WO2011117699A1

WO2011117699A1 - Element made of composite material, plant and method to make said element

Info

Publication number: WO2011117699A1
Application number: PCT/IB2011/000545
Authority: WO
Inventors: Valdi Artico
Original assignee: Friul Filiere Spa
Priority date: 2010-03-22
Filing date: 2011-03-15
Publication date: 2011-09-29
Also published as: IT1402684B1; RU2012144573A; CO6620003A2; EP2550143A1; BR112012024122A2; ITUD20100054A1; ITUD20100146A1; IT1399064B1

Abstract

Element made of composite material (10), plant (11) to produce the element (10) and relative method, in which at least a mixing station (13) is provided in which a first plastic component and a second component are mixed together, and at least a station for extrusion and drawing passage through a draw-plate (15), in which said mixed first and second components are extruded and drawn in order to obtain the element (10) having a shaped cross section. The plant (1 1) comprises at least a dispenser member (32) to dispense a chemical expanding agent in powdered form inside the mixture, in order to determine a closed-cell expansion of the material and a porous internal conformation of the element (10). The element (10) thus obtainable is a composite mono-material which comprises an internal core having a form and cross section shaped in a desired manner and a closed-cell porous conformation, made by means of expansion, and an external covering part or surface having a surface hardness and specific weight higher than the value of hardness and specific weight of the internal core.

Description

ELEMENT MADE OF COMPOSITE MATERIAL, PLANT AND METHOD TO MAKE SAID ELEMENT

FIELD OF THE INVENTION

The present invention concerns an element made of composite material formed by a first component with a plastic base and by a second component. The present invention also concerns the plant and the method to obtain the element made of composite material and to confer upon it determinate structural characteristics. In particular, the element made of composite material according to the present invention is applied widely, but not exclusively, in the building trade and/or in the field of furniture, such as components for fixtures, frames, inserts, skirting boards, shelves, furniture or others, advantageously but not exclusively for interiors.

BACKGROUND OF THE INVENTION

In particular but not only in the field of buildings, it is known to use elements made of composite materials, which substitute natural materials such as wood, metal or others, in order to reduce the costs relating to the construction materials while still guaranteeing the same conditions of mechanical and structural resistance of the components.

In particular, it is known to provide plastic or plastic-based elements, loaded with differing percentages of vegetal fibers, for example wood or others, in order to obtain determinate structural and mechanical characteristics.

In order to guarantee the structural characteristics of the natural materials, the wood-plastic composite materials have a very high specific weight and increased thicknesses, thus increasing the overall production costs and the installation spaces.

It is currently known to make the structural elements made of composite material by extrusion.

The traditional equipment used for the extrusion of known profiles substantially comprises an extrusion screw or Archimedes screw and a draw- plate through which the cross section of the structural element is defined, through specific forced passes. Other types of known plant provide to make slabs of composite material, from which the structural elements are then made by means of mechanical working.

In this second type of known plant it is possible to expand the composite material by blowing in gas during the extrusion process.

The expansion of the material by means of gas is possible, in this solution, since by making slabs of composite material we obtain substantially flattened and compact cross sections. Furthermore, in this known solution, the degree of expansion is not easily managed and the density of the products obtained in this way is in the order of about 20 kg/m .

The application of a similar expansion technique in an extrusion plant with a draw-plate would entail the loss of the conformation induced by the draw-plate at exit therefrom, and would greatly limit the possible productions of the profiles and their potential functional applications.

The document US-A-2007/0054107 is known, which describes a method to make a molded component by disposing a core comprising a slab of expanded material, with a density comprised between 250 kg/m³ and 800 kg/m³, possibly with reinforcement inserts or fibers, in a mold containing a thermosetting plastic material at low temperature and pressure (LPSMC, for example Crystic Impreg 6503 or Crystic Nupreg H 30), which is melted and polymerized. The core is formed by polyurethane or phenolic resins, or mineral fibers bonded with resins, or metals, wood, plastic or ceramic, and has cavities in which, at the appropriate temperature and pressure conditions in the mold, the LPSMC material flows and is distributed. The composite thus obtained is formed by two distinct components, of which a first is the foamed material that is encapsulated in the second component, or the LPSMC material, thus achieving a lattice structure of expanded material covered with LSPMC material.

Document US-A-2004/0142160 describes a method to extrude a composite based on wood fiber, thermoplastic polymer based on acrylo nitrile styrene, acrylo nitrile butadiene styrene resin and an expanding agent. The composite thus obtained has homogeneous physical and mechanical properties, with a density of not more than about 600 kg/m³.

One purpose of the present invention is to achieve an element made of composite material which has good mechanical properties, in particular hardness, but which is also light and is easy and economical to make, and which guarantees an efficient dimensional stability and an overall reduction in the costs and weight of the components of which it is made.

Another purpose of the present invention is to perfect a plant and a method to make, simply and economically, an element made of composite material, and which guarantees an efficient dimensional stability and an overall reduction in the costs and weight of the components of which it is made.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.

In accordance with the above purposes, an element made of composite material according to the present invention comprises at least a first plastic component, preferably a thermoplastic polymer, more preferably in powdered form, and a second component selected from one or more components in a group comprising vegetal fibers, special waste, glass fibers, flour or dust from plastic laminate. The first and second components are mixed together to define a composite mono-material with high structural properties.

According to the present invention, the composite mono-material comprises an internal core having a form and cross section shaped in a desired manner by extrusion and drawing passage through a draw-plate, and a closed-cell porous conformation achieved by means of expansion, and an external covering part or surface having a surface hardness and specific weight higher than the value of hardness and specific weight of the internal core.

The present invention thus obtains a composite mono-material element which has both a specific conformation in relation to operating needs, and also high physical-technical characteristics which make it structural, hard and rigid, and also a lower weight than the elements made of composite material of a known type.

In fact, apart from the known shaping advantages given by production by means of extrusion and drawing passage through a draw-plate, the closed-cell internal porosity determined by the expansion allows on the one hand to reduce the specific weight of the element made of composite material and on the other hand to keep the physical-technical characteristics of the composite material unchanged.

The mono-material obtainable with the present invention not only has a lower specific weight but also an external surface with high mechanical properties, in particular hardness, which avoids the need to apply a supplementary covering film or sheet which increases its mechanical characteristics.

In this way, the invention obtains a simple and economical element made of composite material, with an overall reduction in costs and the weight of the components, while still guaranteeing an effective dimensional stability.

Furthermore, the element made of composite material thus achieved has self- extinguishing characteristics, and thanks to the hydrophobic characteristics of the plastic component, allows a rapid and substantially total discharge of the water, if any humidity or a certain mass of water penetrates between the porosity of the material.

Therefore, unlike in known solutions where the hygroscopic characteristics of the vegetal fiber entail a substantially irreversible swelling of the element made of composite material, with the solution according to the present invention, there is no substantial risk of irreversible swelling and/or deformation of the element made of composite material.

According to the present invention, the expansion achieved to make the closed-cell porosity is of a chemical type, that is, a desired quantity of powdered chemical expanding agents, which during the extrusion step determine the formation of the closed cells in the composite material, are mixed with the first component and the second component.

This variant solution, guaranteeing a closed-cell conformation of the porosity, allows the composite material to be worked substantially with equipment traditionally used for the mechanical working of wood or wood materials.

According to another variant, the percentage in weight of the second component in the composition of the composite material reaches up to about 60%. Advantageously, the surface hardness measured according to the Shore D scale has a value comprised between about 55 and about 75, while in Shore A scale it has a value comprised between about 90 and about 100.

The plant to produce the composite material comprises at least a mixing station, where the first and second components as above are mixed so as to define a composite mono-material, and at least a station for extrusion and drawing passage through a draw-plate where the mixed materials are extruded and drawn to obtain an element made of composite material having a desired form and cross section.

According to the invention, the mixing station comprises at least a member to dispense an expanding agent, advantageously a chemical expanding agent, inside the mixture, to determine a closed-cell expansion of the material during the extrusion step and to determine a porous internal conformation of the element made of composite material.

Furthermore, the station for extrusion and drawing passage through a draw- plate comprises at least an extrusion member provided with an extrusion screw, or Archimedes screw, able to define an amalgam of material, and a draw-plate directly downstream of the extrusion member.

According to the present invention, the station for extrusion and drawing passage through a draw-plate comprises calibration means disposed directly at exit from the draw-plate, gripping the extruded and drawn material as it comes out, which are able to define a desired form and section of the material.

The disposition of the calibration means directly downstream of the draw-plate allows to manage effectively the degree of expansion of the material, which is limited by the substantially zero interspace between the exit from the draw-plate and the entrance to the calibration means.

The calibration means, in direct grip at exit from the draw-plate, allow to achieve the mono-material with different hardness and specific weight between an internal core and an external covering part or surface.

In fact it is possible to adjust the final product to the desired specific weight and expansion, and hence hardness, without the product expanding in an uncontrolled manner at exit from the draw-plate and before the calibration means; in this way the internal core is obtained with a cross section shaped in a desired manner by extrusion and drawing passage through a draw-plate and a closed-cell porous conformation made by means of the above expansion, and the external covering part or surface having a surface hardness and specific weight higher than the value of hardness and specific weight of the internal core.

According to a variant, the invention provides a plurality of extrusion members operatively disposed parallel to each other.

In this way it is possible both to increase productivity and also to produce series of different conformations of elements made of composite material.

Advantageously, the extrusion or Archimedes screw has a conformation such as to define a plurality of operating portions, each having a determinate pitch and/or a relative core diameter.

According to a variant, the extrusion or Archimedes screw has at least an operating portion having a determinate ratio between the pitch and core diameter such as to induce a de-gassing of the composite material treated, before extrusion.

The method according to the present invention comprises at least a mixing step in which the first and second components as above are mixed together, and at least an extrusion and drawing step through a draw-plate, in which the mixed materials are extruded and drawn in order to obtain a element made of composite material having a desired cross section.

According to the invention, the method also comprises an expansion step, substantially simultaneous with the extrusion and drawing through a draw-plate step, in which a closed-cell expansion occurs such as to define a porous internal conformation of the element made of composite material.

Furthermore, according to the present invention, a calibration step is provided, to define a desired form and section of the final profile, directly after it passes through the draw-plate, that is, calibrating the extruded and drawn material as soon as it has been extruded, without providing intermediate movements in which the material might expand uncontrollably.

This allows to manage the degree of expansion of the material efficiently, adjusting the final product to the desired specific weight without it expanding uncontrollably at exit from extrusion and before calibration, thus obtaining an internal core having a cross section shaped in a desired manner by extrusion and drawing passage through a draw-plate, and a closed-cell porous conformation achieved by means of said expansion, and an external covering part or surface having a surface hardness and specific weight higher than the hardness and specific weight of the internal core.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawings wherein:

- fig. 1 is a partial and three-dimensional view of an element made of composite material according to the present invention;

- fig.2 is a schematic view of a lay-out of a plant according to the present invention to make the element made of composite material in fig. 1 ;

- fig. 3 shows schematically a detail of the plant in fig. 2;

- fig. 4 shows schematically an enlarged detail of the plant in fig. 2.

DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF

EMBODIMENT

With reference to fig. 1, the reference number 10 denotes in its entirety an element made of composite material, in this case a cover profile, having a substantially arched shape.

The element 10 is formed by a composite mono-material with good structural properties: by mono-material we mean an element formed by the material in a single piece or body, not by the joining, association or incorporation of several distinct elements made of the same material, nor an element formed by different materials associated together.

The composite mono-material in question comprises an internal core having a closed-cell porous conformation made by expansion, and an external covering part or surface, formed in a single piece by the same material as the internal core, which at least partly surrounds the internal core itself and having a surface hardness and specific weight greater than the value of hardness and specific weight of the internal core.

The external covering part or surface not only has greater properties of hardness but also a consequent greater rigidity than the internal core.

The composite material of the element 10 is formed by a single mixture of a first plastic component, preferably a thermoplastic polymer or resin, more preferably in powdered form, for example a polyvinyl chloride (PVC) resin, or a polyolefin such as polypropylene (PP), and a second component chosen from among vegetal fibers, such as for example wood, flour or dust from MDF (medium density fiber), jute, rice, hemp, possibly loaded with additives, such as calcium or magnesium carbonate, or special waste, glass fibers, flour or dust from plastic laminate, such as melamine resins. The first plastic component may advantageously be in powdered form.

Therefore, according to the present invention, the element 10 is formed by a single mixture of the two components identified above but, between the internal core and the external covering part or surface, following the specific treatment to which it is subjected, has different physical properties -in particular specific weight - and mechanical behavior - in particular hardness - as described above.

Advantageously, the minimum average thickness of the external covering part or surface that has the greater specific weight and hardness is comprised between about 0.2 and 0.5 mm.

Moreover, advantageously, the percentage in weight of the second component in the mixture that forms the element 10 is comprised between about 25% and about 60%.

In this case, the structural composite mono-material is plastic-fibrous vegetal with a PVC and wood fiber base, where the percentage in weight of the wood fiber can vary between about 25% and about 60%, with respect to the percentage in weight of PVC. Advantageously, the wood fiber is comprised between about 30% and about 40% in weight, with respect to the percentage in weight of PVC. In the form of embodiment shown of the element made of composite material 10, both the PVC and the wood fiber are mixed together in powder form, and not in granules or scales, as happens in the state of the art.

In the production steps of the element made of composite material 10, the mixture of the first and second component, for example but not only, of PVC and wood fiber powders, is extruded and drawn so as to confer on the element made of composite material 10 its definitive arched transverse conformation.

The element made of composite material 10 also has an internal closed-cell porous composition, since together with the first and second components, in this case PVC and powdered wood fiber, a plurality of powdered chemical expanding agents are also mixed, in the desired quantity and ratio, which in this case are both endothermic and exothermic.

A typical example of powdered chemical expanding agent that can be used in the present invention is azodicarbonamide, a crystalline powder with a molecular formula C₂H₄O₂N , generally used as an additive to produce expanded or foamed plastics. The thermal decomposition of azodicarbonamide, generally at about 200°C for the pure component or at lower temperatures, for example about 170°C for the component modified with additives, results in the development of nitrogen, carbon dioxide and ammonia gas, which are trapped as bubbles in the material to form the expanded article.

The use of a chemical expanding agent in powdered form is also advantageous because it costs less than an expanding gas.

The powdered chemical agents, during extrusion, due to the effect of the pressure and temperature variations that lead to the plasticization of the mixture, determine an expansion of the material, rendering it porous in a closed-cell conformation. The expansion of the material is controlled to obtain the desired specific weight and hardness of the internal core and the external covering part or surface of the element 10, subjecting the extruded material, directly downstream of the extrusion, to calibration so as to determine the desired form and section of the final profile, without giving space and time to the material to expand uncontrollably.

The closed-cell porous conformation mainly determines a reduction in the weight of the element made of composite material 10, at the same time allowing to not vary excessively the characteristics of mechanical resistance of the composite material obtained.

Applicant has found that the element 10 thus made of composite material has a different specific weight between the internal core and the external covering part or surface: in particular, the internal core has a specific weight comprised between about 400 kg/m³ and about 600 kg/m³, and the external covering part or surface has a specific weight comprised between about 1000 kg/m³ and about 1400 kg/m³.

Applicant has found that the element made of composite material 10 has a surface hardness in Shore D scale comprised between about 55 and about 75, while in Shore A scale it is comprised between about 90 and about 100.

According to some experiments made by Applicant on the element made of composite material 10, it has been found that the closed-cell composite material has the following technical characteristics:

- water absorption at 50°C for 48 hours comprised between about 25% and about 30%, advantageously about 28%;

- resistance to flexion comprised between about 20 MPa and about 30 MPa, advantageously about 26 MPa;

- a Vicat (1 kg in oil) softening temperature comprised between about 80°C and about 85°C, advantageously about 83°C;

- class 94 V-O in self extinguishing;

- heat shrinkage comprised between about 5x10^"5 cm/(cm°C) and about 6x10^~5 cm/(cm°C), advantageously 5.5xl0^"5 cm/(cm°C); and

- coefficient of heat conductibility comprised between about 0.060 Kcal/(mh°C) and about 0.0650 Kcal/(mh°C), advantageously 0.062 Kcal/(mh°C).

The mixture of the mono-material according to the present invention can be integrated with one or more mineral loads, such as the calcium or magnesium carbonates cited above, non-toxic stabilizers, modifiers, lubricants, also depending on the type of plastic component used.

With particular reference to fig. 2, a plant 1 1 is shown in its entirety for the production of the element made of composite material 10 according to the present invention.

In this case, the plant 1 1 comprises in sequence a loading station 12, a mixing station 13 and a station for extrusion and drawing passage through a draw-plate 15.

The loading station 12 comprises a drying device 16 and two loading devices, respectively a first 17 for the first component, for example powdered PVC, and a second 19 for the second component, for example powdered wood fiber.

In one form of embodiment, the powdered PVC and/or wood fiber have a grain size comprised in a range varying between about 150 μπι and about 250 μηι.

The drying device 16 comprises a containing silo 20, into which the powdered wood fiber is poured.

The containing silo 20 is prepared in a substantially known manner, and is fluidically connected at exit to a drying conduit 21, through which the powdered wood fiber is induced to pass until it is poured, in a dried condition, into the second loading device 19.

The movement of the powdered wood fiber inside the drying conduit 21 is induced by a blower 22, suitable to cause it to exit from the containing silo 20.

Furthermore, the drying device 16 comprises a pair of burners 23, which are suitable to blow in hot drying air inside the drying conduit 21.

At the end of the drying conduit 21 and upstream of the second loading device 19, a cyclone member 25 is provided, which carries out an action of removing water from the stream of powdered wood fiber exiting from the drying conduit 21.

In this way, completely dry powdered wood fiber, substantially without any water, is poured into the second loading device 19.

The loading station 12 also comprises two additive dispensers, respectively a first 26 for stabilizing additives and a second 27 for modifying additives.

The first loading device 17, the second loading device 19 and the two dispensers 26 and 27 are connected to each other at exit through a conveyor pipe 29.

The exits of both the first loading device 17 and the second loading device 19, and of the two dispensers 26 and 27, are selectively adjusted by means of relative valve members 30 to adjust the percentages in mixing weight of the first and second component, for example powdered PVC, powdered wood fiber and additives, according to the indications above.

The powders and additives are thus conveyed by the conveyor pipe 29 toward the mixing station 13.

The mixing station 13 comprises a weighing device 31, a dispenser 32, a mixer 33 and a cooler 35.

The weighing device 31 is directly connected to the conveyor pipe 29 of the loading station 12, and is able to weigh all the powders and additives arriving from the loading station 12, before introducing them into the mixer 33.

By weighing the mixture it is possible to rationalize the load inside the mixer 33 and obtain greater accuracy in loading other additives by the dispenser 32, as will be described in more detail hereafter.

The mixer 33 is of a substantially traditional type and allows to uniformly and homogeneously mix the first and second component, for example the powdered PVC, the powdered wood fibers and the additives.

The cooler 35 is disposed immediately downstream of the mixer 33 and is suitable to lower the temperature of the powders and additives after mixing.

The dispenser 32 is disposed operatively parallel with respect to the weighing device 31 and the mixer 33, in order to introduce into the cooler 35, and therefore into the mixture of powders and additives, a further additive of the chemical expanding type.

In particular, the additive introduced has a composition of exothermic parts and endothermic parts, so as to exploit to the utmost the extrusion temperatures so as to determine an optimum closed-cell expansion of the material.

The mixing station 13 also comprises a transport screw 36 through which the mixture of powders, added with the chemical expander, is transported to the station for extrusion and drawing passage through the draw-plate 15.

In particular, the station for extrusion and drawing passage through the draw- plate 15 comprises a plurality of extruders 37 disposed in this case parallel and substantially all the same.

Each extruder 37 comprises at least an extrusion screw or Archimedes screw 39 and a draw-plate 40.

The extrusion screw 39 is advantageously suitable to extrude material in powder form.

Both the extrusion screw 39 and the draw-plate 40 can be provided different from the other extruders 37, so as to allow a parallel production of elements made of composite material 10 of different conformations.

In general, as shown schematically in fig. 3, the extrusion screw 39 is conformed on its length so as to define a plurality of operating portions each having a ratio between the pitch of the coils 39a and the diameter of the core 39b such as to define a determinate operating condition on the composite material being worked.

To give a non-restrictive example, the extrusion screw 39 comprises five distinct operating portions, respectively a first 41, a second 42, a third 43, a fourth 44 and a fifth 45.

In this case, the first operating portion 41 of the extrusion screw 39 is conformed to promote the feed of the mixture of powders and additives arriving from the transport screw 36, and provides a substantially constant pitch of the coils 39a.

The second operating portion 42 of the extrusion screw 39 is conformed to promote the compression of the mixture of powders and additives, and provides a substantially variable pitch of the coils 39a.

The third operating portion 43 of the extrusion screw 39 is conformed to promote the plasticization of the mixture of powders and additives, and provides a very close pitch of the coils 39a and an increased diameter of the core 39b.

The fourth operating portion 44 of the extrusion screw 39 is conformed to promote the de-gassing of the plasticized composite material, and provides a substantially wide and constant pitch of the coils 39a and a reduced diameter of the core 39b.

The fifth operating portion 45 of the extrusion screw 39 is conformed to promote the dosing to the draw-plate 40 of the plasticized composite material, and provides a substantially constant pitch of the coils 39a.

In this way, the de-gassing effected due to the conformation of the fourth operating portion 44 of the extrusion screw 39 determines a further de- humidification of the mixture, and in particular of the wood fibers, allowing to obtain the above technical characteristics of the element made of composite material 10.

The draw-plate 40 is associated in this case with a calibrator-carrying bench, with a length calculated according to the desired form and section of the profile to be cooled, a tracked drawing device, a circular saw cutter and a profile collection bench.

In particular, according to the present invention, a calibration unit 50 is provided in direct cooperation with the draw-plate 40, to determine the desired final form and section of the product (fig. 4).

According to the present invention, the calibration unit 50 is disposed directly downstream of the draw-plate 40, where the arrow F indicates the sense of advance of the material, so that between the exit 40a of the draw-plate 40 and the entrance 50a to the calibration unit 50 the space is limited, substantially zero.

This reciprocal disposition of the draw-plate 40 and the calibration unit 50 determines that the material just drawn is directly gripped by the calibration unit 50, and this allows to prevent an uncontrolled expansion of the material that has just been extruded and drawn; it is thus fed directly to the calibration unit 50 where, instead, the form and section are kept controlled and the desired specific weight and the desired surface hardness are obtained, as described above.

The element made of composite material 10 at exit from the draw-plate 40 passes through these passages so as to be cooled and to reach at the end the dimensional stability laid down by specifications.

The values of surface hardness and the specific weight of the element made of composite material 10 thus obtained, and indicated above, can be increased or decreased proportionally depending on the increase or reduction in compression of the material in transit in the calibrators.

EXAMPLES

Hereafter, all the parts are given in weight and the expression "phr", or "per hundred of resin", is used to indicate parts in weigh for 100 parts in weight of polymer.- The expression "phr" is commonly used in composite materials with resin systems, that is, where there is a resin with additives.

Example 1

Composite mono-material according to the present invention formed by a mixture consisting of the components of the following Table 1.

Component Phr

PVC resin 100

Mineral load 6 - 30

Non-toxic stabilizer 3 - 5

Modifiers 4 - 10

Lubricants 0.1 - 1

Vegetal fiber 20 - 60

Powdered chemical expanders 1 - 3

Example 2

Composite mono-material according to the present invention formed by a mixture consisting of the components in the following Table 2.

Component phr

Polyolefins 100

Modifiers 1 - 5

Lubricants 0.5 - 2

Vegetal fiber 20 - 60

Powdered chemical expanders 1 - 3

It is clear that modifications and/or additions of parts may be made to the element made of composite material 10, the plant 1 1 and the method as described heretofore, without departing from the field and scope of the present invention. It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of element made of composite material, plant to make said element made of composite material and the relative method of production, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

Claims

1. Element made of composite material comprising at least a first plastic component, preferably a thermoplastic polymer, and a second component selected from one or more components from a group comprising vegetal fibers, special waste, glass fibers, flour or dust from plastic laminate, said element having a cross section shaped by extrusion and drawing passage through a draw- plate, characterized in that said first and second component are mixed with each other so as to define a composite mono-material, said mono-material comprising an internal core having a closed-cell porous conformation, made by means of expansion, and an external covering part or surface having a surface hardness and specific weight higher than the value of hardness and specific weight of the internal core.

2. Element as in claim 1, characterized in that the closed cell expansion is of the type induced with powdered chemical expanding agents.

3. Element as in claim 2, characterized in that the powdered chemical expanding agents are endothermic and exothermic.

4. Element as in claim 1, 2 or 3, characterized in that the hardness of the external surface on the Shore D scale has a value comprised between about 55 and about 75.

5. Element as in claim 1, 2, 3 or 4, characterized in that the hardness of the external surface on the Shore A scale has a value comprised between about 90 and about 100.

6. Element as in any claim hereinbefore, characterized in that the percentage in weight of the second component in the composition of the composite material is up to about 60% with respect to the percentage in weight of the first plastic component.

7. Element as in any claim hereinbefore, characterized in that it has a specific weight comprised between about 400 kg/m³ and about 600 kg/m³, and the external covering part, or surface, has a specific weight comprised between about 1000 kg/m³ and about 1400 kg/m³.

8. Element as in any claim hereinbefore, characterized in that it has a water absorption at 50° per 48 hours comprised between about 25% and about 30%.

9. Element as in any claim hereinbefore, characterized in that it has a resistance to flexion comprised between about 20 MPa and about 30 MPa.

10. Element as in any claim hereinbefore, characterized in that it has a Vicat

(1 kg in oil) softening temperature comprised between about 80°C and about

85°C.

1 1. Element as in any claim hereinbefore, characterized in that it is class 94 V-O in self extinguishing.

12. Element as in any claim hereinbefore, characterized in that it has a heat shrinkage comprised between about 5xl0^~5 cm/(cm°C) and about 6xl0^"5 cm/(cm°C).

13. Element as in any claim hereinbefore, characterized in that it has a coefficient of heat conductibility comprised between about 0.060 Kcal/(mh°C) and about 0.0650 Kcal/(mh°C).

14. Element as in any claim hereinbefore, characterized in that the minimum average thickness of the external covering part or surface with the higher surface hardness and specific weight is comprised between about 0.2 mm and about 0.5 mm.

15. Plant to make an element (10) made of composite material comprising at least a mixing station (13), in which a first plastic component, preferably a thermoplastic polymer, and a second component selected from one or more components in a group comprising vegetal fibers, special waste, glass fibers, flour or dust from plastic laminate, are mixed with each other, and at least a station for extrusion and drawing passage through a draw-plate (15), in which the mixed components are extruded and drawn so as to obtain said element (10) having a shaped cross section, characterized in that in the mixing station (13) said first and second component are mixed together so as to define a composite mono-material and in that said plant comprises at least a dispenser member (32) to dispense a powdered chemical expanding agent inside the mixture, so as to determine the closed cell expansion of the material and to determine a porous internal conformation of said element (10), and in that the station for extrusion and drawing passage through the draw-plate (15) comprises at least a draw-plate (40) and calibration means (50) suitable to determine a calibration pressure on the material so as to determine the desired final form and section of the element (10), wherein the calibration means (50) are disposed directly downstream of the draw- plate (40) so as to reduce or cancel the uncontrolled expansion space of the material between the draw-plate (40) and the calibration unit (50) so that the element (10) has an internal core having a closed cell porous conformation made by means of expansion and an external covering part or surface having a surface hardness and specific weight higher than the value of hardness and specific weight of the internal core.

16. Plant as in claim 15, characterized in that the station for extrusion and drawing passage through the draw-plate (15) comprises at least an extrusion member (37) able to define an amalgam of material.

17. Plant as in claim 16, characterized in that the station for extrusion and drawing passage through the draw-plate (15) comprises a plurality of extrusion members (37) operatively disposed in parallel.

18. Plant as in claim 16 or 17, characterized in that the extrusion member (37) comprises at least an extrusion screw, or Archimedes screw (39), which has a conformation such as to define a plurality of operative portions (41, 42, 43, 44, 45), each having a determined pitch (39a) and/or a relative core diameter (39b).

19. Plant as in claim 18, characterized in that the extrusion screw or Archimedes screw (39) has at least an operative portion (44) having a determinate ratio between pitch (39a) and core diameter (39b) such as to induce the degassing of the composite material.

20. Method to make an element made of composite material, comprising at least a mixing step in which at least a first plastic component, preferably a thermoplastic polymer, and a second component selected from one or more components in a group comprising vegetal fibers, special waste, glass fibers, flour or dust from plastic laminate, are mixed with each other, and at least a step of extrusion and drawing passage through a draw-plate, in which the mixed components are extruded and drawn so as to obtain said element (10) having a shaped cross section, characterized in that in the mixing step said first and second component are mixed together so as to define a composite mono-material, in that said method also comprises an expansion step, substantially at the same time as said step of extrusion and drawing passage through a draw-plate, in which a closed cell expansion occurs such as to define a porous internal conformation of said element (10), and in that it provides at least a calibration step exerting a determinate calibration pressure on the material so as to determine the desired final form and section of the element (10), wherein said calibration step is performed directly downstream of the step of extrusion and drawing passage through a draw-plate so as to reduce or cancel the uncontrolled expansion space of the material between the draw-plate and the calibration unit so that the element (10) has an internal core having a closed cell porous conformation made by means of expansion and an external covering part or surface having a surface hardness and specific weight higher than the value of hardness and specific* weight of the internal core.

21. Method as in claim 20, characterized in that upstream of said mixing step at least a drying step is provided to dry the second component.