline 'Saffil' alumina fibres (140 g) of average length 8 mm and average diameter 3 microns were added to water (101) in a plastic container. N 1030 silica sol solution (1000 ml) was added to the container and the mixture was agitated using a Silverson mixer for 1 minute to form a suspension of the fibres.

The suspension was poured into a perforated brass mould of dimensions 20 cm x 20 cm square x 10 cm height and as liquid drained from the mould the fibre mass was compressed to a depth of 6 cm using a lightweight polystyrene block (20 cm x 20 cm). Leaving the polystyrene block in position, the mould was inverted and lifted off the fibrous mass, leaving the fibre mass on the polystyrene block. A square of cloth of open weave and made from polytetrafluoroethylene-coated glass fibre was placed over the wet fibre mass, followed by a perforated stainless steel plate. The assembly was inverted and placed in an oven in which it was dried by heating at 130-1 400C for 12 hours.

The assembly was removed from the oven and the fibre block was separated and cooled. The block had a density of 77.5 kg/m3. The block was heated at 1 4000C for 24 hours, and the linear shrinkage of the block resulting from this treatment was 2.15%.

EXAMPLE 14 A fibre block was produced from the following materials: 'Saffil' alumina fibres 140 g Silica sol solution (N 1030) 1000 ml Hydrated alumina powder 5 9 ("Cerasol" from The British Aluminium Company} Water 1 0 litres Silica sol solution N.1030 is a colloidal dispersion of sub-micron silica particles in water and is available from Nalfloc Ltd:: Silica 30% Na20 0.4 pH 10.2 particle size 11-16 m* surface area 1 90-270 m2/g visccosity (250 C) max. 4 cp The block was prepared by the procedure described in Example 13 except that the silica sol and hydrated alumina powder were mixed together in a Kenwood mixer for 2 minutes prior to addition to the fibre/water mixture in the plastic container.

The dry fibre block had a density of 67.3 kg/m3.

EXAMPLE 1 5 Using the procedure described in Example 13, a 50:50 mixed-fibre block comprising alumina fibres and aluminosilicate fibres was produced from the following materials: 'Saffil' alumina fibres 93 g "Kaowool" aluminosilicate fibres 93 g (av length 20 mm: av diam 3 microns) Silica sol (N 1030) 600 ml Water 101 The dry mixed-fibre block had a density of 111 kg/m3, and its linear shrinkage after heating at 1 4000C for 24 hours was 2.69%.

EXAMPLE 16 Using the procedure described in Example 13, a 70:30 alumina fibre minosilicate fibre block was produced from the following materials: 'Saffil' alumina fibres 115 9 "Kaowool" aluminosilicate fibres 50 g Silica Sol (N 1030) 600 ml Water 10 litres The dry block had a density of 99 kg/m3 and its linear shrinkage upon heating at 1 4000C for 24 hours was 1.67%.

EXAMPLE 17 Using the mixing procedure described in Example 13, two fibre suspensions were produced to the formulations: Suspension I Suspension II 'Saffil' alumina fibres 300 g "Kaowool" aluminosilicate fibres - 1060g Silica Sol (N 1030) 1 500 ml 4000 ml Water 20 litre 20 litre Suspension I was pored into a perforated brass mould of dimensions 30 cm x 30 cm x 25 cm height and liquid was allowed to drain from the mould for 1 minute. The surface of the fibrous mass in the mould was levelled (depth of mass 5 cm) and then Suspension II was added to the mould on top of the wet fibre mass.Suspension II being poured onto a polystyrene square held at an angle so that the suspension was deposited on the fibre mass deposited from Suspension I without greatly disturbing the surface of the fibre mass.

The resulting fibre mass (total height 10 cm) was removed from the mould and dried as described in Example 13. Drying was at 130-1 40OC for 12 hours.

EXAMPLE 18 A fibre block comprising a layer (5 cm thick) of 'Saffil' alumina fibres and a layer (5 cm thick) of "Kaowool" aluminosilicate fibres and provided with a protruding bolt for attaching the block to a furnace wall was produced by the procedure described in Example 17 except that prior to adding Suspension II to the mould there was applied over the wet fibre mass in the mould (i.e. the mass deposited from Suspension I) a square (15 cm x 1 5 cm) piece of stainless steel expamet mesh to the centre of which had been welded a threaded bolt of length 5 cm, the bolt pointing upwards. The layer of aluminosilicate fibres was laid down and the fibre mass was removed from the mould and dried as is described in Example 1 7.

Eight dry blocks, each with a protruding bolt, were made and these were assembled by means of the bolts on the wall of a Shelley furnace with "Kaowool" cement between adjacent blocks. The furnace was heated at 1 4000C for three separate 12 hour periods. At the end of this test the wall was examined and both the blocks and the wall surface they created were found to be completely satisfactory.

EXAMPLE 19 This example describes the production of blocks comprising the basic two layer fibre block fitted with fixing bolt described in Example 1 8 to which had been added one or more non-fibrous layers to form graded temperature furnace building blocks.

A. The procedure described in Example 1 8 was repeated, except that prior to drying of the wet fibre block a layer (thickness 2.5 cm and dimensions 30 cm x 30 m) of a vermiculite board was applied over the surface of the wet aluminosilicate fibre layer, the vermiculite board having a hole bored at its centre through which protruded the fixing bolt embedded in the wet fibre block. The vermiculite board was made by cementing together heat-exfoliated vermiculite granules and is available under the trade name "Vicuclad". The board was pressed lightly onto the wet fibre block and the assembly was dried at 130--1400C for 1 2 hours.The dry composite block consisted in the thickness direction of a face layer of 'Saffil' alumina fibres, an intermediate layer of "Kaowool" aluminosilicate fibres and a backing layer of "Vicuclad" board, the layers being firmly bonded together and the composite block being provided with a fixing bolt.

B. A composite block was made as described in A but using a 2 cm thick block of firebrick instead of the "Vicuclad" board. The layer of firebrick became firmly bonded to the layer of aluminosilicate fibres.

C. The procedure described in Example 1 8 was repeated, except that prior to drying the fibre block a block (3-4 mm thick) made of firebrick was pressed lightly onto the layer of 'Saffil' alumina fibres. The block of firebrick became firmly bonded to the layer of 'Saffil' alumina fibres to provide a composite block consisting in the thickness direction of a facing layer of firebrick, an intermediate insulation layer of high temperature 'Saffil' alumina fibres and a backing layer of aluminosilicate fibres. The composite block is useful for lining furnaces wherein a glazing treatment is carried out, the firebrick facing layers being resistant to refractory glazes.

D. Using a combination of procedures A and C, a composite block was made consisting in the thickness direction of a facing layer of firebrick, a layer of 'Saffil' alumina fibres, a layer of "Kaowool" aluminosilicate fibres and a backing layer of "Vicuclad" board, the block being provided with a threaded fixing bolt.

E. Using procedure A a composite block was produced having as backing layer a 'Saffil' alumina fibre board instead of the "Vicuclad" board. The board is a commercially-available material consisting of 'Saffil' alumina fibres bonded together in a dense material with an alumina-based refractory binder.

EXAMPLE 20 Using the procedure described in Example 13 a 15:85 mixed-fibre block comprising alumina fibres (15%) and aluminosilicate fibres (85%) was produced from the following materials: 'Saffil' alumina fibres 42 g "Rockwool" mineral wool fibres 238 g Silica Sol (N 1030) 1200 mi Water 10 litres The dry block had a density of 141.3 kg/m3. Linear shrinkage tests on blocks made as above indicated a linear shrinkage of 0.33% after 24 hours at 1 0000C, 0.80% after 24 hours at 11 000C and 6.4% after 24 hours at 1200 C.

Claims (39)

1. A handleable low-density fibrous material comprising inorganic fibres and a binding agent which is a phosphate, a silica sol or an aluminium chlorohydrate binding agent.

2. A fibrous material as claimed in claim 1 of density below 100 Kg/m3.

3. A fibrous material as claimed in claim 2 of density below 50 Kg/m3.

4. A fibrous material as claimed in claim 1, 2 or 3 wherein the amount of the binding agent is less than 1 0% by weight based on the total weight of fibres and binding agent.

5. A fibrous material as claimed in any one of claims 1 to 4 wherein the fibres are staple fibres.

6. A fibrous material as claimed in claim 5 wherein the fibres are of length in the range of from Smmto 10 mm.

7. A fibrous material as claimed in any one of the preceding claims wherein the diameter of the fibres is from 2 microns to 20 microns.

8. A fibrous material as claimed in any one of the preceding claims wherein the binding agent is a phosphate binding agent.

9. A fibrous material as claimed in any one of claims 1 to 7 wherein the binding agent is a silica sol binding agent.

10. A fibrous material as claimed in any one of claims 1 to 7 wherein the binding agent is an aluminium chlorohydrate binding agent.

11. A fibrous material as claimed in any one of the preceding claims comprising metal oxide fibres.

12. A fibrous material as claimed in claim 11 comprising alumina fibres.

13. A fibrous material as claimed in any one of claims 1 to 10 comprising aluminosilicate fibres.

1 4. A fibrous material as claimed in any one of the preceding claims comprising fibres of different materials.

1 5. A fibrous material as claimed in claim 14 comprising a mixture of polycrystalline alumina fibres and aluminosilicate fibres.

16. A fibrous material as claimed in claim 14 of layered structure comprising one or more layers of one type of fibre and one or more layers of a different type of fibre.

1 7. A fibrous material as claimed in claim 1 6 comprising one or more layers of polycrystalline alumina fibres and one or more layers of aluminosilicate fibres.

18. A fibrous material as claimed in any one of the preceding claims which contains one or more additives selected from fillers and strength-improving agents.

1 9. A fibrous material as claimed in claim 1 8 comprising alumina fibres and a negatively-charged additive.

20. A fibrous material as claimed in claim 18 or 1 9 wherein the additive is in particulate form.

21. A composite material comprising one or more layers of a fibrous material as claimed in any one of the preceding claims and one or more layers of a non-fibrous material.

22. A fibrous material as claimed in any one of claims 1 to 20 or a composite material as claimed in claim 21 having incorporated therein one or more fixing devices for attaching the material to surfaces.

23. Afurnace or kiln provided with thermal insulation comprising a fibrous material or a composite material as claimed in any of the preceding ciaims.

24. A process for the production of a low-density fibrous material which comprises forming an aqueous suspension of inorganic fibres, incorporating in the suspension a binding agent which is a phosphate, a silica sol or an aluminium chlorohydrate binding agent, and removing water from the suspension.

25. A process as claimed in claim 24 wherein removal of water from the suspension is effected by drainage of free water followed by evaporaton of residual water.

26. A process as claimed in claim 24 or 25 wherein the fibrous material is heated at 1 000C to 1 500C to remove water.

27. A process as claimed in claim 24, 25 or 26 for the production of a fibrous material of layered structure comprising layers of different types of fibres which comprises laying down a suspension of one type of fibre, removing water from the suspension by drainage to form a first layer of fibres, laying down over the first layer of fibres a suspension of a different type of fibres, removing water from the suspension by drainage to form a second layer of fibres, and removing residual water from the layered structure by evaporation.

28. A process as claimed in any one of claims 24 to 27 for the production of a composite material which comprises applying the suspension of fibres to a porous non-fibrous substrate to which the binding agent will adhere, allowing water to drain from the suspension Into the substrate and removing water from the composite material.

29. A process as claimed in any one of claims 24 to 28 wherein the amount of fibres in the suspension is less than 5% by weight of the suspension.

30. A process as claimed in any one of claims 24 to 29 wherein the amount of the binding agent in the suspension is from 5% to 20% by weight based on the weight of fibres in the suspension.

31. A process as claimed in any one of claims 24 to 30 wherein the suspension contains a particulate additive in an amount of from 10% to 50% by weight based on the weight of fibres in the suspension.

32. A process as claimed in any one of claims 24 to 31 wherein the suspension contains an organic polymer and wherein after removal of water from the suspension the resulting fibrous material is heated at an elevated temperature up to the decomposition temperature of the fibres to remove the organic polymer.

33. A process as claimed in any one of claims 24 to 32 wherein the suspension contains a foaming agent.

34. A low-density fibrous material comprising inorganic fibres and a phosphate binding agent substantially as described herein with particular reference to any one of Examples 9 to 12.

35. A low-density fibrous material comprising inoranic fibres and a silica sol binding agent substantially as described herein with particular reference to any one of Examples 1 3 to 20.

36. A low-density fibrous material comprising Inorganic fibres on an aluminium chlorohydrate binding agent substantially as described herein with particular reference to any one of Examples 1 to 8.

37. A process for the production of a low-density fibrous material comprising inorganic fibres and a phosphate binding agent substantially as described herein with particular reference to any one of Examples 9 to 12.

38. A process for the production of a low-density fibrous material comprising inorganic fibres and a silica sol binding agent substantially as described herein with particular reference to any one of Example; 13 to 20.

39. A process for the production of a low-density fibrous material comprising inorganic fibres and an aluminium chlorohydrate binding agent substantially as described herein with particular reference to any one of Examples 1 to 8.