Sheets of thermoplastic material have a wide spread use for producing a great number of products for example as supporting layers for interior lining panels in vehicles, furniture, buildings or for manufacturing panels used as structural elements for prefabricated buildings, for internal separation walls, for doors, or the like, for truck bodies of any kind Such sheets has to be extremely cheap concerning the material used and the process for their production.

At the same time such sheets must be as light as possible and have high mechanical qualities relating to their mechanical resistance to stress and loads.

Furthermore, particularly when used for interior lining panels of vehicles such sheets must show a correct behaviour under rupture, i. e. they do not break forming sharp splitter which might separate from the rest of the panel or sharp edges or veining. Sheets of thermoplastic material must further show a ductile rupture behaviour with nails, this means by fixing nails, the material does not form veining. Another important feature relates to the fact that the composition must be ambient-friendly, this means that it is possible to recycle the material at high degree.

Thermoplastic sheets must also show a very good behaviour relating to shaping according to different techniques, for example by drawing or by moulding and particularly in the case of strong two or three dimensional deformations.

Several attempts has been made in order to achieve compositions that might show all this qualities at the same time.

Compositions comprising thermoplastic material and natural organic fibres are known. Generally the fibres are subject o different kinds of preventive treatments which might comprise separately or in combination premixing with the thermoplastic material itself or with other thermoplastic materials and/or with granulated fillers either of organic or inorganic, natural or synthetic kind, and/or preheating of the fibres itself or of the premixed mass or precoating of the fibres. Other solutions suggests o introduce the fibres in form of woven, non woven or agglomerated sheets which are bonded to or permeated with the thermoplastic material.

This known compositions do not lead to materials showing at the same time all the above mentioned desired features. Generally, very complicated and expensive proceedings for producing the composition are developed or used in the attempt of ensuring a good bonding of the fibres with the thermoplastic material which is thought to be the relevant point in ensuring constant very high mechanical qualities. This leads to high expensive materials which use is very limited due to the high costs, thus blocking the technical improvement which could be reached or aimed in the

different fields of possible use of sheets of thermoplastic materials.

The present invention is based on the problem to provide for a thermoplastic composition particularly for the production of sheets having all the desired qualities listed above.

The invention achieves the above mentioned aims by means of a thermoplastic composition according to claim 1.

In using sisal fibres directly introduced alone in the thermoplastic material, particularly in the extruder, sheets may be obtained which show all the features desired features listed above without needing preventive treatments of the fibres or other particular operation steps.

Surprisingly sisal fibres seems to bond with the thermoplastic material in a very strong way and with constant results.

Using polyolephins resins and particularly polypropylene polymers or copolymers the further advantage is achieved of having a highly recyclable material.

Sisal fibres are generally introduced as singularised fibres in the mass of thermoplastic material already inside the extruder.

Sisal fibres are long fibres and are mixed to the thermoplastic material without having a predefined orientation, his means a isotropic orientation, but with casual orientations so that in the composition in the extruder there is a quite random distribution of the orientation of the fibres.

Extruding the sheets will lead generally to a orientation in a defined direction of a certain amount

of the fibres. Surprisingly it has been found out that when the sheets are further subjected to treatment such as heat treatment for bonding the sheets to other layers and or for shaping in a mould, the major part of the fibres resume the random orientation.

The length of the fibres might vary from 3 to 20 mm, particularly from 5 to 12mm preferably from 6 to8 mm.

The content by weight of Sisal fibres of the composition may vary from 10 to 60%, particularly from 30 to 50% ouf the total mass of the composition.

The content of Sisal fibres in the composition according to the invention may vary in order to regulate the mechanical and weight characteristics of the material. Particularly interesting is also the fact that even using high contents of fibres in the composition a specific weight of the composition may be achieved which is less than 1 g/cm3 this means that the composition may be used for producing floating sheets having improved mechanical resistance and which are relatively cheap.

According to a further feature, to the composition of Sisal and thermoplastic material also an organic filler may be added. This is done separately to the feeding and mixing of the sisal fibres to the thermoplastic material.

The organic filler may consist in wood particles or wood flour. In a particular embodiment the range of the particle size has its maximum concentration at about 250 micron.

The amount of organic filler may vary between about 10 and 40% in weight relating to the total weight of sisal fibres and organic filler.

Particularly advantageous are the use of flour of fir wood or of beech wood.

The present invention does no exclude adding one or more of other kinds of natural, or synthetic fibres to the sisal fibres. In this case the one or more additional fibres may be added separately to the composition or premixed to the sisal fibres.

This additional one or more fibres may have the function of regulating mechanical, physical and/or chemical characteristics of the composition, for example in the case of special use of the composition.

To the composition according to the present invention also inorganic filler may be added, particularly in the form of granulate or powder.

Inorganic fillers may be mineral fillers, such as talc or the like.

The invention relates to a composition for producing sheets of thermoplastic material. In this case, the sheets are obtained by extrusion.

A predetermined amount of thermoplastic material, such as polyolephin resin, especially polypropylene polymer or copolymer is fed alone to the extruder and heated.

Singularised sisal fibres are then fed alone to the extruder.

The sisal fibres are neither preheated, nor premixed with other fibres or other components such as filler or the like.

The sisal fibres are also not pre-coated for improving their adhesion to the thermoplastic material.

A sheet of such material is obtained through a linear extrusion head.

The sheet may be than subjected to further treatment such as shaping through moulding, combination with other sheets for forming composite or laminated panels.

Particularly it has been discovered that during extrusion the fibres of sisal are at least partially oriented parallel to the direction of the extrusion.

Thus in a certain way modifying the mechanical properties due to mutual crossing of the randomly oriented fibres. Surprisingly during the following moulding or laminating process or simply by heating the sheets, the sisal fibres recovers at a certain degree the originally random orientation and the mutual crossing.

According to a further improvement of the present invention, at least further layer may be coupled to a sheet made of the composition according to the invention.

According to a first embodiment the at least further layer is a layer formed by a mat or the like of woven or non woven fabric. The fibres may be synthetic fibres, natural fibres or mixtures thereof. The mat is coupled by means of chemical/physical adhesion to the sheet made of the composition according to the invention.

The at least further layer may be alternatively in the form of a sheet or of a film of plastic material.

Particularly the layer may be in the form of a thin film or sheet of polyolephin resin, as polypropylene or the like. In this case the further layer may have a thickness between 20 and 300 microns, particularly between 20 and 150 microns.

In a preferred embodiment, to the sheet made of the composition according to the invention there are bond two further layers, each one on one side of the said sheet.

In particular, one layer is formed by a mat or the like of fibres, as described above, particularly of non woven fibres. This layer is not impregnated completely with the plastic material of the sheet, but almost only bond at the surfaces in contact with each other.

The second layer is bond to the opposite side of the sheet made of the composition according to the invention and is made of a thin film of polyolephin resin according to the above description.

Obviously the at least further layer may be also of other kind, as for example a sheet of paper, crepe paper, or a gauze, a grid or a net of different materials such as plastic, metal, or fibres or also a layer having a honeycomb structure or a multi-cavity layer.

The sheet according to the invention may be part of a composite multilayer structure comprising one or more of the different preceding layers and at leas one but even more of the sheets made with the composition according to the invention.