The present invention relates to a material based on vulcanized rubber, in particular coming from used tyres. The present invention derives from the field of re¬ cycling processes of used materials and specifically re¬ lates to a process for the recycling of vulcanized rub¬ bers and used tyres . In recent years, problems associated with the dis¬ posal of waste materials have become increasingly more important in industrialized countries. This demand has led to the development of new tech¬ nologies suitable for recycling used and recyclable mate- rials. A flourishing industry in the recycling field of paper, aluminum and plastic products has therefore devel¬ oped. At present, however, there is no suitable technology for processing rubbers and in particular used tyres. This problem is particularly felt in modern society as it is estimated that about 15,000,000 tons of waste products deriving from vulcanized rubber are produced an¬ nually. Approximately half of these products derive from used tyres whereas the remaining quantity relates to the production of new rubber products. The elimination of this enormous quantity of waste material represents a great environmental problem so much so that an improper elimination of tyres increases risks for the environment and of fires. The difficulty in finding a suitable recycling tech¬ nology specifically lies in the properties which make vulcanized rubber so useful and requested in industry. Vulcanized rubber, in fact, does not melt, it does not decompose, it is subject to very slow aging and is also formulated to resist extreme environmental conditions. In particular, the rubber used for tyres is highly elas¬ tic and unyielding within a wide range of temperatures and is also extremely resistant to the attack of the com- monest chemical substances. As a result of this, over half of used tyres are destroyed by incineration and adopted for the generation of energy or for the feeding of cement furnaces. In addition to incineration, recycling technologies have been recently proposed, which envisage the distilla- tion of vulcanized rubber, devulcanization by breakage of the sulfur bridges and fragmenting to produce small sizes to be added to thermoplastic blends. At the present moment however, these technologies have not given satisfactory or acceptable results from an economical point of view. Recycling processes are also known which essentially comprise the pulverization of tyres and the use of the resulting powders as additives for natural raw rubbers to form new tyres. These recycling processes however also have draw¬ backs, such as the necessity of having to resort to the use of glues or aggregating materials which are neither inexpensive nor ecological . Furthermore, the quantity of rubber powders which can be added to the raw rubber during treatment is re¬ stricted by the size of the rubber particles. In particu¬ lar, superfine rubber particles can be added in signifi¬ cantly higher percentages than larger particles. In addi- tion, particles with lower dimensions lose their elastic¬ ity and pulverize more easily, making their use much sim¬ pler. In order to obtain fine particles, a fragmentation technology with high-speed blades can be used. This tech- nology does not allow the formation of micronized powders or powders with uniform particle dimensions . Finer rubber particles can only be produced through a cryogenic fragmentation technology which is effected at extremely low temperatures, for example by resorting to the use of liquid nitrogen. Due to the high energy re¬ quired for cooling the rubbers to these very low tempera¬ tures, however, the formation of powders with the cryo¬ genic technology is not economically advantageous. In the present state, the necessity is felt for new technologies which allow the recycling of rubber and in particular vulcanized rubber. One of the general objectives of the present inven¬ tion consists in providing new uses for a material which can be easily obtained from vulcanized rubber. A further objective of the present invention con¬ sists in providing a materials based on revulcanized rub¬ ber which can be used in various fields of the art . In view of these objectives and others which will appear more evident hereunder to experts in the field, a material based on vulcanized rubber is provided, which can be obtained by the revulcanization of a vulcanized rubber according to the enclosed claims 1-11. The Applicants have found that, contrary to a tech¬ nical preconception existing in the field of rubber recy- cling processes, by increasing the vulcanization degree of a prevulcanized rubber to be recycled, it is possible to obtain a revulcanized material which can be used in various fields of the art. This disclosure is in contrast with those of the known art according to which the chemical recycling of the vulcanized rubber of tyres can be obtained by means of a devulcanization process and consequently that only by the breakage of the S-S bridges is it possible to ob¬ tain a recyclable material . The material of the invention can be conveniently obtained by the revulcanization of vulcanized materials selected from natural rubber (NR) , polybutadiene (BR) , styrene-butadiene rubber (SBR) , ethylene-propylene-diene- terpolymer (EPDM) , isoprene (CR) , nitrile rubber (NBR) . In particular, the material of the invention can be ob¬ tained by the heating and pressing of a prevulcanized rubber to be recycled. The term prevulcanized rubber is meant to comprise a material containing particles of vulcanized rubber, in particular coming from used tyres. In general, vulcanization refers to a chemi¬ cal/physical treatment of a hydrocarbon rubber polymer of a natural or synthetic nature wherein the polymeric long chain molecules cross-link with molecules of a cross- linking agent, typically comprising sulfur. The cross- linking agent can be added in varying quantities depend¬ ing on the vulcanization degree to be reached. In the embodiment of the process of the invention, for example, it is possible to added sulfur in a quantity ranging from 0.01-50% by weight and preferably ranging from 5-20% by weight with respect to the quantity of rub¬ ber to be recycled. The sulfur can be added in the form of elemental sulfur or a substance, compound, component containing sulfur in a suitable form for the vulcaniza- tion of rubbers. According to an embodiment of the invention, the vulcanized rubber is put in contact with sulfur or a sub¬ stance containing sulfur, mixed, pressed for example in a suitable forming mould and heated to the vulcanization temperature. Before being subjected to the revulcanization treat¬ ment, the prevulcanized rubber is ground or crushed into a smaller size, typically with a length in the order of 0.01-2 cm. The revulcanization degree of the recycled rubber mainly depends on the following factors: temperature, pressure and revulcanization times and optionally the quantity of cross-linking agent added. Suitable cross-linking temperatures typically range from 110 to 25O0C, preferably between 150-2200C. The revulcanization treatment times can be estab¬ lished by experts in the field in relation to the cross- linking degree to be reached and typically vary from 30 seconds to 5 hours, preferably from 3 minutes to 20 min- utes . According to an embodiment, the preliminary grinding or crushing phase is effected adopting a blade crusher or with cryogenic grinding and is continued until a size is obtained with medium-small dimensions ranging from 3 cm to 0.01 cm, preferably ranging from 1 cm to 0.01 cm. It is possible, for example, to crush the rubber to be recy¬ cled into simil-pellet elements. The material coming from the mechanical grinding can conveniently consist of more or less spherical particles in the form of a powder/granulate, or fibres. The forma¬ tion of one form or another depends on the tools adopted. Before being formed, the material can conveniently be subjected to an extrusion/calendering process followed by forming in the mould also in various steps. A vulcanized rubber of any origin, natural or syn¬ thetic, coming, for example, from used tyres, worn pip¬ ing, flooring, etc., can be used as starting material. At the end of the treatment of the invention, the resulting product has a varying hardness depending on the vulcanization degree reached. The revulcanized material of the invention typically has a density ranging from 1.0 to 1.4 g/cm3. If low quantities of sulfur are added and milder re- vulcanization conditions are used, a semi-rigid rubber is obtained, whereas if greater quantities of sulfur and used together with higher temperatures, a rigid material of the thermosetting resin type, is obtained. In the process of the invention, it is optionally possible to add one or more fillers to the rubber to be revulcanized, typically in quantities varying from 50% to 0.0001% by weight with respect to the weight of the rub¬ ber to be treated. Dispersing agents, antioxidants, pig¬ ments, natural or synthetic rubbers, reinforcing agents, antioxidants, oils, foaming agents, flame-retardant addi- tives, expanding agents, retardants, accelerators (TBBS, ZMBT, MBT, TMTM) and vulcanization agents can also be op¬ tionally added in varying quantities, typically ranging from 0.001 to 20% by weight. It has also been surprisingly found that the product obtained is a revulcanized rubber which can be further recycled by subjecting it to additional vulcanization cy¬ cles by the addition of further sulfur under vulcaniza¬ tion conditions. The product obtained can be used in various applica- tions such as, for example, as a filling material, as a substitute for wood, for example for producing chip¬ boards, mdf, furniture, pallets or in the building indus¬ try and in carpentry as a filler or for producing insu¬ lating panels. The material of the invention is not fri- able and is suitable for being processed as a substitu¬ tive material for wood as it can be nailed and cut using common cutting means . The rubbers which can be treated according to the process of the invention are generally vulcanized rubbers of a natural or synthetic nature and are typically rubbers coming from used tyres. The revulcanization treatment of the invention therefore allows the advantageous recycling from an eco¬ nomical point of view of a material otherwise destined for disposal or incineration with serious consequences with respect to environmental impact. According to another aspect of the invention, a re- vulcanized rubber product is obtained by means of the re- vulcanization process described above and having a spe¬ cific weight conveniently ranging from 1 to 1.4 g/cm3 and a significant water impermeability. If the cooling of the material/revulcanized end- product is effected immediately after the forming, a product is surprisingly obtained, having higher mechani¬ cal and aesthetical characteristics with respect to a sample obtained by means of air cooling. According to an embodiment, the cooling can be ef¬ fected in the same forming mould or in an already cooled mould and pressure can be applied during the cooling phase, for example with values within the range of 1 to 150 Kg/cm2, preferably from 10 to 100 Kg/cm2. According to an embodiment, the cooling of the vul¬ canized material is effected by wetting the material it¬ self or the mould with water. Alternatively, cooling methods used in the moulding processes of end-products can be adopted. The product of the invention can be superimposed in two or more layers having different vulcanization degrees and consequently a different rigidity. It is possible, for example, to superimpose layers with a pre-established rigidity correlated to the revulcanization degree. The stratification can be advantageously obtained without the use of adhesive or gluing materials, by the superimposition of two or more layers of vulcanized or revulcanized rubber and pressing with thermal treatment. A typical structure with a differentiated rigidity com¬ prises an internal layer of revulcanized rubber lined with two outer layers of revulcanized rubber having a greater rigidity of the inner layer. The product obtained from the revulcanization proc- ess of the invention has high mechanical properties and light resistance with respect to wood together with a high water impermeability. According to an embodiment, the material of the in¬ vention can also be processed in subsequent phases to ob- tain mouldings. According to this embodiment, the neces¬ sary sulfur can be added to the rubber to be recycled and a partial revulcanization can be effected reducing, for example, the time and/or temperature necessary for ob¬ taining the desired product. The prevulcanized product can be subsequently formed in a mould with suitable oper¬ ating conditions. In this way three-dimensional objects can be easily obtained. According to an embodiment, the prevulcanized mate¬ rials according to the invention is subsequently revul- canized. According to another aspect of the invention, a re- vulcanization process of vulcanized rubber is provided according to one of the claims 18-21. The method of the invention typically comprises the heating of vulcanized rubber to a vulcanization temperature, with the optional addition of sulfur and subsequent cooling of the vulcan¬ ized material. According to an embodiment of the invention, a proc¬ ess is envisaged for producing an end-product made of re- vulcanized rubber comprising the feeding of a mould with a vulcanized rubber, the revulcanization of the rubber as referred to above, preferably under temperature condi¬ tions ranging from 180 to 22O0C and a pressure ranging from 10 to 100 Kg/cm2, and the forming of the end- product, after rapid cooling, obtained, for example, by wetting the mould, with or without pressure, or the end- product removed from the mould, with water. The following examples are provided for purely illustrative purposes of the present invention and should in no way be considered as limiting its protection scope as indicated in the en¬ closed claims . Example 1 A sample of 600 g of vulcanized rubber coming from a used tyre was crushed in a blade grinder giving a powder with an average size ranging from 0.0001 to 0.1 cm. The ground material was subsequently mixed with sulfur in a quantity equal to 12% by weight with respect to the total weight of the sample to be treated. The ground sample containing sulfur was fed to an aluminum 30 cm x 30 cm mould and brought to moulding con¬ ditions with a pressure of 55 Kg/cm2, temperatures of 2000C and a duration of 5 minutes. The formed sheet had a high rigidity degree and was suitable for being cut, glued, smoothed, nailed analo- gously to wood. Example 2 A sheet produced according to the method of example 1 and having a thickness of about 1 cm was lined with two outer sheets of revulcanized rubber, also having a thick- ness of 1 cm. The outer layers had a greater hardness with respect to that of the inner layer and were obtained according to the procedure of example 1 but increasing the pressure to 85 Kg/cm2 and applying a temperature of about 2000C for a duration of about 25 minutes. The sand- wich lined sheet was inserted in a press and subjected to a pressure value of about 55 Kg/cm2 until complete adhe¬ sion. A multilayered end-product was obtained, equipped with a particularly rigid lining with characteristics similar to bakelite and subject to further stratifica- tion. The material was used in the building industry. Example 3 20 test samples coming from previously ground tyre vulcanized rubber, prepared with the method described in example 1, in the form of fibres or powder, were sub- jected to revulcanization treatment under the following conditions : The revulcanized samples had the following physico- mechanical characteristics

P1 : powder 0-1 mm P2 : powder 1-2 mm F : tyre fibres Example 4 A sheet of material 5 mm thick, prepared as in exam¬ ple 1 but with a vulcanization T of 1500C, was placed in a 20 cm x 20 cm mould thus allowing a raised edge to be obtained. A pressure of 80 Kg/cm2 was then applied for 5 minutes obtaining a three-dimensional end-product having a high rigidity. Example 5 5 g of mineral oil were added to 100 g of fine tyre powder and the mixture was brought to a temperature of 1500C, 20 g of sulfur were then added and the mixture was passed several times in a calender heated to 18O0C. After 10 passages of material, it was compacted and a 5 mm thick strip was obtained. A piece of strip was then put into a mould and brought to 2000C for 5 minutes obtaining a hard sheet . Example 6 A sample of 600 g of fine powder (0.0001-0.1 cm) coming from used tyres was mixed with sulfur in a quan¬ tity equal to 12% by weight with respect to the total weight of the sample to be treated. 3% of expanding balls were then added (EXPANCEL 092DU120, AKZO) . The sample was then fed to a 30 cm x 30 cm aluminum mould and brought to moulding conditions with a pressure of 55 Kg/cm2, temperatures of 1800C for a duration of 5 minutes, the pressure was then reduced to 20 Kg/cm2 for 3 minutes . The material has a greater volume with respect to the material of example 1. Example 7 A sample of 600 g of material produced following the method of example 1, was ground and mixed with sulfur in a quantity equal to 8% by weight. The ground sample containing sulfur was fed to a 30cm x 30cm aluminum mould and brought to moulding condi¬ tions with a pressure of 55 Kg/cm2, temperatures of 2000C for a duration of 5 minutes. The sheet formed had a high degree of hardness and was suitable for being cut, glued, smoothed, nailed, analogously to wood.