HEATING ELEMENT BASED ON A MIXTURE OF PURE GRAPHENE AND CARBON BLACK AND PROCESS FOR ITS PRODUCTION

FIELD OF THE INVENTION

The present invention concerns a heating element consisting of a dispersion of pure graphene flakes and carbon black particles in a polymeric matrix. The invention also concerns the method for producing the heater.

BACKGROUND OF THE INVENTION

Electric heating (or ohmic heating, or heating by Joule effect) is a process in which electric energy is converted into thermal energy.

The electric heating systems that are ideal for both industrial and domestic uses must have the following technical characteristics:

1) high electric power achievable by applying the selected electric parameters. The voltage limits are preferably selected within very low voltage (ELV) ranges that involve a low risk of electrocution, as defined by the safety standards;

2) homogeneous distribution of the temperature over the area of the heater, preferably with temperature self-monitoring functionality to avoid the need for local temperature monitoring/control through devices such as temperature sensors, multiple independently controlled electrodes in pairs, multi-channel thermostats and the like;

3) high velocity of heat dissipation through radiative, conductive, convective mechanisms or a combination thereof, to avoid the need for additional accessories that promote thermal dissipation (e.g., paints and/or thermally conductive and/or radiative supports/substrates and/or motorized fans, just to name a few) that increase the cost, the footprint and the weight of the plant;

4) mechanical properties that are adequate to withstand the mechanical stresses, including those induced by the thermal stresses, depending on the sites of application (e.g. floor, wall and roof in a building) and on the operating temperature range, while maintaining adequate electrical properties;

5) adequate surface properties and affinities with surrounding components (e.g. water), which typically result in hydrophilic/hydrophobic properties, depending on the desired field of application (e.g. hydrophobic properties for anti-icing and anti-fogging/anti- condensation applications);

6) chemical and electrochemical stability (e.g. anti-corrosion properties) to withstand surrounding oxidizing environments (e.g. air and salt water) at operating temperatures.

The known electric heaters that are already in use do not meet all the requirements exposed above.

Most of the modern electric heating systems are based on metal wires/fi laments as active resistive elements. The metals typically used for such resistive elements include alloys such as Ni-Cr (nichrome), Fe-Cr-AI, as well as high melting point metals (e.g. Mo, W, and Ta). Resistive elements based on these metals or alloys have spiroidal shapes or other more elongated conformations and have resistance values that are adequate to obtain a sufficiently high generated power (characteristic 1). These linear resistive elements, however, do not guarantee homogeneous heating over an area (thus not satisfying characteristic 2) and can easily fail following mechanical stresses or corrosion (thus not satisfying characteristics 4 and 6). Furthermore, the metals have poor radiant properties because of their low emissivity (E), indicatively equal to or less than 0.4, in the visible and infrared regions of the electromagnetic radiation (in particular, the range of wavelengths from 0.5 to 20 pm), which are those of interest for the heating at a temperature of thermal comfort; therefore, metallic or metal alloy resistive elements must be coupled with thermally conductive, electrically insulating and high emissivity coatings and/or substrates (e.g. mica), which absorb the heat generated then dissipating it radiatively with higher efficiency than the standard conformation. In addition, the metals have positive resistance temperature coefficients (TCRs), which means that their electric resistance increases with increasing temperature; this requires the presence of sensors and voltage/temperature regulating devices to prevent localized overheating peaks (for example due to defects in the structure of the metal or to thickness inhomogeneities) that could result in malfunctions of the system. The necessary additional active and passive components lead to increases in the footprint and costs of the final heating system.

To overcome the problems posed by the metallic resistive elements, alternative heaters have been reported in the literature, generally in the form of thin layers consisting of particles of amorphous or allotropic forms of carbon that are dispersed in a polymeric matrix. Examples of these studies (just to name a few) are reported in the articles “Enhanced electrothermal efficiency of flexible graphene fabric Joule heaters with the aid of graphene oxide”, M. Tian et al., Mater. Lett. 234, 101 (2019), “Multiwalled carbon nanotube/polydimethylsiloxane composite films as high performance flexible electric heating elements”, J. Yan et al., Appl. Phys. Lett. 105, 051907 (2014) and “Composites of graphene nanoribbon stacks and epoxy for joule heating and deicing of surfaces”, A.R.O. Raji et al., ACS Appl. Mater. Interfaces 8, 3551 (2016).

These heaters generally have good mechanical performance (e.g., no surface lesions during drying/hardening and mechanical flexibility) but electrical properties not sufficient to provide satisfactory heating performance for a large area at an operating voltage within ELV values when adopting simple connections to the power supply system (e.g., with a limited number of metallic electrodes); with these simple configurations one typically has resistances greater than 100 Q and therefore a low power output. Better performance can be achieved by reducing the distance between the current collector electrodes (e.g. copper, silver foils/strips) or by adopting electrodes with complex shapes (e.g. interdigitated electrodes) that connect these thin layer resistive elements to the power supply systems; in this way, however, in addition to increasing the production costs, there is generally a decrease in the mechanical properties of the heating systems, which are more rigid and subject to breakage.

The electrical performance of the carbon-based thin layer systems can be improved, e.g. by achieving resistivity of the order of 0.01 Q cm, by increasing the content of electrically conductive charges (up to 100% by weight) or by using sheets of expanded graphite/pyrolytic graphite. The systems thus obtained, however, have poor mechanical performance and high costs of the sheets of expanded or pyrolytic graphite, problems that limit their fields of application.

Another possibility to improve the performance of the non-metallic heaters described above is to add carbon nanotubes to the system, which can potentially lead to a stable and uniform distribution of the temperature and prevent localized overheating. However, the carbon nanotubes pose processing problems because they tend to agglomerate and are not easily dispersed in the polymeric matrices, negatively affecting the homogeneity of the composites and the consequent mechanical, thermal and electrical performance. In addition, the cost of the carbon nanotubes is significantly higher than that of other carbonbased conductive charges (e.g., carbon black and graphite).

Patent application CN 109280426 A describes a composition that can be used to make thin layer heating elements, consisting of a mixture of carbon black and graphene oxide dispersed in a matrix comprising at least one silicone or phenolic resin, wherein the weight ratio between carbon black and graphene oxide can vary between 1 :1 and 6:1. Although the text of this application speaks generically about graphene, without further specification, the present inventors have replicated the procedures and the tests described in this document, and have been able to confirm that the material used to produce the described heating elements is graphene oxide.

Similar compositions are also described in patent application CN 110418444 A, and also in this document chemical and structural characteristics of the graphene and carbon black are not specifically provided. In this document the general term graphene is used, which comprises a wide class of materials, and the methods described in the document for the production of this component are for example the electrochemical exfoliation and the redox method, which lead to forms of graphene with functional groups, for example oxygen functionalities, and structural defects in the basal planes. The results obtained according to the teachings of this document are not yet sufficient for many practical applications.

There is therefore still a need in the sector to have resistive heating elements that overcome the limits of those currently available.

Aim of the present invention is to provide advanced resistive heating elements for electric heaters that have the set of technical characteristics required for ideal electric heating systems.

SUMMARY OF THE INVENTION

These objects are achieved according to the present invention which, in a first aspect thereof, concerns a composition comprising an electrically conductive component dispersed in a matrix of an electrically insulating component and wherein:

- the electrically conductive component is present in an amount between 30 and 80% by weight with respect to the sum of the electrically conductive component and the electrically insulating component, and comprises a mixture of pure graphene flakes and carbon black particles, wherein the pure graphene flakes have thickness between 0.34 and 180 nm, the carbon black particles have dimensions between 10 and 100 nm, and the weight ratio between pure graphene and carbon black is between 4:1 and 1 :4;

- the electrically insulating component is present in an amount between 20 and 70% by weight with respect to the sum of the electrically conductive component and the electrically insulating component, and comprises a polymer, a precursor of a polymer, or a mixture thereof.

One or more solvents may also be present in the composition described above.

In a second aspect thereof, the invention concerns a process for the production of the composition described above.

In a third aspect thereof, the invention concerns a process for producing a resistive layer using the composition described above.

Finally, in a fourth aspect thereof the invention concerns a resistive layer obtained by deposition on a support and subsequent drying of the composition of the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be illustrated in the following with reference to the figures, in which:

- Fig. 1 shows the conductivity and resistivity trend of heating elements of the invention produced with a polyphenylsulfone polymeric matrix (PPSU) as the percentage of electrically conductive component varies;

- Fig. 2 shows a micrograph obtained with an atomic force microscope (AFM) of the surface of a heating element of the invention produced with a PPSU polymeric matrix;

- Fig. 3 reports in the graph the trend as the temperature of the resistance, normalized with respect to that at 20 °C, of a heating element of the invention produced with a PPSU polymeric matrix varies;

- Fig. 4 shows the graph of a thermogravimetry test on a composition of the invention, produced with a PPSU polymeric matrix, after drying; - Fig. 5 shows graphs similar to those of Fig. 1 for heating elements produced with a polymeric matrix consisting of a polyurethane: polycarbonate mixture;

- Fig. 6 shows an AFM micrograph of the surface of a heating element of the invention produced with a polymeric matrix consisting of a polyurethane:polycarbonate mixture;

- Fig. 7 shows a graph similar to that of Fig. 3 obtained with a heating element of the invention produced with a polymeric matrix consisting of a polyurethane:polycarbonate mixture;

- Fig. 8 shows the graph of a thermogravimetry test on a composition of the invention, produced with a polymeric matrix consisting of a polyurethane:polycarbonate mixture, after drying;

- Fig. 9 shows graphs similar to those of Fig. 1 for heating elements produced with a polyvinyl butyral polymeric matrix;

- Fig. 10 shows an AFM micrograph of the surface of a heating element of the invention produced with a polymeric matrix consisting of polyvinyl butyral;

- Fig. 11 shows a graph similar to that of Fig. 3 obtained with a heating element of the invention produced with a polyvinyl butyral polymeric matrix;

- Fig. 12 shows the graph of a thermogravimetry test on a composition of the invention, produced with a polyvinyl butyral polymeric matrix, after drying;

- Fig. 13 shows graphs similar to those of Fig. 1 for heating elements produced with a polymeric matrix consisting of an acrylic resin;

- Fig. 14 shows an AFM micrograph of the surface of a heating element of the invention produced with a polymeric matrix consisting of an acrylic resin;

- Fig. 15 shows a graph similar to that of Fig. 3 obtained with a heating element of the invention produced with a polymeric matrix consisting of an acrylic resin;

- Fig. 16 shows the graph of a thermogravimetry test on a composition of the invention, produced with a polymeric matrix consisting of an acrylic resin, after drying;

- Fig. 17 shows graphs similar to those of Fig. 1 for heating elements produced with a polymeric matrix consisting of poly(vinylidene fluoride);

- Fig. 18 shows the conductivity and resistivity trend of heating elements of the invention produced with a PPSU polymeric matrix as the percentage of electrically conductive component varies and with a form of pure graphene different from that used for the resistive elements of Fig. 1 ;

- Figs. 19 and 20 respectively report the temperature trend as a function of the supply time and the dissipated power as a function of the applied voltage measured on heating elements of the invention produced with a PPSU polymeric matrix and complete with support and electrodes;

- Fig. 21 shows four thermographs obtained at different times on a sample with which the graphs of Figs. 19 and 20 have been obtained; and

- Fig. 22 reports the temperature trend as a function of the supply time of heating elements of the invention produced with different polymeric matrices and complete with support and electrodes;

- Fig. 23 shows photographs of four resistive layers obtained with a composition not of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect thereof, the invention concerns a composition consisting of an electrically conductive component comprising a mixture of pure graphene flakes and carbon black particles, which is dispersed in a matrix comprising at least one polymer or a precursor of a polymer. For the purposes of the present invention, the flakes must be of pure graphene; with the definition “pure graphene”, in the description that follows and in the claims, is meant a form of graphene in which all the carbon atoms of the basal faces have hybridization sp ² and the carbon atoms bonded by covalent bonds are all coplanar. For the purposes of the present invention, the graphene can be considered pure even when its basal faces have chemical functional groups deriving from the impurities of the starting graphite; the starting graphite must however have an atomic percentage content (at%) of oxygen or other elements of less than 2%, preferably less than 1 %, more preferably less than 0.5%; the content of elements other than carbon can be determined by X-ray photoelectron spectroscopy (XPS).

In the description that follows, for brevity the composition of the invention is defined as “paste" in the state prior to its drying. By the term “drying” in the description that follows is meant any chemical and/or physical process leading to the formation of a solid film starting from a paste according to the first aspect of the invention; such chemical and/or physical processes comprises, inter alia, evaporation of the solvent, polymerization of monomers and cross-linking of oligomers and/or polymers. By the term “paste” is meant any mixture between the two or three components which is sufficiently fluid such that it can be applied to a support by means of normal deposition methods, for example screen printing, coating, printing, ..., and also includes the paints, which generally have lower viscosities than the pastes, and the inks, which normally have even lower viscosities.

As mentioned, the electrically conductive component comprises a mixture of pure graphene flakes and carbon black particles.

Graphene is an allotropic form of carbon that is obtained by exfoliating the graphite. As is well known, the graphite consists of crystalline planes formed by carbon atoms with hybridization sp ² arranged in hexagonal arrangement; within the plane, each carbon atom is bonded to three other carbon atoms with covalent bonds, while parallel crystalline planes interact with each other only through Van der Waals forces, of weak intensity. It is therefore possible, with various known methods, to separate the graphite in flakes consisting of one or more superimposed atomic planes; the resulting particles are defined in the sector “graphene flakes”, a term that is also adopted in the present invention, and have “basal” faces, i.e. corresponding to the crystalline planes of carbon atoms bonded by covalent bonds, and lateral surfaces consisting of the edges of said crystalline planes.

The chemical-physical and structural properties of graphene flakes depend on the method used for the exfoliation of the graphite. In many cases what is obtained is the so- called “graphene oxide”, that is a derivative of the graphene in which all or part of the carbon atoms exposed on the basal plane are bonded to oxygen atoms or hydroxyl groups; the oxidized carbon atoms lose their hybridization sp ² which determines the planarity of the crystalline planes of the graphite, and with this the availability of mobile electrons that makes graphite an electric conductor. Graphene oxide can be reduced with chemical (e.g., with hydrazine or metallic hydrides), electrochemical or thermal treatments; the material obtained, however, usually has a high number of defects, for example crystalline planes consisting of carbon atoms in a non-planar configuration, or a high number of chemical groups bonded to the surface carbon atoms, which conditions lead to non-optimal electrical, mechanical and thermal properties.

Due to the finite dimension of the graphene flakes, even in pure graphene, structural defects located at the edges of the crystalline planes, and therefore on the non-basal surfaces of the flake, are accepted.

The absence of impurities on the basal faces of the graphene flakes, and the possible presence of bonded chemical groups on the lateral surfaces, can be investigated with spectroscopic methods known in the literature. For example, the article “Raman spectroscopy of graphene-based materials and its applications in related devices”, J. B. Wu etal., Chem. Soc. Rev. 47, 1822 (2018), reports that in the Raman spectrum of the graphite (and, more generally, of the graphitic materials) there are two characteristic peaks defined as D and G, and that the values of the ratio between the intensities of these two peaks (l(D)/l(G)) with respect to the width at half height of the peak G (FWHM(G), Full Width at Half Maximum) can be used to identify the nature of the defects. In particular, the defects associated with the peak D are attributed to the edges of the graphene flakes when the graph of I (D)/I(G) as a function of FWHM(G) does not show a linear correlation (e.g., R ² < 0.6) which is therefore the case of the pure graphene flakes used in the present invention. Conversely, the graph of l(D)/l(G) as a function of FWHM(G) shows a linear correlation when the defects also occur in the basal planes of the graphene flakes, as occurs for the graphene derivatives (e.g. graphene oxide and reduced graphene oxide).

In the remainder of the description and in the claims, when it is talked about graphene as a component of pastes of the invention and of resistive layers and devices produced therewith, it is meant the form of pure graphene having the characteristics indicated above.

The pure graphene flakes useful for the purposes of the present invention have lateral dimensions, i.e. length and width of the basal surfaces, which can vary within wide limits, between about 0.01 pm and about 100 pm. The thickness of the flakes can vary between 0.34 nm (monatomic thickness of a single crystalline layer of the graphite) and 180 nm, typically between 0.9 nm (indicatively, the thickness of three crystalline planes of the graphite) and 150 nm. The ratio between length or width of the basal faces and thickness of the flake is equal to at least 5, while according to the invention there is no upper limit to this ratio, which is determined only by the flake production methodology.

Pure graphene flakes, with characteristics useful for the present invention, are preferably produced by wet-jet micronization of graphite, as described in patent application WO 2017/089987 A1. Briefly, according to the procedure described in this document, a dispersion of powdered graphite in a solvent (e.g., N-methyl-2-pyrrolidone) or a mixture of solvents is pressurized by a high pressure pump (150-250 MPa). The pressurized dispersion generates a jet stream in the liquid phase, which leads to a laminar flow (Reynolds number, Re, ~ 10 ² ). Subsequently, the jet stream is divided into two jet streams that are made to collide with each other causing a turbulent flow (Re > 10 ⁴ ), whose velocity gradient (> 10 ⁶ s ^-1 ) generates the shear forces responsible for the exfoliation of the graphite. Finally, the linear jet streams become swirling jet streams, further increasing the exfoliation process and dispersing the exfoliated material. By varying the number of passages through the wet-jet micronization apparatus, it is possible to control the dimensions (i.e. lateral dimensions and thickness) of the pure graphene flakes, which decrease as the number of passages increases. Other parameters that control the dimensions of the pure graphene flakes are the dimension of the nozzles that produce the two jet streams that are made to collide and the pressure of the dispersion.

Carbon black is an electrically conductive material produced by an incomplete combustion of heavy hydrocarbons lacking oxygen. In more detail, carbon black is a form of paracrystalline carbon with a high surface/volume ratio, corresponding to a surface area typically between 10 and 1500 m ² /g. Carbon black exists in the form of discrete spherical particles typically smaller than 500 nm in size, which typically form aggregates/agglomerates. It is a product that is commercially widely available, sold, just to give a few examples by ORLEN Unipetrol RPA, s.r.o. (Prague, Czech Republic), CABOT Corporation (Boston, USA), or Degussa AG (Essen, Germany). For the purposes of the present invention, carbon black is used in the form of particles with dimensions between 10 and 100 nm and surface area between 30 and 1500 m ² /g; the indicated manufacturers or sellers provide various types of carbon black, among which it is possible to select a material with the desired characteristics. The pure graphene flakes and the carbon black used in the present invention represent “black materials”, i.e., materials that, regardless of the frequency or angle of incidence, strongly absorb the incident electromagnetic radiation; according to Kirchhoff’s law, the emissivity £ of an object is equal to its absorption capacity and the sum of reflectivity, transmissivity and absorbency is equal to 1 in thermal equilibrium. Therefore, £ of pure graphene and carbon black is greater than 0.8 in a very wide wavelength range (0.2- 200 pm), which covers the interesting spectral range for the electric heaters; this results in a high velocity of heat dissipation by irradiation of resistive heating elements produced with these materials compared to those based on metals.

In the present invention, the weight ratio between pure graphene flakes and carbon black is between 1:4 and 4:1 , preferably between 1 :1 and 3:1. Amounts of carbon black greater than those indicated degrade the cohesive properties of the paste, with the consequence that the resistive layers produced with these pastes tend to crack during drying. Conversely, pastes produced with a weight ratio between pure graphene flakes/carbon black greater than 4:1 give rise, after drying, to resistive layers with relatively high porosity, which leads to a decrease in electrical performance (in particular, increasing the electric resistivity).

Optionally, other graphite-based fillers, including graphite chips, having a thickness greater than 200 nm, may be used as electrically conductive additives within the formulation of the electrically conductive component described above. However, in order to exploit the advantages of the disclosed composition, the amount of additives must be less than 15% by weight with respect to the total weight of the electrically conductive component.

The above-described electrically conductive component is dispersed in a matrix which comprises at least one polymer or a precursor of a polymer, which constitutes the electrically insulating component of the paste.

The electrically insulating component comprises organic or inorganic polymers or their precursors (in the latter case all the polymerization reagents are included in the formulation of the component). Since the precursors of the polymers polymerize in the obtained resistive layers by drying the paste, in the following only the term polymer will be used to indicate all the materials that make up the electrically insulating component, even if they refer to precursors of polymers.

A wide variety of polymers can be used depending on the desired electrical, physical and chemical performance of the resistive heating elements. Examples of polymers suitable for the purposes of the present invention are epoxy polymers, acrylic polymers, polymeric organosilicones (e.g., polydimethylsiloxane, (PDMS)), polyurethanes (PU), polyisobutylenes, vinyl polymers, polyvinyl butyral (PVB), ionomers, polyaryletherketones, polyphenylsulfones (PPSU), polyamides, polyimides, polyesters, polycarbonates (PC), polyketones, polyoxymethylene, polyphenylene sulfide, polyethers (e.g. polyphenylene oxide), polysulfones, poly(p-phenylene), fluoropolymers (e.g. polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluoroethylene vinyl ether (FEVE)), polymethyl methacrylate (PMMA), copolymers such as poly(ethylene-vinyl acetate) (EVA) or of the acrylonitrile-butadiene-styrene (ABS) families and mixtures thereof.

In principle it would also be possible to use electrically conductive polymers (e.g., polyaniline) as binding elements in the matrix. However, in the present invention, thanks to the formulation of the conductive component described above, electrically insulating polymers can be used for the formulation of thin films with the desired electrical characteristics for electric heating elements, and at the same time provide advantages in terms of mechanical properties, processability and costs compared to the electrically conductive polymers.

Preferably, PVBs, acrylic polymers, PU:PC mixture, PPSU and PVDF are used in the present invention to produce the matrix of the electrically conductive component, due to the wide range of physical and chemical properties obtainable through their use, as well as their solubility dispersibility in different types of solvents.

Components known in the industry of plastics and paint products can be added to the electrically insulating component. The additives for polymeric materials can be both organic and inorganic compounds and include, for example, plasticizers, antiplasticizers, stabilizers and dispersants. The plasticizers are non-volatile compounds that improve the softness and the flexibility of the polymers, increasing plasticity, decreasing viscosity or decreasing friction during handling in the production step; examples of plasticizers are dicarboxylic/tricarboxylic esters, adipates, sebacates, maleates, trimellitates, azelates, benzoates, sulfonamides, organophosphates, glycols, polyethers and phthalate esters. The antiplasticizers have opposite functionalities; examples of antiplasticizers are tricresyl phosphate and butylphthalide and pentachlorobiphenyl. The stabilizers are for example antioxidants, anti-ozonants, photostabilizers (e.g. UV absorbers, excited state quenchers and blocked amine-based photostabilizers), acid sequestrants, chelating agents, thermostabilizers, flame retardants and biocides. The dispersants are polymeric compounds with variable molecular weight useful for improving the surface interaction between fillers and liquid phase (i.e. mixture of polymers or prepolymers and, if necessary, solvent) and therefore the efficacy of the process of dispersion of the fillers and the stability of the dispersion itself. Such substances also have the effect of reducing the viscosity of the dispersions, further facilitating their processing and reducing the need for a solvent.

The weight ratio between the electrically conductive component and the electrically insulating component is between 30:70 and 80:20, preferably between 35:65 and 55:45.

An excessive amount of the electrically insulating component, greater than 70% by weight, causes a decrease in the electrical performance of the paste and of the resistive layers obtained with it, which show a high electric resistivity (in some cases, even greater than 1 O cm) due to a poor interconnection of the conductive particles. An excessive amount of the electrically conductive component, greater than 80% by weight, instead decreases the compactness of the final resistive layers and deteriorates their mechanical properties (e.g. cohesion).

In order to obtain optimal electrical performance (i.e., electric resistivity), regardless of the polymer used to make the electrically insulating component, the preferred weight ratio between pure graphene flakes and carbon black is between 1 :1 and 3:1.

Depending on the physical state of the electrically insulating component, as well as the desired rheological properties of the paste, a liquid component (solvent or mixture of solvents) can be added to the mixture of electrically conductive and insulating components for the production of the same; this measure is used in particular when the polymer that will constitute the electrically insulating component is solid, or in any case when the mixture of only the electrically conductive and electrically insulating components (including any additives) is excessively viscous, in which case the use of a liquid component allows to achieve a better dispersion of the electrically conductive component and therefore improve the homogeneity of the paste while regulating its rheological properties. As a general rule, increasing the amount of carbon black in the paste leads to an increase in its viscosity, thus requiring the addition of solvents.

The liquid component may be water, organic solvents, or a combination thereof, and must homogeneously dissolve or disperse the components of the paste without being involved in any polymerization or cross-linking reactions. The organic solvent can be protic or aprotic, polar or non-polar, depending on the other substances (e.g. binders) that make up the overall mixture. A person skilled in the art can identify a suitable solvent from the generic manual and from the published literature, also taking into account the correspondence of the Hildebrand or Hansen solubility parameters of a polymer with those of a solvent (in this regard, see for example the article “Critical assessment of the Hildebrand and Hansen solubility parameters for polymers”, S. Venkatram et al., J. Chem. Inf. Model., 59, 4188 (2019).

Examples of solvents useful for the purposes of the present invention are ketones (e.g. acetone, methylethylketone, hexanone, cyclohexanone), aromatic solvents (e.g. toluene, xylene, benzonitrile, chlorobenzene), glycol ethers (e.g. polyethylene glycol, polypropylene glycol, propylene glycol butylether, diethylene glycol butylether, propylene glycol methyl ether, etc.), esters (e.g. ethyl acetate, butyl acetate, ethylene glycol butylether acetate, diethylene glycol butyl ether acetate, etc.), mineral spirit, alcohols (methanol, ethanol, terpineol, 1-propanol, 2-propanol, 1-butanol, ethylene glycol, propylene glycol, etc.), hydrocarbons (e.g. n-hexane, naphtha), terpenes, oils, acetonitrile, N-methyl-2- pyrrolidone, N,N-dimethylformamide, chloroform, 1 ,3-dioxolane, tetrahydrofuran, carboxylic acids (e.g., acetic acid) and water.

The solvents, when used, are added in weight ratio not greater than 10: 1 with respect to the weight of the mixture of electrically conductive and insulating components described above.

Preferably, the electrically conductive pastes of the present invention have a Hegman value greater than 4. The Hegman value defines the fineness of the particles within paints, pastes and inks. From its determination by means of a grindometer, it is possible to identify the suitability of the products for the standard painting/printing processes.

In a second aspect thereof, the invention concerns a process for the production of the paste described above. In such a process, in the presence of a solid or excessively viscous electrically insulating component, it is first dissolved or diluted in a solvent or mixture of solvents. The latter are those able to dissolve or disperse the (pre)polymers, without causing coagulation, flocculation and/or precipitation/sedimentation effects. In some embodiments, the electrically insulating component may consist of liquid phase (pre)polymers (e.g. epoxy resins and polyols), and the addition of solvent may not be necessary. Preferably, the electrically conductive component is premixed with at least one of the above solvents, and a degassing process with mixers operating in “vacuum” mode (i.e. low pressure less than 1 kPa) or with planetary centrifuges operating in degassing mode (i.e. in “vacuum” and/or with centrifugal accelerations greater than 100 g) is used to remove the air trapped in the materials constituting the conductive component. In such degassing process it may be advantageous to add (pre)polymeric additives which, through a control of the solubility parameters of the solvent (single solvent or a mixture), act as dispersants to obtain a suitable chemical interaction between the materials of the electrically conductive component and the solvent or mixture of solvents, avoiding phenomena of aggregation of the solid components. Additional possible additives may then be taken into consideration, including rheological additives (e.g. thickeners) and defoamers to facilitate the subsequent process steps. Such a premixing process of the electrically conductive component limits the formation of foam and bubbles during the subsequent process steps, particularly critical for water-based products as a solvent. Subsequently, the electrically conductive component, possibly premixed as described above, is added to the electrically insulating component. The resulting mixture is homogenized by means of mixers, homogenizers or grinders, possibly by heating the mixture below the decomposition, polymerization or cross-linking temperature of its substances to facilitate the dissolution thereof. Subsequently, additives for polymeric materials (e.g., thickeners, retardants, antioxidants and dispersants) and solvent can be added allowing also a final control of the viscosity of the paste.

In a third aspect thereof, the invention concerns a process for producing a resistive layer using the above-described paste.

A paste of the invention may be deposited by conventional painting and printing techniques to form resistive heating elements in the form of coatings that are dried at low temperatures (< 150 °C), compatible with a wide variety of substrates, including plastics, fabrics, mica, plasterboard, mineral wool panels, wood and plywood panels, and cement composites, just to name a few.

The viscosity of the paste must be regulated according to the deposition technique chosen; as indicated above, this can be achieved by adding solvents or additives to the paste. The preferred viscosity ranges are: 1-50 mPa s for inkjet printing, 50-150 mPa s for spray painting, 50-500 mPa s for gravure and flexographic printing and 1000-10000 mPa s for screen printing. Depending on the deposition technique, a person skilled in the art may find suitable additives, such as retardants, diluents, thickeners to obtain the desired rheological properties (e.g., thixotropy for screen printing pastes) from the print technology manual and from the published literature.

The electrically conductive pastes can be deposited on both flat and curved substrates or supports. Optionally, a primer may be deposited on the support to provide the desired adhesion performance of the resistive heating elements to the support, as well as to reduce the absorption of the electrically conductive paste used to make the resistive heating elements. This practice is particularly recommended for cementitious composites, porous fabrics, mineral wool panels, wood and plywood panels. In fact, the application of a primer prevents uneven absorption of the paste by the support, thus avoiding local overheating (e.g. hot-spot effects). The use of electrically insulating primers can also be used to deposit the resistive heating elements on metallic substrates, avoiding the risk of short circuits.

The resistive layers of the invention can also be produced with the technique of phase inversion by dip precipitation. Once the paste is deposited on a suitable support, the whole is immersed in a coagulation bath containing a non-solvent for the polymer which constitutes the electrically insulating component. Due to the exchange between solvent and non-solvent, precipitation occurs, forming a self-supporting layer (also referred to as a self- supporting membrane).

Finally, in a fourth aspect thereof, the invention concerns a resistive layer obtained by deposition on a support and subsequent consolidation of the paste described above. In the following, “resistive layer” means the active element consisting of the polymeric matrix in which the electrically conductive component is distributed, while “resistive element” means a complete heating device, comprising the electrodes and any support of the resistive layer.

Preferably the resistive heating elements of the invention have square shapes, since these trivial shapes limit the number of electrodes, connections of the electric terminals and wires. However, it will be apparent to the person skilled in the art that other shapes may be used to minimize the distance between the electrodes, thereby increasing the deliverable thermal power of the electric heater. In fact, assuming to maintain the same area of the resistive heating element, the resistance of a resistive layer is proportional to the distance between the electrodes. In addition, the resistance of the resistive layers is inversely proportional to their thickness. However, it is generally preferable to minimize the thickness of the resistive heating element to be adaptable in applications where little space is available, for example those of interior design or interior arrangements, as well as to avoid mechanical defects, such as splits and/or delamination of the high thickness resistive heating elements. Therefore, preferably the resistive heating layers have a thickness of less than 100 pm. Even more preferably, the resistive heating layers of the invention have a thickness of less than 50 pm; resistive layers of these thicknesses exhibit remarkable mechanical properties, such as flexibility and elasticity, preserving the electrically conductive properties in the presence of strong mechanical stresses (e.g. tensile deformations even higher than 50%). Resistive layers comprising only carbon black do not have mechanical properties comparable to those of the invention.

In particular, the preferred resistive layers have a surface RMS roughness > 100 nm. This property is advantageous over non-nanostructured metallic resistive heating layers, as it inherently increases both radiative and convective heat dissipation rates, which are both proportional to the contact area between the resistive layer and the surrounding environment. In fact, according to Stefan-Boltzmann law, the thermal power of the radiation transferred between the electrode and the surrounding environment (dQ _r /dt) is directly proportional to the surface of the heat-emitting element.

In some embodiments, depending on the type of polymer used for forming the electrically insulating component, the resistive heating element has hydrophobic and even impermeable surfaces. These surface properties allow water to be quickly removed from the vertically positioned resistive heating elements for antifreeze, anti-fogging or anticondensation applications. The preferred polymers for making resistive heating elements of this type are PPSU and mixtures between PC and PU, as shown in the Examples section. Furthermore, the impermeability of said resistive heating elements also provides excellent anticorrosive properties, which are also associated with the chemical inertia of the composite materials and the barrier properties of the graphene flakes (pure in the case of the invention). As described in the Examples section, resistive layers in which the matrix is produced with PPSU effectively protect the underlying structural steel, providing a corrosion rate of less than 10' ³ mm per year (mmpy), as assessed by electrochemical methods according to standard ASTM G5-14. The resistive layers described are used to form resistive elements. The main block of the electrical resistance is formed by the resistive layer described above and by the electrodes that are electrically connected to terminals and, through them, to the external wiring connected to the electrical power supply source. The electrodes are metallic bands that are covered or embedded within the resistive layers during deposition of the latter. As mentioned, a primer or even an adhesive can be used to make the resistive layers adhere to the support of the resistive heating element. For the purposes of the present invention, the preferred terminal connections of the resistive heating elements are common aluminium or copper sheets or bands. Other possible materials for the realization of the electrodes are aluminium coated with copper, silver-plated copper and tantalum. Copper or aluminium strips 3M ^TM are preferred here, which have a thickness of 0.07 mm and a width of 12.7 mm, but a person skilled in the art can use foils, strips or even wires with different geometries. The electrodes are preferably placed parallel to the sides of the resistive heating elements over the entire length of the sides thereof. The electrodes must have sufficient length to allow their connection to the contact terminal and then to the external electrical wiring. It is possible to use standard or special commercially available terminals and connectors, such as pins, nuts, bolts, rings, hooks, flat fork-tipped or serrated or smooth terminals. Finally, the terminals described are connected to the power supply sources with normal electrical wiring, possibly through voltage regulators (transformers) that allow to regulate the electrical power in input supplied to the resistive heating elements, allowing the control of the heating power of the electric heater.

Other elements can also be coupled to the heaters of the invention, such as for example:

- infrared radiation sensors to monitor the temperature of the resistive heating element, possibly connected in feedback mode to the power supply source to activate/deactivate the power supply provided to the resistive element. It will be clear to the person skilled in the art that other control components, such as voltage waveform modifiers, timers, rheostats and thermostats, can also be used for this purpose;

- thermochromic coatings deposited on the resistive heating elements of the invention, to visually show the temperature distribution on the heated area;

- integrated systems with phase change material for latent thermal storage, in order to store the thermal energy provided by the electric heaters during the favourable period, wherein the cost of electricity is lower or when any renewable energy sources are fully operational. The phase change materials may be organic (paraffinic or non-paraffinic), inorganic or eutectic metallic mixtures, depending on the desired transition temperature and the corresponding phase change. The phase change materials can be integrated into both the supports and finishes used for the electric heaters of the invention by direct incorporation, dip encapsulation and stabilization methods. In particular, the phase change materials can be integrated into mortars, concrete, bricks, plasterboard, plastic, just to name a few, and used in combination with wall, roof, ceiling, floor, window, continuous facade electric heaters.

An important feature of the resistive layers of the present invention is that they have a nearly zero or even negative coefficient of temperature resistance (TCR). This is due to the presence of pure graphene which, unlike the graphene derivatives (e.g. graphene oxide or reduced graphene oxide), behaves like a zero-gap semiconductor with a negative TCR at room temperature, because resistivity is dominated by the concentration of charge carriers and decreases with temperature as the availability of “hot carriers” increases. Meanwhile, the electrically insulating component, consisting mainly of polymeric materials, provides a positive contribution to the TCR of the resulting layers, because its thermal expansion as a result of heating can cause the breakage of the percolative paths within the electrically conductive component. The prior art reported that the polymers filled with carbon black or filled with graphite powder form electrically conductive composites with positive TCR. This effect of the matrix causes heating elements of the prior art, based on carbon forms but not comprising graphene, to present a positive TCR; this effect is instead compensated in the resistive layers of the present invention by the negative TCR of pure graphene, which leads as said to TCR values close to zero or slightly negative. The nearzero or even negative TCR of the resistive heating is fundamental to providing easy or even self-regulating temperature control. This technical feature avoids the need for active accessories (such as temperature sensors, multi-channel thermostats and connecting cables).

The invention will be further illustrated by the following examples.

MATERIALS, TOOLS AND METHODS

The following chemical compounds and materials were used in the examples described below:

- ultrapure water produced through a Milli-Q® water purification system;

- N-methyl-2-pyrrolidone (NMP), purity > 97% (Sigma Aldrich);

- N,N-dimethylformamide (DMF), purity 99.8% (Sigma Aldrich);

- 1-methoxy-2-propanol (MOP), purity 98% (Sigma-Aldrich);

- butyl acetate, purity > 99,0% (Sigma Aldrich);

- isopropyl alcohol (IPA), purity > 99.8% (Sigma Aldrich);

- tetrahydrofuran (THF) (Chromasolv, Sigma Aldrich);

- chlorobenzene, purity > 99,5% (Sigma Aldrich);

- graphite chips (+100 mesh, Sigma Aldrich);

- carbon Black (CB) (Super P® Conductive, 99+%, Alpha Aesar); - polyvinyl butyral (PVB) powder (Sigma Aldrich);

- polyphenylsulfone powder (PPSU) (Oppanol N80, BASF);

- aqueous-based acrylic resin (Acrilem RP6015, Icap-Sira Chemicals and Polymers);

- polyurethane:polycarbonate resin (PU:PC) (Larithane LS2251 , Novotex);

- poly(vinylidene fluoride) (PVDF) (Solef 6020, Solvay Specialty Polymers);

- vinyl resin (Omnialux, Quasar Inks);

- cellulosic resin (Seriprop, Quasar Inks);

- polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning);

- epoxy resin kit (Setra Vernici);

- polyacrylic acid (PAA) powder (Sigma Aldrich);

- polyvinyl alcohol powder (Mw 89000-98000, 99+% hydrolyzed, Sigma Aldrich);

- poly(ethylene-co-vinyl acetate) (Eva) (vinyl acetate 40% by weight, Sigma Aldrich);

- two-component acrylic resin (Icroacryl TOP DTP Satin 2K, Icro Coatings);

- water-based primer for cement/mortar (Primer 3296, Mapei);

- copper strip 3M (RS Components Italia).

The transmission electron microscopy (TEM) images were obtained with a JEM 1011 transmission electron microscope (JEOL) operating at 100 kV. The samples for TEM measurements were prepared by depositing the material dispersions onto ultra-thin C-on- holey coated Cu grids under nitrogen atmosphere. The samples were then rinsed with deionized water and subsequently dried overnight under vacuum. The statistical analysis of the lateral dimension of graphene flakes was performed using ImageJ software (NIH) and OriginPro 2020 software (OriginLab), respectively. The lateral dimension of a flake has been estimated as its maximum lateral dimension. The statistical distribution of the lateral dimension data was calculated on several dozen flakes.

The atomic force microscopy (AFM) images were acquired with an NX10 AFM (Park System) instrument in non-contact mode (NCM) using a 10 M non-contact PPP-NCHR cantilever (resonance frequency ~ 330 k, force constant 42 N/m). The samples were prepared by depositing the material dispersions onto mica substrates and by heating at 50 °C for 10 minutes to dry the sample. The images were collected on different areas (512 x 512 data points), with a scanning frequency of 0.15 Hz and keeping the working set point around 6 nm. The Gwyddion software (64-bit) was used to process the images, calculate the RMS roughness and acquire the height profiles, while the data were analysed using the OriginPro 2020 software. The statistical distribution of the thickness was calculated on several dozen flakes of material.

The X-ray photoelectron spectroscopy measurements were performed with a Kratos axis UltraDLD spectrometer, using a monochromatic Al Ka source (15 kV, 20 mA). The spectra were acquired over an area of 300 x 700 pm ² . Extensive scans were collected with a constant-pass energy of 160 eV and an energy difference of 1 eV. The high-resolution spectra were acquired with a constant-pass energy of 10 eV and an energy difference of 0.1 eV. The binding energy scale was referred to the peak C 1s at 284.8 eV. The spectra were analysed using CasaXPS software (version 2.3.17). The samples were prepared by deposition of the material dispersions onto a Si/SiOs substrate (LDB Technologies Ltd.) and dried under vacuum.

The Raman spectroscopy measurements were performed using a Send 1000 Renishaw micro-Raman with a 50* lens, an excitation wavelength of 514 nm and an incident power on the samples of 0.5 mW. The samples were prepared by deposition of the material dispersions onto Si/SiC>2 substrates under nitrogen atmosphere and subsequently dried under vacuum overnight.

The volume resistivity and the resistance of the sheet of the resistive heating elements were determined according to standard ASTM F1711-96. More in detail, four-probe measurements were performed using a Jandel RM3000 test unit. The thickness of the samples was measured using a contact profilometer (XP-200, Ambios).

The th e rm ogravi metric analysis (TGA) was performed on an alumina crucible with 4- 5 mg of mass sample for each composite, using a simultaneous thermal analyser (SDT 650, TA instruments) controlled by the TRIOS software. The tests were carried out under nitrogen atmosphere (flow of 100 mL/min), in double-sample TGA mode, in a temperature range of 30-600 °C and using a heating velocity of 10 °C/min.

Copper electrodes and a benchtop direct current (DC) power supply (triple EL302RT, Aim-TTi power supply) were used for resistive heating.

The heating and antifreeze performance of the resistive heating elements were measured using an infrared thermal camera (A655sc, FLIR).

The electrochemical measurements for the evaluation of the anti-corrosion properties and of water and ion impermeability of the films of the invention were performed using a potentiostat/galvanostat station (VMP3, Biologic) in a three-electrode configuration mode, following standard ASTM G5-14 for both the assembly of the sample and for the measurement protocols. More in detail, a structural steel cylinder (S275) with a diameter of 1.2 cm was coated with a resistive heating element with a thickness of 25 pm by deposition with doctor blade, forming a cylindrical sample used as a working electrode. For the assembly of the working electrode described in ASTM G5-14, the cylindrical sample is pressed with a metallic support with a 3-48 UNC thread, screwed onto the support rod covered with polytetrafluoroethylene (PTFE). Leak-free sealing has been ensured with PTFE compression seals. The linear polarization voltammetry and the Tafel diagram analysis were performed as described in ASTM G5-14.

The powder surface area measurements (BET measurements) were carried out with a 3Flex instrument from Micromeritics, operating under nitrogen.

EXAMPLE 1

This example refers to the production of pure graphene flakes, which are used in the preparations of the examples below, with a single exfoliation step of the wet jet exfoliation process.

The pure graphene flakes were produced starting from graphite chips (200 g) by wet jet exfoliation method (in NMP) described in patent application WO 2017/089987 A1.

The pure graphene flakes thus obtained have a flat (not wrinkled) morphology with a thickness between 1.5 and 163.0 nm. The statistical distribution of the thickness of the flakes obeys a log-normal distribution with the maximum peak at 3.82 nm. The statistical analysis of the data on the lateral dimensions indicates that the flakes have lateral dimensions between 250 and 22000 nm. The data on the lateral dimensions follow a lognormal distribution with a peak at 983 nm.

At an XPS measurement, the pure graphene flakes thus obtained show an atomic oxygen content of less than 2%.

EXAMPLE 2

This example refers to the production of pure graphene flakes, which are used in the preparations of the examples below, with five exfoliation steps of the wet jet exfoliation process.

The procedure of Example 1 was repeated, with the difference that the first dispersion produced was subjected to four further exfoliation steps under the same conditions reported above.

The pure graphene flakes thus obtained have a thickness between about 0.3 nm and 94.6 nm. The statistical distribution of the thickness of the flakes obeys a log-normal distribution with the maximum peak at 2.2 nm. The statistical analysis of the lateral dimension indicates that the flakes have lateral dimensions between 150 and 7000 nm.

At an XPS measurement, the pure graphene flakes thus obtained show an atomic oxygen content of less than 2%.

The pure graphene flakes thus obtained were also subjected to Raman spectroscopy measurements, which excluded the presence of structural defects in the basal plane of the flakes themselves.

EXAMPLE 3

This example refers to the production of resistive layers with pure graphene flakes obtained in Example 2 and Super P® Conductive carbon black, with surface area 62 m ² /g measured by physioabsorption measurements of N ₂ in accordance with standard ASTM D6556 (BET method).

In this example, PPSU is used for the production of the electrically insulating component (matrix) of the resistive layer.

A total of thirty compositions were produced, varying the ratio between electrically insulating component and electrically conductive component and, with the same ratio, with different ratios between pure graphene flakes and carbon black. Specifically, compositions were produced with weight ratios between electrically conductive component and electrically insulating component equal to 10:90, 17.5:82.5, 25:75, 37.5:62.5, 50:50, and 75:25; for each of these ratio values between electrically insulating component and conductive component, five compositions were produced with ratios between pure graphene flakes and carbon black equal to 100:0, 75:25, 50:50, 25:75, and 0:100. The samples with pure graphene flakes:carbon black ratio equal to 100:0 and 0:100 (i.e., containing only pure graphene flakes or only carbon black) are not part of the invention and have been prepared for comparison purposes. Hereinafter, the electrically insulating component and the electrically conductive component are also cumulatively referred to as “solid components”.

Each of the thirty compositions described above was produced in the form of a paste using NMP as a solvent; the amount of solvent was 5 litres per kg of solid components, in order to have pastes with viscosity suitable for the production of resistive layers as described below; the exact amount of solvent used is however not relevant for the purposes of the final results, since this component of the paste is completely evaporated in order to achieve the production of the resistive layers, and can vary between 3 and 12 litres per kg of solid components.

A resistive layer was produced with each of the thirty pastes, depositing the paste on glass support with the spreading method, i.e. “doctor blading” (MTI Corp, instrument, Richmond, CA, USA), obtaining layers of thickness 25 ± 10 pm.

The layers thus obtained were subjected to resistivity measurements, the results of which are graphically reported in Fig. 1; in particular, in the upper part of the figure the resistivity is reported as a function of the percentage by weight of electrically conductive component with respect to the solid components, while in the lower part of the figure the same results are reported in terms of conductivity. The two graphs in the figure each report five curves, one for each ratio between pure graphene flakes and carbon black, identified by the symbols reported in the inserts.

The results of these tests indicate that the preferred compositions for the resistive layers of the invention made with PPSU comprise the electrically conductive component in an amount between 30 and 60% by weight with respect to the solids content, and that the preferable amounts of pure graphene flakes are between 40 and 80% with respect to the weight of the electrically conductive component. It is interesting to note that the reference samples consisting of only carbon black or only pure graphene flakes provide unsatisfactory electrical performance.

EXAMPLE 4

Another paste was produced using pure graphene flakes obtained in Example 2 and PPSU as electrically insulating component. In this case, the solid components of the paste consist, by weight, of 18.75% of pure graphene flakes, 18.75% of Super P® Conductive carbon black and 62.5% of PPSU; 5 litres of NMP per kg of solid components were used for the preparation of the paste.

With this paste, a resistive layer on glass support, and a self-supporting resistive layer obtained through deposition on glass followed by precipitation by immersion in water (nonsolvent for PPSU) were produced by means of the doctor blading method of Example 1.

On the resistive layer on glass a surface roughness measurement was carried out with AFM technique; the result is the image reproduced in Fig. 2, and the measured RMS roughness value was equal to 335 nm.

On the same sample on glass, after preconditioning at 100 °C for 30 minutes, resistance measurements were carried out at -40 °C, 20 °C and 100 °C, and the values obtained were normalized with respect to the value at 20 °C. The trend of the normalized resistance for these three temperature values is reported in Fig. 3; from the graph data a negative TCR at 20 °C of -9.94 x10 ^-5 °C' ¹ is estimated. By lowering the temperature to -40 °C, PPSU can approach its ductile-fragile transition temperature, causing local cracks and fractures (not visible to the eye), which increase the overall resistance of the film.

Finally, Fig. 4 shows the result of a thermogravimetric test in nitrogen on the self- supporting resistive layer obtained with the antisolvent method (dashed curve); for comparison, the weight loss curve (continuous curve) of the PPSU alone used to produce the sample is also reported in the same graph. The sample of the invention shows at the end of the test (800 °C) a weight loss of 42.88%, while the sample of PPSU shows a weight loss of 57.95%. The weight loss of the sample between 200 and 300 °C is attributable to the evaporation of the solvent used for the production of the paste. The curve relative to the sample indicates that it is preferable to subject the resistive layers of the invention to heat treatment for solvent evaporation prior to their use, and that layers produced with PPSU can withstand operating temperatures up to about 500 °C.

EXAMPLE 5

The test of Example 3 was repeated under identical conditions, however using a polyurethane: polycarbonate resin (PU:PC) as a polymer for the realization of the electrically insulating component of the resistive layers and MOP as a solvent for the preparation of the pastes.

The results of the resistivity and conductivity measurements of the obtained resistive layers are shown in graphical form in Fig. 5, respectively as values of resistivity (upper part of the figure) and conductivity (lower part of the figure) of the layers as a function of the percentage by weight of electrically conductive component with respect to the total of the solid components.

Even with PU:PC, the results obtained with PPSU are confirmed, that is, the best electrical results are obtained with an amount of electrically conductive component between 30 and 60% by weight with respect to the solids content, and amounts of pure graphene flakes are between 40 and 80% with respect to the weight of the electrically conductive component; and that the reference samples consisting of only carbon black or only pure graphene flakes provide poor electrical performance, and mechanical performance similar to those observed with PPSU-based samples.

EXAMPLE 6

The test of Example 4 was repeated under identical conditions, using in this case 62.5% of PU:PC resin instead of PPSU.

On the samples thus obtained, measurements of surface roughness with AFM technique, of resistance and thermogravimetry similar to those of Example 4 were carried out.

The image obtained with AFM is reproduced in Fig. 6, and the test result is an average RMS roughness of 291.2 nm.

The resistance values measured at -40 °C, 20 °C and 100 °C, and normalized with respect to the value at 20 °C, are reported in the graph in Fig. 7; from the data reported in the graph a negative TCR at 20 °C of -2.03*1 O’ ⁴ °C’ ¹ is estimated. By decreasing the temperature from 20 °C to -40 °C, the resistance decreases slightly. Contrary to PPSU, the elasticity of the PU:PC polymeric mixture avoids local mechanical degradation effects at low temperatures, preserving an optimal interconnection of the electrically conductive component materials. Therefore, the PU:PC based films are products preferred by the present invention for both applications at low temperature (< 0 °C) and at temperatures > 100 °C (and up to about 250 °C, see thermogravimetric test).

Finally, Fig. 8 shows the result of a thermogravimetric test in nitrogen on the self- supporting resistive layer obtained by the antisolvent method (upper curve); for comparison, the same graph also shows the weight loss curve (lower curve) of the PU:PC resin alone used to produce the sample. The sample of the invention shows at the end of the test (800 °C) a weight loss of 62.4%, while the resin-only sample shows the total disappearance of the sample (weight loss of 100%). Since the resin is completely decomposed at high temperatures, the weight loss of 62.4% measured on the sample of the invention optimally matches with the theoretical amount of resin of 62.5%. The degradation of the resin begins at about 260 °C.

EXAMPLE 7 The test of Example 3 was repeated under identical conditions, however using polyvinyl butyral (PVB) as a polymer for the realization of the electrically insulating component of the resistive layers and DM F as a solvent for the preparation of the pastes.

The results of the resistivity and conductivity measurements of the obtained resistive layers are shown in graphical form in Fig. 9, respectively as values of resistivity (upper part of the figure) and conductivity (lower part of the figure) of the layers as a function of the percentage by weight of electrically conductive component with respect to the total of the solid components.

Also with PVB the results obtained with the polymers of the previous examples are confirmed, namely that the best electrical results are obtained with amount of electrically conductive component between 30 and 60% by weight with respect to the solids content, and amount of pure graphene flakes are between 40 and 80% with respect to the weight of the electrically conductive component; and that the reference samples consisting of only carbon black or only pure graphene flakes provide poor electrical performance. Furthermore, in this case it was impossible to produce resistive layers containing more than 50% by weight of electrically conductive component when it consisted of only carbon black.

EXAMPLE 8

The test of Example 4 was repeated under identical conditions, using in this case 62.5% of PVB instead of PPSU.

On the samples thus obtained, measurements of surface roughness with AFM technique, of resistance and thermogravimetry similar to those of Example 4 were carried out.

The image obtained with AFM is reproduced in Fig. 10, and the test result is an average RMS roughness of 364.2 nm.

The resistance values measured at -40 °C, 20 °C and 100 °C, and normalized with respect to the value at 20 °C, are reported in the graph in Fig. 11 ; from the data reported in the graph, a positive TCR at 20 °C of 2.29x10 ^-3 °C' ¹ is estimated, however a much lower value than the typical ones for pastes based on metallic conductors (which have TCR values > 10’ ² °C’ ¹ ).

Finally, Fig. 12 shows the result of a thermogravimetric test in nitrogen on the self- supporting resistive layer obtained with the antisolvent method (upper curve); for comparison, the weight loss curve (lower curve) of the PVB alone is also reported in the same graph. The sample of the invention shows at the end of the test (800 °C) a weight loss of 61 .1%, while the sample of PVB only shows the almost total disappearance of the sample (weight loss of about 98%). The degradation of PVB begins at approximately 260 o/^

EXAMPLE 9 The test of Example 3 was repeated under identical conditions, but using an aqueousbased acrylic resin (Acrilem RP6015) as a polymer for the realization of the electrically insulating component of the resistive layers and water as a solvent for the preparation of the pastes; in this case no pastes containing 10% by weight of electrically conductive component were produced, and the pastes were produced only with the ratios pure graphene flakes:carbon black 75:25 and 50:50, and a series of pastes with only pure graphene flakes as a comparison.

The results of the resistivity and conductivity measurements of the obtained resistive layers are shown in graphical form in Fig. 13, respectively as values of resistivity (upper part of the figure) and conductivity (lower part of the figure) of the layers as a function of the percentage by weight of electrically conductive component with respect to the total of the solid components.

Also with the acrylic resin the results obtained with the polymers of the previous examples are confirmed, namely that the best electrical results are obtained with amount of electrically conductive component between 30 and 60% by weight with respect to the solids content, and amount of pure graphene flakes are between 40 and 80% with respect to the weight of the electrically conductive component; and that the reference samples consisting of only carbon black or only pure graphene flakes provide poor electrical performance. Moreover, in this case it was impossible to produce resistive layers starting from pastes with 100% of carbon black due to the poor cohesive properties found.

EXAMPLE 10

The test of Example 4 was repeated under identical conditions, using in this case 62.5% of acrylic resin instead of PPSU.

On the samples thus obtained, measurements of surface roughness with AFM technique, of resistance and thermogravimetry similar to those of Example 4 were carried out.

The image obtained with AFM is reproduced in Fig. 14, and the test result is an average RMS roughness of 306.8 nm.

The resistance values measured at -40 °C, 20 °C and 100 °C, and normalized with respect to the value at 20 °C, are reported in the graph in Fig. 15; from the data reported in the graph, a positive TCR at 20 °C of 2.09x10 ^-3 °C' ¹ is estimated, however a much lower value than the typical ones for pastes based on metallic conductors (which have TCR values > 10’ ² °C’ ¹ ).

Finally, Fig. 16 shows the result of a thermogravimetric test in nitrogen on the self- supporting resistive layer obtained by the antisolvent method (upper curve); for comparison, the weight loss curve (lower curve) of the acrylic resin alone is also reported in the same graph. The sample of the invention shows at the end of the test (800 °C) a weight loss of 62.7%, while the sample of PVB alone shows the almost total disappearance of the sample (weight loss of about 97.8%). The degradation of the resin begins at about 250 °C.

EXAMPLE 11

The test of Example 3 was repeated under identical conditions, however using poly(vinylidene fluoride) (PVDF) as a polymer for the realization of the electrically insulating component of the resistive layers and water as a solvent for the preparation of the pastes.

With the pastes thus prepared, only eight resistive layers were produced in this case, obtained from pastes with weight ratios between electrically conductive component and electrically insulating component equal to 25:75, 37.5:62.5, 50:50 and 75:25 and, for each of these values of ratio between electrically insulating and conductive component, with ratios between pure graphene flakes and carbon black equal to 75:25 and 50:50.

The results of the resistivity and conductivity measurements of the obtained resistive layers are shown in graphical form in Fig. 17, respectively as values of resistivity (upper part of the figure) and conductivity (lower part of the figure) of the layers as a function of the percentage by weight of electrically conductive component with respect to the total of the solid components.

Also with PVDF the results obtained with the polymers of the previous examples are confirmed, namely that the best electrical results are obtained with amount of electrically conductive component between 30 and 60% by weight with respect to the solids content, and amount of pure graphene flakes are between 40 and 80% with respect to the weight of the electrically conductive component.

EXAMPLE 12

The test of Example 3 was repeated but using pure graphene flakes produced in Example 1 (i.e. with a single exfoliation step).

With the pastes thus prepared, sixteen resistive layers were produced in this case, obtained from pastes with weight ratios between electrically conductive component and electrically insulating component equal to 25:75, 37.5:62.5, 50:50 and 75:25 and, for each of these values of ratio between electrically insulating and conductive component, with ratios between pure graphene flakes and carbon black equal to 100:0, 75:25, 50:50 and 25:75.

The results of the resistivity and conductivity measurements of the obtained resistive layers are shown in graphical form in Fig. 18, respectively as values of resistivity (upper part of the figure) and conductivity (lower part of the figure) of the layers as a function of the percentage by weight of electrically conductive component with respect to the total of the solid components.

Also in this case, the results obtained in the previous examples are confirmed, namely that the best electrical results are obtained with amount of electrically conductive component between 30 and 60% by weight with respect to the solids content, and amount of pure graphene flakes are between 40 and 80% with respect to the weight of the electrically conductive component.

The electrical performances of the products shown in the present example are higher than those of the products described in Example 3. The pure graphene flakes used in this example are, however, larger in size than those of Example 3, and the use of pastes comprising them requires suitable dimensions of the nozzle and screen for spray coating and screen printing respectively, which may limit the printing resolution. The use of pastes made with pure graphene flakes of Example 1 or 2 is therefore at the choice of the technician, and depends on the type of product to be made and the technique used for its production.

EXAMPLE 13

Following the same procedures as the previous examples, further resistive layers were produced, using different polymers as an electrically insulating component always in an amount of 62.5% with respect to the total solid content of the product, and with a weight ratio between pure graphene flakes and carbon black equal to 1 :1. The volume resistivities obtained were 0.21 Q cm with vinyl resin, 0.16 Q cm with cellulosic resin, 0.17 Q cm with PDMS, 0.17 Q cm with epoxy resin, 0.07 Q cm with polyacrylic acid, 0.15 Q cm with polyvinyl alcohol and 0.40 Q cm with two-component acrylic resin.

EXAMPLE 14

This example refers to the preparation of complete resistive elements, which may find use in heating, deicing, antifreeze or similar applications.

A type II calcareous Portland cement tile was obtained with lateral dimensions 12 cm x 12 cm and thickness 2 cm.

The tile was covered with a universal acrylic base (Primer 3296) in water dispersion with strong penetrating action for the consolidation of porous surfaces. Before the deposition of the resistive heating layers, metallic bands in 15 cm long copper strip were deposited on the acrylic base, placed parallel to the sides of the tiles over their entire length and at a distance of 10 cm, defining an area of 10 cm x 10 cm for the resistive heating elements. A composition paste corresponding to that of Example 4 was deposited on the electrodes and on the primer-coated surface by spray deposition. The resistance of the resistive layer between the two electrodes was found to be 6.7 Q.

Fig. 19 reports in the graph the heating performance of the resistive element thus obtained as a function of time and at different values of applied electrical voltage, while Fig. 20 reports power and power density of the resistive element at the different electrical voltage values tested.

Fig. 21 reproduces thermographic images obtained at different times with applied electrical voltage of 15 V, illustrating the heating properties of the resistive element described above.

EXAMPLE 15

Resistive elements similar to that of Example 14 were produced and measured, however depositing resistive layers obtained with pastes of compositions corresponding to those of Examples 6 (polymer PU:PC), 8 (polymer PVB) and 10 (polymer acrylic resin).

In particular, the resistive element with electrically insulating component PU:PC was produced with roller technique and had a resistance between the electrodes of 11.8 Q, the one with electrically insulating component PVB was produced with spray technique and had a resistance between the electrodes of 38.1 Q, and the one with electrically insulating component acrylic resin was produced with roller technique and had a resistance between the electrodes of 11.8 Q. These three resistive elements reached, after 15 minutes, maximum temperatures of 116.1 °C respectively with applied electrical voltage 20 V, 81.2 °C with applied electrical voltage 25 V, and 90.1 °C with applied electrical voltage 15 V.

Fig. 22 reports in the graph the temperature evolution curves for the resistive element of Example 14 and for the three resistive elements of this example as a function of time, with applied electrical voltage of 24 V.

(COMPARATIVE) EXAMPLE 16

The procedure of Example 3 was repeated, however using graphene oxide (GO) and reduced graphene oxide (RGO) instead of the pure graphene of the invention. GO was produced starting from graphite powder (Sigma Aldrich, +100 mesh) using the modified Hummer method, as described in the article “Graphene-based hole-selective layers for high-efficiency, solution-processed, large-area, flexible, hydrogen-evolving organic photo cathodes”, S. Bellani eta!., J. Phys. Chem. C 2017, 121, 40, 21887-21903. RGO was produced starting from GO, produced as described above, through a thermal reduction process in reducing atmosphere Ar/H2 (90:10) inside a tubular furnace (PSC 12/-/600H, Lenton, UK), as described in the cited article. These procedures are representative of the methodology generally classified in patent application CN 110418444 A as “redox methods", although the synthesis parameters used are not reported in the aforesaid application.

With each of these two forms of graphene two compositions were produced with weight ratios between electrically conductive component and electrically insulating component equal to 37.5:62.5 and 50:50.

For the samples with a 37.5:62.5 ratio, a weight ratio between graphene and CB equal to 50:50 was used for the conductive component; for the samples with a 50:50 ratio, a weight ratio between graphene and CB equal to 75:25 was instead used for the conductive component. Due to the different properties of GO and RGO compared to the pure graphene of the invention, the resulting resistive layers showed delamination from the glass substrate and formation of cracks, making subsequent electrical characterization impossible. The results are shown in Fig. 23, wherein C:l indicates the weight ratio between electrically conductive component and electrically insulating component, and GO:CB or RGO:CB indicates the weight ratio between the graphene form used and carbon black.

(COMPARATIVE) EXAMPLE 17

The procedure of Example 3 was repeated, replacing the Super P® Conductive carbon black with carbon black with a specific surface area of 8 m ² /g (THERMAX® 990, CANCARB Ltd.), as measured experimentally by measurements according to the BET method. The weight percent content of the solid components of the paste was 18.75 for the graphene flakes prepared as described in Example 1, 18.75 of carbon black and 62.5 of PPSU.

The film produced according to Example 3 using Super P® Conductive showed an average volumetric resistivity of 0.054 Q cm. The film produced using THERMAX® 990 showed a significantly increased average volumetric resistivity by two orders of magnitude and equal to 9.317 Q cm, losing the technical characteristics necessary for the products to which the present invention is directed.