Carbon-based conductive ink

Abstract

The present invention provides a conductive film comprising graphene nanoplatelets packed in a substantially Apollonian manner. The conductive film may also include carbon nanotubes to enhance electrical conductivity. The present invention also provides a liquid composition that can be used to print the conductive film described herein.

Description

[Technical field]

[0001] The present invention relates to conductive inks containing carbon nanomaterials, methods for making such inks, their uses, and substrates printed with the conductive inks. [Background technology]

[0002] Two-dimensional (2D) materials are crystalline materials composed of several layers or monolayers (monolayers) of atoms or molecules. A wide range of 2D materials are known, including graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs). The chemical formula of TMDs is MX2, where M is a transition metal and X is a chalcogen atom (S, Se, or Te). Such TMDs include, for example, molybdenum disulfide (MoS2), niobium diselenide (NbSe2), and tungsten disulfide (WS2).

[0003] 2D materials are known to have many interesting and potentially useful properties that differ from those of their bulk 3D counterparts: for example, graphene is highly conductive and has applications in conductive composites as well as electrode structures.

[0004] Interesting functional properties of many materials are often only observed when the materials are in a single- or few-layer (i.e., 2D) form. However, exfoliation of bulk three-dimensional (3D) materials to form their 2D counterparts requires overcoming strong interlayer dispersion forces.

[0005] Carbon nanotubes are nano-sized tubes composed of rolled sheets of graphene. The tubes typically range from 1 to 50 nanometers in diameter, but may be of lengths in the micrometer range. Carbon nanotubes can be either single-walled (i.e., formed from a single rolled sheet of graphene) or multi-walled (i.e., formed from multiple concentric rolled sheets of graphene). Carbon nanotubes have attracted significant interest due to their physical properties, namely high tensile strength and high electrical conductivity.

[0006] Dispersions containing carbon nanomaterials (e.g., carbon nanotubes, carbon nanographite, graphene, and mixtures thereof) have been considered as inks that can be used to deposit conductive films. Such films have the advantage of being "metal-free" for certain commercial applications, yet conductive. However, to date, the use of such inks has been limited to printed films with low electrical conductivity. For example, copper has a conductivity of 6×10 7 Although carbon nanomaterials have conductivities in the range of 100 S / m, reported films made from carbon nanomaterials usually have conductivities much lower than 100 S / m (see U.S. Pat. No. 10,244,628). Furthermore, the printed carbon-containing inks that do exist can only be printed on a limited range of substrates, such as aluminum and plastics (especially polyethylene terephthalate, PET). These materials cannot be recycled.

[0007] The formulation of printable inks based on dispersions of carbon nanomaterials in water has suffered from aggregation issues due to the non-polar nature of these materials, which limits their industrial applications due to the need for settling of the nanocarbon materials and excess organic solvents.

[0008] Khan et al., “The preparation of hybrid films of carbon nanotubes and nano graphite / graphene with excellent mechanical and electrical properties”, Carbon 48 (2010), pp. 2825-2830, report that hybrid films containing both carbon nanotubes and nano graphite have higher electrical conductivity than films containing only one component. However, Khan et al. only describe the dispersion of nano graphite and carbon nanotubes in N-methylpyrrolidone solvent. The solvent is removed by vacuum filtration to form a film of carbon nanomaterial. The electrical conductivity of the film is up to 2×10 4S / m, this fluid is not suitable for printing.

[0009] Pan et al., “Sustainable production of highly conductive multilayer graphene ink for wireless connectivity and loT applications”, Nature Comm. (2018), 9:5197, describes inks containing graphene, dihydrolevoglucosenone, and NMP. The conductivity of films printed from these inks is only 7.13 × 10 4 S / m.

[0010] Ferrari et al. (WO2017 / 060497A1) reported that 4 We describe the fabrication of liquid-phase exfoliated GNP / carboxymethylcellulose films that exhibit electrical conductivity of 1.0 S / m. These films were printed onto a PET substrate and used to fabricate UHF RFID tags with a read range of 1.4 m at 2 W incident radiation.

[0011] The applicant's earlier patent application (International Patent Application No. PCT / EP2021 / 055458) describes, inter alia, an ink comprising graphene nanoplatelets and carbon nanotubes that can be printed to form a film with good electrical conductivity. With a ratio of nanoplatelets to nanotubes of about 2:1, electrical conductivity of up to 500 kS / m can be achieved.

[0012] Recycling mixtures in an environmentally friendly manner is a perennial challenge, especially in the electronics industry. The implementation of electronic systems with less negative environmental impact stimulates new innovations in the combination of materials used in the manufacture of these devices. Mass-produced UHF RFID tags are composed of mixed materials (plastics, metals, silicon, and paper). Moving towards materials with increased environmental reliability and acceptable performance is interesting for many stakeholders. In some cases, metals are not preferred due to strict requirements for product screening to protect the interests of consumers.

[0013] High solids inks are an essential requirement to reduce the environmental impact of printing due to the drying process. Stabilizing nanocarbon dispersions using synergistic binders increases the potential thickness of screen printed films. This helps reduce resistive losses, which are essential for a variety of printed electronics applications. For efficient carbon-based RF antenna applications, the printed film thickness must be thinner, typically limited to less than 100 μm by process and ink solids considerations (Jordan, Edward Conrad (1968), Electromagnetic Waves and Radiating Systems, Prentice Hall, ISBN 978-0-13-249995-8).

[0014] There remains a need for alternative structures based on alternative carbon-based conductive inks, preferably with improved electrical conductivity and / or that can be printed on recyclable substrates. Summary of the Invention [Problem to be solved by the invention]

[0015] The inventors of the present application have discovered that by applying the principles of Apollonian packing, the electrical conductivity of films including graphene nanoplatelets can be enhanced. [Means for solving the problem]

[0016] Thus, in a first aspect, the present invention provides a conductive film comprising substantially Apollonian packed graphene nanoplatelets.

[0017] The conductive film may also include carbon nanotubes that act as fillers to fill gaps between the graphene nanoplatelets in the film to further enhance the conductivity of the film, however, as a result of the Apollonian packing of the nanoplatelets in the film and the resulting enhanced conductivity, a higher conductivity can be obtained using a lower amount of carbon nanotubes (compared to that described in applicant's earlier published International Patent Application No. PCT / EP2021 / 055458 (WO2021 / 175989)).

[0018] Percolation theory describes the connectivity of objects (e.g., graphene nanoplatelets, carbon nanotubes) in a network and their effect on macroscale material properties such as electrical conductivity. According to conventional electrical percolation theory, for a mixture of dielectric and metallic components, the electrical conductivity and permittivity of the mixture exhibit critical behavior when the fraction of the metallic component reaches a percolation threshold.

[0019] The inventors of the present invention have found that the percolation threshold (with respect to the amount of carbon nanotubes) of a mixture of graphene nanoplatelets and carbon nanotubes can be reduced by decreasing the porosity in the matrix of graphene nanoplatelets, which can be achieved by applying the principles of Apollonian packing.

[0020] Figure 1 shows a conductivity percolation data set obtained by measuring the sheet resistance and thickness of films prepared from coatings containing carbon nanotube (CNT) content for a given total carbon nanotube and graphene nanoplatelet content. The optimal CNT content was calculated based on a conversion of the data to estimate the printed sheet resistance using the ink conductivity and solids content. The normalized single-pass sheet resistance S is calculated by:

[0021]

number

[0022] where σ0 and φ0 are the reference conductivity and CNT mass fraction values, and σ and φ are the conductivity and CNT mass fraction of a selected point on the percolation curve. The optimal CNT content of the CNT-nanoplatelet blend film is defined as the x value of the local minimum of S that correlates with the percolation threshold of the CNTs.

[0023] Figure 2 plots two sets of calculated experimental values ​​of S using graphene nanoplatelet fillers with different relative packing densities, ρ. The local minima of S are obtained by applying a quadratic fit to the data. As can be seen, GNPs with higher relative packing densities have a lower optimal CNT content, as indicated by the vertical dashed lines.

[0024] The "Apollonian approach" significantly reduces the volume fraction of packing material required for electrical percolation. The principle of Apollonian packing originates from mathematical work done by Apollonius of Perga over 2000 years ago, who showed that successive smaller circles fill the gaps of larger circles. In the early 20th century, Furnas revisited the Apollonian circular geometry while trying to explain the granular systems in mines in the United States. Furnas realized that when considering hard spheres, it was possible to follow a similar configuration as Apollonius described, and that by using a bimodal particle system, it was possible to pack spherical particles in a similar way to that described by Apollonius, thus creating a material with high density and low porosity.

[0025] Furnas established some relatively simple equations that explain how a bimodal distribution of spherical particles pack together when the smaller particles are small enough to fill the voids between the larger particles. Using these equations, the density of the entire system can be maximized, and therefore the porosity can be minimized. Applying Furnas' theory to compositions containing graphene nanoplatelets and carbon nanotubes, films can be produced with a very low percolation threshold, since the reduced voids can be forced to be filled by the filler, forming a connected network with a much smaller amount of nanotubes.

[0026] A major problem with conventional carbon nanotube-containing composites is that the random distribution of carbon nanotubes within the matrix requires relatively high loadings to achieve electrical percolation.

[0027] The present invention is based on the principle of isolated networks. In these isolated systems, the design of the matrix becomes a major factor influencing the electrical properties, whereas previously it was of little importance. To create an isolated network of conductive particles, the matrix must be carefully selected to ensure the proper structure, where the filler particles overlap in the required way.

[0028] Utilizing a mixed population of two or more GNPs, with a distribution of graphite nanoplatelets typical of those produced by industrial liquid-phase graphite exfoliation processes, leads to inks that are highly conductive and economically favorable for high-volume printing and coating applications.

[0029] The theory of segregation percolation has been shown to depend on the mesoscale crystalline structure that the conductive filler forms within the matrix used. By combining theories on particle packing and percolation threshold, it is possible to optimize the final system to create a dense, low percolation system. If the porosity of the system is considered as the space that the conductive filler can occupy, the "shape" of the voids defines the "shape" of the network that is formed when the filler particles are trapped inside the system. Therefore, the electrical percolation threshold of a composite is directly linked to the porosity of the matrix.

[0030] The Apollonian packed films described herein can be produced from liquid compositions containing two or more populations of graphene nanoplatelets with different sizes. These populations can also occur within the same material source, commonly known as bimodal distributions. For example, a mass of graphene nanoplatelets can contain two or more populations of nanoplatelets with different sizes. Extending this concept, populations of multiple particle sizes within a single material can be superimposed to create a broad size distribution where fitting of small and large particles occurs easily. In the present invention, 2D nanoplatelets can exist in a broad particle distribution ranging from a few nanometers to several microns. This allows for an optimized particle packing morphology that helps lower the percolation threshold of fibrous carbon nanotubes. Conversely, a narrow size distribution of the material results in relatively non-optimal packing and a higher percolation threshold.

[0031] An additional benefit of the optimized packing morphology is the densification of the film upon printing and drying. Industrially, this eliminates the need for an additional compression step to increase the overall film conductivity.

[0032] Thus, the present invention also provides a film comprising: (i) a first population of graphene nanoplatelets; and (ii) a second population of graphene nanoplatelets having an average size up to 25% of the average size of the first population of graphene nanoplatelets.

[0033] The film may further include carbon nanotubes and / or thickeners, as described herein.

[0034] The film may further include a third population of graphene nanoplatelets having an average size up to 25% of the average size of the second population of graphene nanoplatelets.

[0035] Highly conductive films can be produced, for example, by printing such liquid compositions.

[0036] Thus, the present invention also provides a liquid composition comprising: (i) a first population of graphene nanoplatelets; (ii) a second population of graphene nanoplatelets having an average size up to 25% of the average size of the first population of graphene nanoplatelets; (iii) a thickening agent, and (iv) Solvent.

[0037] The liquid composition may further comprise a third population of graphene nanoplatelets having an average size up to 25% of the average size of the second population of graphene nanoplatelets.

[0038] The liquid composition may further include carbon nanotubes, as described above in relation to the film.

[0039] The thickener can adequately bind and adhere the carbon nanomaterial to a substrate, such as a cellulosic or other suitable hydrophilic substrate. The thickener can be or include a cellulose derivative. The inventors have also found that carbon nanomaterial-containing inks can be prepared and printed and deposited on recyclable substrates, particularly paper.

[0040] The thickening agent provides a means to adequately separate and encapsulate the carbon nanotubes, distributing a maximum number of individual conductive pathways between the nanotubes and the conductive carbon particles.

[0041] The liquid composition (after printing) dries to form a conductive film that can adhere to cellulose-containing substrates. When the solvent is an aqueous solvent, the composition may more accurately be called a hydrogel ink, due to the nature of the interaction between the cellulose derivative thickener and the solvent. In this specification, references to the liquid composition of the present invention encompass hydrogel inks, unless the context requires otherwise.

[0042] The above liquid compositions may also be provided in dry powder or aerogel compositions in the absence of solvent.

[0043] In a further aspect of the invention, there is provided a substrate (eg, a cellulosic substrate) printed with a conductive ink that is a liquid composition as described herein.

[0044] The present invention also provides a method of printing a conductive ink, which is a liquid composition described herein, onto a substrate (eg, a cellulosic substrate).

[0045] It is believed that the liquid compositions described herein can be printed onto a stretchable substrate in a similar manner to the liquid compositions described in the applicant's previous patent application (International Patent Application No. PCT / EP2021 / 055458, published as WO2021 / 175989). Also provided are compositions as described in more detail herein that can be printed onto a stretchable substrate.

[0046] According to the present invention, it is possible to produce films having electrical conductivity comparable to those described in PCT / EP2021 / 055458 (WO2021 / 175989), but using a lower proportion of carbon nanotubes.

[0047] The concentration of ink solids and the use of screen printing also facilitate the thick film formation required to achieve good electrical conductivity (0.1 Ohm / Sq / mil), giving the film suitable antenna properties and the electromagnetic "skin depth" characteristics required for radiating antennas within the UHF band.

[0048] Printing conductive structures onto substrates enables the integration of surface mounted electronic components for a variety of applications. Examples of potential commercial applications such as RFID tags, micro-heaters and sensors are discussed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] The term "carbon nanomaterial" as used herein refers to a nanomaterial that includes or consists of carbon (i.e., a material that has one critical dimension with an average size between 1 nm and 100 nm). Typically, carbon nanomaterials include at least 90% by weight or more, preferably at least 95% by weight or more, for example 99% by weight or more, of carbon. The term includes materials such as graphene, graphite nanoplatelets, single-walled carbon nanotubes, multi-walled carbon nanotubes, crystalline diamond, diamond-like carbon (see ISO standard ISO / TS80004-3:2020). The dimensions of the nanomaterials can be measured by transmission electron microscopy. The carbon nanomaterial-containing films and liquid compositions described herein may include graphene nanoplatelets themselves, or may include graphene nanoplatelets with single-walled carbon nanotubes, multi-walled carbon nanotubes, or both. In particular, the films and liquid compositions described herein include a mixture of (i) graphene nanoplatelets and (ii) single-walled carbon nanotubes.

[0050] It has been found that there is a cooperative effect on electrical conductivity in compositions containing both graphene nanoplatelets and single-walled carbon nanotubes. Without wishing to be bound by theory, it is believed that the carbon nanotubes provide a conductive bridge between individual graphene nanoplatelets, thus reducing the "patch resistance" of the individual nanoplatelets. The patch resistance is caused by finite tunneling of electrons between adjacent sheets, which is much higher than the movement within the internal structure of the sheet (graphene nanoplatelet) or rod (carbon nanotube). Furthermore, without wishing to be bound by theory, the inventors believe that the junction resistance between a graphene nanoplatelet and a carbon nanotube is lower than the junction resistance between two nanoplatelets or two nanotubes. Thus, the intimate mixing of nanoplatelets and nanotubes enhances the electrical conductivity of films formed from the liquid compositions described herein that contain both graphene nanoplatelets and carbon nanotubes (specifically single-walled carbon nanotubes).

[0051] To maximize this effect, the carbon nanotubes are preferably individualized. Typically, more than 75% by weight of the nanotubes in the composition are individualized, such as more than 80% by weight, preferably more than 85% by weight. Individualized nanotubes can be seen in FIG. 3. The degree of nanotube individualization can be determined by UV-Vis spectroscopy, as individualized single-walled carbon nanotubes exhibit Van Hove singularities (peaks) at specific wavelengths (Alafogianni et al., Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol 495, (2006), pp. 118-124). These UV-Vis absorptions are not visible for bundled carbon nanotubes, so the prominence of these peaks is a measure of exfoliation / individualization.

[0052] By loading particles of different insoluble geometric shapes and sizes, a range of physical properties can be enhanced, depending on the nature of those particles. This effect extends to the nanoscale. By carefully combining different particle sizes and shapes, the overall physicochemical properties of the formulated system can be tuned to achieve the desired properties. In commercial applications, the cost factor of the most active elements requires the system to be loaded with a significant percentage (>50%) of lower cost fillers that do not affect performance to unacceptable levels, or fillers added to impart other properties such as thermal conductivity, mechanical strength, and / or chemical reactivity. In the present invention, cost-effective formulations can be obtained by concentrating the most conductive elements within the loaded voids within the matrix of larger conductive carbon particles (which may exhibit one dimension at the nanoscale). Blending the thixotropic single-walled carbon nanotube hydrogel with conductive carbon particles ensures that high conductivity is maintained throughout the printing and drying process, resulting in excellent film conductivity.

[0053] As mentioned above, the film may include a first population of larger nanoplatelets and a second population of smaller nanoplatelets. The first population of larger nanoplatelets may form a close-packed matrix, and the second population of smaller nanoplatelets may fill the interstitial positions (or voids) within the matrix of the larger nanoplatelets. In order to fit within the interstitial positions formed by the packing of the larger nanoplatelets, the second population of smaller nanoplatelets has an average size of up to 25% of the size of the first population of nanoplatelets. Typically, the second population of nanoplatelets has a size of 20% or less, typically 17% or less, for example 15% or less, of the size of the first population of nanoplatelets.

[0054] The film may further comprise a third population of nanoplatelets filling interstitial sites formed by the packing of the second population of nanoplatelets (with the first population of nanoplatelets), the third population of nanoplatelets having a size up to 25%, typically up to 20%, preferably up to 17%, e.g., up to 15%, of the size of the second population of nanoplatelets.

[0055] The first population of graphene nanoplatelets typically constitutes 40% or more (w / w), such as 50% or more (w / w), such as 60% or more (w / w) of the total amount of nanoplatelets in the film.

[0056] The second population of graphene nanoplatelets typically constitutes no more than 50% (w / w) of the total amount of nanoplatelets in the film, such as no more than 35% (w / w), such as no more than 25% (w / w).

[0057] Throughout, references to the size of a first, second, or third population of nanoplatelets typically refer to the maximum lateral dimension of the nanoplatelets (e.g., the longer of the length and width of the nanoplatelets shown in FIG. 4).

[0058] Apollonian packing of graphene nanoplatelets increases the packing efficiency of the nanoplatelets. In the films described herein, the nanoplatelets are typically packed with a packing efficiency of more than 5%, preferably more than 10%, for example more than 15%, and in some cases up to 40%. Alternatively, the packing efficiency can be determined by measuring the voids in the packed matrix of nanoplatelets. In the films described herein, the Apollonian packed nanoplatelets typically have a void of 95% or less, preferably 90% or less, for example 85% or less. In general, the voids are greater than 60%. The packing efficiency is determined by calculating the ratio of the measured film density to the density of ideally packed single crystal graphite. The voids can be calculated as the volume not occupied by the material.

[0059] As a result of the tighter packing, the density of the resulting film increases. Thus, the films described herein have a density of 200 kg / m 3 More than 250kg / m 3 More preferably, 300 kg / m 3 More than 350kg / m 3 or more than 375kg / m 3 The film has a density of about 160 kg / m or more. The density is calculated by measuring the weight and the volume occupied by the film. The volume of the film can be measured by profilometry to measure the thickness of the film. Alternatively, the density can be calculated from buoyancy measurements. For comparison purposes, the films described in PCT / EP2021 / 055458 (WO2021 / 175989) have a density of about 160 kg / m 3 It is.

[0060] The term "graphene nanoplatelet" (also referred to herein as "graphite nanoplatelet") as used herein refers to a nanoparticle of graphite consisting of small stacks of graphene. The term "few-layer" nanoplatelet refers to a nanoplatelet having an average of 30 layers or less, suitably 20 layers or less, typically 15 layers or less, preferably 10 layers or less. The number of layers can be determined by UV-Vis spectroscopy (see C. Backes et al., 'Spectroscopic metrics allow in-situ measurement of mean size and thickness of liquid-exfoliated graphene nanosheets', Nanoscale, 2016, doi: 10.1039 / C5NR08047A).

[0061] Graphite nanoplatelets typically contain more than 80% by weight carbon, preferably more than 90% by weight carbon, for example more than 95% by weight carbon. In some compositions described herein, the conductive carbon particles consist of carbon (i.e., to a significant extent, contain carbon and no other elements).

[0062] Graphite nanoplatelets are electrically conductive. Therefore, sp 2 The proportion of carbon atoms in the conductive carbon particles that are in a hybridized state is typically 50% or more, such as 75% or more, preferably 90% or more.

[0063] Graphite nanoplatelets typically have an average thickness of less than 50 nm, typically less than 30 nm, for example less than 20 nm. As used herein, the term "thickness" refers to the dimension of the nanoplatelet along the stacking axis of the layers within the nanoplatelet. The terms "length" and "width" refer to the long and short dimensions, respectively, of the nanoplatelet along orthogonal axes in the plane of the sheet of layered material (see FIG. 4).

[0064] The nanoplatelets (e.g., the first population of nanoplatelets) typically have an average (median) length and / or width of 30 nm or more, preferably 100 nm or more or 500 nm or more. Preferably, the nanoplatelets in the first population have an average length of 1 μm or more, such as 2 μm or more, such as 3 μm or more. The nanoplatelets typically have an average (median) length and / or width of 50 μm or less, or 30 μm or less, such as 10 μm or less, typically 9.0 μm or less, such as 8.0 μm or less. The dimensions of the nanoplatelets can be measured using a scanning or transmission electron microscope. The nanoplatelets are typically micron-sized in only two dimensions (i.e., their length and width are micron-sized, and their thickness is less than 1 μm, e.g., less than 100 nm). As mentioned above, these dimensions can be measured by transmission electron microscopy.

[0065] The total graphite nanoplatelets are usually present in the film in an amount of 85% (w / w), typically from 90% (w / w), preferably from 92% (w / w), such as from 94% (w / w) and / or up to 99% (w / w), preferably up to 98% (w / w), such as up to 97% (w / w).

[0066] The carbon nanotubes may be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), but preferably comprise or consist of single-walled carbon nanotubes. Carbon nanotubes typically have an outer average diameter (determined by transmission electron microscopy) of 1 nm to 5 nm, preferably 1 nm to 2 nm, and may have a length of more than 200 nm, or more than 3 μm, typically more than 5 μm, for example more than 10 μm, or more than 15 μm. Carbon nanotubes may have an aspect ratio of 50 or more, typically more than 100. Here, aspect ratio refers to the ratio of the length of the nanotube compared to the diameter of the nanotube. Graphene nanoplatelets are micron-sized in two dimensions, while carbon nanotubes are only micron-sized in one dimension (i.e. along their length).

[0067] The amount of carbon nanotubes present can be defined relative to the amount of carbon nanoplatelets present. Carbon nanotubes can be present in the films described herein in a weight ratio (carbon nanotubes:graphene nanoplatelets) of less than 1:15 or less than 1:20, typically less than 1:30 or less than 1:40, such as less than 1:50 or less than 1:60, preferably more than 1:100, suitably more than 1:90, relative to the amount of graphite nanoplatelets.

[0068] The carbon nanotubes are typically present in the film in a weight ratio (carbon nanotubes:graphene nanoplatelets) of 1:15 to 1:100, such as 1:30 to 1:80 or 1:50 to 1:70, preferably 1:55 to 1:65, relative to the amount of graphite nanoplatelets.

[0069] As discussed above, the Apollonian packing of the nanoplatelets enhances the conductivity of the films, so that less carbon nanotubes need to be added to achieve a comparable level of conductivity.

[0070] The Apollonian-filled films described herein can be fabricated from a liquid composition that includes two or more populations of graphene nanoplatelets having different sizes. The Apollonian-filled films can be fabricated, for example, by printing such a liquid composition.

[0071] Thus, the present invention also provides a liquid composition comprising: (i) a first population of graphene nanoplatelets; (ii) a second population of graphene nanoplatelets having an average size up to 25% of the average size of the first population of graphene nanoplatelets; (iii) a thickening agent, and (iv) Solvent.

[0072] The liquid composition may further comprise a third population of graphene nanoplatelets having an average size up to 25% of the average size of the second population of graphene nanoplatelets.

[0073] To form an Apollonian loaded film, the second population of nanoplatelets can have a relative size compared to the size of the first population as larger nanoplatelets described above for the film. The liquid composition may also optionally include a third population of nanoplatelets having a relative size to the second population of nanoplatelets described above for the film.

[0074] The graphite nanoplatelets are typically present in the liquid composition in an amount of from 5% (w / w), preferably from 7%, such as from 8%, up to 20% (w / w), preferably up to 15%, such as up to 10% (w / w).

[0075] As discussed above in relation to the film, the first population of graphene nanoplatelets typically comprises 40% or more (w / w), such as 50% or more (w / w), such as 60% or more (w / w) of the total amount of nanoplatelets in the liquid composition.

[0076] The second population of graphene nanoplatelets typically comprises no more than 50% (w / w) of the total amount of nanoplatelets in the liquid composition, such as no more than 35% (w / w), such as no more than 25% (w / w).

[0077] The amount of carbon nanotubes in the composition can be defined relative to the weight of the entire composition. For example, carbon nanotubes may be present in the liquid composition in an amount of from 0.01% (w / w), preferably from 0.025% or 0.05%, for example from 0.1% to a maximum of 1% (w / w), preferably from 0.5% (w / w), for example up to 0.2% (w / w). When the liquid composition is dried to form a dry film, carbon nanotubes are typically present in an amount of from 0.5% (w / w), preferably from 1% (w / w) to a maximum of 10% (w / w), for example up to 5% (w / w) or up to 3% (w / w), preferably up to 2.5% (w / w), for example up to 2% (w / w).

[0078] As discussed above in relation to the film of the present invention, the amount of carbon nanotubes present can be defined relative to the amount of carbon nanoplatelets present. Carbon nanotubes can be present in the liquid composition described herein in a weight ratio (carbon nanotubes:graphene nanoplatelets) of less than 1:15 or less than 1:20, typically less than 1:30 or less than 1:40, such as less than 1:50 or less than 1:60, preferably more than 1:100, suitably more than 1:90, relative to the amount of graphite nanoplatelets.

[0079] The carbon nanotubes are typically present in the liquid composition in a weight ratio (carbon nanotubes:graphene nanoplatelets) of 1:15 to 1:100, such as 1:30 to 1:80 or 1:50 to 1:70, preferably 1:55 to 1:65, relative to the amount of graphite nanoplatelets.

[0080] The solvent may be aqueous or non-aqueous. However, the solvent is preferably water (necessary for hydrogel formation) or contains water. Alternatively, the solvent may be a dipolar aprotic solvent. Examples of such dipolar aprotic solvents include cyclopentanone, cyclohexanone, N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylsulfoxide (DMSO), dimethylacetamide (DMAc), sulfolane, dihydrolevoglucosenone (Cyrene), and lactones such as γ-valerolactone. It has been found that a solvent system comprising a combination of water and γ-valerolactone results in an ink suitable for printing on stretchable substrates (see Example 4 below). γ-valerolactone, when present, may be present in an amount of 1% to 10% (w / w), for example 1% to 5% (w / w).

[0081] The composition also includes a thickener (which may also act as a gelling agent) to increase the viscosity of the composition, which ensures that the composition is suitable for printing and also reduces the tendency of the carbon nanomaterials to agglomerate.

[0082] The thickener is preferably a hydrogel-forming thickener. As discussed above, the formation of a hydrogel matrix containing carbon nanotubes and conductive carbon particles results in a highly conductive ink. Hydrogel-forming thickeners are generally hydrophilic polymer chains that form colloidal gels in water through extensive hydrogen bonding networks.

[0083] The thickener also preferably binds to cellulose, for example, when the ink / liquid composition of the present invention is printed and dried on a cellulose-containing substrate (such as paper).

[0084] Examples of suitable thickening agents include: Cellulose derivatives such as carboxymethylcellulose (CMC), methylcellulose, hydroxyethylcellulose, and carboxyethylcellulose, and their salts (sodium salts, etc.), Polymers such as polyethylene oxide (PEO), polypropylene oxide (PPO), polyanaline (PANI), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), and poly N-isopropylacrylamide (PNIPAAm); Cyclodextrin, Natural gelling agents such as xanthan gum, gelatin, glycerol, alginates, chitosan, etc. Inorganic silica and clays such as bentonite, montmorillonite, laponite, nanosilica, titania, and Fibrous or rod-like materials, e.g. those with an aspect ratio greater than 100 (e.g. carbon nanotubes).

[0085] In a preferred embodiment, the thickener is a cellulose derivative, such as carboxymethylcellulose. The term cellulose derivative as used herein refers to a chemical derivative of cellulose formed by functionalization (e.g., by etherification or esterification reactions) of some or all of the hydroxyl groups present in cellulose. The derivatives can be formed by incorporating one or more or all of the carboxy, hydroxy, methyl, ethyl and / or propyl groups. Examples of cellulose derivatives include hydroxypropylmethylcellulose, hydroxypropylcellulose, methylethylcellulose, methylcellulose, and carboxymethylcellulose, or combinations thereof, as well as cellulose itself. Liquid ink compositions containing this type of binder have been found to adhere well to paper substrates. CMC is available in several forms (e.g., depending on the degree of substitution and functionality) and can be covalently crosslinked with some chemicals or through hydrogen bond networks with others to give new properties that can be tailored according to requirements (Gels 2018, 4, 54; doi: 10.3390 / gels4020054).

[0086] Cellulose derivatives readily form hydrogels that are utilized in many industrial applications. These materials can also act as surfactants to stabilize nanocarbon materials in aqueous media. Hydrogels exhibit ideal thixotropic behavior due to extended hydrogen bonding or supramolecular network formation behavior. These networks help provide long-range order to improve rheological behavior.

[0087] The total concentration of thickener may be in the range 0.05% to 2% by weight of the total composition (including solvent), typically 0.07% to 1% by weight, for example 0.1% to 0.5% by weight of the total composition.

[0088] The thickener is believed to increase the viscosity of the composition and also enable the carbon nanotubes, if present, to form a pre-ordered supramolecular network, thereby increasing the electrical conductivity of a film printed from the composition.

[0089] The viscosity of the composition is important to be able to print and form a film. Additionally, the composition must be viscous enough to prevent agglomeration of the carbon nanomaterials within the composition. Of course, the exact viscosity depends on the application of the composition (and the resulting film). The thickener also ensures that the ink has a suitable viscosity for printing, e.g., screen printing. Inks suitable for screen printing are typically thixotropic, and therefore their viscosity depends on the shear rate. As shown in Figure 7, the ink may have a viscosity of 100-1000 Pa·s at a shear rate of 0.1 / s, and / or a viscosity of 1-10 Pa·s at a shear rate of 100 / s.

[0090] The composition may also include one or more surfactants. The composition is typically a non-ionic surfactant. Examples of suitable non-ionic surfactants include polyethylene oxide (PEO) surfactants (e.g., Triton X-100), polypropylene oxide (PPO) surfactants, cyclodextrin and polyvinylpyrrolidone (PVP) surfactants. However, ionic surfactants such as sulfate surfactants (e.g., sodium dodecyl sulfate) or cholate (e.g., sodium cholate) can also be used.

[0091] The total concentration of surfactant may range from 0.01% to 1% by weight of the total composition (including solvent), or from 0.01% to 0.5% by weight, for example from 0.05% to 0.2% by weight of the total composition.

[0092] The composition may also include one or more solvents and / or adhesives to improve adhesion of the dry film (formed by printing the ink) to the substrate. The nature and combination of adhesives will of course vary depending on the substrate.

[0093] The composition may also include one or more crosslinkers to improve the rheological parameters of the ink and / or the properties of the resulting film. This may include a wide range of functional organic acids or bases, such as ascorbic acid. Additional examples of crosslinkers include di- and tricarboxylic acids, such as glutaric acid and trimesic acid. This crosslinking serves to stabilize the film against rapid redissolution and the effects of ambient humidity on electrical conductivity.

[0094] In addition, the composition may further include a hardener, which is a material that cures upon exposure to heat or radiation to harden the liquid ink composition into a solid film. These include photocurable monomers or infrared activators, such as epoxides (which may undergo ring-opening reactions), aldehydes, or acids such as citric acid (which may undergo esterification reactions). Alternatively, a film formed with a monovalent binder, such as sodium carboxymethylcellulose, may be treated with an aqueous solution of a divalent, trivalent, or tetravalent ion salt, such as calcium(II) chloride or calcium(II) sulfate, to form an insoluble film by ion exchange.

[0095] Other additives may be included in the final ink formulation to aid in the printability and robustness of the present invention. These may include humectants to ensure that the wetting and drying properties are suitable for screen printing, and crosslinkers to immobilize the resulting coating so as to impart some additional functional performance (resistance to moisture and other solvents to which the printed or coated film may be exposed). In water-based compositions, the addition of glycols such as urea, glycerin, or polypropylene glycol as humectants slows the drying process of the ink, resulting in stable and reproducible prints.

[0096] In an exemplary embodiment, the present invention provides a liquid composition comprising: (a) Graphite nanoplatelets in the weight range of 5%–15% (w / w); (b) carbon nanotubes in the weight range of 0.05% to 0.5% (w / w); (c) carboxymethylcellulose in the weight range of 0.1% to 1.0% (w / w); (d) sodium cholate in the weight range of 0.01% to 0.5%; and (e) Water.

[0097] In a further exemplary embodiment, the present invention provides a liquid composition comprising: (a) Graphite nanoplatelets in the weight range of 5%–15% (w / w); (b) carbon nanotubes in the weight range of 0.05% to 0.5% (w / w); (c) carboxymethylcellulose in the weight range of 0.1% to 1.0% (w / w); (d) sodium cholate in the weight range of 0.01% to 0.2%; and (e) Water.

[0098] A preferred component of the liquid composition is a cellulose derivative, as described elsewhere. Ethyl cellulose, methyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, and hydroxyethyl cellulose are suitable. Carboxymethyl cellulose (CMC) and its derivatives are particularly suitable. Salts of carboxymethyl cellulose, such as the sodium salt, can also be used.

[0099] In the present testing, CMC was found to give the composition a strong binding affinity to cellulosic materials such as paper and card, and is expected to bind to cotton as well, making it ideal for these substrates. During use, CMC formed a stable hydrogel with water, resulting in a printable, highly conductive ink that adhered to paper.

[0100] In a further aspect, the present invention provides a method of producing an ink as defined herein, the method comprising: (i) obtaining exfoliated graphite nanoplatelets as defined herein; (ii) obtaining exfoliated single-walled carbon nanotubes as defined herein; and (iii) dispersing the exfoliated graphite nanoplatelets, the exfoliated single-walled carbon nanotubes, the thickener, and optionally a surfactant in a solvent.

[0101] To ensure a uniform mixture of the nanoplatelets and carbon nanotubes, the mixture of step (iii) may be subjected to a high shear mixing stage. Additionally, a further step of compressing (e.g., roll-milling) the ink may be performed to degas the ink, which facilitates printing of the ink onto a substrate.

[0102] The compositions described above can be used as inks for printing on a variety of substrates, including soft polymers (such as polyethylene terephthalate, polypropylene, and polyimides), elastomers (such as silicones and polyurethanes), metal foils and films (such as aluminum, copper, gold and platinum foils / films), and rigid substrates (such as silicon wafers, glass, quartz, polycarbonate, etc.).

[0103] The inks described herein can be printed onto a cellulosic substrate material, such as paper.

[0104] Thus, in a further aspect of the present invention there is provided a substrate (eg a cellulosic substrate) printed with a conductive ink which is a liquid composition as defined herein.

[0105] The present invention also provides a method of printing a conductive ink, which is a liquid composition as defined herein, onto a substrate (eg, a cellulosic substrate).

[0106] The cellulosic substrate is typically paper or card.

[0107] The ink can be printed using a variety of printing techniques, such as screen printing or inkjet printing.

[0108] The ideal behavior of a screen-printable ink requires a thixotropic rheology profile that undergoes shear thinning within the printing process, followed by elastic recovery and stabilization of the printed structures at the resolution required for drying or curing. Such behavior is beneficial for high-resolution printing of interconnects and connecting lines for printed electronics applications. When building electronic circuits for "bare die" or unencapsulated silicon parts, print fidelity better than 125 microns is typically preferred for automated die-attach methods.

[0109] As mentioned above, the carbon nanomaterial may be graphite nanoplatelets, single-walled carbon nanotubes, or a mixture thereof, and the cellulosic binder may be carboxymethyl cellulose. The film may also include graphite particles as the conductive carbon particles.

[0110] The conductive ink may also have additional components or properties as described herein.

[0111] The present invention combines the high electrical conductivity of a combination of nano-carbon materials with the thixotropic rheology required for good printing properties.

[0112] Conductive inks can be used to print a wide range of applications including, but not limited to, microwave antennas, RFID tags, biosensing electrodes, printed heaters, wireless inductive coils, metasurfaces for tunable low emissivity and reflectivity coatings, strain sensors, surface acoustic wave devices, temperature sensors, energy storage electrodes and electrolytes for supercapacitors, batteries, capacitance sensors, flexible, stretchable or structural electronic conductors, low-density aerogels for catalysis, power storage and chemical repair, self-healing coatings and drug delivery platforms.

[0113] In a further aspect, the present invention provides an RFID tag comprising an antenna deposited (e.g., printed) onto a substrate from the liquid composition described herein. The substrate may be a plastic polymer substrate (such as PET) or a cellulosic substrate (such as paper).

[0114] Also provided herein is a fabric substrate printed with a conductive ink, the conductive ink comprising: (i) conductive carbon particles (e.g., carbon nanomaterials), and (ii) a binder which binds to cellulose, suitably a cellulose derivative.

[0115] The present invention also provides a method for printing a conductive ink on a textile, the conductive ink comprising: i) conductive carbon particles (e.g., carbon nanomaterials); ii) cellulose derivatives, and iii) Solvent.

[0116] The ink may have the properties described above in relation to other aspects of the invention.

[0117] Fabrics can be woven or nonwoven. For example, woven fabrics can be woven from fibers that can achieve greater breaking strain properties and relax strain applied at an angle. This gives the printed film additional mechanical integrity when stretched. The pitch of the weft and warp threads of woven fabrics is usually at least 100 threads per cm so that the print resolution is not limited by surface roughness. To improve the print resolution, flattening of the substrate by application of a surface coating to reduce surface roughness may be added. Surface chemical treatment of the fabric by a corona discharge process may be applied to improve the wettability and printability of the substrate.

[0118] The fibers can be natural fibers such as cotton, silk, or synthetic fibers such as polyester, nylon, polyurethane, polyolefins such as polyethylene and polypropylene, polymers including modified and reconstituted cellulose such as viscose, etc. These fibers may be spun or woven in combination with each other to provide additional performance.

[0119] Depending on the selection of the woven material, some bleeding of the ink into the body of the woven substrate may occur, which may increase the thickness of the printed film. The desired thickness of the printed film is preferably greater than 5 microns, for example greater than 10 microns. This reduces the sheet resistance to less than 10 Ω / Sq, providing sufficient conductivity for RFID applications.

[0120] Also provided herein is a thermoplastic substrate having a conductive ink printed thereon, the conductive ink comprising: (i) conductive carbon particles (e.g., carbon nanomaterials), and (ii) a binder which binds to cellulose, suitably a cellulose derivative.

[0121] The present invention also provides a method of printing a conductive ink on a thermoplastic substrate, the conductive ink comprising: i) conductive carbon particles (e.g., carbon nanomaterials); ii) cellulose derivatives, and iii) Solvent.

[0122] The ink may have the properties described above in relation to other aspects of the invention.

[0123] As described herein, the inks can be used to print an RFID antenna. Accordingly, also provided herein is an RFID tag comprising a fabric or thermoplastic substrate having printed thereon an RFID antenna, the RFID antenna comprising conductive carbon particles (e.g., carbon nanomaterials such as carbon nanoplatelets and / or carbon nanotubes) and a binder (such as a cellulose derivative, e.g., carboxymethyl cellulose).

[0124] RFID antennas can have the properties of films printed from the inks described herein. Similarly, fabrics and thermoplastic substrates can have the properties described above.

[0125] The tag typically comprises a flat portion on which the RFID antenna is printed, and a loop or a means for forming a loop (e.g., an opening through which the end of the tag cab is threaded) that allows the tag to be attached to an object.

[0126] The tag may be of substantially planar shape in that its thickness may be substantially less than its length or width. Typically the tag has a thickness of 3mm or less, such as 2mm or less, for example 1mm or less.

[0127] The tag may be formed from a strip of material (e.g., fabric or thermoplastic) whose length is greater than its width. An opening may be provided at one end of the tag's length through which the feed end of the tag's length may be inserted to provide a "loop lock tag" (see Figures 8A and 8B). The size of the opening shape allows for gentle reversible retention of the loop tag.

[0128] As an alternative to the "loop lock tag" described above, the tag may be a flat tag as described above with complementary Velcro® sections attached so that portions of the tag can be secured together to form a loop.

[0129] The tag may be encapsulated with an overcoat, such as polyurethane or silicone, to improve the tag's resistance to abrasion and damage, for example, by water.

[0130] Alternatively, the tags may be provided in a continuous, elongated form as a tape format with the tags assembled at regular intervals.

[0131] Thus, the present invention utilizes water-based, highly conductive inks in the manufacture of RFID tags that achieve the sheet resistance required for use as good antennas for RFID. Conventional RFID tags use metal materials that make them difficult to recycle, but the carbon-based RFID tags of the present invention provide a more environmentally friendly alternative. Furthermore, the combination of a cellulose-based binder with carbon nanomaterials provides a degree of flexibility and stretchability to overcome stress fatigue that may be present in metal-based antennas. Additionally, carbon-based RFIDs exhibit non-magnetic properties, allowing for their use in food production environments.

[0132] In a further aspect, the present invention provides a printed heater comprising a heating element printed onto a substrate from the liquid composition described herein.

[0133] Measurement of surface strains can be exploited for many industrial applications. Nanocarbon-based printed structures show strain-dependent electrical conductivity when applied to substrates above the percolation threshold. Polymer binder-based films show reproducible elastic properties, going beyond the use of conductive metals that may break before reaching the elastic limit of the substrate. Thus, by utilizing the present invention, measurements of high strain (>2%) conditions on elastic substrates are possible with good reproducibility. Furthermore, this elastic behavior can be extended to modify the resonant properties (frequency and Q) of the antenna. A new example is presented where the resonant behavior of a UHF RF antenna printed on an elastic substrate can be monitored without the need for an internal power source or processing circuitry.

[0134] In a further aspect, the present invention provides a deposited liquid composition as described herein, deposited (eg, printed) onto a stretchable substrate. [Brief description of the drawings]

[0135] [Figure 1] 1 is a plot showing the percolation relationship between electrical conductivity and carbon nanotube (CNT) weight fraction (wt %) of dry carbon nanotube (CNT)-graphene nanoplatelet hybrid films. [Diagram 2]FIG. 13 is a plot of normalized single-pass sheet resistance as a function of CNT weight fraction for films prepared using GNPs of different relative densities, ρ. [Diagram 3] 1 is an atomic force microscope (AFM) image showing individualized carbon nanotubes in a film of the present invention. [Figure 4] FIG. 1 is a schematic diagram showing the length, width, and thickness parameters of few-layer carbon nanoplatelets. [Diagram 5] The size distribution of the graphene nanoplatelets used in Example 1 below is shown. [Figure 6] 1 is a scanning electron microscope (SEM) image showing the location of carbon nanotubes within a packed matrix of graphene nanoplatelets. [Figure 7] FIG. 2 is a rheology trace showing the viscosity of the ink described in Example 2 below. [Figure 8A] 1 shows a tag having an RFID tag printed thereon using the inks described herein. [Figure 8B] 1 shows a tag having an RFID tag printed thereon using the inks described herein. EXAMPLES

[0136] Example 1 - Characterization of graphene nanoplatelets Samples of liquid-phase exfoliated graphene nanoplatelets with lateral sizes up to 8 μm (or up to 50 μm), average lateral sizes of about 5 μm, and thicknesses up to 30 nm were obtained.

[0137] The hydrodynamic radius of the nanoplatelets is measured by dynamic light scattering (DLS) of the GNP dispersion. Conversion to length is performed using literature indices (Lotya et al., DOI: 10.1088 / 0957-4484 / 24 / 26 / 265703). The size distribution of the graphene nanoplatelets used in Example 2 below is shown in Figure 5.

[0138] Due to the wide particle size distribution, the obtained nanoplatelets were considered useful for the fabrication of Apollonian-filled films.

[0139] Example 2 - Ink Formulation The compositions of the batches of ink prepared are shown in the table below: The total solids content of the inks prepared (including binders etc.) was about 9% by weight.

[0140] [Table 1]

[0141] To make the ink, the ingredients were weighed into a suitable container. A NutriBullet NB-WL076G-23 blender was used to mix the ingredients in a sealed container under ambient laboratory conditions for 1 minute before high pressure homogenization using equipment of the type described in WO2020 / 074698.

[0142] As described in Example 1, the graphite nanoplatelets have a lateral size distribution between 700 nm and 8000 nm and a maximum thickness of about 20 nm.

[0143] Structural characterization by SEM revealed the presence of a dense network of carbon nanotubes in the gaps between the packed graphite nanoplatelets (see Figure 6 ).

[0144] The viscosity of the ink was measured over a shear rate range of 0.1 / s to 100 / s. The ink was found to be thixotropic and the rheological trace is shown in Figure 7. As shown in Figure 7, the ink may exhibit a viscosity of 100-1000 Pa·s at a shear rate of 0.1 / s and / or have a viscosity of 1-10 Pa·s at a shear rate of 100 / s.

[0145] The ink was successfully printed on a variety of substrates, including several grades of polyethylene terephthalate (PET) substrates (DuPont Tejin ST504 & Felix Scholler F40100) and paper substrates.

[0146] The electrical conductivity of the printed films was measured using a four-point probe according to the International Electrotechnical Commission standard IEC TS62607-2-1:2012. The thickness of the films was measured by SEM cross-sectional analysis or scanning probe profilometry, and the electrical conductivity and thickness were used to calculate the specific conductivity. For the printed films, the electrical conductivity was measured at a maximum of 3.8 (±0.1) × 10 4 Sm -1 A conductivity of was observed.

[0147] Accordingly, the present invention provides highly conductive inks formed from carbon nanomaterials, and in particular highly conductive inks formed from carbon nanomaterials that can be printed onto a substrate.

[0148] Example 3 - RFID Tag RFID tags were fabricated using the ink described in Example 2 above.

[0149] The tags were cut from recycled tightly woven polyester having the general shape shown in Figure 8B. As shown in Figure 8B, the tags are generally rectangular with a circular hole near one end of the tag. Semicircular notches are provided on each of the long sides of the tag.

[0150] On the end of the tag opposite the circular hole, an RFID antenna was printed using the ink described in Example 2. This ink was shown to have good adhesion to a woven polyester substrate.

[0151] In use, the end of the tag with the RFID antenna printed on it can be threaded through the circular hole on the other end of the tag to form a loop as shown in Figure 8 A. The semicircular notch is positioned within the circular hole to secure the loop in place.

[0152] The tag can therefore be attached to animals or other objects that would benefit from being equipped with an RFID tag. The tag is resistant to aqueous liquids. The tag also has the advantage that it does not contain any metal materials and is flexible and removable.

[0153] Similar tags were made using PET silk or PET film. The read range of the resulting RFID tags using these materials was measured to be approximately 1.6 m when attached to a willing person. These results therefore validate the use of the tags in meat processing applications.

Claims

1. A conductive film comprising graphene nanoplatelets packed in a substantially Apollonian manner.

2. 10. The film of claim 1, wherein the graphene nanoplatelets have a packing efficiency greater than 10%.

3. 3. The film of claim 1 or 2, wherein the graphene nanoplatelets comprise a first population of nanoplatelets having a first size and a second population of nanoplatelets having a second size up to 25% of the first size.

4. 4. The film of claim 3, wherein the first population of nanoplatelets have a length / width of 1 μm or greater.

5. 5. The film of claim 3 or 4, wherein the first population of graphene nanoplatelets comprises 40% or more (w / w) of the total amount of nanoplatelets in the film.

6. The film is 250 kg / m ３ 6. The film of claim 1, having a density greater than

7. The film of claim 1 , further comprising carbon nanotubes.

8. 8. The film of claim 7, wherein the carbon nanotubes are present in the film in an amount of up to 10%.

9. 8. The film of claim 7, wherein the carbon nanotubes are present in the film in an amount of up to 5%.

10. 10. The film of any one of claims 1 to 9, wherein the graphite nanoplatelets are present in the film in an amount of 92% (w / w).

11. (i) a first population of graphene nanoplatelets; (ii) a second population of graphene nanoplatelets having an average size up to 25% of the average size of said first population of graphene nanoplatelets; (iii) a thickening agent, and (iv) solvent A liquid composition comprising:

12. The liquid composition of claim 11 further comprising carbon nanotubes.

13. 13. The film of any one of claims 1 to 10, or the liquid composition of claim 11 or 12, wherein the number of layers of the graphite nanoplatelets is 30 or less.

14. 14. The liquid composition of any one of claims 11 to 13, wherein the first population of nanoplatelets have a length / width of 1 μm or greater.

15. 14. The liquid composition of any one of claims 11 to 13, wherein the first population of graphene nanoplatelets comprises 40% or more (w / w) of the total amount of nanoplatelets in the film.

16. 13. The film of claim 9, or the liquid composition of claim 12, wherein the carbon nanotubes are single-walled carbon nanotubes, optionally having an average diameter of 1 nm to 5 nm and / or a length of more than 3 μm.

17. 17. The liquid composition of any one of claims 11 to 16, wherein the graphite nanoplatelets are typically present in the liquid composition in an amount of 5% (w / w), optionally up to 15% (w / w).

18. 18. A liquid composition according to any one of claims 12 to 17, comprising carbon nanotubes, which may be present in the liquid composition in an amount of up to 0.5% (w / w).

19. 18. A liquid composition according to any one of claims 12 to 17, comprising carbon nanotubes, which may be present in the liquid composition in an amount of up to 0.2% (w / w).

20. 20. The film or liquid composition of any one of claims 1 to 19, comprising carbon nanotubes, said nanotubes being present in a weight ratio relative to the amount of graphite nanoplatelets of less than 1:

30.

21. 20. The film or liquid composition of any one of claims 1 to 19, comprising carbon nanotubes, the nanotubes being present in a weight ratio relative to the amount of graphite nanoplatelets of less than 1:

40.

22. 20. The film or liquid composition of any one of claims 1 to 19, comprising carbon nanotubes, the nanotubes being present in a weight ratio relative to the amount of graphite nanoplatelets of 1:50 to 1:70 (carbon nanotubes:graphite nanoplatelets).

23. (a) graphite nanoplatelets in the weight range of 5% to 15% (w / w); (b) carbon nanotubes in the weight range of 0.05% to 0.5% (w / w); (c) carboxymethylcellulose in the weight range of 0.1% to 1.0% (w / w); (d) sodium cholate in the weight range of 0.01% to 0.5%, and (e) water; A liquid composition comprising:

24. 24. A textile or thermoplastic substrate printed with a liquid composition according to any one of claims 11 to 23.

25. 24. A method of printing a liquid composition according to any one of claims 11 to 23 onto a textile or thermoplastic substrate.

26. 24. An RFID tag comprising an RFID antenna formed from the film or liquid composition of any one of claims 1 to 23.

27. 27. The RFID tag of claim 26, wherein the tag comprises a flat portion on which the RFID antenna is printed and a loop or means for forming a loop that allows the tag to be attached to an object.

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