GB1590481A - Sheet materials - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO SHEET MATERIALS (71) We, SOCIETE D'EXPLOITATION DES BREVETS GRANOFIBRE (S.E.B.R.E.G.), a French Body Corporate, of Le Verenay, Ampuis, 69420 Condrieu, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - The present invention relates to sheet material and to a process for manufacturing same.

Sheet materials have numerous applications, some of which are set forth hereinbelow.

Paper and cardboard of between 0.1 and 2mm thickness. These materials are conventionally obtained by treating vegetable fibres in aqueous phase. Considerable quantities of water and energy are consumed by this technique. In addition, the effluents which are rejected into the rivers are pollutants which are particularly aggressive with respect to the environment by their nature, volume and temperature.

Rigid or semi-rigid panels of large thickness, of between 2 and 10 mm. Such panels produced by conventional techniques are of limited dimensions which leads to difficulties in assembly. Moreover, their density is relatively high and cannot be less than certain values and their thermal and acoustic insulating properties are poor.

A substrate for receiving a coating to form a floor covering. At present this type of floor covering is conventionally made from a sheet of asbestos paper on which are deposited one or more layers of plastic products, and more particularly polyvinyl chloride foam. The normally coated asbestos paper contributes virtually nothing as far as the acoustic attenuation with respect to the noise of impacts between floor and ceiling is concerned. On the other hand, it has been discovered that asbestos presents a certain danger to public health since the inhalation of the fibres may lead to serious illnesses such as fibrosis, pleural tumours, and cancer.

Preferred embodiments of the invention to be described hereinafter by way of example provide sheet materials for each of the above applications and which avoid the disadvantages of the conventional materials.

According to the present invention, there is provided sheet material comprising juxtaposed fibrous granules joined together by binder and compressed to form lenticular particles, the particles lying in the opposed surfaces of the sheet being cut in the plane of the surface.

Further according to the present invention, there is provided a process for manufacturing the above material comprising mixing fibrous granules with a binder, compressing the mixture substantially uniformly to obtain a homogeneous block, slicing the block to form at least one sheet, and subjecting the sheet to heat treatment to promote the migration of the binder.

The invention will be described by way of example only with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view showing sheet material sliced from a cylindrical block which has been compressed axially; Figure 3 is a fragmentary transverse section, to an enlarged scale, taken on line II-II of Figure 1; Figure 3 is a view similar to Figure 1 showing sheet material sliced from a cylindrical block which has been compressed radially; and Figure 4 is a fragmentary transverse section, to an enlarged scale, taken on line IV-IV of Figure 3.

There are described in our French Patent Specifications Nos. 1 422 835, 1 540 382, 1 583 783, 2 193 350 processes for the manufacture of fibrous granules. The granules have particularly interesting properties. Each granule is composed of entangled fibres constituting a network in which they are virtually free so that a relative mobility is ensured therefor. On the other hand, the fibres of each granule are orientated in any manner and consequently the properties of each granule are substan tially isotropic. Finally, the fibres of the same granule remain spaced apart from one another, this resulting in the granules having an open porosity, being very light, having a supple and elastic resistance as well as a high heat and sound insulating power.

Due to their properties recalled hereinabove and to their low cost price per unit of mass, these granules constitute the fibrous granules used in the materials to be described hereinafter.

There is described in our French Patent Specification No. 1 568 187, the manufacture of flexible sheet by mixing granules with a binder and by subjecting the mixture to compression and, subsidiarily, to heating.

The granules are flattened in the form of lenticular particles, parallel to the faces of the plate.

When a long, continuous sheet of small thickness is to be manufactured, the mixture is distributed between heated calendering rollers which enable substantially the same structure to be obtained.

The use of fibrous granules in place of free, loose fibres offers numerous advantages, for example: -the gluing and distribution of the dry fibres are difficult if not impossible, whereas these operations applied to fibres assembled as granules raise no problem.

-the fibres are orientated in preferential manner during manufacture of the article whereas the granules, due to their structure, present fibres orientated in all directions.

However, the possibilities of the technique described in French Patent Specification No. 1 568 187 are limited since the thickness of the sheet cannot be less than 2.2 mm. Certain applications are therefore excluded. On the other hand, the surface state is not satisfactory, since this surface is neither smooth nor flat due to the roughness and nodules which inevitably appear.

As will be described hereinafter in greater detail, the material in accordance with preferred embodiments of the invention is obtained by mixing the fibrous granules with a binder of appropriate flexibility and compressing the mixture.

However, in contrast to the technique described in French Patent Specification No.

1 568 187 compression is effected substantially uniformly in order to obtain a homogeneous block of large dimensions, and the block is sliced into at least one relatively thin sheet, the sheet then being subjected to a heat treatment which promotes the migration of the binder. This heat treatment may be combined with a compression, which may be easily maintained by hot-pressing.

The unexpected effect of this technique is that the sheet obtained has a good strength and does not tear - easily.

Although, it may be thought that by cutting the flattened fibrous granules across their thickness or their largest dimensions, they lose the advantageous qualities discussed above, it has been found that this is not the case especially after passage in an oven or hot-pressing (calendering), since the heat treatment ensures the migration or distribution of the binder, deposited on the outside of the granules, in the mass of the material and the calendering improves the surface state.

It is then ascertained that the sheet is resistant to traction and to tearing; its cohesion is excellent; its heat and sound insulating properties are much better than that of the products which it is intended to replace; it may be of any thickness, particularly very thin; its density may be relatively low and in any case lower than the other products; its mode of manufacture may orientate the elasticity either in the thickness or in the width; it may be made with a bread width and delivered as a roll, when it is made using flexible binders. Finally, its surface state is excellent and is perfectly suited to all desired applications. This surface state is, moreover, improved by the hot calendering which further makes it possible to obtain a better cohesion of the fibrous structures of the sheet and a more uniform quality of the sheet.

Referring now to Figures 1 to 4, in these Figures arrow F denotes direction of slicing of a cylindrical block, and arrows CA and CR denote the directions in which the block is compressed. Arrow CA denotes the direction of axial compression and CR the direction of radial compression.

The block is made from fibrous granules and binder. It is very simple to manufacture fibrous granules from natural (vegetable, animal, mineral) or synthetic fibres, with suitable properties (slze, flexibility, elasticity, porosity, insulating power, lightness, etc . . .). Such processes are described in the French Specifications discussed earlier. These granules are mixed with a binder of appropriate flexibility and the mixure is subjected to a unidirectional compression which tends to flatten the granules to give them, individually, the form of lenticular particles.

The compression is exerted in a mould to obtain a block of large dimensions which has a certain homogeneity in its mass, although its texture is fibrous with the fibres being orientated parallel to the compression surfaces.

Sheet material is then cut from the block by slicing. In the case of a cylindrical block, a single continuous sheet can be produced by slicing the block spirally. This operation has for its effect to produce a surface of separation between adjacent sliced layers, the surface of separtion pass ing through the superficial lenticular particles of these layers at random, leav ing in one layer a portion, of any volume, of each of these particles and in the other layer, the remaining portion.

Consequently, each face of the sliced material has the appearance of a flat juxtaposition of fibrous structure, delimited by closed curves, these round, elliptic or ovoidal structures themselves being coplanar but of different dimensions.

To produce individual sheets, the block is preferably parallelepipedic in form rather than cylindrical and its base then corresponds to the contour of the sheets to be cut.

To produce a continuous sheet, the block is necessarily cylindrical. The cylin der may be solid, but it is more advanta geous if it is hollow; in fact, this avoids the usual loss of a central core and, more over, the hollow interior made during casting makes it possible to effect the centering, support and rotation of the cylinder under easier and more reliable conditions by means of a mandrel, which also very effectively resists the buckling of this cylinder under the cutting effect.

The hollow interior may be grooved co operate with a ribbed mandrel or may be provided with a smooth sleeve made of hard cardboard to cooperate with an expansible mandrel.

The following description refers to the moulding and slicing of a cylindrical block but it will be apparent that similar considerations apply to the moulding and slic ing of a parallelepipedic block.

According to a first embodiment, the mixture (granules and binder) is poured into a cylindrical mould in successive layers which are axially compressed one after the other; the depth of each layer is chosen so that the rate of compression is substantially uniform for each layer. The fibrous granules are therefore flattened under the effect of pressure to form lenti cular particles joined together by the binder and extending flat in planes perpendicular to the axis of the cylinder; conse quently, by tangentially slicing this cylinder, particles are cut perpendicularly to their largest surface so that their section in thickness is visible. As shown in Figs. 1 and 2, the sliced sheet 1 may contain, if its thickness is sufficient, the particles being orientated substantially perpendicularly to the surfaces of the sheet whole lenticular particles; in any case, its superficial zones are constituted by partial lenticular particles 2, of which the portion incorporated in the layer in question is separated from the portion remaining incorporated in the adjacent layer, by the cutting surface 3- which has the appearance of a juxtaposition of substantially elliptical fibrous structures 4 which are in fact the visible cut part of said superficial lenticular particles. These structures are orientated with their major axes in the longitudinal direction of the sheet 1 and are substantially parallel to one another; under these conditions, this sheet has a certain transverse elasticity. The elliptical structures obviously have different dimensions since they correspond to different sections of cut of the particles.

According to a second embodiment, the mixture (granules and binder) is deposited progressively around a central mandrel and at the same time, Is compressed radially against said latter.

The radial compression has for its effect to flatten the fibrous granules to form lenticular particles joined together by the binder and extending concentrically with respect to the central mandrel, i.e. flat and parallel to the periphery of the cylinder obtained; consequently, by tangentially slicing the cylinder, the particles which are orientated substantially parallel to the surfaces of the sheet are cut perpendicularly to their thickness of their section of largest surface to be visible. As shown in Fig. 3, the sliced sheet 5 is composed in particular of partial lenticular particles of which the incorporated portion of each allows a firbous structure 7 of substantially circular form to appear on the cut surface 6 of this sheet, all the structures being juxtaposed on the same surface. The sheet 5 in question has, as before, a certain elasticity, but, in this case, in its thickness.

In the above embodiments, before slicing the cylinder or slicing the compressed block, polymerisation and/or gelling and/or cross-linking of the or each binder is effected. This operation is carried out by means of a suitable heat treatment for an appropriate length of time. The heat treatment may consist of drying in a hurnid atmosphere in order to enable the cylinder or compressed block to maintain a sufficient moisture content to facilitate subse quent slicing.

Similarly, it may be advantageous, for certain applications, to add waterproof products, fungicidal products, fireproof, products, etc . . . to the mixture.

Furthermore, it is necessary, when the sheet has been sliced off, to subject it to a further treatment with the particular purpose of improving the surface state and promoting the migration of the binder in the whole mass to obtain a better cohesion and- a more uniform quality. This treat ment may consist in passing the sheet between heated calendering rollers, or simply in an oven.

Some specific examples of materials in accordance with the invention, their method of manufacture, and their possible applications, will now be given.

EXAMPLE I A sheet for forming a substrate which is subsequently coated to act as a floor covering, the sheet being intended to replace asbestos paper. The sheet is as thin as asbestos paper but is lighter, less expensive and has an excellent heat and sound insulating power, thus allowing the thickness of the layer of plastics material deposited to be reduced.

A cylinder is formed by using, for example, fibrous granules of hardwood, with a diameter of about 1.8 mm and by agglomerating these granules by pressure in a cylindrical mould by means of a thermoplastics binder of the latex type, previously deposited about said granules, at a rate of 15% by weight with respect to the dry granules.

If the binder is incorporated a waterproof product (such as for example a paraffin emulsion) at a rate of 6% by weight with respect to the dry granules and also a fungicidal product, at a rate of 0.72%.

The granules thus coated with the binder and other products as mentioned are then compressed in successive layers under a pressure of the order of 7 kg/cm2 with a view to obtaining a cylinder of which the final density is of the order of 0.45 g/cm3.

The cross-linking of the binder is obtained by placing the cylinder in an oven with circulation of hot, damp air at a temperature of 140"C for a length of time depending on the mass to be treated; for example, if the cylinder has a diameter of 2 mm and a length of 2.4 m, the duration of the treatment is 6 hours.

After cooling, the cylinder is ready to be sliced and slicing may be effected without the assistance of a pressure bar.

From this cylinder a sheet whose thickness is 0.8 mm may be sliced off, this corresponding to a weight of 360 g/m2; a cylinder of 2 m diameter therefore allows a continuous sheet having 3 800m of developed length to be obtained.

When the sheet is to constitute a substrate for a coating, it is necessary to glue on one of the faces thereof a reinforcing network which may be composed of glass fibres and having a mesh opening of 0.5 x 0.5 cm, the network providing resistance to traction for the passage through gelling ovens and also enabling the dimensional stability to be improved.

The addition of this network may be made either at the same time as the slicing or during a separate operation and in this latter eventually, the network, which is glued or not, is applied onto the sliced off sheet by the above-mentioned heating calendering rollers.

This example gives precise values to the various parameters, but it is clear that these values may be included within broader limits, to provide products which are slightly different as regards their particular properties but which satisfy the requirements imposed by the application chosen.

In this way, the average diameter of the granules may be included between 0.8 and 2.5 mm; the proportion of binder may be included between 7% and 30% by weight with respect to the dry granules; the pressure applied on the mixture may be included between 1.5 and 15 kg/cm2 to obtain a cylinder whose final density is between 0.15 and 1 g/cm3; the heating temperature may be included between 120 and 1600C for a duration of between 3 and 10 hours; the thickness of the peeled off sheet may be between 0.1 and 2 mm.

In this example, the sheet does not undergo any separate heat treatment after slicing since the coating operations which follow require several passages through ovens.

EXAMPLE 2 Dry manufacturer of a sheet similar to paper and preferably to corrugated paper.

In this case a thermoplastics binder of the latex type is used, with a high styrene content, for example 80% by weight. This binder is mixed with the granules at a rate of 15% by weight with respect to these dry granules; it may be used alone but may also include additives, for example for waterproofing.

The cylinder is formed as described in Example I, but the pressure exerted is of the order of 4 kg/cm2, this enabling a density of about 0.40 g/cm3 to be obtained.

The cross-linking of the binder is obtained by drying at a temperature of 1200C for a suitable length of time, which is 5 hours when the diameter of the cylinder is 2 m and its length 2 m.

The thickness of the sheet sliced from the cylinder is 0.25 mm, its weight per m2 is 150 g and its length is about 8000 m long.

The above-mentioned values of the parameters in question may be included within broader limits: -for the average diameter of the granules: between 0.8 and 2.5 mm -for the proportion of styrene in the binder: between 75 and 90% by weight -for the proportion of the total binder with respect to the dry granules: between 7 and 20% by weight -for the pressure applied onto the mixture between 1.5 and 6 kg/cm2, the final density then being between 0.15 and 0.50 g/cm3.

-for the heating temperature: between 110 and 1300C -for the duration of heating: between 2 and 8 hours -for the thickness of the sheet between 0.1 and 2 mm.

By reheating the sheet thus obtained to a temperature of between 150 to 1700C preferably 160"C, the sheet may be shaped by pressing, stamping, embossing, or other technique. For example, this sheet may be passed through a series of heated rollers which are grooved complementarily with respect to one another, so as to obtain a conventional corrugated paper with corrugations in one direction only or a paper with corrugations intersecting in the longitudinal direction and in the transverse direction. This latter type of intersecting corrugations is advantageous for packing, since the corrugated paper thus obtained and which cannot be obtained from conventional paper is particularly rigid. As another example, the sheet may be pressed against an embossing die to produce motives.

The sheet obtained according to Example 2 may also serve to manufacture a compact, light cardboard simply by interposing and gluing it between two sheets of kraft paper.

For example, if the sliced sheet has a thickness of 2 mm and if a sheet of kraft paper weighing 200 g/m2 is applied to each of its faces and glued thereon, a semi-rigid cardboard is obtained of about 2 mm thickness and weighing 550 g/m2 whilst a conventional cardboard of the same thickness weighs substantially 2 kg/m2.

EXAMPLE 3 Manufacture of a thin sheet similar to non-woven glass fibre fabrics.

In this case, granules of insulating glass fibres are used and a thermoplastics binder of the latex type, at a rate of 20% by weight with respect to these dry granules.

The procedure is the same as before, but applying a pressure of about 2.2 kg/cm2, this enabling a density close to 0.30 g/cm3 to be obtained.

The cross-linking of the binder is effected by drying in hot damp air at about 140"C.

To obtain a sheet of which the weight per m2 is 45 g( a thickness of 15/10 mm is sliced off and if the cylinder measures 2 m in diameter, the continuous length sliced off at this thickness is about 20 000 m.

The above-indicated values may, in this example, also be chosen within broader limits. Thus the average diameter of the granules may be between 3 and 6 mm, the proportion of the binder between 15 and 40% the pressure applied between 1.5 and 10 kg/cm2, the final density between 0.15 and 0.70 g/cm3, the cross-linking temperature between 120 and 1600C, the duration of cross-linking between 3 and 10 hours, and the thickness of the sheet between 0.1 and 1 mm.

EXAMPLE 4 Manufacture of a thick sheet which can be cut up in rigid panels.

A heat-setting binder of the ureaformaldehyde type is now used, at a rate of 10% by weight with respect to the dry granules of hardwood, which have a diameter of 5 mm.

To obtain a density of about 0.70 g/cm3, a pressure of the order of 10 g/cm2 is applied.

The cross-linking of the binder is effected by drying in hot, damp air at around 1500C for 8 hours.

To manufacture a sheet whose weight per m2 at the density indicated is 6.3 kg, a thickness of 9 mm is sliced off and if the cylinder has a diameter of 2 m, the total length is about 340 m.

Of course, as the product obtained is rigid, the slicing device cooperates with a transverse sawing device which cuts the sheet into panels to the desired length.

This type of panel is especially applicable to the insulation of buildings (floor, ceiling, roof, etc . . .

As in the other examples, the values indicated for the parameters in question are not limiting as they may be included within broader limits. In fact, the diameter of the granules may be between 0.8 and 8 mm, the proportion of the binder between 7 and 20%, the pressure applied between 1.5 and 15 kg/cm2, the density obtained between 0.15 and 1 g/cm3, the polymerisation temperature between 110 and 1700C, the duration of polymerisation between 3 and 10 hours, the thickness of the sheet between 5 and 30 mm.

The invention is thus generally applicable to the production of sheet-like materials of any thickness, having previously defined properties; it is more particularly applicable to the dry manufacture of various paper and cardboards, substrate to be coated for floor coverings, wall coverings, non-woven fabrics, and panels.

The sheet material described has perfectly controllable rigidity or flexibility, which is continuous for any length, low density and good power of insulation and can be produced in broad widths.

WHAT WE CLAIM IS: - 1. Sheet material comprising juxtaposed fibrous granules joined together by a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (21)

**WARNING** start of CLMS field may overlap end of DESC **. -for the pressure applied onto the mixture between 1.5 and 6 kg/cm2, the final density then being between 0.15 and 0.50 g/cm3. -for the heating temperature: between 110 and 1300C -for the duration of heating: between 2 and 8 hours -for the thickness of the sheet between 0.1 and 2 mm. By reheating the sheet thus obtained to a temperature of between 150 to 1700C preferably 160"C, the sheet may be shaped by pressing, stamping, embossing, or other technique. For example, this sheet may be passed through a series of heated rollers which are grooved complementarily with respect to one another, so as to obtain a conventional corrugated paper with corrugations in one direction only or a paper with corrugations intersecting in the longitudinal direction and in the transverse direction. This latter type of intersecting corrugations is advantageous for packing, since the corrugated paper thus obtained and which cannot be obtained from conventional paper is particularly rigid. As another example, the sheet may be pressed against an embossing die to produce motives. The sheet obtained according to Example 2 may also serve to manufacture a compact, light cardboard simply by interposing and gluing it between two sheets of kraft paper. For example, if the sliced sheet has a thickness of 2 mm and if a sheet of kraft paper weighing 200 g/m2 is applied to each of its faces and glued thereon, a semi-rigid cardboard is obtained of about 2 mm thickness and weighing 550 g/m2 whilst a conventional cardboard of the same thickness weighs substantially 2 kg/m2. EXAMPLE 3 Manufacture of a thin sheet similar to non-woven glass fibre fabrics. In this case, granules of insulating glass fibres are used and a thermoplastics binder of the latex type, at a rate of 20% by weight with respect to these dry granules. The procedure is the same as before, but applying a pressure of about 2.2 kg/cm2, this enabling a density close to 0.30 g/cm3 to be obtained. The cross-linking of the binder is effected by drying in hot damp air at about 140"C. To obtain a sheet of which the weight per m2 is 45 g( a thickness of 15/10 mm is sliced off and if the cylinder measures 2 m in diameter, the continuous length sliced off at this thickness is about 20 000 m. The above-indicated values may, in this example, also be chosen within broader limits. Thus the average diameter of the granules may be between 3 and 6 mm, the proportion of the binder between 15 and 40% the pressure applied between 1.5 and 10 kg/cm2, the final density between 0.15 and 0.70 g/cm3, the cross-linking temperature between 120 and 1600C, the duration of cross-linking between 3 and 10 hours, and the thickness of the sheet between 0.1 and 1 mm. EXAMPLE 4 Manufacture of a thick sheet which can be cut up in rigid panels. A heat-setting binder of the ureaformaldehyde type is now used, at a rate of 10% by weight with respect to the dry granules of hardwood, which have a diameter of 5 mm. To obtain a density of about 0.70 g/cm3, a pressure of the order of 10 g/cm2 is applied. The cross-linking of the binder is effected by drying in hot, damp air at around 1500C for 8 hours. To manufacture a sheet whose weight per m2 at the density indicated is 6.3 kg, a thickness of 9 mm is sliced off and if the cylinder has a diameter of 2 m, the total length is about 340 m. Of course, as the product obtained is rigid, the slicing device cooperates with a transverse sawing device which cuts the sheet into panels to the desired length. This type of panel is especially applicable to the insulation of buildings (floor, ceiling, roof, etc . . . As in the other examples, the values indicated for the parameters in question are not limiting as they may be included within broader limits. In fact, the diameter of the granules may be between 0.8 and 8 mm, the proportion of the binder between 7 and 20%, the pressure applied between 1.5 and 15 kg/cm2, the density obtained between 0.15 and 1 g/cm3, the polymerisation temperature between 110 and 1700C, the duration of polymerisation between 3 and 10 hours, the thickness of the sheet between 5 and 30 mm. The invention is thus generally applicable to the production of sheet-like materials of any thickness, having previously defined properties; it is more particularly applicable to the dry manufacture of various paper and cardboards, substrate to be coated for floor coverings, wall coverings, non-woven fabrics, and panels. The sheet material described has perfectly controllable rigidity or flexibility, which is continuous for any length, low density and good power of insulation and can be produced in broad widths. WHAT WE CLAIM IS: -

1. Sheet material comprising juxtaposed fibrous granules joined together by a

binder and compressed to form lenticular particles, the particles lying in the opposed surfaces of the sheet being cut in the plane of the surface.

2. Material as claimed in claim 1, wherein the lenticular particles are orientated substantially perpendicularly to the opposed surfaces of the sheet whereby the cut particles lying in the opposed surfaces have a substantially elliptical appearance.

3. Material is claimed in claim 1, wherein the lenticular particles are orien tatted substantially parallel to the opposed surfaces of the sheet whereby the cut particles lying in the opposed surfaces have a substantially circular appearance.

4. A process for manufacturing the material according to claim 1 comprising mixing fibrous granules with a binder compressing the mixture substantially uniformly to obtain a homogeneous block, slicing the block to form at least one sheet and subjecting the sheet to heat treatment to promote the migration of the binder.

5. A process as claimed in claim 4, for obtaining the material as claimed in claim 2, wherein the mixture is poured in successive layers in a cylindrical mould and said layers are subjected, one after the other, to an axial compression, and the cylindrical block thus obtained is sliced spirally.

6. A process as claimed in claim 4, for obtaining the material as claimed in claim 3, wherein the mixture is deposited progressively around a central mandrel, and, concomitantly, is subjected to a radial compression, and the cylindrical block thus obtained is sliced spirally.

7. A process as claimed in any one of claims 4 to 6, wherein the heat treatment is applied to the sheet concomitantly with a compression.

8. A process as claimed in claim 5, for producing a sheet to act as a substrate for a coating, wherein granules of hardwood of an average diameter of between 0.8 and 2.5 mm are mixed with a thermoplastics binder of the latex type in a proportion of between 7% and 30% by weight with respect to the dry granules, the mixtureis poured into a cylindrical mould in successive layers which are compressed one after the other under a pressure of between 1.5 and 15 kg/cm2 to obtain a cylindrical block whose final density is between 0.15 and 1 g/cm3, the block is heated to a temperature of between 120 and 1600C for a duration of between 3 and 10 hours, and the dried block is sliced spirally in order to produce a sheet of a thickness between 0.1 and 2 mm.

9. A process as claimed in claim 8, wherein the granules have a diameter of 1.8 mm and are mixed with the thermoplastics binder in a proportion of 15% by weight, the pressure exerted on the mixture is substantially equal to 7 kg/cm2 to obtain a final density of about 0.45 g/cm3 and the cylindrical block is dried at a temperature of 140"C for 6 hours.

10. A process as claimed in claim 8 or 9, further comprising the step of applying to at least one of the surfaces of the sheet, a reinforcing network to increase the resistance of the sheet to traction forces.

11. A process as claimed in claim 5, for producing a sheet similar to paper, wherein granules of hardwood of an average diameter of between 0.8 and 2.5 mm are mixed with a thermoplastics binder in a proportion of between 7% and 20% by weight with respect to the dry granules, the binder being of the latex type with a styrene content of between 75% and 90% by weight, the mixture is poured into a cylindrical mould in successive layers which are compressed one after the other under a pressure of between 1.5 and 6 kg/cm2 to obtain a cylindrical block whose final density is between 0.15 and 0.50 g/cm3, the cylindrical block is heated to a temperature of between 110 and 1300C for a duration of between 2 and 8 hours, and the dried cylindrical block is sliced spirally to produce a sheet of a thickness between 0.1 and 2 mm.

12. A process as claimed in claim 11, wherein the granules have a diameter of 1.8 mm and are mixed with the thermoplastics binder in a proportion of 15% by weight, the binder has a styrene content of 80% by weight, the pressure exerted on the mixture is substantially equal to 4 kg/ cm2 and the drying of the cylindrical block is effected at 120"C for 5 hours.

13. A process as claimed in claim 11 or claim 12, wherein the sliced sheet is reheated to a temperature of between 150 and 1700C and is shaped.

14. A process as claimed in claim 13, wherein the reheated sheet is continuously pressed between heated grooved rollers to obtain corrugations.

15. A process as claimed in claim 5, for producing a thin sheet similar to non-woven fabrics of glass fibre, wherein granules of insulating glass fibres whose average diameter is between 3 and 6 mm are mixed with a thermoplastics binder of the latex type in a proportion of between 15 and 40% by weight with respect to dry granules, the mixture is poured into a cylindrical mould in successive layers which are compressed one after the other under a pressure of between 1.5 and 10 kg/cm2 to obtain a cylindrical block whose final density is between 0.15 and 0.7g/cm3, the cylindrical block is heated to a temperature of between 120 and 1600C for a duration of between 3 and 10 hours, and the dried cylindrical block is sliced spirally to produce a sheet whose thickness is between 0.1 and 1 mm.

16. A process as claimed in claim 15, wherein the granules have a diameter of 5 mm and are mixed with the thermoplastics binder in a proportion of 20% by weight, the pressure exerted on the mixture is substantially equal to 2.2 kg/cm2 to obtain a final density of about 0.30 g/cm3 and the cylinder is dried at a temperature of 1600C for 6 hours.

17. A process as claimed in claim 5, for producing a thick sheet which can be cut up into rigid panels, wherein granules of hardwood of an average diameter of between 0.8 and 8 mm are mixed with a heatsettable binder of the urea-formaldehyde type in a proportion of between 7 and 20% by weight with respect to the dry granules, the mixture is poured into a cylindrical mould in successive layers which are compressed one after the other under a pressure of between 1.5 and 15 kg/cm2 to obtain a cylindrical block whose final density is between 0.15 and 1.0 g/cm3, the cylindrical block is heated to a temperature of between 110 and 1700C for a duration of between 3 and 10 hours, the dried cylindrical block is sliced to produce a sheet whose thickness is between 5 and 30 mm, and the sheet is cut into panels.

18. A process as claimed in claim 17, wherein the granules have a diameter of 5 mm and are mixed with the heat-settable binder in a proportion of 10% by weight, the pressure exerted on the mixture is substantially equal to 10 kg/cm2 to obtain a final density of about 0.70 g/cm3, and the cylindrical block is dried at a temperature of 1500C for 8 hours.

19. Sheet material substantially as hereinbefore described and illustrated in the accompanying drawings.

20. A process for manufacturing sheet material, substantially as hereinbefore described with reference to any one of the Examples.

21. Sheet material manufactured by a process as claimed in any one of claims 4 to 18 or claim 20.