The present invention relates to a composite material with improved energy-absorbing properties and to a method for producing this material. The material according to the invention can, for example, be used for the interiors of motor vehicles, furnishing parts, padding, etc. For some time using expanded polyurethane to make parts of the interiors of motor vehicles, padding for seats, helmets or other similar products has been known in the prior art. The polyurethane, obtained by making isocyanate react with substances provided with active hydrogen (such as, for example, polyhydric alcohols), is injected into the mould, where it is left to expand until it takes on the final shape desired for the product. This material is characterized by relatively low resistance to compression, yielding even to small compression force. This feature enables a soft-touch surface effect to be achieved that is very much appreciated for the impression of softness to the touch. Furthermore, open-cell expanded polyurethane is permeable to air, as is often required in the applications mentioned above (furnishing, padding, car interiors, ... ) . Nevertheless, the use of polyurethane alone results in rather poor absorption of energy in the event of a violent impact (think of a road accident). In fact, the pronounced compliance of the material leads to great compression deformation with absorption of a relatively small amount of energy where dissipation of the energy caused by blows is required. Deformable materials have been proposed with high energy- absorption consisting of a mass of granules with relatively low deformability that are bound by polyurethane with greater deformability. These materials are obtained by completely filling a mould with the granules (for example of expanded polypropylene) and injecting the polyurethane that has not yet expanded into the gaps between the particles. The polyurethane is then left to expand inside the mould to form a continuous material. The particles, having already filled the mould before injection of the polyurethane, cannot move away from one another because they remain in contact with one another even after expansion of the polyurethane. With this configuration of the particles, the compression forces on the material are transmitted directly from granule to granule, without affecting the expanded polyurethane, which in fact mainly acts as a binding means between the particles. The response to the compression of the material is thus linked to the rigidity of the balls of polypropylene, even in the presence of minimum force on the surface of the object and not to resistance to compression (typically low) of the flexible polyurethane. In this way, on the one hand a material with great capacity of absorption of energy is obtained but on the other hand those qualities of softness (or "soft touch") are lost that are typical of products made entirely of polyurethane. The features of the polyurethane are retained only superficially in part. Substantially, where material that is pleasantly yielding to the touch is desired, thus also if slight force is exerted, material is used that is capable of deforming itself with low absorption of energy, i.e. with unsatisfactory ability to absorb violent shocks. If greater energy absorption is required, a more rigid material is required, with the drawback of the loss of the required softness, above all when the material comes into contact with the human body. The general object of the present invention is to obviate the aforementioned drawbacks by providing a composite material that has great energy-absorbing qualities whilst maintaining softness under relatively low deformation stress such as to produce a soft-touch effect. A further object of the invention is to provide a material that is both cheap and light, whilst having good mechanical properties. Another object of the invention is to provide a rapid and cheap method to produce a material with the above features. In view of this object, it has been decided to create, according to the invention, a composite material comprising a matrix of expanded polyurethane in which granular particles of plastic material having greater rigidity than that of the expanded polyurethane are dispersed isotropically, characterized in that the granular particles are spaced apart from one another by the interposition of expanded polyurethane. Also according to the invention it has been decided to devise a method for the production of composite material comprising the phases of: - mixing granular particles of plastic material with a mixture of reagents suitable for giving rise to expanded polyurethane, - arranging the formed mixture in a mould having a volume greater than the gross volume of the granular particles contained in the mixture, - letting the reagents react to achieve the expansion of the polyurethane in the mould until complete filling thereof, the particles moving away from one another due to the effect of the expansion of the polyurethane between one particle and another. In order to make clearer the explanation of the innovative principles of the present invention and its advantages in relation to the prior art, with the help of attached drawings a possible embodiment applying such principles will be disclosed. In the drawings: - figure 1 shows a graph of the resistance/compression of composite materials curves according to the invention and of expanded polyurethane, - figure 2 shows two stages of the process of formation of the material according to the invention. The material according to the invention consists of a matrix of flexible polyurethane foam into which granular particles are dispersed (for example in polyethylene, polypropylene, expanded polystyrene or their copolymers, or in rigid or semirigid expanded polyurethane, or in expanded resins of other families of plastics such as acrylics, PET etc.) having greater rigidity than that of foam. In particular, the granular particles are dispersed isotropically in the polyurethane matrix in such a way as not to touch one another by the interposing of a layer of expanded foam between one granule and another. This solution enables yielding behavior of the material with low-compression and high energy-absorbing capacity in the event of a violent blow to be obtained. In fact, when the material is subject to light compression (think of the touch of a hand or the weight of a person sitting on the seat of a motor vehicle), the force spreads in the polyurethane matrix, which deforms without the balls interacting between themselves or undergoing noticeable deformation. In the presence of great compression of the material (think of an impact or violent blow, as in the case of a road accident), the polyurethane matrix is compressed to the point of almost making the balls come into contact with one another. In this situation, the forces are transmitted directly between the rigid balls, which are deformed to oppose a resistance to compression that is substantially greater than the polyurethane foam. The deformation of the granules (much more rigid than the polyurethane foam) comprises high energy absorption, so that the material has rigidity that in normal conditions (or because of low compression force) it did not display. The material according to the invention thus has softness and soft-touch features at low compression whereas it is rigid and capable of absorbing considerable quantities of energy in the event of great compression. In figure 1 the M5, MlO, Mil and Ml2 curves are shown that express the resistance (expressed in MPa) provided by different types of material in function of the percentage volumetric compression. The M12 curve refers to a material (the M12 material) made only of expanded polyurethane foam; the MlO and Mil curves refer to material made with a dispersion of spherical particles of expanded polystyrene (PSE) in a matrix of expanded polyurethane (PUR) foam; the M5 curve refers to a material made from granules of expanded polyethylene (PEE) in a polyurethane matrix. In table 1 here below the data on the composition of the M5, MlO, Mil and M12 materials are set out.

- Table 1

The resistance/compression curve gradient, expressed in KPa/%, can be used as an indicative parameter of the rigidity of the materials. As can be seen, the M12 material, which is made only of polyurethane foam, has a relatively low resistance/compression gradient, around 0.2 KPa/% up to about 50% of volumetric compression. With such low rigidity, even under low-intensity pressure the expanded polyurethane is compressed, noticeably varying its volume without opposing great resistance. This features translates into a soft-touch effect that is well known in the prior art. In order to have compliance that is pleasant to the touch, the resistance/compression gradient of the material must advantageously not exceed 0.5 KPa/%. Nevertheless, the M12 material is not able to absorb great quantities of energy, yielding easily without ever opposing great resistance to compression up to 70% in volume. The Mil and MlO materials according to the invention are made with isotropic dispersion of granules (or balls) of polystyrene in the polyurethane foam. The balls have greater rigidity than the foam (for example around 2 KPa/%), and are dispersed therein in such a way as not to come into contact with one another. Their percentage in net volume (respectively 8% and 14%) is in fact relatively low to the point that each ball is "suspended" in the polyurethane matrix. The gradients of the Mil and MlO curves for low volumetric compression percentages are comparable with those of the M12 curve of the expanded polyurethane (below 0.5 KPa/%). In fact, at the start of the compression of the composite material the foam is compressed without the rigid balls interacting with one another or being noticeably deformed. For this reason, the MlO, Mil materials first have softness that is comparable with that of the expanded polyurethane (M12). When the compression of the MlO and Mil composite materials then increases (around 20-30% of volumetric compression) the characteristic MlO7 Mil curve rises up, displaying very- steep gradients (up to 2 KPa/% or more) . This behavior is linked to the fact that the foam is compressed to the point where the balls are compacted and almost enter in contact with one another. In this situation the forces no longer propagate through the foam but directly from ball to ball. As the latter are more rigid than the foam, this explains the greater capacity of absorption of energy that the composite material displays when it is subjected to blows or violent pressure. It should be noted that the MlO material has a net volume percentage of PSE balls (around 14%) that is greater than the Mil material (about 8%). This higher PSE balls content explains the fact that the MlO curve "rises up" at lower volumetric compression than the Mil curve. As the balls in the MlO material are denser, a lesser volumetric compression percentage will be sufficient to make them come into contact with one another, thus increasing the energy absorption capacity of the material. The MlO and Mil materials, in addition to maintaining the special soft-touch of the polyurethane, are also permeable to air, as is the expanded foam. The M5 material is made of granular particles in expanded polyethylene (PEE), and not in polystyrene (PSE) as in the case of MlO and Mil materials. Note from figure 1 how in this case the composite material displays conspicuous resistance to compression only when its volume is reduced by about 50%. At low compression, the material M5 even displays greater compliance than the (M12) simple polyurethane. By varying the material used to make the granules and their volumetric percentage it is thus possible to obtain different responses to compression from the materials, depending on the type of feature desired and the particular technical application. A method for making the above material will now be disclosed. Initially, a chamber 12 is filled with granules (or balls) 13 made from a material chosen for example from between polyethylene, polypropylene or polystyrene. The chamber 12 must have a volume that is equal to the gross volume of the granules (or the apparent volume of the amassed granules, including the gaps between one ball and another) in such a way as to be completely filled by the particles 13. Into the chamber 12 non-expanded polyurethane is then injected ("non-expanded polyurethane" is defined as a mixture of isocyanate and polyhydric alcohol that, by reacting, give rise to expanded polyurethane). Possibly, together with the reagents that are suitable for forming polyurethane, catalyzing agents can also be injected to promote the expansion reaction. The polyurethane 14 is made to expand inside the chamber 12 in such a way as to occupy all the gaps present between the granules 13. It should be noted that at this stage of the process the balls 13 are still in contact with one another, as they cannot move away from one another as long as they are inside the chamber 12. Once the polyurethane 14 has been evenly distributed in the gaps between the balls 13, the mixture that has been created is placed in a mould 15 having a greater volume than the gross volume of the granules 13 (and therefore also of the chamber 12). In the mould 15 the polyurethane 14 continues to expand until it fills the entire volume. During this expansion phase, the polyurethane drags the balls 13 away from one another, interposing itself between one granule and another to give rise to isotropic dispersal of the particles 13 in the foam matrix. Alternatively, instead of transferring the mixture to the chamber 12 to a distinct final mould 15, the geometry of the chamber 12 could be varied once the polyurethane has completed the first expansion phase going to occupy the gaps between the granules 13. The geometry of the chamber 12 is varied in such a way as to increase its volume and make it take on the desired shape for the molding of the material. Advantageously, the total net volume of the granules 13 is less than 60% of the total volume of the material, preferably less than 30%. As already said, the granules may furthermore have the shape of small spheres having a diameter that is preferably less than 10% of the minimum characteristic dimension of the final mould 15. Nevertheless, it should be noted that the granules could also have a cylindrical shape, like a grain of rice or the like. The granules could also be treated superficially to obtain improved properties of adhesion to the polyurethane matrix. The treatment, for example, could lead to surface roughness of the granule suitable for promoting the adhesion of the polyurethane at the moment of the molding of the composite material. At this point it is clear how the objectives of the present invention have been achieved. A composite material has in fact been created having a soft-touch property and which at the same time is able to absorb great quantities of energy. Furthermore, the material according to the invention is permeable to air, similarly to expanded polyurethane foam. It should also be noted that the composite material is very light and has relatively low costs if it is considered that the granular particles are made from a material that is typically cheaper than expanded polyurethane. A simple and cheap method is also provided to create the material according to the invention. Naturally, the above disclosure of an embodiment applying the innovative principles of the present invention is given by way of example of such innovative principles and must not therefore be taken to be a limitation in civil law of what is claimed here. In particular, the mixture between granular particles and non-expanded polyurethane need not necessarily occur in the manner disclosed above. For example, this mixing may occur directly inside a mould-injection device that is supplied with isocyanate, polyhydric alcohol and granules in the due proportions.