TW202323453A - Carbon-based conducting inks - Google Patents

Abstract

The invention provides liquid compositions comprising conductive carbon particles and/or carbon nanoparticles, a thickening agent, and a solvent. The carbon nanoparticles are preferably a mixture of graphite nanoplatelets and carbon nanotubes and the thickening agent is preferably a cellulose derivative. The liquid compositions can be used as ink to print highly conductive films that adhere to paper substrates.

Description

Carbon-Based Conductive Ink

The present invention relates to a conductive ink containing nano carbon material, a method for manufacturing the ink, its application and a substrate on which the conductive ink is printed.

Two-dimensional (2D) materials are crystalline materials composed of several layers or even a single layer (monolayer) of atoms or molecules. Most 2D materials are known, including graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs). TMDs have the molecular formula _MX2 , where M is a transition metal and X is a chalcogen atom (S, Se or Te). Examples of such TMDs include molybdenum disulfide (MoS ₂ ), niobium diselenide (NbSe ₂ ), and tungsten disulfide (WS ₂ ).

2D materials are known to possess many interesting and potentially useful properties that differ from those of their bulk 3D counterparts. For example, graphene has high electrical conductivity and can be used in electrode structures and conductive composites.

Many of the attractive functional properties of materials are usually only observed when the material is in the form of a single layer or several layers (ie, 2D). However, strong interlayer dispersion forces must be overcome in order to exfoliate bulk three-dimensional (3D) materials to form corresponding 2D materials.

Carbon nanotubes are nanoscale tubes composed of rolls of graphene sheets. The diameter of the tube is typically in the range of 1 to 50 nm, but the length can be in the micron range. The carbon nanotubes can be single-walled (that is, formed by a single roll of graphene sheets), or multi-walled (that is, formed by multiple concentric rolls of graphene sheets). Carbon nanotubes have attracted much attention due to their physical properties, namely high tensile strength and high electrical conductivity.

Liquid dispersions containing carbon nanomaterials (eg, carbon nanotubes, carbon nano-graphite, graphene, and mixtures thereof) are considered inks that can be used to deposit conductive films. For certain commercial uses, the film has the advantage that it is "metal-free" but still conducts electricity. However, until today, the use of such inks has been limited to low conductivity printed films. For example, although the conductivity of copper is in the range of 6 x ¹⁰⁷ S/m, it has been reported that the conductivity of thin films made of carbon nanomaterials is generally much less than 100 S/m (see US 10,244,628). Furthermore, currently existing carbon-based inks for printing can only be printed on a limited range of substrates, such as aluminum and plastics (especially polyethylene terephthalate (PET)), and these substrates The material is not recyclable.

The formation of printable inks based on dispersions of carbon nanomaterials in water suffers from flocculation problems due to the non-polar nature of these materials. Due to the settlement of carbon nanomaterials and the need for excess organic solvents, their industrial applications are reduced.

Khan et al., "The preparation of hybrid films of carbon nanotubes and nano-graphite/graphene with excellent mechanical and electrical properties"properties"), Carbon 48 (2010), pp. 2825-2830, describing that hybrid films containing both carbon nanotubes and nanographite have greater electrical conductivity than films containing each component alone. However, Khan et al. only described the dispersion of nano-graphite and carbon nanotubes in N-methyl pyrrolidone (N-methyl pyrrolidone) solvent. The solvent is removed by vacuum filtration to form a thin film of carbon nanomaterials. The conductivity of the film is only as high as 2 x 10 ⁴ S/m, and this liquid formulation is not suitable for printing.

Pan et al., "Sustainable production of highly conductive multilayer graphene ink for wireless connectivity and IoT applications", Nature Comm. (2018), 9:5197, described inks containing graphene, dihydrolevoglucosenone and NMP, with which the conductivity of films printed with these inks was only 7.13 x 10 ⁴ S/m.

Ferrari et al. (WO2017/060497A1) describe the preparation of liquid phase exfoliated GNP/carboxymethyl cellulose films with a conductivity of 7.14 x 10 ⁴ S/m. These films were printed on PET substrates and used to fabricate UHF RFID tags with a read distance of 1.4 m at 2W incident radiation.

A growing number of industries are adopting radio frequency identification (RFID) to assist in the digital management and tracking of inventory. Many industries have discovered the benefits of using these technologies to reduce inventory loss and improve inventory control without using labor-intensive verification and counting methods.

RFID can also be combined with tamper-resistant devices to achieve several goals, including secure stock tracking as goods are transferred between different modes of transport and storage media (crates, cages, and pallets) in typical supply chain operations. It is expected that the adoption of RFID technology will become ubiquitous, and billions of items and packages will be given an identity. Considering the number of applicable labels, the greater the need for sustainable material solutions.

In recent years, the demand for sustainable and recyclable packaging solutions has increased dramatically, thereby seeking to recycle packaging materials into other products. The current method of constructing UHF RFID is to use a combined continuous polymer substrate, metal and adhesive. These combinations are used to produce a range of form factors including flat adhesive labels, swing labels or inserts.

The ecological recycling of mixed-material objects is a long-term challenge, especially in the electronics industry. The implementation of environmentally sound electronic systems stimulates novel innovations in the combinations of materials used to produce these devices. Mass-produced UHF RFID tags are composed of mixed materials (plastic, metal, silicon and paper). Many stakeholders are interested in adopting acceptable performance materials with higher environmental credentials. In some cases, metals are not the first choice due to the stringent requirements to screen goods to protect the interests of consumers.

Inks with high solid content are necessary to reduce the environmental burden on printing during the drying process. Stabilizing the nanocarbon dispersion with a cooperative binder increases the possible thickness of screen printing films, which helps reduce resistive losses, which are indispensable for various printed electronics applications. For high-efficiency carbon-based radio frequency (RF) antenna applications, the thickness of the carbon-based RF antenna should be smaller than the thickness of the printed film. Considering the process and solid content in the ink, the thickness is usually limited to <100 µm (Jordan, Edward Conrad ( 1968), Electromagnetic Waves and Radiating Systems, Prentice Hall, ISBN 978-0-13-249995-8).

Therefore, there remains a need for alternatives based on carbon-based conductive inks, preferably with improved conductivity and/or printable onto recyclable substrates.

The inventors of the present application have found that printable inks containing graphite nanosheets or particles and single-walled carbon nanotubes have very high electrical conductivity (up to 5×10 ⁵ kS/m, see Example 2 and Example 6). The ink could be used in a wide range of applications, including making "metal-free" antennas for RFID tags and printing heaters.

Therefore, in the first aspect, the present invention provides a liquid composition, comprising: (i) carbon nanomaterials; (ii) a thickener; and (iii) A solvent.

The thickener can be properly combined with carbon nanomaterials and adhered to the substrate, such as cellulose base or other suitable hydrophilic substrates. The thickener may be or include a cellulose derivative. The inventors also found that it is possible to prepare inks containing carbon nanomaterials that can be printed and adhered to recyclable substrates, especially paper substrates.

The composition may include carbon nanotubes as the carbon nanomaterials or as one of the carbon nanomaterials. The composition may also include conductive carbon particles. Preferably, the composition includes a mixture of carbon nanotubes and additional conductive carbon particles.

Therefore, in the second aspect, the present invention provides a liquid composition, comprising: (i) conductive carbon particles; (ii) carbon nanotubes; (iii) a thickener; and (iv) A solvent.

The thickener properly separates and encapsulates the carbon nanotubes, thereby providing a means of dispersion for a maximum number of nanotubes and a single conductive path between the conductive carbon particles.

In some embodiments, the conductive carbon particles are graphite particles, such as micron-sized graphite particles.

In another embodiment, the conductive carbon particles are graphite nanosheet particles. The inventors have advantageously found that when the nanocarbon material is a mixture of graphite nanosheets and single-wall carbon nanotubes, and the thickener is a cellulose derivative, the films printed by these liquid ink compositions have high conductivity .

Therefore, in a third aspect, the present invention provides a liquid composition comprising: (i) Graphite nanosheets; (ii) carbon nanotubes; (iii) a cellulose derivative; and (iv) A solvent.

The liquid composition is dried (once printed) to form a conductive film that can adhere to a cellulose-containing substrate. When the solvent is an aqueous solvent, due to the characteristic of the interaction between the cellulose derivative thickener and the solvent, the composition can also be correctly called a hydrogel ink (hydrogeink). Herein, unless the context requires otherwise, references to liquid compositions of the present invention include hydrogel inks.

The above-mentioned liquid composition may also be provided in the form of a solvent-free dry powder or an aerogel composition.

In yet another aspect of the present invention, there is provided a substrate (such as a cellulose-based substrate) printed with a conductive ink thereon, the conductive ink comprising: (i) Conductive carbon particles (e.g. carbon nanomaterials); and (ii) A binder for binding to cellulose, suitably a cellulose derivative.

The present invention also provides a method of printing a conductive ink onto a substrate, such as a cellulose-based substrate, the conductive ink comprising: (i) Conductive carbon particles (such as carbon nanomaterials); (ii) a cellulose derivative; and (iii) A solvent.

It has also been shown that the liquid compositions described herein can be printed onto stretchable substrates. Also provided are the compositions described in further detail herein that can be printed onto a stretchable substrate.

As mentioned above, the ink may also include carbon nanotubes, and may also include graphite particles as conductive carbon particles. A similar improvement in conductivity has been observed with graphite particles as with graphite nanoplatelets (see Example 6 below).

Compared to the films described in WO 2017/060497, the present invention provides a significant improvement through the addition of carbon nanotubes with a higher overall film conductivity of up to 5×10 ⁵ Sm ⁻¹ .

It has also been shown that films printed according to the present invention can be printed on common cellulose based substrates such as paper substrates as well as stretchable substrates with good film formation and stability.

It has also been shown that the use of size-selected graphene nanosheets (with a median _D50 diameter of less than 2 µm) can produce fine-scale inks that enable high-resolution printing, e.g. for printing and micromanufacturing electronics RFID connection of the device.

Also described herein are textile or thermoplastic substrates printed with conductive inks and RFID tags, the construction of which includes RFID antennas printed with the inks described herein. These aspects of the invention are described in further detail below.

The concentration of ink solids and the use of screen printing also facilitates the thick film formation required to achieve good conductivity (0.1 Ohm/Sq/mil), allowing the film to provide suitable antenna characteristics and use in the UHF band The electromagnetic "skin depth" characteristics necessary for the radio frequency antenna inside.

To aid the printability and robustness of the present invention, other additives may be included in the final ink mixture. This may include humectants to ensure wet and dry characteristics suitable for screen printing, and crosslinkers to cure the resulting coating, imparting a degree of additional functional performance (to humidity and other solvents to which the printed or coated film is exposed) resistance).

Printed conductive structures on substrates enable a range of applications through the integration of surface-mounted electronic components, describing examples of possible commercial applications (eg RFID tags, micro heaters and sensors).

The term "conductive carbon particles" refers to particles that include carbon and are conductive, for example having a conductivity of 750 S/m or higher or eg 1000 S/m or higher.

The conductive carbon particles typically comprise greater than 80 wt% carbon, preferably greater than 90 wt% carbon, eg greater than 95 wt% carbon. In some compositions described herein, the conductive carbon particles consist of carbon (ie, contain largely carbon and no other elements).

As noted above, conductive carbon particles are electrically conductive. Therefore, the proportion of carbon atoms in the conductive carbon particles in the sp ² mixed orbital region is usually above 50%, such as above 75%, preferably above 90%.

Examples of conductive carbon particles include graphite and graphene (eg, graphite nanoplatelets). Therefore, the average particle size may be in the order of micrometers or nanometers, respectively.

When the conductive carbon particles, such as graphite particles, are micron-sized, all three dimensions (length, width, and thickness) typically have dimensions above 1 μm, such as above 2 μm or above 3 μm. However, the longest dimension of the micron-sized conductive carbon particles is usually below 50 μm, usually below 30 μm, for example below 25 μm or below 20 μm.

Herein, the term "nanocarbon material" refers to a nanomaterial including or composed of carbon (ie, a material having a critical dimension with an average size of 1 nm to 100 nm). Usually, the nanocarbon material includes at least 90% by weight of carbon, preferably at least 95% by weight, such as more than 99% by weight of carbon. The term includes materials such as graphene, graphite nanosheets, single-walled carbon nanotubes, multi-walled carbon nanotubes, crystalline diamond and diamond-like carbon (see ISO standard ISO/TS 80004-3:2020). Generally, carbon nanomaterials are electrically conductive nanocarbon materials. Preferably, the carbon nanomaterials include (i) graphene nanosheets and (ii) single-walled carbon nanotubes, multi-walled carbon nanotubes or a mixture of both. In particular, the carbon nanomaterial preferably includes a mixture of (i) graphite nanosheets and (ii) single-walled carbon nanotubes. The size of nanomaterials can be determined by transmission electron microscopy.

A synergistic effect on electrical conductivity has been found in compositions containing both graphite particles or graphite nanosheets and SWNTs. It is theorized that carbon nanotubes provide a conductive bridge between individual graphite particles or graphite nanosheets, thereby reducing the "patch resistance" of individual nanosheets/particles, but do not wish to be bound by theory. Sheet resistance is due to the finite tunneling of electrons between adjacent sheets, which is much larger than the movement within the internal structure of sheets (in graphite) or rods (in carbon nanotubes) . Furthermore, without wishing to be bound by theory, the inventors believe that the junction resistance between graphite nanosheets or particles and carbon nanotubes is less than the junction between two nanosheets/particles or two nanotubes resistance. Therefore, the intimate mixing of nanosheets/graphite particles and nanotubes will lead to improved conductivity of films made of graphite nanosheets/particles and carbon nanotubes (especially single-walled carbon nanotubes) at the same time. formed from the liquid compositions described herein.

To maximize this effect, carbon nanotubes are preferably individualized. Typically, greater than 75%, such as greater than 80%, preferably greater than 85%, of the carbon nanotubes by weight of nanotubes in the composition are individualized. Individualized nanotubes can be seen in Figures 1 and 2. The degree of individualization of nanotubes can be determined through UV-Vis spectroscopy, since individualized SWNTs exhibit Van Hove singularities (peaks) at specific wavelengths (Alafogianni et al., Colloidal and Surfaces A: Colloids and Surfaces A: Physical Chemical and Engineering Aspects, Vol. 495, (2006), pp. 118-124). For bundled carbon nanotubes, these UV-Vis absorptions are not visible, so the prominence of these peaks shows the degree of exfoliation/individualization.

Packing of particles of different insoluble geometries and sizes can lead to enhancement of various physical properties depending on the nature of those particles, and this effect is also prevalent at the nanoscale. By carefully combining different particle sizes and geometries, the overall physicochemical properties of the formulation system can be tuned to achieve desired properties. In commercial applications, due to the cost considerations of the most active elements, the system is often required to be filled with a significant portion (>50%) of low-cost filler materials that do not affect performance to an unacceptable level; or that are added to impart additional properties (eg thermal conductivity, mechanical strength and/or chemical reactivity). In the present invention, the concentration of the most conductive hydrogel elements within the filled voids of a matrix of larger conductive carbon particles (which may exhibit one dimension on the nanometer scale) results in a cost-effective formulation. The blend of rheological SWNT hydrogels with conductive carbon particles ensures high conductivity throughout the printing and drying process, resulting in excellent film conductivity.

As used herein, the term "graphite nanosheets" (also referred to herein as "graphene nanosheets") refers to nanoparticles of graphite consisting of small stacks of graphene. The term "few-layer" nanosheets refers to nanosheets with an average of less than 20 layers, usually less than 15 layers, preferably less than 10 layers. 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" ('Spectroscopic metrics allow in-situ measurement of mean size and thickness of liquid-exfoliated graphene nanosheets'), Nanoscale, 2016, doi:10.1039/C5NR08047A).

Nanosheets typically have an average thickness of less than 30 nm, such as less than 20 nm. As used herein, the term "thickness" refers to the dimension of the nanosheet along the axis of the layer stack within the nanosheet. The terms "length" and "width" refer to the longer and shorter dimensions of the nanosheets, respectively, along the plane of the layered material sheet perpendicular to the axis (see Figure 14).

The average length and/or width of the nanosheets is usually above 30 nm, preferably above 50 nm, or above 100 nm. The average length and/or width of the nanosheets is usually less than 10 μm; usually less than 3.0 μm, such as less than 2.0 μm; usually less than 1.5 μm, preferably less than 1 μm, such as less than 800 nm. In some cases, depending on the specific exfoliation process of these materials, graphitic nanosheets have larger lateral dimensions, greater than 1 micron and less than 50 microns. The amount and size distribution of these materials can be broad or narrow, depending on the exfoliation technique employed and/or any subsequent size selection process. The dimensions of the nanosheets can be measured using a scanning or transmission electron microscope.

Compared with the above-mentioned micron-sized particles, the micron-sized particles are all micron-sized in three dimensions (that is, length, width, and thickness are all above 1 μm), while nanosheets are usually in two dimensions (that is, length and thickness). In terms of width, its thickness is obviously less than 1 μm, such as less than 100 nm), and its thickness is only on the order of microns. As mentioned above, these dimensions can be measured by transmission electron microscopy.

When the conductive carbon particles include graphite nanosheets, the graphite nanosheets are usually present in the liquid composition in an amount above 0.5% (w/w), preferably above 0.75% (w/w), such as 1% to 5% (w/w); preferably at most 3% (w/w), such as at most 2% (w/w). When the liquid composition has been dried to form a dry film/powder, the graphite nanosheets are usually present in an amount above 25% (w/w), preferably above 30% (w/w), such as 35% (w/w). w) or more; and/or up to 50% (w/w), preferably up to 45% (w/w), for example up to 40% (w/w).

When the conductive carbon particles comprise micron-sized graphite particles, the graphite particles are typically present in the liquid composition in an amount of 0.5%, for example 1% up to 5% (w/w); preferably up to 3% (w/w ), for example up to 2% (w/w). When the liquid composition has been dried to form a dry film/powder, graphite particles are usually present in an amount above 30% (w/w), preferably above 40% (w/w) and up to 70% (w/w) , preferably up to 55%, for example up to 60% (w/w).

The carbon nanotubes can be single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), but preferably include or consist of single-walled carbon nanotubes. The average outer diameter of carbon nanotubes is usually 1 nm to 5 nm, preferably 1 nm to 2 nm (determined through a transmission electron microscope), and the length can be greater than 200 nm, or greater than 3 μm, usually greater than 5 μm, For example greater than 10 µm or greater than 15 µm. The aspect ratio of the carbon nanotubes can be more than 50, usually more than 100. Here, the aspect ratio refers to the ratio of the length of the nanotube to its diameter. Whereas the aforementioned micron-sized particles are micron-sized in three dimensions and nanosheets are micron-sized in two dimensions, carbon nanotubes are micron-sized in only one dimension (ie, along their length).

Carbon nanotubes may be present in the compositions described herein in a weight ratio relative to the amount of graphite nanoplatelets or graphite particles greater than 0.05:1 (carbon nanotubes:graphite nanoplatelets/particles), for example greater than 0.10 :1 or 0.15:1, preferably greater than 0.2:1, and the ratio is at most 1:1, suitably at most 0.75:1 or at most 0.5:1, for example at most 0.4:1 or at most 0.35:1.

For example, when the conductive carbon particles are graphite nanoplatelets, the carbon nanotubes are typically present in a weight ratio of 0.05:1 to 0.7:1 (carbon nanotubes:graphite nanoplatelets) relative to the amount of graphite nanoplatelets In the preparation, the preferred ratio is 0.05:1 to 0.6:1.

Alternatively, when the conductive carbon particles are micron-sized graphite particles, carbon nanotubes are usually present in the formulation in a weight ratio of 0.02:1 to 0.2:1 (carbon nanotubes:graphite particles) relative to the amount of graphite particles. Among them, the preferred ratio is 0.05:1 to 0.15:1.

Alternatively, the amount of carbon nanotubes in a composition can be defined relative to the weight of the total composition. For example, carbon nanotubes may be present in the liquid composition in an amount above 0.1% by weight (w/w), preferably above 0.25% (w/w), such as above 5% and up to 1.5% (w/w), Preferably at most 1.25% (w/w), such as at most 1% (w/w). When the liquid composition has been dried to form a dry film/powder, the carbon nanotubes are usually present in an amount above 5% (w/w), preferably above 10% (w/w), such as 15% (w/w). w) or more and up to 30% (w/w), preferably up to 25% (w/w), for example up to 20% (w/w).

The solvent may be aqueous or non-aqueous, but preferably the solvent is or includes water (necessary for hydrogel formation). 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), dimethylethyl Amides (DMAc), sulpholanes, dihydrolevoglucosenone (Cyrene) and lactones such as gamma-valerolactone. It was found that a solvent system comprising a combination of water and gamma-valerolactone produced inks suitable for printing on stretchable substrates (see Example 4 below). When gamma-valerolactone is present, it may be from 1% to 10% (w/w), for example up to 5% (w/w) or from 5% (w/w) to 10% (w/w), preferably The liquid composition is present in an amount of 6% to 10% (w/w).

The composition may also include thickeners (also useful as gelification agents) to increase the viscosity of the composition. The increased viscosity ensures that the composition is suitable for printing and also reduces the tendency of the carbon nanomaterials to flocculate.

The thickener is preferably a hydrogel-forming thickener. As mentioned above, the formation of a hydrogel matrix containing carbon nanotubes and conductive carbon particles produced a highly conductive ink. Hydrogel-forming thickeners are generally hydrophilic polymer chains that form colloidal gels through a large network of hydrogen bonds in water.

The thickener is also preferably associated with cellulose, for example when the ink/fluid composition of the invention is printed and dried onto a cellulose-containing substrate such as paper.

Examples of suitable thickeners include: - Cellulose derivatives such as carboxymethyl cellulose (CMC), methyl cellulose, hydroxy ethyl cellulose and carboxy ethyl cellulose and its salts (such as its sodium salt); - Polymers, such as polyethylene oxide (polyethylene oxide, PEO), polypropylene oxide (polypropylene oxide, PPO), polyaniline (polyaniline, PANI), polyvinylpyrrolidone (polyvinylpyrrolidone, PVP), polyvinyl alcohol (polyvinyl alcohol) alcohol, PVA) and poly N-isopropylacrylamide (poly N-isopropylacrylamide, PNIPAAm); - Cyclodextrins; - Natural gelling agents such as xanthan gum, gelatine, glycerol, alginates and chitosan; - Inorganic silica and clays such as bentonite, montmorillonite, laponite, nano-silica and nano-titanium dioxide; and - Filamentous or rod-like materials, such as materials with an aspect ratio greater than 100 (such as carbon nanotubes).

In a preferred embodiment, the thickener is a cellulose derivative, such as carboxymethyl cellulose. As used herein, the term "cellulose derivative" refers to a chemical derivative of cellulose formed by functionalizing some or all of the hydroxyl groups present in cellulose (eg, by etherification or esterification). Derivatives can be formed by combining one or more or all of carboxyl, hydroxyl, methyl, ethyl and/or propyl groups. Examples of cellulose derivatives include hydroxypropylmethylcellulose, hydroxypropylcellulose, methylethylcellulose, methylcellulose, and carboxymethylcellulose (carboxymethylcellulose) or combinations thereof, and cellulose itself. Liquid ink compositions containing this type of binder have been found to facilitate adhesion to paper substrates. CMC has many forms (for example, changes with the degree of substitution and function), and can be covalently cross-linked with a variety of chemical reagents or cross-linked with other reagents through a hydrogen bond network structure to endow novel and customizable Properties (Gels 2018, 4, 54; doi:10.3390/gels4020054).

Cellulose derivatives readily form hydrogels that are useful in many industrial applications. These materials can also be used as surfactants to stabilize carbon nanomaterials in aqueous solvents. Hydrogels exhibit ideal thixotropic behavior due to their extended hydrogen bonding or supramolecular network formation properties. These networks provide long range ordering for improved rheological behavior.

The total concentration of the thickener may be in the range of 0.5% to 2% by weight of the total composition (including solvent), such as 1% to 1.75% by weight of the total composition.

Thickeners increase the viscosity of the composition, and it is envisaged that the thickener also enables the carbon nanotubes, if present, to form a predetermined supramolecular network, thereby increasing the conductivity of films printed from the composition.

The viscosity of the composition is important to ensure that it can be printed to form a film. In addition, the composition should be viscous enough to prevent flocculation of carbon nanomaterials in the composition. Of course, the exact viscosity will depend on the application of the composition (and resulting film). Thickeners also ensure that the ink has a suitable viscosity for printing, such as screen printing. Inks suitable for screen printing are usually thixotropic, so their viscosity depends on the shear rate. As shown in FIG. 15 , the ink may have a viscosity of 100 to 1000 Pa.s at a shear rate of 0.1/s, and/or may have a viscosity of 1 to 10 Pa.s at a shear rate of 100/s.

The composition may also include one or more surfactants. Surfactants are generally nonionic surfactants. Examples of suitable nonionic surfactants include polyethylene oxide (PEO) based surfactants (e.g. Triton X-100 (Triton X-100)), polypropylene oxide (PPO) based surface active agents Active agents, cyclodextrins and polyvinylpyrrolidone (PVP) surfactants. However, it is also possible to use ionic surfactants, for example sulfate-based surfactants (eg sodium cholate, sodium abietate or sodium dodecyl sulphate).

The total concentration of surfactants may be in the range of 0.01% to 1% or 0.01% to 0.1% by weight of the total composition (including solvent), for example in the range of 0.02% to 0.05% by weight of the total composition.

The composition may also include one or more solvents and/or binders to improve the adhesion of the dried film (printed with the ink) to the substrate. Of course, the nature and combination of adhesives will depend on the substrate.

The composition may also include one or more cross-linking agents to improve the rheological parameters of the ink and/or the properties of the resulting film. This can include various functional organic acids or bases such as ascorbic acid. Examples of other crosslinking agents include di-carboxylic acids and tri-carboxylic acids, such as glutaric acid and trimesic acid. This crosslinker stabilizes the film against rapid redissolution and the effect of ambient humidity on conductivity.

The composition may also contain one or more humectants to aid the printability of the ink in industrial processes. In water-based compositions, the addition of urea, glycerin or glycols such as propylene glycol, hexylene glycol, and butylene glycol, or their polymers, such as polypropylene glycol, slows down the ink drying process, allowing for consistent and repeatable printing. Alternative humectants include glyceryl triacetate, lithium halide salts such as LiCl, sugar alcohols such as glycerin, sorbitol, xylitol, and maltitol, polymeric polyols, polymeric sugars ( Examples include polydextrose), polysaccharide alcohols, sodium hexametaphosphate (E452i), and castor oil. In a particular embodiment of the invention, the humectant is selected from urea and polypropylene glycol (preferably polypropylene glycol).

In addition, the composition may further include a setting (crosslinking) agent (setting (crosslinking) agent), which is a material that cures when exposed to heat or radiation, so as to cure the liquid ink composition And curing (set) into a solid film. These curing (crosslinking) agents include photocurable monomers or infrared activators such as epoxides (which may undergo ring-opening reactions), aldehydes or acids (which may undergo esterification reactions), such as citric acid.

Alternatively, a binder containing monovalent ions (such as sodium carboxymethylcellulose) can be treated with an aqueous solution of a salt of a divalent, trivalent or tetravalent ion (such as calcium (II) chloride or iron (III) sulfate). Formed films to form ionically cross-linked insoluble films through an ion exchange process.

In an exemplary embodiment, the invention provides a composition comprising: (a) graphite nanosheets or graphite particles; (b) carbon nanotubes; (c) cellulose derivatives; (d) surfactants; (e) humectants; and (f) water.

In one embodiment, the present invention provides a composition comprising: (a) graphite nanosheets or graphite particles; (b) carbon nanotubes; (c) carboxymethyl cellulose; (d) polypropylene glycol; (e) sodium cholate; and (e) water.

In yet another embodiment, the present invention provides a composition comprising: (a) Graphite nanosheets with a weight percentage ranging from 0.5% to 3% (w/w); (b) carbon nanotubes in the range of 0.1% to 1.5% (w/w) by weight; (c) Carboxymethylcellulose in the range of 0.5% to 2% (w/w) by weight; (d) polypropylene glycol in the range of 5% to 30% (w/w) by weight; (e) Sodium cholate in the range of 0.01% to 0.1% (w/w) by weight; and (f) water.

In yet another embodiment, the present invention provides a composition comprising: (a) Graphite nanosheets with a weight percentage ranging from 0.5% to 3% (w/w); (b) carbon nanotubes in the range of 0.1% to 1.5% (w/w) by weight; (c) Carboxymethylcellulose in the range of 0.5% to 2% (w/w) by weight; (d) γ-valerolactone in the range of 1% to 10% (w/w) by weight; (e) Triton X-100 in the range of 0.01% to 0.1% (w/w) by weight; and (f) water.

As described elsewhere, a preferred component of the liquid composition is a cellulose derivative. Ethylcellulose, methylcellulose, hydroxypropylcellulose, carboxymethylcellulose and hydroxyethylcellulose are suitable. Carboxymethylcellulose (CMC) and its derivatives are particularly suitable. Salts of carboxymethylcellulose, such as the sodium salt, may also be used.

In tests of the present invention, it has been found that CMC provides compositions with strong binding affinity to cellulosic materials such as paper and cardboard, and is expected to bind similarly to cotton, making it ideal for use on these substrates of. During use, CMC forms a stable hydrogel with water and provides a printable, highly conductive ink that adheres to paper.

In yet another aspect, the present invention provides a method of manufacturing ink, the method comprising: (i) Obtain exfoliated graphite nanosheets; (ii) obtain well-dispersed single-walled carbon nanotubes; and (iii) Disperse exfoliated graphite nanosheets, exfoliated single-wall carbon nanotubes, thickener and optional surfactant in a solvent.

In order to ensure uniform mixing of nanosheets and carbon nanotubes, the mixture in steps i), ii), and iii) may be subjected to a high shear mixing stage. One example of such mixing is the use of a modified homogeniser (see eg the device described in WO 2020/074698).

In addition, a further step of compressing (eg, roll milling) the ink may be performed to degas the ink, which facilitates printing of the ink onto the substrate.

The above compositions can be used as inks for printing onto various substrates, including flexible polymers such as polyethylene terephthalates, polypropylenes, and polyimides , elastomers (such as silicone and polyurethane), metal foils and films (such as aluminum, copper, gold, and platinum foils/films), and rigid substrates (such as silicon wafers, glass, quartz, and polycarbonate).

In addition to the substrates listed above, the inventors have surprisingly found that the inks described herein can be printed onto cellulosic substrate materials, such as paper.

Therefore, in another aspect of the present invention, there is provided a substrate (such as a cellulose-based substrate) on which a conductive ink is printed, the conductive ink comprising: (i) nanocarbon materials; and (ii) Cellulose derivatives.

The present invention also provides a method of printing a conductive ink onto a substrate, such as a cellulose-based substrate, the conductive ink comprising: (i) nanocarbon materials; and (ii) Cellulose derivatives.

As mentioned above, the conductive ink may include carbon nanotubes and graphite particles or nanosheets, among other components.

Cellulose-based substrates are usually paper or cardboard.

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

The ideal behavior of screen printing inks requires thixotropic rheology for shear thinning to occur during printing followed by elastic recovery for the required resolution after drying or curing. Stable printing structure at high temperature. This behavior is beneficial for high resolution printing of lines and connections for printed electronics applications. For the construction of electronic circuits suitable for "bare-die" or unpackaged silicon components, printing fidelity is generally better than 125 microns for automated die attach methods.

As mentioned above, the nanocarbon material can be graphite nanosheets, single-walled carbon nanotubes or a mixture thereof, and the cellulose-based binder can be carboxymethyl cellulose, and the film can also contain carbon dioxide as conductive carbon particles. graphite particles.

The conductive ink may also have other components or properties as described herein.

Also provided herein is a textile substrate having printed thereon a conductive ink comprising: (i) Conductive carbon particles (e.g. carbon nanomaterials); and (ii) A binder for binding to cellulose, suitably a cellulose derivative.

The present invention also provides a method of printing conductive ink onto textiles, the conductive ink comprising: i) Conductive carbon particles (such as carbon nanomaterials); ii) a cellulose derivative; and iii) a solvent.

The ink may have the properties described above with respect to other aspects of the invention.

Textiles can be woven or non-woven textiles. For example, woven textiles can be woven from fibers to achieve greater strain-rupture properties and allow for diagonally applied strain relief. This imparts additional mechanical integrity to the printed film when stretched. The spacing of the weft and warp yarns in the woven fabric is usually at least 100 yarns per centimeter so that the resolution of the printing is not limited by surface roughness. The substrate can be planarized by an applied topcoat to reduce surface roughness and thus improve print resolution. Textiles can also be surface chemically treated by the corona discharge process to improve substrate wettability and printability.

Fibers can be natural fibers (e.g. cotton, silk) or synthetic fibers (e.g. polymers including polyester, nylon, polyurethane, polyolefins (e.g. polyethylene and polypropylene), modified and regenerated cellulose (e.g. viscose ( viscose)). These fibers can be spun and woven in combination with each other to provide additional properties.

The choice of woven material allows some ink to penetrate into the bulk of the woven substrate to 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 ensures that the sheet resistance (sheet resistance) can be reduced to below 10 ohms/square, thus providing sufficient conductivity for RFID applications.

Also provided herein is a thermoplastic substrate printed thereon with a conductive ink comprising: (i) Conductive carbon particles (e.g. carbon nanomaterials); and (ii) A binder for binding to cellulose, suitably a cellulose derivative.

The present invention also provides a method of printing conductive ink onto a thermoplastic substrate, the conductive ink comprising: (i) Conductive carbon particles (such as carbon nanomaterials); (ii) a cellulose derivative; and (iii) A solvent.

The ink may have the properties described above with respect to other aspects of the invention.

When the conductive ink has a median diameter (D ₅₀ ) of graphene nanoplatelets below 1 μm, finer screen printing screens (typically greater than 100T) can be used, compared to inks containing larger nanomaterials, to Higher resolution printing inks. Through the appropriate use of anisotropic conductive adhesives, this ink enables interconnection to unpackaged microfabricated devices.

As described herein, the ink can be used to print RFID antennas. Therefore, this paper also provides a kind of RFID label, and this RFID label comprises the textile or thermoplastic base material that RFID antenna is printed on it, and wherein RFID antenna comprises conductive carbon particle (for example, nano carbon material (for example carbon nano sheet and/or carbon nanotubes) and a binder (such as cellulose derivatives (such as carboxymethyl cellulose)).

An RFID antenna can have the properties of a film printed with the inks described herein. Similarly, textiles and thermoplastic substrates can have the properties described above.

Tags typically include a flat portion on which the RFID antenna is printed and a loop or element for forming a loop (eg, a hole through which the end of the tag can pass) to allow the tag to be adhered to an object of interest. .

The shape of the label may be substantially planar in that its thickness may be substantially less than its length or width. Typically, the label has a thickness of less than 3 mm, such as a thickness of less than 2 mm (eg, a thickness of less than 1 mm).

A label may be formed from a long strip of material, such as textile or thermoplastic, where its length is greater than its width. At one end of the tag length, a hole may be provided through which the sending end of the tag length can be inserted to provide a "loop lock tag" (see Figures 16A and 16B). The size of the shape of the hole allows the ring tab to be held together in a gentle and reversible manner.

As an alternative to the "loop lock label" described above, the label may be a flat label as described above fitted with complementary ^Velcro® sections to allow the sections of the label to be secured together to form a loop.

Labels can be encapsulated with an overcoat, such as polyurethane or silicone, to increase their abrasion resistance and resistance to damage such as water.

Alternatively, the labels may be provided in continuous extensions as strips and the assembled labels produced at regular intervals.

Therefore, the present invention uses water-based, high-conductivity ink in the production of RFID tags to achieve the required sheet resistance for use as an excellent RFID antenna. Although conventional RFID tags are made using metallic materials, making them difficult to recycle, the carbon-based RFID tags of the present invention provide a more environmentally friendly alternative. In addition, the combination of cellulose-based binder and carbon nanomaterials allows some flexibility and stretchability to overcome stress fatigue, which may exist in metal antennas. In addition, carbon-based RFIDs may exhibit non-magnetic properties, enabling their use in food production environments.

The present invention combines the high electrical conductivity of the carbon nanomaterial combination with the thixotropic rheology required for good printing properties, giving numerous embodiments of devices and circuits that demonstrate applicability for printed electronics applications. This example (Example 3) outlines a UHF RFID tag. Likewise, (Example 5) exemplifies a micro heater device.

Conductive inks can be used for printing in a variety of applications including, but not limited to, microwave antennas, RFID tags, biosensing electrodes, printed heaters, wireless induction coils, metasurfaces for tunable low-emissivity and reflectivity coatings ( metasurfaces), strain sensors, surface acoustic wave devices, temperature sensors, energy storage electrodes and electrolytes for supercapacitors, batteries, capacitive sensors, flexible, stretchable or structured electronic conductors, for Low-density aerogels for catalysis, electrical energy storage and chemical repair, self-healing coatings, and drug delivery platforms.

In yet another aspect, the present invention provides an RFID tag comprising an antenna deposited (eg, printed) from a liquid composition described herein onto a substrate. The substrate may be a plastic polymer substrate such as PET, a cellulosic substrate such as paper, a textile substrate such as a woven or nonwoven textile substrate, or a continuous thermoplastic substrate.

In yet another aspect, the present invention provides a printed heater comprising a heating element printed onto a substrate from the liquid composition described herein.

The measurement of surface strain can be used in many industrial applications. When applied to a substrate at greater than or equal to its percolation threshold, the carbon nanoprinted structure exhibits strain-dependent electrical conductivity. Films based on polymer binders exhibit reproducible elasticity beyond that using conductive metals, which can fracture before reaching the elastic limit of the substrate. By utilizing the present invention therein, high strain (>2%) state measurements can be performed on elastic substrates with good reproducibility. Furthermore, this elastic behavior can be extended to modify antenna resonance properties (frequency and Q-factor). A novel embodiment is presented in which the resonant behavior of printed UHF RF antennas on elastic substrates is monitored without the need for internal power sources or processing circuits.

In yet another aspect, the invention provides a deposited liquid composition as described herein deposited (eg, printed) on a stretchable substrate.

Example

Example 1- Exfoliation of graphite to form graphite nanosheets

Graphite flakes were exfoliated using the equipment and method described in International Patent Application No. PCT/EP2019/077579 to obtain graphite nanosheets with an average lateral size distribution of about 1 μm and an average thickness of about 10 layers.

In summary, fine graphite powder (flake size 1–50 μm obtained by air classification of ground powder) was dispersed into a surfactant-water system and added to a high pressure homogenizer (e.g. International Patent Application No. PCT/EP2019 /077579 equipment described in the feed tank). The fluid is then pressurized and accelerated under reduced pressure before passing from the process chamber of the homogenizer into the heat exchanger. Once the fluid has cooled to the temperature maintained by the external chiller system, it can be collected or recirculated depending on the system configuration.

After processing the graphite, the exfoliated mixture was centrifuged at 5000 g for 20 minutes to remove all unexfoliated crystallites and larger fragments. These parameters lead to the almost complete settling of several layers of nanosheets (ie graphite nanosheets) present. The graphite nanoplatelets obtained have distributions of lateral dimensions and thicknesses of 50 to 2000 nm and up to about 20 nm, respectively.

Example 2- ink preparation

Ink A

The table below gives the composition used to prepare the ink batches. The total solid content of the prepared ink (including binder, etc.) is about 3.7 wt%. Material mass (g) Dry film fraction (wt%) Graphite nanosheets (obtained as described in Example 1) 3.10 40 Single-walled carbon nanotubes (Tuball Batt-H ₂ O SWCNTs provided by OCSiAl) 1.54 20 Carboxymethyl Cellulose (Sodium Salt) 2.70 34 Triton X-100 0.50 6

Ink B

The table below gives the composition used to prepare the ink batches. The total solid content of the prepared ink (including binder, etc.) is about 2.8 wt%. Material mass (g) Dry film fraction (wt%) Graphite nanosheets (obtained as described in Example 1) 1.34 47.4 Single-walled carbon nanotubes (Tuball Batt-H ₂ O SWCNTs provided by OCSiAl) 0.58 20.3 polypropylene glycol 22.86 margin Carboxymethyl Cellulose (Sodium Salt) 0.86 30.4 Sodium cholate or other 0.05 1.9 water 73.90 margin

To make the ink, the components are weighed into suitable containers. To reduce the viscosity enough to mix the components, a Silverson L5M-A laboratory high shear mixer was used running at 5000 rpm, the mixture was heated under mixing conditions (60°C hot plate), and the mixture was mixed for 5 minutes.

Graphite nanoplatelets have a lateral size distribution from 50 nm to 800 nm and have a thickness up to about 20 nm.

Structural characterization by SEM indicated the presence of a tight network of carbon nanotubes in the gaps between the stacked graphite nanosheets (see Figures 1 and 2).

The viscosity of ink A was measured at a shear rate of 0.1/s to 100/s, and it was found that these inks had thixotropy and rheological curves, as shown in Figure 15.

The ink has been successfully printed on a variety of substrates including several grades of polyethylene terephthalate (PET) substrates (DuPont Tejin ST504 & Felix Scholler F40100) and paper substrates.

The conductivity of the printed film was measured using a four-point probe according to the International Electrotechnical Commission standard IEC TS 62607-2-1:2012. The film thickness was measured by SEM cross-sectional analysis or scanning probe profilometry, and the specific conductivity was calculated using the conductivity and thickness.

The conductivity of the printed film was observed to be as high as 500 kSm ⁻¹ .

Therefore, the present invention provides a highly conductive ink formed of carbon nanomaterials, especially a highly conductive ink formed of carbon nanomaterials that can be printed onto paper substrates.

Example 3 : UHF RF Tag integration

An appropriately designed UHF antenna was screen printed using Ink A as described in Example 2 above, and screen printed onto a paper substrate (standard uncoated label stock) using a 32T mesh to a dry thickness of approx. to 3 microns.

An image of the printed antenna is shown in Figure 3. Figure 4 shows the flexibility of the printed antenna, where it can be seen that the printed antenna can be crimped on a narrow diameter without compromising the integrity of the printed antenna. Figure 5 shows the printing resolution of the ink.

The resulting antenna is integrated with a bare-die RFID integrated circuit (Impinj Monza 6 or NXP UCODE 8) through an anisotropic conductive film (ACF) thermode process. The resulting RFID tags were analyzed using a handheld Zebra (Model MC3300) reader in an unshielded office environment, achieving a typical read distance of 3 meters.

Example 4 : Printed on a stretchable substrate

During formulation, 5 wt% of the binder gamma-valerolactone (GVL) was added to the water content of ink A as described in Example 2.

When printing, films printed with this GVL-containing ink have comparable sheet resistance (for example, 2-3 Ohm/ sq).

Inks containing GVL were also printed onto two different thermoplastic polyurethane (TPU) elastomers and vulcanized polyisoprene rubber.

The linearity curve of the ink deposited on the TPU was used to quantify the response of the ink to strain. For strain measurements, linear curves were printed onto dogbone shaped substrates mounted in a TA Systems Texture Analyzer. Sample resistance was monitored in situ using a Keithley 2614B SourceMeter.

As shown in Figure 6, the response of the sheet resistance is linear up to a strain of 5%, after which the resistance begins to rise rapidly. The slope of the linear region (shown by the dashed line) is uniform, which is difficult to understand since most anisotropic materials exhibit a gauge factor (Gauge Factor, G) > 2 due to deformation of the material. The mechanism by which this occurs has not been determined.

A bow-tie antenna pattern (as shown in Figure 7) was printed on a commercially available TPU elastomeric substrate using the inks described above for GVL-containing inks. The antenna design has been optimized for resonance in the UHF RFID frequency band (860 to 960 MHz).

Once installed with a suitable RFID integrated circuit (IC), the read distance of the printed antenna shown in Figure 7 is about 80-85 cm.

To compare the antenna behavior with the measured resistive response in Figure 7, a single antenna was connected to a vector network analyzer (VNA, Pico Technologies PicoVNA 106) through the SMU-A connector and the spectral response was monitored as the antenna was strained. Figure 8 shows the results, together with Figure 9 interpolated measurements of the resonance position as a function of strain.

其中

是0 Hz時反射波損耗(return loss)的外推值。數據與圖6的數據一起繪製於圖10中。可以看出兩種方法在天線電阻隨應變的相對變化方面有合理的一致性。 The data show that the response of the antenna's resonant frequency to applied strain is weak, even beyond the region where the conductivity of the film is expected to vary linearly. The data can equally be "fitted" to a constant mean value at 890 MHz. The antenna resistance can be estimated by extrapolating the data on the left hand side of Figure 6 to 0 Hz;

in

is the extrapolated value of the return loss at 0 Hz. The data are plotted in Figure 10 along with the data from Figure 6 . It can be seen that the two methods are in reasonable agreement with respect to the relative change in antenna resistance with strain.

Finally, the individual assembled tags (antenna and IC) were tested under a cyclic load of 5% strain, and the read range was measured before and after the test. The initial read range was 70 cm, while the final read range (after 10,000 strain cycles) was also 70 cm.

Example 5 : Printable Heater

Ink A of Example 2 was printed on label paper in the pattern shown in FIG. 11 . Printing involved three passes, and the sheet resistance of the printed film was 2 Ω/sq (ohms/square). The resulting film was flexible and adhered well to paper substrates and could conform to a 2 mm roll diameter without delamination.

Applying a 10 V DC potential difference to the printed film (about 0.08 A, 0.8 W), as determined by infrared thermography, resulted in a temperature increase of about 20°C (see Figures 12A and 12B).

Example 6 : Ink preparation containing graphite

It has also been found that when using micron-sized graphite instead of the graphite nanoplatelets of Example 2 above, an increase in conductivity is still observed through the addition of carbon nanotubes.

Figure 13 shows the electrical conductivity of a mixture of graphite and carbon nanotubes at different mass fractions of carbon nanotubes.

Example 7 : RFID Label

RFID labels were produced using the ink described in Example 2 above.

A label was cut from recycled tightly woven polyester, generally in the shape shown in Figure 16B. As shown in Figure 16B, the tag is generally rectangular in shape with a circular hole near one end of the tag. There is a semi-circular cutout on each long side of the label.

At the end of the label opposite the circular hole, the RFID antenna was printed with the ink described in Example 2, which showed excellent adhesion to the woven polyester substrate.

During use, one end of the tag printed with the RFID antenna can pass through the round hole at the other end of the tag to form a loop as shown in FIG. 16A . A semi-circular cutout sits in the round hole to locate the ring.

Thus, the tag can be affixed to crates, pallets, animal carcasses, living plants and trees or other objects where it would be beneficial to provide RFID tags so as not to cause damage to the object itself. The label is resistant to water-based liquids. The tag also has the advantage of not containing any metallic material and being both flexible and detachable.

Similar labels can be produced using PET filament or PET film. RFID tags made with these materials had a read range of about 1.6 m when attached to a volunteer. Therefore, these results validate the label's use in meat processing applications.

Figures 1 and 2 show scanning electron microscope (SEM) images of the printing inks described in Example 2 below. Figure 3 shows an image of the printed antenna described in Example 3 below. FIG. 4 shows an image of the flexibility of the printed antenna shown in FIG. 3 . Figure 5 shows an image of the printing resolution of the ink described in Example 2 below. FIG. 6 shows the strain response of the resistance of the printed film as described in Example 4. FIG. FIG. 7 shows the shape of the antenna pattern printed in Example 4. FIG. FIG. 8 shows the resonant frequency of films printed in Example 4 with different GVL contents. Figure 9 shows the resonant frequency of the film described in Example 4 as a function of strain. Figure 10 shows the strain response of the resistance of the printed films as described in Example 4 (see Figure 6) and the estimated resistance of these printed films. Figure 11 shows a printed film as described in Example 8, used as a printed heater. 12A and 12B are infrared thermal images of the printed film in FIG. 11 , wherein FIG. 12A shows that no potential difference is applied to the film, and FIG. 12B shows that a potential difference of 10V is applied to the film. Figure 13 shows the electrical conductivity relative to the mass fraction of carbon nanotubes from films printed from inks containing graphite particles and carbon nanotubes, as described in Example 6 below stated. Figure 14 shows a schematic diagram of the respective width, length and thickness parameters of graphite nanosheets. Figure 15 is a rheological curve showing the viscosity of the ink described in Example 2 below. Figures 16A and 16B show a label with an RFID tag printed thereon using the inks described herein.

Claims (19)

A liquid composition comprising: (i) Graphite nanosheets; (ii) carbon nanotubes; (iii) a thickener; (iv) a humectant; and (v) A solvent. The liquid composition according to claim 1, wherein the humectant is selected from urea, glycerin or glycols such as polypropylene glycol. The liquid composition according to claim 1 or claim 2, wherein the graphite nanosheets have a thickness of 30 nm or less. The liquid composition according to any one of claims 1 to 3, wherein the thickener is a hydrogel-forming thickener. The liquid composition according to any one of claim 1 to claim 4, wherein the thickener is a cellulose derivative, such as carboxymethyl cellulose. The liquid composition according to any one of claim 1 to claim 5, further comprising single-walled carbon nanotubes, optionally having an average diameter of 1 nm to 5 nm and/or a length greater than 3 μm. The liquid composition as described in any one of claim 1 to claim 6, wherein the conductive carbon particles are graphite nanosheets, and the weight ratio of the carbon nanotubes to the graphite nanosheets is 0.05:1 to 0.6:1 (carbon nanotubes:graphite nanosheets). The liquid composition as described in any one of claim 1 to claim 7, wherein the carbon nanotubes are present in the composition in an amount of 0.1% to 1.5% by weight (w/w). The liquid composition according to any one of claim 1 to claim 8, further comprising a surfactant. The liquid composition as described in claim 9, wherein the surfactant is sodium cholate. The liquid composition according to any one of claim 1 to claim 10, wherein the solvent is an aqueous solvent (such as water). The liquid composition according to any one of claim 1 to claim 11, further comprising a binder containing monovalent ions. The liquid composition as claimed in claim 12, wherein the binder comprises carboxymethyl treated with an aqueous solution of a divalent, trivalent or tetravalent ion salt (such as calcium chloride (II) or iron sulfate (III)). cellulose. The liquid composition as described in claim 1, comprising: (a) Graphite nanosheets; (b) carbon nanotubes; (c) carboxymethyl cellulose; (d) polypropylene glycol; (e) sodium cholate; and (f) water. The liquid composition as described in claim 1, comprising: (a) Graphite nanosheets with a weight percentage ranging from 0.5% to 3% (w/w); (b) Carbon nanotubes with a weight percentage ranging from 0.1% to 1.5% (w/w); (c) Carboxymethylcellulose in the range of 0.5% to 2% (w/w) by weight; (d) polypropylene glycol in the range of 5% to 30% (w/w) by weight; (e) Sodium cholate in the range of 0.01% to 0.1% by weight; and (f) water. A textile or thermoplastic substrate, on which the liquid composition according to any one of claim 1 to claim 15 is printed. A method of printing onto a textile or thermoplastic substrate using the liquid composition described in any one of claim 1 to claim 15. An RFID label comprising an RFID antenna printed by the liquid composition as described in any one of claim 1 to claim 15. The RFID tag as claimed in claim 18, wherein the tag includes a flat portion on which the RFID antenna is printed and a loop or elements for forming a loop to allow the tag to be adhered to an object of interest.

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