US20230295870A1

US20230295870A1 - Method for making a conductive coating containing graphene on natural or synthetic leather, and natural or synthetic leather thus obtained

Info

Publication number: US20230295870A1
Application number: US18/018,612
Authority: US
Inventors: Giulio Giuseppe Cesareo; Laura Giorgia RIZZI; Alessandra Bambini
Original assignee: Directa Plus SpA
Current assignee: Directa Plus SpA
Priority date: 2020-07-29
Filing date: 2021-07-21
Publication date: 2023-09-21
Also published as: WO2022023137A1; EP4189156A1; CN116157570A; KR20230048068A

Abstract

Method for making a thermally and electrically conductive coating on natural or synthetic leather comprising the deposition of several layers based on aliphatic polyurethanes, with an intermediate layer, having a thickness from 10 to 50 μm, comprising graphene nano-platelets, wherein at least 90% has a lateral dimension (x, y) from 50 to 50000 μm and a thickness (z) from 0.34 to 50 nm, and wherein the C/O ratio is ≥100:1.

Description

The present invention relates to a method for making a conductive coating containing graphene on natural or synthetic leather, and to the natural or synthetic leather thus coated.

BACKGROUND OF THE INVENTION

It is known in the art to make substrates that are not naturally conductive, such as natural leather, tanned leather or natural or synthetic textile articles, electrically and/or thermally conductive by applying conductive substances such as graphene.
WO 2015/103565 A1 describes the application of conductive inks containing graphene on a wide variety of substrates, including leather and tanned leather. The graphene can be present in any form, for example also as oxide, or can contain oxygen in any ratio with carbon, from 1:1 to 2000:1, or can be functionalized with hydroxyl, carboxyl or epoxy groups, or can be mixed with graphite in any ratio, from 1 to 99% by weight. The conductive ink can be applied with any method, such as painting, dyeing, powder coating, lamination, extrusion, electrodeposition, lithography and many others, but no specific examples of application are provided.
WO 2020/002979 A1 describes a tanned leather treated so as to be thermally dispersive and electrically conductive, and the related treatment method. The method comprises direct impregnation of the tanned leather with an aqueous bath containing graphene particles after tanning the leather and before the finishing treatment of the tanned leather, by means of mixing in a drum. The graphene particles are thus trapped inside the voids of the collagen matrix of the tanned leather, which is then dried by heating to 70° C.
CN 108330704A discloses a graphene superfine fibre polyurethane synthetic leather and a preparation method thereof. The graphene superfine fibre polyurethane synthetic leather comprises a surface layer, a middle layer and an adhesive layer. The synthetic leather is prepared from the following components: high-performance waterborne non-yellowing polyurethane resin, nylon, graphene, superfine fibre polyurethane synthetic substrate, abrasion-resistant agent, light-resistant agent, flame-retardant master batch and water. The preparation method comprises the steps of weighing the raw materials; treating the graphene; mixing and stirring the raw materials; filtering the solution by a filtering net, and preparing the leather by applying the components on a base cloth.
CN 105735002A discloses polyurethane modified in-situ with graphene and a method for manufacturing synthetic leather with high physical properties. The graphene-modified polyurethane is characterized in that graphene is firstly subjected to lipophilic modification, then the modified graphene is dispersed in a polyurethane solution, and the modified polyurethane is prepared with traditional dry and wet processes. By means of the introduction of the graphene, the polyurethane can be strengthened and toughened, and is applicable to manufacturing of various synthetic leather.
US 2020/062914 A1 discloses a polyurethane film comprising a polyurethane resin and graphene, characterized in that said graphene is present in an amount of 1 to 30% by weight on the total weight of the film.
However, the prior art embodiments do not provide an optimal solution to the problem of effectively maintaining the thermal and electrical dispersion properties of the tanned leather or of the synthetic leather over time. In fact, the graphene applied to the surface can be removed by rubbing and surface wear.
Moreover, with the prior art solutions it is not possible to obtain articles having surface finishes with particular aesthetic effects, both with regard to the presence of ornamental surface decorations and with regard to the morphological structure of the surface. This problem is particularly significant in the case of synthetic leather, which must imitate the tanned leather both in appearance and in surface morphology, so as to give a “handle” provided with a slight roughness appreciable to the touch.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a method for making a thermally and electrically conductive coating on natural or synthetic leather which is stable and maintains its properties over time.
Another object of the present invention is to provide a method for making a thermally and electrically conductive coating on natural or synthetic leather which is also adapted to give it surface finishes having particular aesthetic effects, both with regard to the presence of decorative patterns on the surface and with regard to the morphological structure of the surface.
A further object of the invention is to provide a natural or synthetic leather provided with a thermally and electrically conductive coating, and optionally with decorative patterns on the surface and/or with a particular surface morphology.
Therefore, an aspect of the present invention concerns a method for making a thermally and electrically conductive coating containing graphene on natural leather or on a textile substrate for the production of synthetic leather, characterized in that it comprises the following steps:

- a) formation of an internal layer comprising one or more aliphatic polyurethanes, having a thickness from 10 to 50 applied on said natural leather or said textile substrate;
- b) formation of one or more intermediate layers, each having a thickness from 10 to 50 applied on said internal adhesive layer (A), said intermediate layers each comprising one or more aliphatic polyurethanes and graphene nano-platelets, wherein at least 90% of said graphene nano-platelets has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, and wherein the C/O ratio is ≥100:1;
- c) formation of an external layer comprising one or more aliphatic polyurethanes, having a thickness from 2 to 10 applied on said intermediate layer (B).

Another aspect of the invention concerns a natural or synthetic leather comprising a coating characterized in that it comprises:

- (A) an adhesive internal layer comprising one or more aliphatic polyurethanes, having a thickness from 10 to 50 μm, in contact with said natural leather or with a textile substrate;
- (B) an intermediate layer, having a thickness from 10 to 50 μm, superimposed on said adhesive internal layer (A), said intermediate layer (B) comprising one or more aliphatic polyurethanes and graphene nano-platelets, wherein at least 90% of said graphene nano-platelets has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, and wherein the C/O ratio is ≥100:1;
- (C) an external layer comprising one or more aliphatic polyurethanes, having a thickness from 2 to 10 μm, superimposed on said intermediate layer (B).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below also with reference to the accompanying drawings, wherein:

FIG. 1 is a sectional schematic view of a first embodiment of a synthetic leather according to the invention; and

FIG. 2 is a sectional schematic view of a second embodiment of a synthetic leather according to the invention.

DESCRIPTION OF THE INVENTION

In the present description, the term “tanned leather” or “natural leather” is meant as the final product obtained by processing animal skin.
In the present description, the term “synthetic leather” or “leatherette” or “faux leather” is meant as a natural or synthetic textile substrate coated with a film of plastic material with a surface finish that imitates tanned leather or natural leather.
In the present description, the term “textile substrate” is meant as a substantially flat substrate such as a fabric, a nonwoven, a polymer film or membrane. With regard to fabric, it can be made of natural, artificial or synthetic fiber. With regard to the nonwoven and the polymer film or membrane, these are typically made with artificial fibers or synthetic resins.
The natural or synthetic leather with the coating according to the present invention can be used in the manufacture of garments or other articles, for example in the furniture sector, such as seats, sofas and the like, including seats and seating in the automobile sector, where thermal and electrical dissipation properties are particularly advantageous.
With regard to thermal dissipation, natural or synthetic leather with the coating according to the invention is capable of distributing the heat absorbed evenly, but also of ensuring breathability, so as to maximize the comfort of the user.
With regard to electrical dissipation, it leads to the effective dissipation of the electrical charges that form mainly through rubbing and tend to accumulate on the surface of the article, reducing the comfort of the user or causing even greater drawbacks.
In order to increase the electrical and thermal conductivity of natural or synthetic leather, and to make these properties of conductivity stable and long lasting, the invention provides for applying a multi-layer coating as defined above, either directly on the natural leather, or tanned leather, or on a textile substrate adapted to produce synthetic leather.
With regard to the material with which the textile substrate is made, it has been said that it can be a natural, artificial or synthetic material. When the textile substrate is a nonwoven or a polymer film or membrane, the material of which it is formed is, as a rule, artificial or synthetic.
Useful natural fibers include, for example, wool, silk and cotton. Useful artificial fibers include modified or regenerated cellulose fibers, such as viscose and cellulose acetate. Useful synthetic fibers comprise polyamide, including aromatic polyamides (aramids), polyester, polyurethane, polyacrylonitrile, polycarbonate, polypropylene, polyvinyl chloride and mixtures thereof. Moreover, fabric obtained from blends of natural, artificial and synthetic fibers can advantageously be used.
According to an embodiment of the invention referring, for example, to natural leather, or tanned leather, shown schematically in FIG. 1 , a coating is applied to a substrate 10 consisting of a textile or tanned leather substrate according to the following steps:

- a) formation of an adhesive internal layer (A) comprising one or more aliphatic polyurethanes, having a thickness from 10 to a 50 μm, in contact with said natural leather or with a textile substrate, followed by heating to a temperature between 70 and 100° C. for a time between 1 and 10 minutes;
- b) formation of one or more intermediate layers (B), having a thickness from 10 to 50 superimposed on said adhesive internal layer (A), said intermediate layers (B) comprising one or more aliphatic polyurethanes and graphene nano-platelets, wherein at least 90% of said graphene nano-platelets has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, and wherein the C/O ratio is ≥100:1, followed by heating to a temperature between 70 and 100° C. for a time between 1 and 10 minutes;
- c) formation of an external layer (C) comprising one or more aliphatic polyurethanes, having a thickness from 2 to 10 μm, superimposed on said intermediate layer (B), followed by heating to a temperature between 100 and 160° C. for a time between 1 and 10 minutes.

In the method of the invention in each of said steps a), b) and c) said aliphatic polyurethanes are applied in the form of aqueous dispersion.
According to another embodiment of the invention, for example referring to synthetic leather, shown schematically in FIG. 2 , a coating comprising the following layers is applied to a suitable textile substrate 20:

- (A) an adhesive internal layer (A) comprising one or more aliphatic polyurethanes, having a thickness from 10 to 50 μm, in contact with said natural leather or with a textile substrate;
- (B) a first intermediate layer (B), having a thickness from 10 to 50 μm, superimposed on said adhesive internal layer (A), said first intermediate layer (B) comprising one or more aliphatic polyurethanes and graphene nano-platelets, wherein at least 90% of said graphene nano-platelets has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, and wherein the C/O ratio is ≥100:1;
- (B1) a second intermediate layer (B1), having a thickness from 10 to 50 μm, applied to the first intermediate layer (B). As in the first intermediate layer (B), the second intermediate layer (B1) also comprises one or more aliphatic polyurethanes and graphene nano-platelets, wherein at least 90% of said graphene nano-platelets has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, and wherein the C/O ratio is ≥100:1
- (C) an external layer (C) shaped with depressions and/or roughness that imitate the surface morphology of tanned leather, comprising one or more aliphatic polyurethanes, having a thickness from 2 to 10 μm, superimposed on said second intermediate layer (B1).

In this second embodiment, the intermediate layer (B) is advantageously present when the outermost layer (C) is formed with depressions and/or roughness that imitate the surface morphology of tanned leather. As the depressions expose, or can expose, areas of the second intermediate layer B1 containing graphene, to the atmosphere, the presence of two intermediate layers (B) and (B1) ensures the permanence of a sufficient amount of graphene in the coating, ensuring that the features of electrical and thermal dissipation are maintained. With regard to the method of formation of the coating on the tanned leather and on the textile substrate, it is possible to carry out deposition of the various layers (A), (B), (B1) and (C) both directly on the tanned leather or on the textile substrate, and indirectly, i.e., by carrying out deposition of the layers in reverse order (C), (B1), (B) and (A) on an external support, for example a release paper. The layers thus deposited are then coupled to the tanned leather or to the textile substrate, with the layer (A) in contact therewith, and finally the release paper is detached, exposing the external layer (C).
Therefore, according to an aspect, the invention relates to a method for making a thermally and electrically conductive coating containing graphene on natural leather or on a textile substrate for the production of synthetic leather, characterized in that it comprises the following steps:

- i. spreading an aqueous dispersion of one or more aliphatic polyurethanes on a paper substrate for producing said external layer (C), having a thickness from 2 to 10 μm, followed by heating to a temperature between 70 and 90° C. for a time between 1 and 10 minutes;
- ii. spreading on said layer (C) an aqueous dispersion of one or more aliphatic polyurethanes and graphene nano-platelets, wherein at least 90% of said graphene nano-platelets has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, and wherein the C/O ratio is ≥100:1, for the formation of said intermediate layer (B) having a thickness from 10 to 50 μm, followed by heating to a temperature between 70 and 100° C. for a time between 1 and 10 minutes;
- iii. spreading on said intermediate layer (B), an aqueous dispersion of one or more aliphatic polyurethanes for the formation of said adhesive internal layer (A), having a thickness from 10 to 50 μm;
- iv. coupling said coating consisting of said paper substrate, said layers (C), (B) and (A) with said natural or synthetic leather, wherein said natural or synthetic leather is in contact with said adhesive internal layer (A), followed by heating to a temperature between 100 and 160° C. for a time between 1 and 10 minutes; and
- v. separating said paper substrate from said external layer (C).

According to an embodiment, the method also provides for a step ii′. after the step ii., as follows:

- ii′. spreading on said layer (B) an aqueous dispersion of one or more aliphatic polyurethanes and graphene nano-platelets, wherein at least 90% of said graphene nano-platelets has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, and wherein the C/O ratio is ≥100:1, for the formation of an intermediate layer (B1) having a thickness from 10 to 50 μm, followed by heating to a temperature between 70 and 100° C. for a time between 1 and 10 minutes.

The method comprising the step ii′. leads to obtaining a coating comprising two intermediate layers (B) and (B1), as defined above.
The indirect deposition method is preferable in the case of producing the coating on the textile support. In this case, the method is in actual fact a method for making an artificial leather with features of thermal and electrical conductivity.
Moreover, with regard to the formation of the coating on the tanned leather or on the textile substrate, in the first case it is not necessary to produce a surface finish that imitates tanned leather, although it might be desirable to give the tanned leather a particular surface finish. Instead, in the second case a surface finish that imitates leather must be created.
The surface finish that imitates tanned leather can be produced with various known methods. FIG. 2 schematically shows the production of a surface finish of artificial leather on the outermost intermediate layer B1, represented schematically by a series of triangles. In this case, the external layer (C) is applied by means of a cylinder engraved with a punctiform pattern filled with the material of the layer (C), which is deposited on the layer (B1).
Each of the layers (A), (B), (B1) and (C) comprises as main component one or more aliphatic polyurethanes, present in an amount of at least 80% by weight of each single layer, preferably at least 90% by weight of each single layer.
Each of the layers (A), (B), (B1) and (C) further comprises one or more of the following components selected in the group consisting of: leveling agents, cross-linking agents and thickening agents. Preferably, each layer comprises all of these agents.
Leveling agents advantageously usable are fluorinated surfactants, or mixtures thereof.
Cross-linking agents advantageously usable are polyethylene amines, or mixtures thereof.
Thickening agents advantageously usable are natural or synthetic thickening agents.
Examples of inorganic natural thickening agents are laminar silicates such as bentonite.
Examples of organic natural thickening agents are proteins, such as casein or polysaccharides.
Natural thickening agents chosen from agar agar, gum arabic and alginates are particularly preferred.
Examples of synthetic thickening agents are generally liquid solutions of synthetic polymers, in particular polyacrylates.
Preferably the graphene is present in the layer (B) or in the layer (B1) in an amount from 0.5 to 15% by weight of the total weight of the layer, more preferably between 1.5 and 10% by weight, more preferably between 2 and 8% by weight.
The graphene consists of graphene nano-platelets, wherein at least 90% has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm.
Preferably, at least 90% of the graphene nano-platelets has a lateral dimension (x, y) from 100 to 10000 nm and a thickness (z) from 0.34 to 10 nm, more preferably a lateral dimension (x, y) from 200 to 8000 nm, even more preferably between 500 and 5000 nm, and even more preferably a thickness (z) from 0.34 to 8 nm, more preferably from 0.34 to 5 nm.
The graphene nano-platelets have a C/O ratio ≥100:1, preferably a C/O ratio ≥200:1.
Graphene is a material consisting of a monatomic layer of hybridized carbon atoms in the form sp². Therefore, they are arranged in close-packed, highly crystalline and regular hexagonal honeycomb structures.
Several methods for the preparation of graphene are known from the scientific and patent literature, such as chemical vapor deposition, epitaxial growth, chemical exfoliation and chemical reduction of the oxidized form of graphene oxide (GO).
The Applicant Directa Plus S.p.A. is the owner of patents and patent applications relating to methods for producing structures comprising layers of graphene, such as EP 2 038 209 B1, WO 2014/135455 A1 and WO 2015/193267 A1. The last two patent applications describe methods for producing dispersions of pristine graphene, from which it is possible to obtain graphene nano-platelets with the dimensions required for the implementation of the present invention, and with a C/O ratio ≥100:1. This ratio is important as it defines the maximum amount of oxygen bonded to the carbon forming the graphene. In fact, the best properties of graphene, which derive from its high crystallographic quality are obtained when the amount of oxygen is minimum.
A pristine graphene, i.e., with a C/O ratio ≥100. and having the dimensional features defined above is produced and marketed by Directa Plus S.p.A. with the brand name G+®.
The C/O ratio in the graphene used in the method according to the invention is determined by means of elemental analysis performed by elemental analyzer (CHNS O), which provides the weight percentage of the various elements. By normalizing the values obtained with respect to the atomic weight of the C and O species and finding their ratio, the C/O ratio is obtained.
It was found that the graphene in oxidized form, just as the graphene in the form obtained through reduction of graphene oxide (“GO”), has different features and properties to pristine graphene. For example, the features of electrical and thermal conductivity and those of mechanical strength of the pristine graphene are higher than those of GO and than the reduction product obtained therefrom, also due to the presence of numerous reticular defects and imperfections of the crystalline structure caused by the reduction reaction.
The reticular defects of the nano-platelets can be evaluated using Raman spectroscopy by analyzing the intensity and shape of the Peak D positioned at 1350 cm⁻¹.
According to embodiments described in the above-mentioned patent documents by the Applicant Directa Plus S.p.A., the process for producing pristine graphene is carried out continuously, by continuously feeding the graphite flakes to the high temperature expansion step, continuously discharging the so-obtained expanded graphite in an aqueous medium and continuously subjecting the expanded graphite dispersed in the aqueous medium to exfoliation and size reduction treatment carried out with the methods of ultrasonication and/or homogenization at high pressure.
As described in these patent documents, the final dispersion of the graphene nano-platelets obtained can be concentrated or dried, depending on the final form required for the graphene. The object of drying the dispersion is to obtain a dry powder that is easily redispersible in various matrices, both solvents and polymers, where liquid is not desirable or manageable at process level, or where water cannot be used due to chemical incompatibility.
A significant advantage of the production processes described in the patent documents WO 2014/135455 A1 and WO 2015/193267 A1 consists in the possibility of operating without using surfactants. In fact, the graphene nano-platelets thus obtained are highly pristine, both due to the high C/O ratio and due to the absence of foreign substances that could contaminate them, such as surfactants. In fact, it has been seen that the absence of surfactants allows graphene having an electrical conductivity substantially higher than the graphene obtained with processes that use surfactants to be obtained. This improves the performance of the graphene in many applications.
The pristine graphene nano-platelets, at least 90% of which has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, having a C/O ratio ≥100:1, have a high electrical conductivity. It has also been seen that when a dispersion of graphene nano-platelets is formed in the presence of surfactant, this deposits on the surface thereof and tends to promote its agglomeration.
In the present description, the dimension of the graphene nanoplatelets is defined with reference to a system of Cartesian axes x, y, z, it being understood that the particles are substantially flat platelets but can also have an irregular shape. In any case, the lateral dimension and the thickness provided with reference to the directions x, y and z must be intended as the maximum dimensions in each of the aforesaid directions.
The lateral dimensions (x, y) of the graphene nano-platelets are determined, within the scope of the production process described above, with direct measurement using the scanning electron microscope (SEM), after diluting the final dispersion in a ratio of 1:1000 in deionized water and pouring it dropwise onto a silicon oxide substrate arranged on a plate heated to 100° C.
Alternatively, if the nano-platelets are available in dry state, SEM analysis is carried out directly on the powder deposited on a double-sided carbon adhesive tape. In both cases, the measurement is carried out on at least 100 nano-platelets.
The thickness (z) of the graphene nano-platelets is determined with the Atomic Force Microscope (AFM), which is essentially a profilometer with subnanometric resolution, widely used for characterization (mainly morphological) of surfaces and of nanomaterials. This type of analysis is commonly used to evaluate the thickness of graphene flakes, produced with any method, and thus detect the number of layers forming the flake (single layer=0.34 nm).
The thickness (z) can be measured using a dispersion of nano-platelets diluted in a ratio of 1:1000 in isopropanol, from which 20 ml is collected and subjected to sonication in an ultrasonic bath (Elmasonic S40) for 5 minutes. The nano-platelets are then deposited as described for SEM analysis and are scanned directly with an AFM tip, where the measurement provides a topographical image of the graphene flakes and their profile with respect to the substrate, enabling precise measurement of the thickness. The measurement is carried out on at least 50 nano-platelets.
Alternatively, if nano-platelets in dry state are available, the powder is dispersed in isopropanol at a concentration of 2 mg/L, from which 20 ml is collected and subjected to sonication in an ultrasonic bath (Elmasonic S40) for 30 minutes. The nano-platelets are then deposited as described for SEM analysis and scanned using AFM.
In the concentrated final dispersion or in the dry form obtained after drying, at least 90% of the graphene nano-platelets preferably has a lateral dimension (x, y) from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, and a C/O ratio ≥100:1. Preferably, at least 90% of the graphene nano-platelets has a lateral dimension (x, y) from 100 to 10000 nm and a thickness (z) from 0.34 to 10 nm, more preferably a lateral dimension (x, y) from 200 to 8000 nm, and even more preferably between 500 and 5000 nm, and preferably a thickness (z) from 0.34 to 8 nm, more preferably from 0.34 to 5 nm.
The graphene nano-platelets having the aforesaid features of dimension and purity, hence having a very low oxygen content, as defined by the aforesaid C/O ratio and not functionalized with other molecules, proved to be particularly suitable to be applied on the tanned leather or on the textile substrate in the method according to the invention, in order to form a thermal and electrical circuit capable of distributing the heat evenly along the circuit and of dissipating the electrostatic charges.
The dispersion for application on the tanned leather or on the textile substrate of the invention for the formation of each of the layers (A), (B), (B1) and (C) is in thick liquid or paste form, where the dispersing liquid is preferably water or a mixture of water with other solvents and/or dispersants.
The terms “thick liquid or paste” are meant as an aqueous dispersion containing from 20 to 70% by weight of solids, preferably from 25 to 60% by weight of solids, more preferably from 30 to 50% by weight of solids. In this dispersion, at least 90% by weight of the solids consists of aliphatic polyurethane and of the graphene nano-platelets.
The aqueous dispersion for the formation of each of the layers (A), (B), (B1) and (C) further comprises, as said above, one or more components selected in the group consisting of leveling agents, cross-linking agents and thickening agents. Preferably, the aqueous dispersion of aliphatic polyurethane comprises al these agents.
The aqueous dispersion for the formation of the layer (B) or of the layer (B1) comprises from 0.5 to 15% by weight of graphene nano-platelets, more preferably from 1 to 10% by weight, more preferably from 1.4 to 8% by weight.
The aqueous dispersion is prepared by mixing the aliphatic polyurethane or the aliphatic polyurethanes with one or more of the other components, as defined above.
Preparation of the aqueous dispersion is carried out preferably introducing the polyurethane pre-dispersed in water into a receptacle stirred with a rotary stirrer, into which the graphene and the leveling, cross-linking and thickening agents are then introduced. The dispersion is stirred until obtaining a uniform dispersion. Typically, stirring takes place at a rotation speed of the stirrer between 1000 and 2500 rpm (rounds per minute) for a time from 1 to 2 hours. According to an aspect of the invention, the dispersion has a viscosity from 4000 to 30000 cPs, or mPa·s, measured with a Fungilab series Viscolead PRO rotational Viscometer, impeller R6 speed 10 rpm (rounds per minute). Measurement at T=20° C. The viscosity of the composition is preferably in the range between 10000 and 20000 cPs, or mPa s. The viscosity is regulated also by means of the amount of thickening agent.
Application of the dispersion on the substrate and/or on each layer of the previously formed coating is followed by a step of heating the layer to an increasing temperature between 70 and 100° C., for a time between 2 and 10 minutes. After producing the last layer, heating takes place at a higher temperature, from 100 to 160° C., preferably from 120 to 160° C., to carry out cross-linking of the components and curing of the layer.
According to an operating mode, the heating step is carried out in two ovens in succession.
In the case in which the layers are deposited on a release paper in the order (C), (B), (A), then coupled to the substrate through the layer (A), coupling takes place before the last heating step. This is then carried out maintaining the two ovens at different temperatures, so that the substrate already coupled is first inserted into a first oven at the temperature between 70 and 100° C. to carry out evaporation of the liquids, then inserted into a second oven at a temperature between 120 and 160° to carry out cross-linking of the components and curing of the layer. The total duration of the heat treatment is a time between 1 and 10 minutes.
With regard to the method of formation of the coating on the tanned leather and on the textile substrate, in the case in which deposition of the various layers is carried out in reverse order (C), (B1), (B) and (A) on an external support, such as a release paper, coupling of the preformed layers to the tanned leather or to the textile substrate is carried out by means of a roller that presses the layer (A) on the substrate before it is introduced into the second oven at a higher temperature, i.e., before the step of cross-linking the layer (A). In this way, cross-linking of the layer (A) takes place after contact with and adhesion to the substrate. After exiting from the second oven the release paper is detached, uncovering the external layer (C). In the case of wishing to give the external layer (C) a particular three-dimensional morphology, for example in order to imitate natural leather, the external layer (C) is produced with depressions or roughness suitable to provide a handle similar to that of tanned leather. According to an embodiment, imitation of tanned leather through the formation of depressions or roughness takes place directly on the intermediate layer (B) or (B1), as for example shown in FIG. 2 , depositing this layer on a release paper provided with depressions and/or roughness. Again, with reference to FIG. 2 , a protective external layer (C) is then deposited on the layer (B1) thus formed, for example with a treatment consisting in applying the external layer (C) by means of a cylinder engraved with a punctiform pattern filled with the material of the layer (C), which is deposited on the layer (B1), as shown in FIG. 2 .
The total thickness of the coating comprising all the layers is between 42 and 160 μm, preferably between 45 and 120 μm, more preferably between 47 and 90 μm.
Application of the dispersion on the textile substrate or on the tanned leather gives rise to the formation of a thermally and electrically dispersive coating, characterized by the following parameters:

- i. Thermal conductivity greater than 1 W/mK. It must be considered that the thermal conductivity of a metal is generally >20 W/mK, and that of the insulating polymers is generally <0.1 W/mK.
- ii. Electrical conductivity, expressed as surface resistivity, in the order of 10-10⁸Ω, i.e., the coating is substantially conductive.

Application of the dispersion on the tanned leather or on the textile substrate makes it possible to obtain a coating that is substantially electrically and thermally dissipative.
In the case of application on a textile substrate, the coating also makes it possible to imitate tanned leather, hence to obtain an electrically and thermally dissipative imitation leather.
Therefore, the article thus obtained can advantageously be used for the production of articles in the clothing and furniture sector, including seats and seating for the automobile sector.
The examples below illustrate some embodiments of the invention and are provided by way of non-limiting example.

EXAMPLES

Two water-based aliphatic polyurethanes are used to produce each layer of the coating:

- PU 1—Viscosity=1346 cP (20.5° C.); ˜35% solid content.
- PU 2—Viscosity=53.4 cP (20.5° C.); ˜34% solid content.

The graphene consisted of nano-platelets produced by Directa Plus, dispersed in paste form (Paste G+) in the aqueous dispersion of the polyurethanes PU 1 and PU 2.
The Paste G+ utilized was composed of 23.5% water and 76.5% graphene nano-platelets (Pure G+). Moreover, a cross-linking agent, X-LINK 100 (dry residue 99%; polyaziridine) and a leveling agent, LEV 3F (dry residue 3.5-5.5; mixture of fluorinated surfactants) were also used. In the dispersion step, the Paste G+ was dispersed in the polyurethanes using a Cowles Dissolver DISPERMAT® CN40 disperser. The specifications of the instrument are: power 2.2 kW; rotation speed 0-5500 rpm (rounds per minute); torque 7.2 Nm. The material was treated for 15 minutes at 4000-5000 rpm (rounds per minute). The dispersion produced contained 5% of graphene G+.

Example 1

Formation of a Coating as Shown in FIG. 1
Textile substrate 10: nonwoven in polyester microfiber.
Layer (C): Thickness 5 μm; weight 3 g/m²; oven temperature: 85° C.
Composition:


		Parts by weight

	PU 1 + PU 2	100
	Leveling agent	1
	Cross-linking agent	2
	Thickening agent	2

The layer (C) is spread on a release paper giving an aesthetic effect with organic geometries imitating a natural leather. Coating takes place on a release paper.
Layer (B): Thickness 20 μm; weight 17 g/m²; oven temperature: 85° C.
It is spread on the layer (C).
Composition:


		Parts by weight

	PU 1 + PU 2	150
	Graphene	5
	Leveling agent	1
	Cross-linking agent	2
	Thickening agent	1.5

Layer (A): Thickness 22 μm; weight 38 g/m²;
Composition:

It is spread on the layer (B). It is heated in a first oven at 110° C., coupled to the textile substrate and heated in a second cross-linking and curing oven at 150° C.
The total weight of the coating is 58 g/m². The amount of graphene in the coating is 0.51 g/m², equal to 0.9%.

Example 2

Formation of a Coating as Shown in FIG. 2
Textile substrate 20: nonwoven in polyester microfiber.
Layer (C): Thickness 4 μm; weight 7 g/m²; oven temperature: 85° C.
Composition:


		Parts by weight

	PU 1 + PU 2	140

The layer (C) is applied on the layer (B1) with the “mille punti” method, as described above, as final treatment when the other layers have been produced and coupled to the textile substrate.
Layer (B1): Thickness 25 μm; weight 40 g/m²; oven temperature: 85° C.
It is spread on a release paper structured with depressions that give a three-dimensional effect imitating a natural leather.
Composition:


		Parts by weight

	PU 1 + PU 2	150
	Graphene	5
	Leveling agent	1
	Cross-linking agent	2
	Thickening agent	2

Layer (B): Thickness 33 μm; weight 45 g/m²; oven temperature: 95° C.
It is spread on the layer (B1).
Composition:

Layer (A): Thickness 35 μm; weight 50 g/m²; oven temperature: 95° C.
Composition:

It is spread on the layer (B). It is heated in a first oven to 110° C., coupled to the textile substrate and heated in a second cross-linking and curing oven to 150° C.
The total weight of the coating is 142 g/m². The amount of graphene in the coating is 2.55 g/m², equal to 1.8%.

Claims

1-10. (canceled)

11. A method for making a thermally and electrically conductive coating containing graphene on a leather substrate, the method comprising:

a) formation of an internal layer, comprising one or more aliphatic polyurethanes, having a thickness from 10 to 50 μm, and applied on the substrate;

b) formation of one or more intermediate layers, each of the one or more intermediate layers comprising a thickness from 10 to 50 μm and one or more aliphatic polyurethanes and graphene nano-platelets, wherein at least 90% of the graphene nano-platelets have a lateral dimension from 50 to 50000 nm and a thickness from 0.34 to 50 nm, and wherein a carbon-oxygen (C/O) ratio is ≥100:1; and

c) formation of an external layer comprising one or more aliphatic polyurethanes, the external layer having a thickness from 2 to 10 μm and applied on one of the one or more intermediate layers.

12. The method of claim 11, wherein the substrate is a natural leather substrate or a textile substrate for production of synthetic leather.

13. The method of claim 11, wherein in each of steps a), b), and c), the aliphatic polyurethanes are applied in the form of an aqueous dispersion.

14. The method of claim 13, wherein the aqueous dispersion further comprises one or more components selected from a group consisting of: leveling agents; cross-linking agents; and thickening agents.

15. The method of claim 11, further comprising a heating treatment at a temperature between 70 and 100° C. after each of steps a) and b), and a heating treatment at a temperature between 100 and 160° C. after step c), wherein each of the heating treatments are carried out for a time between 1 minute and 10 minutes.

16. The method of claim 11, wherein:

steps a), b), and c) are carried out in sequential order;

a deposition of the respective layers is carried out in reverse order with respect to the sequential order on an external support; and

the layers thus deposited are then coupled to the substrate, wherein the internal layer is in contact with the substrate, and the external support is detached, exposing the external layer.

17. A leather substrate comprising a coating, wherein the coating comprises:

an internal adhesive layer comprising one or more aliphatic polyurethanes, having a thickness from 10 to 50 μm, in contact with the substrate;

one or more intermediate layers, each of the one or more intermediate layers having a thickness from 10 to 50 μm, superimposed on the internal adhesive layer, each of the one or more intermediate layers comprising one or more aliphatic polyurethanes and nano-platelets of graphene, wherein at least 90% of the graphene nano-platelets have a lateral dimension from 50 to 50000 nm and a thickness (z) from 0.34 to 50 nm, and wherein a carbon-oxygen (C/O) ratio is ≥100:1; and

an outer layer comprising one or more aliphatic polyurethanes, having a thickness from 2 to 10 μm, and superimposed on one of the intermediate layers.

18. The leather substrate of claim 17, comprising natural leather or a textile substrate for production of synthetic leather.

19. The leather substrate of claim 17, wherein each of the internal adhesive layer, the one or more intermediate layers, and the external layer respectively comprises as its main component one or more aliphatic polyurethanes, present in amounts of at least 80% by weight of each individual layer.

20. The leather substrate of claim 19, wherein each of the internal adhesive layer, the one or more intermediate layers, and the external layer respectively comprises as its main component one or more aliphatic polyurethanes, present in amounts of at least 90% by weight of each individual layer.

21. The leather substrate of claim 17, wherein the graphene is present in each of the one or more intermediate layers in an amount from 0.5 to 15% by weight on a total weight of the respective layer.

22. The leather substrate of claim 21, wherein the graphene is present in each of the one or more intermediate layers in an amount between 1.5 and 10% by weight.

23. The leather substrate of claim 22, wherein the graphene is present in each of the one or more intermediate layers in an amount between 2 and 8% by weight.

24. The leather substrate of claim 17, wherein the graphene comprises graphene nano-platelets, and wherein at least 90% of the graphene nano-platelets have a lateral dimension from 100 to 10000 nm and a thickness from 0.34 to 10 nm.

25. The leather substrate of claim 24, wherein the at least 90% of the graphene nano-platelets have a lateral dimension from 200 to 8000 nm.

26. The leather substrate of claim 25, wherein the at least 90% of the graphene nano-platelets have a lateral dimension between 500 and 5000 nm.

27. The leather substrate of claim 24, wherein the at least 90% of the graphene nano-platelets have a thickness from 0.34 to 8 nm.

28. The leather substrate of claim 27, wherein the at least 90% of the graphene nano-platelets have a thickness from 0.34 to 5 nm.

29. The leather substrate of claim 17, wherein the graphene comprises graphene nano-platelets having a CIO ratio ≥100:1.

30. The leather substrate of claim 29, wherein the graphene comprises graphene nano-platelets having a CIO ratio ≥200:1.