A composite material, in particular laminated composite material

Abstract

Disclosed herein is a composite material (1) comprising a polymer matrix (2) and fibres (4) arranged in said polymer matrix (2), wherein: - the polymer matrix (2) comprises a polyphenol in an amount equal to or greater than 15% by weight with respect to the weight of the polymer matrix (2), the fibres (4) have aa tensile modulus of elasticity, measured according to ASTM C1557, greater than 50 GPa, and are present in an amount equal to or greater than 80% by weight with respect to the weight of the polymer matrix (2).

Description

[0001] "A composite material , in particular laminated composite material"

[0002] ★ ★ ★ ★

[0003] TEXT OF THE DESCRIPTION

[0004] Field of the Invention

[0005] The present invention regards composite materials . It was developed with reference to the use of multilayer laminated composite materials used to produce structural components .

[0006] Prior Art

[0007] Polymer-matrix composite materials are commonly used in various f ields , such as aeronautics , automotive , building and transportation, thanks to their unique combination of properties of mechanical strength, chemical and physical stability, low weight .

[0008] However, polymer materials show poor fire and flame resistance and they tend to generate toxic combustion products ( gases or fumes ) .

[0009] Speci fically, the common thermosetting matrixes used in the production of polymer-matrix composite materials release , during a fire , a huge amount of heat , thereby making it necessary to use flame retardants to employ such materials - for example - for covering interior or outer room walls or ceilings .

[0010] Some of the most common flame retardants are halogenated retardants , phosphor-based retardants and metal hydroxide-based retardants . The halogenated retardants , such as brominated compounds , mainly act by inhibiting the phase of flame propagation by forming free radicals . However, they pose serious problems to health and environment , including the production of toxic and corrosive fumes during combustion .

[0011] Phosphor-based retardants , such as phosphates and phosphonates , are ef fective both in the gaseous and in the condensed phase , by forming a protective layer which prevents further combustion . Although they are less toxic than halogenated flame retardants , phosphor-based retardants may anyway release harmful substances under given fire conditions . Metal hydroxides , such as aluminium hydroxide and magnesium hydroxide , mainly act by absorbing heat and releasing water vapour, thereby contributing to cool the material and dilute combustion gases . A signi ficant disadvantage of the phosphor-based retardants is their low ef ficiency per mass unit , which requires the use of large amounts of flame retardant in order to achieve the desired ef fect , thus j eopardi zing the mechanical properties of the composite .

[0012] The problem is even more apparent in the presence of polyolefin polymer fibres , such as UHMWPE (ultra-high molecular weight polyethylene ) fibres . Such fibres are known for being used in composites having a protective function against impacts , bullets , splinters and the like .

[0013] However, due to their substantially nonpolar nature , such fibres exhibit a poor adhesion to the commonly used polymer matrixes , which adhesion may be further reduced in the presence of high amounts of fire retardants in the matrix .

[0014] A partial solution to the problem is a plasma treatment of the polyolefin fibres in order to functionali ze the surface thereof , to improve the adhesion of the resin; this , however, signi ficantly weakens the fibres .

[0015] Obj ect of the Invention

[0016] The invention aims at solving the technical problems outlined in the foregoing . Speci fically, the obj ect of the invention consists in providing a laminated composite material , characteri zed by high mechanical strength, specifically high tensile strength and high impact strength, low brittleness at breaking and good flame resistance . A further obj ect of the invention consists in providing a laminated composite material with improved adhesion between the matrix and the reinforcing fibres .

[0017] Summary of the Invention

[0018] The obj ect of the invention is achieved by means of a composite material having the features set forth in the claims that follow, which form an integral part of the technical disclosure provided herein in relation to the invention .

[0019] Brief Description of the Figures

[0020] The invention will now be described with reference to the annexed Figures , which are provided by way of non-limiting example only and wherein :

[0021] - Figure 1 is a schematic i sometric view of the composite material according to the invention,

[0022] - Figure 2 shows the results o f a TGA analysis o f UHMWPE (ultra-high molecular weight polyethylene ) fibres , and of the UHMWPE fibres treated with polyphenol ,

[0023] - Figures 3 , 4 show the results of a TGA and DTG analysis of a matrix for composite material cross-linked with amine , and of the analysis of a matrix for composite material according to the invention which is crosslinked with polyphenol ,

[0024] - Figures 5, 6 show the data of a calorimeter test for the composite with a matrix of epoxy resin crosslinked with amine and of the composite material with a matrix of epoxy resin cross-linked with polyphenol ( Figure 5 , Hrr = Heat Release Rate , kW / m2, Figure 6, THR = Total Heat Release , MJ / m2) ,

[0025] Figure 7 shows comparative data of tensile strength of composite materials with UHMWPE (ultra-high molecular weight polyethylene ) fibres in specimens which are denoted as "White" (matrix with amine cross-linker, according to the prior art ) , "TA resin" (matrix with polyphenol cross-linker, speci fically tannic acid, according to the invention) , "TA resin and fibres" (matrix with polyphenol cross-linker, speci fically tannic acid, and fibres functionali zed with polyphenol , speci fically tannic acid, according to the invention) ,

[0026] Figure 8 shows comparative data of tensile strength of composite materials with glass f ibres ( Glass ) and aramid fibres (Kevlar ) , in specimens of composite materials denoted as "White" (matrix with amine cross-linker, according to the prior art ) , "TA resin" (matrix with polyphenol cross-linker, speci fically tannic acid, according to the invention ) , "TA resin and fibres" (matrix with polyphenol crosslinker, specif ically tannic acid, and fibres functionali zed with polyphenol , speci fically tannic acid, according to the invention) ,

[0027] - Figure 9 shows comparative data of tensile modulus of elasticity of composite materials with glass fibres ( Glass ) and aramid fibres (Kevlar ) , in specimens of composite materials denoted as "White" (matrix with amine cross-linker ) , "TA resin" (matrix with polyphenol cross-linker, speci fically tannic acid) , "TA res in and fibres" (matrix with polyphenol cross-linker, speci fically tannic acid, and fibres functionali zed with polyphenol , speci fically tannic acid) ,

[0028] Figure 10 shows comparative data of flexural strength of composite materials with UHMWPE (ultra-high molecular weight polyethylene ) fibres in specimens of composite materials denoted as "White" (matrix with amine cross-linker, according to the prior art ) , "TA resin" (matrix with polyphenol cross-linker, speci fically tannic acid, according to the invention) , "TA resin and fibres" (matrix with polyphenol crosslinker, specif ically tannic acid, and fibres functionali zed with polyphenol , speci fically tannic acid, according to the invention) ,

[0029] Figure 11 shows comparative data of flexural modulus of elasticity of composite materials with UHMWPE (ultra-high molecular weight polyethylene ) fibres in specimens of composite materials denoted as "White" (matrix with amine cross-linker, according to the prior art ) , "TA resin" (matrix with polyphenol cross-linker, speci fically tannic acid, according to the invention) , "TA resin and fibres" (matrix with polyphenol crosslinker, speci fically tannic acid, according to the invention, and f ibres functionali zed with polyphenol , speci fically tannic acid, according to the invention) ,

[0030] Figure 12 shows comparative data of flexural strength of composite materials of composite materials with glass fibres ( Glass ) and aramid fibres (Kevlar ) in specimens of composite materials denoted as "White" (matrix with amine cross-linker, according to the prior art ) , "TA resin" (matrix with polyphenol cross-linker, speci fically tannic acid, according to the invention) , "TA resin and fibres" (matrix with polyphenol crosslinker, specif ically tannic acid, and fibres functionali zed with polyphenol , speci fically tannic acid, according to the invention) ,

[0031] Figure 13 shows comparative data of flexural modulus of elasticity of composite materials with fibres of composite materials with glass fibres ( Glass ) and aramid fibres (Kevlar ) in specimens of composite materials denoted as "White" (matrix with amine crosslinker ) , "TA resin" (matrix with polyphenol crosslinker, speci fically tannic acid) , "TA resin and fibres" (matrix with polyphenol cross-linker, speci fically tannic acid, and fibres functionali zed with polyphenol , speci fically tannic acid) ,

[0032] - Figure 14 shows comparative data of resilience ( Charpy) of composite materials with UHMWPE (ultra-high molecular weight polyethylene ) fibres in specimens of composite materials denoted as "White" (matrix with amine cross-linker, according to the prior art ) , "TA resin" (matrix with polyphenol cross-linker, speci fically tannic acid, according to the invention) , "TA resin and fibres" (matrix with polyphenol crosslinker, specif ically tannic acid, and fibres functionali zed with polyphenol , speci fically tannic acid, according to the invention) ,

[0033] - Figure 15 shows comparative data of resilience ( Charpy) of fibres of composite materials with glass fibres ( Glass ) and aramid fibres (Kevlar ) in specimens of composite materials denoted as "White" (matrix with amine cross-linker ) , "TA resin" (matrix with polyphenol cross-linker, speci fically tannic acid) , "TA res in and fibres" (matrix with polyphenol cross-linker, speci fically tannic acid, and fibres functionali zed with polyphenol , speci fically tannic acid) ,

[0034] - Figures 16 and 17 are similar to Figures 11 and 10 , but they regard comparative data of specimens of composite materials denoted as "White" (matrix with amine cross-linker ) and "TA Resin" (matrix with polyphenol cross-linker, speci fically tannic acid, and carbon fibres ) .

[0035] Detailed Description

[0036] Reference 1 in figure 1 generally and schematically shows a composite material according to various embodiments of the invention . The composite material 1 is of the laminated type , and in various embodiments of the invention it comprises a polymer matrix 2 and fibres 4 arranged in the matrix 2 , wherein :

[0037] - the polymer matrix 2 has a compressive modulus of elasticity, measured according to ASTM D695 , greater than 0 . 5 GPa, and comprises a polyphenol in an amount equal to or greater than 15% by weight with respect to the weight of the polymer matrix 2 ,

[0038] - the fibres 4 have a tensile modulus of elasticity, measured according to ASTM C1557 , greater than 50 GPa, and are present in an amount equal to or greater than 80% by weight with respect to the weight of the polymer matrix 2 .

[0039] Preferably, there may be present one or more layers of fibres 4 , with the same weight proportions with respect to the ensuing matrix 2 .

[0040] The fibres 4 are preferably continuous across the matrix 2 , and more preferably they are organi zed in such a way as to form a woven structure of fibres . The material 1 may comprise a plural ity of layers of woven fibres 4 with di f ferent orientations , bound together by the matrix 2 .

[0041] For the production of the material 1 it is possible to use , for example , the following features of fibre weave : plain, 2x2 twill , 4x4 twill , 2x2 double twill , four-thread satin, five-thread satin, eight-thread satin, unidirectional weave and biaxial weave .

[0042] For the production of the material 1 it is preferably possible to use continuous polymer fibres with a high elastic modulus , even more preferably polyolefin fibres , and in particular ultra-high molecular weight fibres ( known as UHMWPE fibres , or with the trade names Dyneema or Spectra ) or, as an alternative to polyolefin fibre , aramid fibres ( commercially known as Kevlar ) . The use of such fibres , which is common in the ballistic sector as well , enhances the impact strength of the material 1 .

[0043] Alternative embodiments envisage the use of mineral fibres with a high elastic modulus , in particular glass fibres . Such fibres exhibit an optimal behaviour as regards flame resistance , and therefore they are employed to optimi ze the flame resistance of the material 1 . The two types of fibres (polymer and mineral ) may also be used together, thereby obtaining a laminated material by employing hybrid woven structures ( formed by various types of fibres ) or woven layers of mutually di f ferent nature , to strike a compromise between the optimi zation of the two di f ferent aspects ( impact strength and fire resistance ) .

[0044] In yet further embodiments , the fibres 4 are carbon fibres .

[0045] As already mentioned in the foregoing, the fibres 4 of the material 1 according to the invention have a tensile modulus (measured according to the standard ASTM C1557 ) greater than 50 GPa . Preferably, it is possible to use fibres 4 with a tensile modulus of elasticity (ASTM C1557 ) greater than 60 GPa, even more preferably greater than 80 GPa (ASTM C1557 ) . Such values of tensile modulus of elasticity confer to the ( continuous ) fibres 4 the ability to transmit tensile stresses without causing an excessive deformation in the material 1 as a whole .

[0046] As already stated in the foregoing, according to the invention the fibres 4 are present in an amount equal to or greater than 80% by weight with respect to the weight of the polymer matrix 2 . Preferably, the fibres 4 may be present in an amount equal to or greater than 95% by weight with respect to the weight of the polymer matrix 2 , more preferably equal to or greater than 110% , in order to increase the tens ile strength of the material , which is mainly due to the presence of the fibres 4 .

[0047] However, there i s a limit to the percentage by weight of the fibres 4 which may be present in the material 1 , mainly in order to avoid having a spatial density of fibres which would hamper the penetration of a suf ficient amount of matrix 2 between the fibres during the production of the material 1 , while ensuring suf ficient compressive strength and impact strength of the laminate . In particular, the fibres 4 are present in an amount lower than 300% by weight with respect to the weight of the polymer matrix 2 , preferably lower than 250% by weight with respect to the weight of the polymer matrix 2 , even more preferably lower than 200% by weight with respect to the weight of the polymer matrix 2 .

[0048] In some preferred embodiments , the fibres 4 are treated by means of a surface functionalization based on a polyphenol solution, such as e . g . tannic acid, in order to increase the chemical compatibility thereof with the polymer matrix 2 and in order to ensure a better interaction between fibres and matrix, as well as good flame resistance . Preferably, it is possible to use polyphenols with a percentage by weight of tannins ( the percentage by weight is referred to the weight of the dry substance , and therefore , as known to the skilled in the art , does not consider the water content of the natural extracts , which may vary) greater than 50% , and GAE ( Gallic Acid Equivalent ) greater than 55% , more preferably - by way of a specific example - with a percentage by weight of tannic acid greater than 70% and GAE greater than 60% . Mixtures / solutions having lower percentages ( either percentage by weight of tannins , or GAE ) , indeed, are prone to exhibit excessive amounts of impurities , which make the production process of the pre-preg composite very di f ficult .

[0049] Further examples of polyphenols which may be used for the surface functionali zation of the fibres 4 may comprise one or more synthetic polyphenols and / or one or more polyphenol-based extracts of natural origin, such as e . g . quebracho extract , quebracho sulphite , chestnut extract , grapeseed extract .

[0050] Said treatment is particularly useful i f polyolefin fibres are used, since the nonpolar nature thereof weakens the adhesion of the matrix to the fibres. The polyphenols such as tannic acid are known for their thermostability, obtained thanks to the formation of intramolecular and intermolecular hydrogen bonds, which enhance the stability of the cross-linked polymer, as shown by the data of thermal gravimetric analysis (TGA) of Figure 2, diagram 10. The diagram shows, on the X- axis, the temperature (unit of measurement [°C] ) and on the Y-axis the percentage by weight of the (dimensionless) specimen. Specifically, it is to be noted that the specimen of the material 1, only comprising UHMWPE fibres which are not functionalized with polyphenol (curve 12) maintains its own weight up to about 300°C, then has a first abrupt drop beyond 300°C and a second final drop at about 450°C. The specimen of the material 1 with UHMWPE fibres 4 functionalized with a polyphenol (curve 14) undergoes a first slight decrease (about 5%) already at approximately 200°C, but beyond 200°C the weight remains substantially constant (the loss is limited to a total of 10%) , up to a collapse at a temperature of about 500°C. Not only is the latter temperature higher than the temperature of the final weight collapse for the curve 12, but up to 500°C the specimen is substantially unchanged.

[0051] As anticipated in the foregoing, the polymer matrix 2 has a compressive modulus of elasticity greater than 0,5 GPa. Preferably, the compressive modulus of elasticity is chosen to be greater than 1 GPa, more preferably greater than 1.5 GPa. The compressive modulus of elasticity may be determined by means of the ASTM D695 test, on a specimen having the same matrix formulation (including the possible presence of additives and hardening agents) , and subjected to the same curing cycle used for the production of the object of the invention, but not having reinforcing fibres . In combination with said values of the elastic modulus , in preferred embodiments the matrix 2 , is that of a thermosetting polymer .

[0052] A high compres sive modulus of elasticity of the matrix enables avoiding or retarding events of instability to compression of the fibres of the composite , while ensuring the necessary rigidity and compressive strength of the laminate as a whole . Moreover, it allows the pressure wave generated by an impact to spread more rapidly, by enlarging the si ze of the area subj ect to energy dissipation phenomena . For the production of the laminate 1 it is possible to use epoxy matrixes o f the type bisphenol A, bisphenol F or phenolic resins (novolacs ) . For the resin cross-linking it is possible to use polyphenols , preferably tannins , more preferably hydrolysable tannins , such as e . g . tannic acid . The latter performs not only the function of a cross-linker, but is also a flame retardant . According to the invention, the percentage of polyphenol in the matrix 2 (used as a cross-linker ) is equal to at least 15% by weight with respect to the weight of the matrix, preferably greater than 20% , more preferably than 30% .

[0053] Preferably, in accord with what has been stated in the foregoing, it is possible to use polyphenols with a weight percentage of tannins ( the weight percentage is again referred to the weight of the dry substance , for the reasons outlined in the foregoing) greater than 50% , and GAE ( Gallic Acid Equivalent ) greater than 55% , more preferably - by way of a speci fic example - with a weight percentage of tannic acid greater than 70% and GAE greater than 60% . Mixtures / solutions with lower percentages ( as regards both the weight percentage of tannins and GAE ) are indeed prone to contain excessive amounts of impurities , which make the production process of the pre-preg composite very di f ficult .

[0054] Further examples of polyphenols which may be used for cross-linking the resin of the matrix may comprise one or more synthetic polyphenols and / or one or more extracts based on polyphenols of natural origin, such as e . g . quebracho extract , quebracho sulphite , chestnut extract , grapeseed extract .

[0055] A high percentage of polyphenol in the resin guarantees a good polymeri zation degree and an increase of the flame retarding properties of the resin . A percentage of polyphenol lower than indicated leads to slow and costly production processes ( since it requires long curing times ) and makes the flame resistance properties negligible , also due to the high percentage of fibres in the material (which tends to dilute the percentage of cross-linker with respect to the overall mass of the material ) . Moreover, the presence of a high percentage of polyphenol tends to improve the interaction between fibres 4 and matrix 2 , thereby enabling a good interaction ( and thus a high strength of the composite material 1 ) also in the presence of a small percentage of matrix with respect to the fibres .

[0056] This is particularly evident for polyolefin fibres which, due to their nonpolar nature , tend to have a less- than-optimal adhesion to a matrix of epoxy resin in the absence of the use of polyphenol as a cross-linker .

[0057] Moreover, with re ference to Figure 3 and Figure 4 , the use of polyphenols as cross-linking agents in thermosetting resins of fers many advantages in comparison to the cross-linkers which are commonly used, such as amines , as regards both the performances of the materials and the impact on the environment . Their chemical structure , which is rich in phenolic groups , enables an ef ficient cross-linking with epoxy resins , thereby improving the thermal and mechanical properties of the final material .

[0058] In comparison with the amines , which may be toxic and irritant , the natural polyphenols have a far lower toxicity, thereby reducing health risks while handling and using the composite materials . Moreover, since they have a natural origin and thus are biodegradable , they contribute to lessen the environmental impact , both during the production and at the end of the li fe cycle of the product .

[0059] The natural polyphenols are known for their properties of fire resistance , in particular since they form a protective carbon layer during combustion, thus acting as a natural flame retardant of the composite . In short , polyphenols of fer a sustainable and less toxic alternative to traditional cross-linkers , while also improving the thermal and fire-resisting properties of the thermosetting resins and of the composite materials wherein they are employed .

[0060] In this regard, Figure 3 ( diagram 20 ) shows the data of thermal gravimetric analysis of a composite material of the prior art ( curve 22 ) with a matrix of epoxy resin cross-linked with an amine , and of a material 1 according to the invention, with a matrix 2 of epoxy resin cross-linked with polyphenol ( curve 24 ) . It is easy to notice that the weight loss of the material , due to the increase in temperature , takes place at substantially lower temperatures in the material of the prior art . The following diagram of Figure 4 moreover shows an evolution of the derivative of the weight as a function of the temperature , which also proves that in the material of the prior art ( curve 32 ) the weight loss takes place earlier and at a remarkably higher rate in comparison with the material 1 according to the invention

[0061] ( curve 34 ) . The diagrams 40 and 50 of the following Figures 5 and 6 further show the difference in the performances of a known material (curve 42, Figure 5 and curve 52, Figure 6) and of the material 1 according to the invention (curve 44, Figure 5 and curve 54, Figure 6) . A known material shows a maximum HRR value which is remarkably higher than the material 1, which is a sign of a rapid and poorly controlled combustion. On the contrary, the material 1 shows a profile of heat release which is flatter and more distributed in time, which is a sign of a prolonged resistance to combustion.

[0062] Similar results may be inferred from the curves of total heat release THR of Figure 6: the heat release by the known material (curve 52) is more rapid and stronger, and reaches its peak at around 250°C, at which temperature the material 1 (curve 54) still resists combustion .

[0063] Also the duration of the curing (cross-linking) process of the thermosetting matrixes, and the temperatures at which the process is carried out, may influence the mechanical properties of the laminate 1 (including the compressive modulus of elasticity of the matrix) ; therefore, the curing process is preferably carried out in such a way as to increase the elastic modulus .

[0064] When the material 1 uses UHMWPE fibres, the forming process (including the resin cross-linking) is carried out at a temperature lower than 140°C: higher temperatures might damage the UHMWPR fibres which are used as a reinforcement of the material 1.

[0065] In other embodiments of the laminate 1, however, further layers of traditional metal or composite material (e.g. carbon fibres in an epoxy matrix without flame retardants) in order to further improve the characteristics required by the application. Speci fically, it may be advantageous to envisage the presence , on at least one o f the opposite outer surfaces of the material 1 ( denoted in Figure 1 by the references 2A and 2B, and corresponding to the opposite faces of the laminate , according to what has been described in the foregoing) of a metal sheet or generally of a layer of a metal material . The layer of metal material improves flame resistance , thus reducing the portion of the laminate which can contact the oxidi zer, and at the same time it of fers a compressive strength greater than in the same material without a layer of metal material . Examples of metal materials which may be used to implement such a layer comprise : aluminium and alloys thereof , iron and alloys thereof ( in particular stainless steel or galvani zed steel ) and titanium alloys .

[0066] The material 1 according to the invention of fers higher impact and tensile strength per mass unit , thus contributing to reduce the weight . Therefore , it may be convenient to position the layer of metal material facing towards the direction from which an impact is expected to come , or in any case from which an external force is expected which may generate a bending moment on the material 1 in such a way as to compress the metal and to pull into traction the assembly of matrix 2 and fibres 4 ( a stress against which it of fers better resistance ) .

[0067] I f the material 1 according to the invention is bound to a layer of traditional composite material , the material 1 may of fer a protection against fire to the traditional composite material , by being interposed between the latter and the flames . At the same time, it can reduce the brittleness and improve the poor impact strength of the traditional composite material : especially i f the matrix 2 includes UHMWPE or aramid fibres , indeed, the material 1 of fers an impact and tensile strength per mass unit which are greater than in the traditional composite , although it involves a reduction in compressive strength .

[0068] As already discussed with re ference to bonding the material 1 with a layer of metal material , it may be convenient to position the layer of traditional composite material facing towards the direction from which an impact or an external force may presumably come , in such a way as to use the greater compressive strength thereof . In the case of an impact , the ductility of the material 1 according to the invention prevents the fragments generated by the brittle break of the traditional composite from being expelled or proj ected towards people or obj ects .

[0069] On the whole , the material 1 is characteri zed by very good performances of mechanical strength, in particular high tensile and impact strength (per mass unit of the composite ) , with a non-brittle breaking behaviour and good resistance to combustion . The features listed above make this material adapted to be used for structural purposes in the presence o f high safety requirements , where the need is also present to reduce the weight of the structures : for example , for the protection of the passengers in means of transport , as well as for the use in components of tanks , pressuri zed containers or for the transport of toxic and flammable liquids and gases ; moreover, for producing structural components the sudden collapse whereof (breaking due to brittleness or to fire ) may lead to safety problems .

[0070] With reference to Figures 7 to 15 , in the preferred embodiments , said technical ef fect is achieved thanks to the synergy of : a ) a high percentage of continuous fibres 4 , which are preferably polymeric fibres with a high elastic modulus (higher than 50 GPa, preferably 60 GPa, even more preferably 80 GPa) , preferably polyolefin (e.g. UHMWPE) or aramid fibres, and preferably being organized in a woven structure, characterized by high tensile performances . b) a high percentage of tannic acid (which is a polyphenol) which simultaneously acts as a flame retardant, a cross-linking agent for the matrix 2, an adhesion promoter of the matrix 2 to the fibres 4 and a plasticizer of the matrix 2.

[0071] Each diagram shows a comparison between performance data of specimens of materials denoted as "White" (composite material comprising a matrix with amine cross-linker, according to the prior art) , "TA Resin" (composite material comprising a matrix with polyphenol cross-linker, specifically tannic acid, according to the invention) , "TA Resin and Fibres" (composite material comprising a matrix with polyphenol cross-linker, specifically tannic acid, and fibres functionalized with polyphenol, specifically tannic acid, according to the invention) . The fibres of the materials as per the diagrams of the Figures 7 to 15 comprise UHMWPE fibres (Figures 7, 10-12, 14) , glass fibres and aramid fibres (Figures 8, 9, 12, 13, 15) .

[0072] As regards tensile strength Ofm, Figure 7, with UHMWPE fibres the best result is obtained for the "TA Resin and Fibres" material according to the invention, wherein the combined action of the tannic acid in the matrix (as a cross-linker) and of the surface functionalization of the fibres 4 with a solution of tannic acid increases the tensile strength both with respect to the "White" material and with respect to embodiments wherein the fibres 4 are not functionalized ("TA Resin") . In the case of glass fibres and aramid fibres, the material 1 with fibres 4 without surface functionali zation ("TA Resin" ) exhibits better performances than the others , with an appreciable increase of the tensile strength Ofm. As regards the tensile modulus of elasticity Et, the performances of the "TA Resin" and " TA Resin and Fibers" materials 1 according to the invention are substantially similar, and in the case of aramid fibres they are definitely better than the "White" prior art ( and the same is true for the "TA Resin" material ) .

[0073] As regards flexural strength Ofm, Figure 10 ( diagram 90 ) , with UHMVPE fibres the best result is obtained for the material "TA Resin and Fibres" according to the invention, albeit achieving a strength lower than the known "White" material ( and the same is true for the "TA Resin" material ) . As regards the flexural modulus of elasticity Ef ( Figure 11 , diagram 100 ) , the highest value is again shown by the state of the art ("White" ) , whereas the "TA Resin" and "TA Resin and Fibres" materials 1 according to the invention have slightly lower values .

[0074] Similar considerations apply to Figures 12 ( diagram 110 , Ofm) and 13 ( diagram 120 , Ef ) , with the exception of known materials with a matrix cross-linked with amines and aramid fibres , which has values of flexural strength and flexural modulus of elasticity that are sensibly lower .

[0075] Finally, Figures 14 ( diagram 130 ) and 15 ( diagram 140 ) show the resilience data of the specimens in the case of UHMWPE fibres - Figure 15 , glass fibres ( Glass ) and aramid fibres (Kevlar ) - Figure 16 . Whereas the values of resilience for the specimens with UHMWPE fibres are substantially comparable in the specimens "White" , "TA Resin" and "TA Resin and Fibres" (with an appreciable increase essentially in the latter specimen) , in the case of Figure 16 a greater dispersion is observed in the specimen with a matrix 2 filled with glass fibres . In the case of a matrix filled with aramid fibres , the results are substantially similar to what is shown in Figure 15 , with the only exception of the values of resilience for the "White" specimen, which has values appreciably lower with respect to the "TA Resin" and "TA Resin and Fibres" specimens , whereas in the case of a matrix filled with glass fibres there is a peak in the value of resilience in the "TA Resin" specimen, i . e . in the specimen wherein the treatment with polyphenol ( tannic acid in the present case ) only regards the polymer matrix .

[0076] Finally, referring to the Figures 16 ( diagram 150 , Ef ) and 17 ( diagram 160 , Ofm) , in the case of a material 1 with fibres 4 made of carbon the values of flexural modulus of elasticity and flexural strength are always higher with respect to the prior art ("White" ) , i . e . to the composite materials with a matrix with amine crosslinker . The reference "TA Resin", as described in the foregoing, denotes the material 1 according to the invention with a matrix with polyphenol cross-linker and, in this case , carbon fibres . A high percentage of fibres 4 and tannic acid ensures a "ductile" break of the material 1 , with a remarkable plastic deformation; on the contrary, high percentages of resin of the matrix 2 , low percentages of fibres 4 ( especially i f they are non-polyolef in, non-continuous and non-woven fibres ) and low percentages of tannic acid impart a basically brittle behaviour .

[0077] This is due at least to the following reasons : i ) the increase in the amount of fibres , which represent the main element configured to transmit forces macroscopically and to resist traction, increases the tensile strength per mass unit , although it is more di f ficult for the matrix 2 to permeate all the spaces between the fibres 4 to prevent the mobility thereof ; ii ) a high percentage of tannic acid contributes to create a cross-linked structure of fibres and matrix which improves the trans fer and the distribution of forces among the fibres , thereby reducing the possibility of a sliding thereof and compensating for the di f ficulty of the resin to penetrate the matrix 2 between the fibres , as mentioned at i ) . Moreover, this leads to high flame resistance , while the e f fect of the presence of tannic acid is negligible below a given percentage . Finally, a high percentage of tannic acid acts as a plastici zer of the matrix, thereby increasing the ductility of the composite ; iii ) the continuous fibres have a large surface contacting the matrix, in comparison with short fibres , thereby contributing to improve an interaction between fibres and matrix which is not local but distributed throughout the length of the fibre , thus compensating for the di f ficulty of penetration of the resin between the fibres mentioned at i ) above ; iv) the organi zation of the f ibres into woven structures improves the transmission of forces among the fibres and the distribution of the stresses thereamong, thereby improving impact resistance ; v) the polymer fibres with a high elastic modulus , especially in the case of UHMWPE or aramid fibres , ensure high mechanical performances while exhibiting high impact strength, due to their particular internal structure . Said fibres , speci fically UHMWPE fibres , remarkably improve the capability o f interaction with the matrix in the presence of tannic acid, thereby increasing the resistance of the material also in the presence of a limited amount of matrix ; vi ) The pre-treatment of the fibres with tannic acid ( the molecules whereof bond to the surface of the f ibres ) further improves the interaction between fibres and matrix, since the molecules of tannic acid bonded to the fibre surface also bond to the matrix while curing the composite .

[0078] On the other hand, in comparison to the traditional composite materials with flame retardants, the use of tannic acid enables reducing, under the same performances, the weight of the material 1 according to the invention, since it replaces the cross-linking agent which would be necessary in any case for cross-linking the matrix 2 (made of epoxy resin) , and it improves the interaction with the fibres 4, therefore reducing the amount of matrix 2 needed to produce the material 1.

[0079] Finally, referring again to flame resistance, the material 1 according to the invention produced in the same way as the "TA Resin" specimens as per Figures 16 and 17 (thus with fibres 4 made as carbon fibres) was subjected to a flammability test according to the TL 1010 standard (TL 1010 Materials for Vehicle Interiors Burning Behavior, Material Requirements, 2008) , moreover comparing it with a material of the prior art corresponding to the "White" specimen as per Figures 16, 17. As is commonly known, the TL 1010 test examines the flammability of materials with a specimen having a predetermined size, which is arranged horizontally and exposed to a flame with a height of 38 mm for 15 seconds. The horizontal dimension (length) of the specimen is divided into five subsequent areas (I, II, III, IV, V) of predefined extensions (I: 0 mm, II: 38.09 mm, III: 38.09 mm, IV: 216.00 mm and V: 38.09 mm) . The flame is applied at the area I, i.e. at an end of the specimen. The duration of the flame application is recorded from the moment when the area III starts burning and, as a function of the extension of the flame along the length of the material, the test provides a flammability class of the material. Flammability classes are defined as follows:

[0080] BR (burn rate) : the flame runs along the whole length of the specimen in a given time (e.g. the specimen burns completely) . The burn rate is calculated as 60 * length covered by the flame / time [mm / min] .

[0081] SE / BR ( self-extinguishing / burn rate) : the flame self-extinguishes before covering the whole length of the specimen, e.g. in the area IV, but it burns beyond the 38 mm of the area III and for more than 50 mm.

[0082] SE / NBR ( self-extinguishing / no burn rate) : the material catches fire, but the flame self-extinguishes before 60 s from the application and does not cover more than 50 mm. Neither burn time nor burn rate are recorded.

[0083] SE = (self-extinguishing) : the material catches fire, but the flame self-extinguishes within the area II .

[0084] For the material 1 according to the invention, produced in the same way as the specimen "TA Resin" as per Figures 16 and 17 (thus with fibres 4 made as carbon fibres) , the TL 1010 test provides an SE classification, and thus the material exhibits a self-extinguishing behaviour. On the contrary, the material of the prior art, corresponding to the "White" specimen of Figures 16, 17, exhibits a strongly flammable behaviour - BR mm / min .

[0085] Of course, the implementation details and the embodiments may amply vary with respect to what has been described and illustrated, without departing from the extent of the present invention, as defined by the annexed claims.

Claims

CLAIMS1. A composite material (1) comprising a polymer matrix (2) and fibres (4) arranged in said polymer matrix ( 2 ) , wherein :- the polymer matrix (2) includes a polyphenol in an amount equal to or greater than 15% by weight with respect to weight of the polymer matrix (2) , the fibres (4) have a tensile modulus of elasticity, measured according to ASTM C1557, greater than 50 GPa, and are present in an amount equal to or greater than 80% by weight with respect to the weight of the polymer matrix (2) .

2. The composite material (1) according to claim 1, wherein said polymer matrix (2) is a thermosetting polymer matrix.

3. The composite material (1) according to claim 1 or claim 2, wherein the polymer matrix (2) has a compressive modulus of elasticity, measured according to ASTM D695, greater than 0.5 GPa, preferably greater than 1 GPa, even more preferably greater than 1.5 GPa.

4. The composite material (1) according to any of the preceding claims, the polymer matrix (2) comprising said polyphenol in an amount equal to or greater than 20% by weight with respect to the weight of the polymer matrix (2) , preferably equal to or greater than 30% by weight with respect to the weight of the polymer matrix (2) .

5. The composite material (1) according to any of the preceding claims, wherein the fibres have a tensile modulus of elasticity, measured according to ASTM C1557, greater than 60 GPa, preferably greater than 80 GPa.

6. The composite material (1) according to any of the preceding claims, wherein the fibres (4) are present in an amount equal to or greater than 95% by weight with respect to the weight of the polymer matrix (2) ,preferably in an amount equal to or greater than 110% by weight with respect to the weight of the polymer matrix (2) .

7. The composite material (1) according to any of the preceding claims, wherein the fibres (4) are present in an amount lower than 300% by weight with respect to the weight of the polymer matrix (2) , preferably lower than 250% by weight with respect to the weight of the polymer matrix (2) , even more preferably lower than 200% by weight with respect to the weight of the polymer matrix ( 2 ) .

8. The composite material (1) according to any of the preceding claims, wherein said fibres (4) are continuous fibres across the polymer matrix (2) .

9. The composite material (1) according to any of the preceding claims, wherein said fibres (4) are treated by means of a surface functionalization based on a solution of polyphenol, preferably tannic acid.

10. The composite material (1) according to any of the preceding claims, wherein said polyphenol in the polymer matrix (2) comprises tannic acid.

11. The composite material (1) according to any of the preceding claims, wherein the compressive modulus of elasticity of the polymer matrix (2) , measured according to ASTM D695, is measured in the absence of fibres within the polymer matrix (2) .

12. The composite material (1) according to claim 1, the material comprising a pair of opposite outer surfaces (2A, 2B) and comprising a layer of metal material applied on at least one outer surface (2A, 2B) of said pair of opposite outer surfaces (2A, 2B) .

13. The material according to any of the preceding claims, wherein said polyphenol has a weight percentage of tannins greater than 50% and GAE (Gallic Acid Equivalent) greater than 55%.14 . The material according to claim 13 , wherein said polyphenol comprises tannic acid in a weight percentage greater than 70% and GAE greater than 60% .15 . The material according to any of the claims 1 to 12 , wherein said polyphenol comprises one or more synthetic polyphenols and / or one or more extracts based on polyphenols of natural origin, in particular quebracho extract , quebracho sulphite , chestnut extract , grapeseed extract .

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