GB2613548A - Rfid - Google Patents

Abstract

An RFID tag antenna 2 comprising a carbon filler of functionalised graphene in a polymer matrix. The functionalised graphene particles may be amine-functionalised, amide-functionalised, pyrrole-functionalised, pyridine-functionalised, nitrogen functionalised, silane functionalised, mercapto-functionalised, vinyl-functionalised or halogen-functionalised. The functionalised graphene particles may have a nitrogen content of 3-20 wt% and an oxygen content of less than 4 wt%. The RFID antenna may be substantially metal free (less than 3 wt%) and be microwave safe. The functionalised graphene particles may be graphene nanoplatelets having an average of two to five graphene layers per particle. The carbon filler has a multimodal size distribution. The carbon filler may be coated or printed, via an ink, onto a substrate which is part of microwaveable packaging for a food product. The polymer matrix may comprise polyvinyl, polyamide, polyvinylidene fluoride (PVDF) or a fluoropolymer resin. The RFID antenna may have a multilayer (Fig.2) structure of functionalised graphene in a polymer matrix layers (13, Fig.2) and electrically insulating layers of vinyl polymer (11, Fig.2). The RFID antenna may have a sheet resistance of 0.06 to 5 Ω per square 25μm.

Description

RFID

FIELD OF THE INVENTION

The present invention relates to radio-frequency identification (RFID) antennae and to radio-frequency tags (RFID tags) comprising such RFID antennae. The present invention also relates to methods for manufacturing such RFID antennae and RFID tags, as well as articles incorporating those antennae and tags.

BACKGROUND

Radio-frequency identification is the use of electromagnetic energy to stimulate a response device (known as an RFID tag or an RFID transponder) to identify itself, and in some cases, provide additional data, which is stored in the tag. RFID tags are normally manufactured with a unique identification number and attached to an article. Once the tag is attached to the article the serial number of the tag is associated with the article in a computer database.

RFID tags typically consist of an antenna and a microchip.

The presence of an RFID tag (and therefore the presence of the article to which the RFID tag is attached) may be checked and monitored by devices known as "readers" or "reader panels". Readers usually transmit radio frequency signals which are received by the RFID antenna. The RFID tags are then able to respond to these radio frequency signals, providing information stored on the RFID tag to enable the reader to identify the article.

RFID tags are used in a wide range of applications such as in electronic keys, toll collection systems, contactless payment, self-check-out systems, electronic article surveillance systems, tracking goods during manufacture and timing sports events. The present invention relates, in particular, to RFIDs suitable for use with self-check-out systems and with electronic article surveillance systems.

Using RFID tags for self-check-out systems is advantageous because, unlike barcodes, the RFID tags do not need to be in the line of sight of the reader/scanner at the check-out, this helps to enable a faster checkout procedure, as customers do not have to individually scan all of the items they wish to purchase, but instead can simply stack the items on top of the reader at the checkout. In addition, RFID tags may be used to facilitate check-out free shopping, such as at Amazon Go® stores. In these systems, reader panels may be mounted at gates at the exit to the shop. Wien a customer passes through the gate when leaving the shop, the reader panels identify the RFID tags on the items they are carrying with them and automatically take payment for these items from a registered credit card. For these applications it is important that the RFID tag has a read range of at least 1 metre.

RFID tags can also be used in electronic article surveillance systems. These systems are used to track products and to prevent shoplifting (the unauthorized removal of products from a shop by thieves without paying). Most shops have alarm systems, these consist of gates mounted at the exit to the shop, which comprise a reader panel. These panels are able to detect the RFID tags on the products from the shops Therefore, when a thief tries to remove a product from the shop which has not been paid for, the reader panel detects this as they try to exit the shop and causes an alarm to sound. For this application it is also important that the RFID tag has a read range of at least 1 metre, as this is realistically the minimum size of the gates at the exit to the shop, in practice it is desirable to have read ranges of over 2 metres.

Given the advantages listed above for using RFID tags instead of barcodes as identification technology on food packaging, there is a need for cheap and reliable RFID tags for food packaging. It is also desirable that RFID tags on food packaging are microwave safe.

Conventional RFID antennae are made of metallic aluminium. A chemical etching process is required to produce aluminium RFID antennae. This results in toxic waste chemicals, which are harmful to the environment. Conventional RFID antennae are also relatively costly to manufacture. In addition, conventional RFID antenna are not microwave safe, because when conventional RFID antennae are heated in a microwave electrons can become concentrated at the edges of the RFID antenna pattern, which can cause the electrons to arc to the air, which causes sparking or flames. This not only damages the RFID tag itself but may also cause damage to the microwave.

RFID tags said to be suitable for microwave use are described in CN209895366U and in WO 2020/006219. However, both of these documents make use of metal antennae and rely on shielding components to avoid sparking in the microwave. CN209895366U describes the use of a sacrificial metal conductor (usually a metal foil) to produce RFID tags which are microwave safe. WO 2020/006219 describes RFID antennae with a split ring (or shield) conductor formed on one side of a substrate (or dielectric) and a coil antenna conductor formed on the opposite side of the substrate. The split ring conductor comprises a gap which allows the microwave current to flow through the coil antenna conductor, yet no part of the coil antenna conductor in the gap interacts with the microwave current, thus preventing arcing (which leads to sparks and flames). However, the RFID antennae described in these documents requires complicated additional components to shield the antennae from the microwave energy, meaning that production of these components is more complicated and hence that these antennae are more expensive to produce.

There remains a need in the art to provide identification technology, which is cheaper and more adaptable, particularly for food packaging.

SUMMARY OF THE INVENTION

The present inventors have developed an RFID antenna and an RFID tag, which helps to address the practical problems outlined above.

Broadly, the present inventors have developed an RFID antenna incorporating functionalised graphene particles dispersed in a polymer matrix material. The antennae can have a read range of at least 1 metre, be microwave safe and can be produced without the need for complex chemical etching processes. Surprisingly, the inventors have found that the functionalised graphene particles can be used to produce RFID antennae with the required level of conductivity for the RFID antenna whilst also demonstrating excellent dispersibility in the matrix material. The use of functionalised graphene leads to better dispersal and hence higher conductivity than non-functionalised graphene, as the functionalised graphene is better able to interact with the polymer matrix material.

Accordingly, in a first aspect the present invention provides an RFID antenna comprising a carbon filler dispersed in a polymer matrix material, the carbon filler including functionalised graphene particles.

In a second aspect, the present invention relates to an RFID tag comprising an antenna according to the present invention.

In a further aspect, the present invention relates to the use of a printed carbon filler dispersed in a polymer matrix material as an RFID antenna, wherein the carbon filler comprises functionalised graphene.

These proposals also encompass an article comprising an RFID tag according to the present invention. The article is preferably a piece of microwaveable packaging.

Further aspects relate to a method of manufacturing an RFID antenna according to the present invention.

In a further aspect, the present invention relates to a microwave capable of reading an RFID tag according to the present invention and using the information on the RFID tag to generate a response.

Within the meaning of this invention an "RFID antenna" is considered to define a component of an RFID tag which enables the tag to receive and transmit a signal.

By "microwave safe" we mean that the RFID antenna is able to be heated in a microwave at a power of 900 W for 5 minutes without causing sparks or flames to appear in the microwave. Preferably, the microwave sage RFID antenna is able to be heated in a microwave at a power of 900 W for 10 minutes without causing sparks or flames to appear in the microwave. The term "microwaveable" is used interchangeably with the term "microwave safe". Generally such microwave safe antennae are tolerant to the high-field emissions that are present in the microwave meaning that the antennae do not need to be removed from a product such as a food item, before cooking, thawing, heating and/or reheating in an apparatus such as in a microwave.

By "microchip" we mean a semiconductor device on which is formed a memory and operating circuitry. In general the microchip stores a unique identifier code associated with the tag. The microchip may also include information about the product it is attached to such as use by date or ingredients included in a food product.

The present invention has a number of advantageous features.

Firstly, the RFID tag has a read range of at least 1 metre. As mentioned above RFID tags have advantages over barcodes as they can be read through opaque outer packaging materials, such as paper and plastic, facilitating an easier checkout and meaning that articles tagged with the RFID tags can be effectively tracked by electronic article surveillance systems in shops.

Secondly, the RFID antenna according to the present invention comprises graphene meaning that it can be produced cheaply compared to conventional RFID antennae produced using aluminium etching. The production process also avoids the environmentally damaging chemical etching processes required to manufacture RFID antenna from aluminium. This means that the chemical waste produced at the end of the reaction can be reduced and that the RFID antennae are more environmentally friendly than conventional RFID antennae.

Thirdly, the RFID antennae are microwave safe and will not spark or catch fire in the microwave. This means that the RFID antenna or tag does not need to be removed from the product before initiating the heating process in the microwave.

Fourthly, the RFID antennae according to the present invention are flexible and can be printed onto flexible paper and plastic substrates. This means that they can be used on a variety of packaging materials such as cardboard boxes or sachets for food packaging. In addition, the RFID antennae retain their activity even when frozen meaning that they are suitable for use on packaging for frozen food. Furthermore, the antennae do not lose their flexibility when frozen meaning that antennae on the packaging of frozen products are not likely to crack and break during storage or transport.

Fifthly, the RFID antennae can be manufactured using a number of printing methods including screen printing, flexographic printing and gravure printing. This means that production of these RFIDs is flexible and can easily be scaled up to meet demand.

Carbon filler The RFID antenna according to the present invention comprises a carbon filler dispersed in a polymer matrix material, the carbon filler including functionalised graphene particles.

Preferably, the RFID antenna comprises from 10 wt.% to 50 wt.% carbon filler, preferably from 20 wt.% to 40 wt.% carbon filler, more preferably from 25 wt.% to 35 wt.% carbon filler, based on the total weight of carbon filler and polymer matrix material.

Preferably, the carbon filler consists of functionalised graphene particles or a combination of functionalised and unfunctionalized graphene particles. Preferably, the carbon filler consists of functionalised graphene particles. Alternatively, the carbon filler may comprise functionalised graphene particles and an additional carbon filler material.

Preferably the carbon filler comprises at least 1 wt.% functionalised graphene particles, preferably at least 2 wt.% functionalised graphene particles, preferably at least 3 wt.% functionalised graphene particles, preferably at least 4 wt.% functionalised graphene particles based on the total weight of the carbon filler. Preferably the carbon filler comprises from 1 wt.% to 50 wt.% functionalised graphene particles, more preferably from 2 wt.% to 45 wt.% functionalised graphene particles, more preferably from 3 wt.% to 40wt.% functionalised graphene particles, most preferably from 4 wt.% to 35 wt.% functionalised graphene particles based on the total weight of the carbon filler.

Preferably, the carbon filler has a multimodal (e.g. bimodal) size distribution. The particle sizes in the carbon filler may be determined using a light scattering analyser e.g., dynamic light scattering particle size distribution analyzer LB-550 (available from HORIBA).

An RFID antenna comprising a carbon filler having a multimodal size distribution may be achieved by admixing multiple sets of graphene particles with different volume average mean particle sizes. Without being bound by any theory it is believed that having different sizes of conductive particles in the polymer matrix material leads to improved conductivity as the smaller particles are able to fill the holes or voids in the matrix material formed by the larger particles. Preferably, all of the sets of graphene particles are sets of functionalised graphene particles.

When the RFID antenna comprises sets of graphene particles with different volume average mean particle sizes one set of particles (A-1) may have an average diameter of 5 pm (determined by light scattering) or less and one set of particles (A-2) may have an average diameter of 8 pm of more. The weight ratio of (A-1) to (A-2) may be from 1:14 to 2:1, more preferably from 1:10 to 3:2, more preferably from 1:7 to 1:1. The volume ratio of (A-1) to (A2) may be from 2:1 to 1:2, preferably about 1:1.

Alternatively, the peaks in the multimodal distribution obtained using a light scattering analyser in the case of two sets of graphene particles with different volume average particle sizes may be modeled as log-normal distributions. This can be used to obtain an area under the curve for both peaks, (B-1) for the particles with a smaller volume average particle size and (B-2) for the particles with a larger volume average particle size. The ratio of (B-1) to (B2) is preferably from 1:14 to 2:1, more preferably from 1:10 to 3:2, most preferably from 1:7 to 1:1.

Alternatively, the carbon filler may comprise graphene particles with a multimodal size distribution having a first peak maximum of from 5 -7 pm (C-1) and a second peak maximum of from 8-10 pm (C-2). The ratio of (C-1) to (C-2) may be from 1:14 to 2:1, more preferably from 1:10 to 3:2, most preferably from 1:7 to 1:1.

Alternatively, the RFID antenna may comprise one or more additional carbon fillers which have a volume average mean particle size which is different to that of the graphene particles described above. This may be achieved by the particles of the additional carbon filler(s) having a relatively smaller volume average mean particle size and the graphene particles having a relatively larger volume average mean particle size or by the additional carbon filler(s) having a relatively larger volume average mean particle size and the graphene particles having a relatively smaller volume average mean particle size.

The additional carbon filler(s) may be selected from the group of carbon black, acetylene black (ACB), carbon nanorods, carbon nanotubes, SWCNT (single wall carbon nanotubes), MWCNT (multi-wall carbon nanotubes), nano wires or graphite (e.g. graphitic platelets) or a combination of these materials. Preferably, the additional carbon filler(s) comprise or consist of carbon black and/or graphite. Without wanting to be bound by theory, it is believed that the carbon black and/or graphite works synergistically with the graphene particles to improve the conductivity of the RFID antenna. The use of these materials as additional carbon fillers gives an RFID antenna with high levels of conductivity at low cost. The use of compositions comprising carbon black is particularly preferred because it is relatively low cost in comparison to other forms of carbon filler.

These additional carbon filler(s) may comprise from 5 to 40 wt.%, preferably from 5 to 35 wt.%, more preferably from 5 to 30 wt.% of the weight of the RFID antenna based on the total weight of carbon filler and polymer matrix material. The additional carbon filler(s) are generally particulate materials.

Preferably, the RFID antenna may comprise 5 to 30 wt.% carbon black, or 5 to 30 wt.% graphite, or mixtures of these fillers, wherein preferably the total amount of additional carbon filler in all cases is in the range of 5 to 30 wt.% based on the total weight of carbon filler and polymer matrix material.

Preferably, the RFID antenna comprises from 1 -15 wt.% carbon black, more preferably from 5-10 wt.% carbon black based on the total weight of the of carbon filler and polymer matrix material.

The type of carbon black is not particularly limited. For example, the carbon black may be channel black, furnace black, lamp black or thermal black. Carbon black is generally obtained by the incomplete combustion of heavy petroleum products, for example FCC tar, coal tar or ethylene cracking tar. The carbon black may have a paracrystalline or amorphous structure. The carbon black may be acidic, neutral or basic.

Carbon black is commercially available, for example as CABOT BP 2000, Degussa Printex XE-2B Mitsubishi MA-7 and Orion FW 200.

Preferably, the carbon black has a volume average mean particle size of 5-15 pm. Volume average mean particle size can be determined using any suitable method known to a skilled person in the art such as light scattering (mean size = mean hydrodynamic diameter of the particles) or laser distribution particle size. Preferably, light scattering is used. Light scattering can be measured using a light scattering analyser e.g., dynamic light scattering particle size distribution analyzer LB-550 (available from HORIBA).

Preferably the ratio of graphene to carbon black is 1:14 to 2:1, more preferably from 1:10 to 3:2, more preferably from 1:7 to 1:1. The volume ratio of graphene to carbon black may be from 2:1 to 1:2 and is preferably about 1:1.

Optionally, the RFID antenna comprises from 5 to 20 wt.% graphite, more preferably from 10 to 20 wt.% graphite based on the total weight of carbon filler and polymer matrix material.

Graphite may be obtained as powder or flakes from chemical suppliers such a Sigma Aldrich. Preferably, the graphite has a volume average mean particle size of 5-10 pm. The use of particles with a mean particle size of less than 10 pm, means that the particles are more easily dispersed in the polymer matrix material.

In embodiments containing both graphene and graphite, the ratio of graphene to graphite may be 2:1 to 20:1, for example, from 1:1 to 10:1. The volume ratio of graphene to graphite may be from 2:1 to 1:2 and is preferably about 1:1.

Optionally, there are a limited number of particles in the carbon filler having a size considerably larger than the volume average mean particle size. For example the D90 value (the value wherein the portion of particles with diameters below this value is 90%, determined using dynamic light scattering) may be less than 200%, less than 180%, or less than 160% of the mean particle size. In instances where the carbon particles have a multimodal distribution, the Do value may be less than 200%, less than 180%, or less than 160% of the peak of the distribution occurring at the highest size value. The median particle size may be within 80% to 120% of the value of the mean particle size. Optionally, the Dio value (the value wherein the portion of carbon particles with diameters smaller than this value is 10%; determined using dynamic light scattering) is at least 40% of the mean particle size. In instances where the particles have a multimodal distribution, the Dlo value may be within 80 to 120% of the peak of the distribution occuring at the lowest size value. For example, for a mean particle size of about 6.5 pm, the median is preferably from 5.2 -7.8 pm. The Dio value is at least 2.6 pm and the Da° value is less than 10.4 pm.

In an alternative embodiment, the carbon filler comprise 95 wt.% graphene, or 97 wt.% graphene, or 99 wt.% graphene based on the total weight of carbon filler. The carbon filler may comprise less than 5 wt.%, preferably less than 3 wt %, preferably less than 1 wt.% of additional carbon filler based on the total weight of carbon filler. For example, the RFID antenna may comprise less than 5 wt.%, preferably less than 3 wt.%, preferably less than 1 wt.% of graphite and carbon black based on the total weight of carbon filler.

Graphene particles The RFID antenna according to the present invention comprises a carbon filler dispersed in a polymer matrix material, the carbon filler including functionalised graphene particles.

The graphene particles are conductive and allow the transfer of electrical energy to a microchip, as well as backscatter from the microchip to the antenna. When the RFID antenna is operated as part of an RFID tag, it is energised by a time-varying electromagnetic radio frequency wave that is transmitted by a reader. When the radio frequency field passes through the antenna, there is an AC voltage generated across the antenna. This voltage is rectified to supply power to the microchip. The information stored in the microchip is then transmitted back to the reader via the antenna. This is referred to as backscatter.

Graphene particles are particularly advantageous for microwave applications because they are "self-regulating", specifically, their resistance goes up as they heat up meaning that RFID antennae comprising graphene particles are less likely to cause sparking or flames.

The graphene particles may be randomly dispersed in the polymer matrix material. Providing carbon in this form instead of, for example, in the form of woven carbon microfibre sheets encased within a polymer matrix material, simplifies manufacture and reduces expense. Furthermore, the conductivity of graphene particles (which is higher than, for example carbon black and graphite) means that the RFID antenna can be formed at relatively low loadings of graphene. In addition, using graphene particles in this form allows the RFID antenna to be applied using known coating or printing techniques, which simplifies manufacture compared to the use of woven carbon microfibres; particularly when used to form complex shapes on the substrate.

Suitably, the graphene particles have a high aspect ratio. Advantageously, graphene particles having a high aspect ratio can form conductive paths at relatively low loading levels, helping to improve the flexibility of the RFID.

The graphene particles (which can be referred to as "graphene-material particles", or "graphene-based particles") may take the form of monolayer graphene (i.e. a single layer of carbon) or mulfilayer graphene (i.e. particles consisting of multiple stacked graphene layers). Multilayer graphene particles may have, for example, an average (mean) of 2 to 100 graphene layers per particle. "Mien the graphene particles have 2 to 5 graphene layers per particle, they can be referred to as "few-layer graphene". Preferably the graphene particles used in the RFID antenna according to the present invention comprise few-layer graphene.

Advantageously, these forms of carbon nanoparticles provide extremely high aspect ratio conductive particles. This high aspect ratio allows the formation of conductive paths at relatively low loading levels, decreasing the volume of the RFID antenna occupied by the carbon nanoparticles and, thus, increasing the flexibility/stretchability of the antenna. This is important for when the RFID antenna is printed directly onto flexible forms of packaging such as sachets and bags which may be bent and deformed. In addition, flexibility is important as it means that it is less easy for potential thieves to damage the RFID antennae by deforming the packaging material (for example by snapping the RFID antenna meaning that it is no longer able to transmit the signal).

The graphene particles may take the form of plates/flakes/sheets/ribbons of multilayer graphene material, referred to herein as "graphene nanoplatelets" (the "nano" prefix indicating thinness, instead of the lateral dimensions).

The graphene nanoplatelets may have a platelet thickness of less than 100 nm and a major dimension (length or width) perpendicular to the thickness. The platelet thickness is preferably less than 70nm, preferably less than 50 nm, preferably less than 30 nm, preferably less than 20 nm, preferably less than 10 nm, preferably less than 5 nm. The major dimension is preferably at least 10 times, more preferably at least 100 times, more preferably at least 1,000 times, more preferably at least 10,000 times the thickness. The length may be at least 1 times, at least 2 times, at least 3 times, at least 5 times or at least 10 times the width.

The loading of graphene particles in the polymer matrix material may be, for example, 0.25 wt.% or more, 0.5 wt.% or more, 1 wt% or more, 2 wt.% or more, 5 wt.% or more, 10 wt.% based on the total weight of carbon filler and polymer matrix material. The upper limit for the loading of graphene particles in the polymer matrix material may be, for example, 0.5 wt.%, 1 wt.%, 2 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%, 30 wt.% or 40wt.% based on the total weight of carbon filler and polymer matrix material. If the loading of graphene particles is too low then the resistance of the RFID antenna will be high, meaning that the antenna must be illuminated with higher power levels in order to achieve active signal transmission. If the loading is too high, then this can adversely affect the mechanical properties of the RFID. For these reasons, it is preferable for the loadings of the graphene particles to be in the range of, for example, 1 to 30 wt.%, 1 to 20 wt.%, or 1 to 10 wt.% based on the total weight of carbon filler and polymer matrix material. In some applications the loading of graphene particles in the polymer may be 30 wt.% or more, 40 wt.% or more based on the total weight of carbon filler and polymer matrix material.

Preferably the RFID antenna has a sheet resistance of less than 5 0/sq/25 pm, or less than 3 f2/sq/25 pm, or less than 1 Q/sq/25 pm, or less than 0.5 Q/sq/25 pm, or less than 0.1 Q/sq/25 pm, or less than 0.05 Q/sq/25 pm, or less than 0.01 Q/sq/25 pm. Optionally, the RFID antenna has a sheet resistance of from 5 0/sq/25 pm to 0.006 0/sq/25 pm, or from 3 Q/sq/25 pm to 0.006 Q/sq/25 pm, or from 1 Q/sq/25 pm to 0.006 Q/sq/25 pm, or from 5 Q/sq/25 pm to 0.06 Q/sq/25 pm, or from 3 Q/sq/25 pm to 0.06 Q/sq/25 pm, or from 1 Q/sq/25 pm to 0.06 0/sq/25 pm, or from 5 Q/sq/25 pm to 0.5 0/sq/25 pm, or from 3 0/sq/25 pm to 0.5 Q/sq/25 pm, or from 1 Q/sq/25 pm to 0.5 Q/sq/25 pm. Optionally the sheet resistance is at least 0.006 0/sq/25 pm. The sheet resistance values are given in units of ohms per square, normalised to 25 pm (0/sq/25 pm).

The use of functionalised graphene particles helps to achieve uniform dispersion throughout the polymer matrix material. This is important, since aggregates (clumps) of material may decrease the conductivity of the RFID antenna, such particles have a powerful tendency to agglomerate and are difficult to disperse in solvents and polymer materials.

In the present invention, the graphene particles comprise or consist of functionalised graphene particles, e.g. functionalised graphene or functionalised graphene nanoplatelets. That is, the graphene particles incorporate functional groups which improve the affinity of the nanoparticles for the solvents and/or polymer matrix material used to form the RFID antenna, thus allowing a more uniform distribution of particles to be achieved.

Preferably, the functionalised graphene particles are amine-functionalised, amidefunctionalised, pyrrole-functionalised, pyridine-functionalised, nitrogen functionalised, silane functionalised, mercapto-functionalised, vinyl-functionalised or halogen-functionalised.

More preferably, the functionalised graphene particles are amine-functionalised, amide-functionalised, pyrrole-functionalised, pyridine-functionalised or nitrogen functionalised.

Preferably, the graphene particles have a low oxygen content (less than 4 at%) and are nitrogen functionalised. The nitrogen functionality can be any suitable form such as amine, pyrrolic, pyridinic etc. If desired, other functionality could be incorporated. For example, the graphene particles may also be halogen functionalised. Other functionalities incorporating oxygen (such as hydroxy functionalisation) are considered unsuitable for the present invention.

Preferably, the functionalised graphene particles are plasma-functionalised graphene particles (i.e. particles which have been functionalised using a plasma-based process). Advantageously, plasma-functionalised graphene particles can display high levels of functionalisation, and uniform functionalisation.

Without being bound by any theory it is believed that amine groups help to boost conductivity and dispersion of the particles in the polymer matrix material, whilst helping to keep the material printable.

For the present purposes the degree of chemical functionalisation of the graphene particles is selected for effective compatibility at the intended loadings with the selected polymer matrix material. A typical upper limit is 21 at% nitrogen, because higher loadings indicate the presence of impurities or loss of sp2 carbon content (and therefore sub-optimal conductivity). A suitable lower limit is at least 3 at% of nitrogen, at least 5 at% of nitrogen, at least 10 at% of nitrogen, or at least 15 at% of nitrogen. Accordingly, appropriate ranges of nitrogen-functionalisation include nitrogen at 3-20 at%, such as 5-20 at% or 10-20 at%, preferably 5-19 at%, more preferably 10-18 at%. Other endpoints can be combined appropriately.

The inventors have also found that when graphene particles are prepared using agitation in low-pressure plasma, such as described in W02010/142953 and W02012/076853 and especially preferably PCT/EP2021/074727, they are readily obtained in a format enabling dispersion in solvents and subsequently in polymer matrices, or directly in polymer melts, at good uniformity and at levels more than adequate for the purposes set out above. This is in contrast to conventional processes for separating and functionalising graphene particles, which are extreme and difficult to control, as well as damaging to the particles themselves.

Specifically, the starting carbon material -especially graphitic carbon bodies -is subjected to a particle treatment method for disaggregating, de-agglomerating, exfoliating, cleaning or functionalising particles, in which the particles for treatment are subject to plasma treatment and agitation in a treatment chamber. Preferably the treatment chamber is a rotating container or drum. Preferably the treatment chamber contains or comprises multiple electrically-conductive solid contact bodies or contact formations, the particles being agitated with said contact bodies or contact formations and in contact with plasma in the treatment chamber.

The particles to be treated are carbon particles, such as particles which consist of or comprise graphite, or other nanoparticles.

Preferably the contact bodies are moveable in the treatment chamber. The treatment chamber may be a drum, preferably a rotatable drum, in which a plurality of the contact bodies is tumbled or agitated with the particles to be treated. The wall of the treatment vessel can be conductive and form a counter-electrode to an electrode that extends into an interior space of the treatment chamber.

During the treatment, desirably glow plasma forms on the surfaces of the contact bodies or contact formations.

Suitable contact bodies are metal balls or metal-coated balls. The contact bodies or contact formations may be shaped to have a diameter, and the diameter is desirably at least 1 mm and not more than 60 mm.

The pressure in the treatment vessel is usually less than 500 Pa. Desirably during the treatment, gas is fed to the treatment chamber and gas is removed from the treatment chamber through a filter. That is to say, it is fed through to maintain chemical composition if necessary and/or to avoid build-up of contamination.

The treated material, that is, the particles or disaggregated, deagglomerated or exfoliated components thereof resulting from the treatment, may be chemically functionalised by components of the plasma-forming gas, forming e.g. amine functionalities on their surfaces. Plasma-forming gas in the treatment chamber may be or comprise e.g. nitrogen, ammonia, amino-bearing organic compounds, halogen such as fluorine, halohydrocarbon such as CF4.

and noble gas. Most preferred is ammonia. Oxygen-functionalised materials, plasma-processed in oxygen, or oxygen-containing gas, are advantageously avoided for preparing materials according to the present invention.

Any other treatment conditions disclosed in the above-mentioned W02010/142953 and W02012/076853 and especially preferably PCT/EP2021/074727 may be used, additionally or alternatively. Or, other means of functionalising and/or disaggregafing carbon particles may be used for the present processes and materials, although we strongly prefer plasma-treated materials.

Preferably, the graphene particles comprise less than 10 ppm ferric impurities, preferably less than 1 ppm ferric impurities, most preferably no ferric impurities. Without being bound by any theory it is believed that ferric impurities may increase the likelihood of flames or sparking occurring in the microwave.

In the present invention, XPS is used to determine the extent (degree) of N functionalization i.e. nitrogen content.

XPS uses monochromatic x-rays to eject core electrons from surface atoms in a sample. These core electrons have specific and well-documented binding energies, which are affected by an atom's chemical environment. As the electrons are ejected from the sample, they are counted, and the kinetic energy measured. This results in peaks in the output spectrum. As each electron is from a single atom, XPS is quantitative. The peak areas can be fitted to give distributions of area within the peaks at different binding energies. Thus, XPS is qualitative as well as quantitative, giving highly detailed and accurate chemical information of a material's surface.

Any suitable XPS spectrometer can be used to determine nitrogen and oxygen content. Such methods are well within the purview of the skilled person. The inventors use a K-a X-ray photoelectron Spectrometer System using an aluminium X-ray source. The sample area is usually in the shape of an ellipse having a maximum width of 400 pm and a measuring depth of up to 9 nm.

Other methods of characterising the oxygen and nitrogen contents of the graphene particles may be used. W02015/150830 describes a method of characterising surface chemistry by monitoring changes in dispersion. Other measurements that can be made include zeta potentials, which correlate with the degree of nitrogen functionalization but do not show precisely the amount of nitrogen present in the sample. The inventors find that nitrogen-functionalised graphene particles having less than 4 at% of oxygen and at least 3 at% of nitrogen show a zeta potential at pH 3 of more than 3 my, such as at least 10 mV, at least 25 mV, at least 35 mV, preferably more than 40 mV.

The skilled person will be aware of suitable methods for measuring zeta potentials. An exemplary method involves dispersing 10 mg of functionalised graphene particles in 20 mL of pH 3 solution, adding aliquots of the dispersion in a cell which is then placed in a Malvern Zetasizer Nano-Z instrument. During the measurement, a potential difference is applied at either end of the cell and the voltage is measured and recorded. The results may then be

cross-referenced against a standard.

Similar to the zeta potential, measurement of the acid number can be used to confirm nitrogen funcfionalisation of the particles. The skilled person will be aware of suitable methods for measuring the acid number. An exemplary method involves measurement with a Mettler Toledo InMofion Pro fitrator and autosampler, where the sample is neutralised with potassium hydroxide and titrated against e.g. HCI (hydrogen chloride) giving the equivalence points of any acids present. In particular, the acid number for unfunctionalized graphene particles is typically a positive value, while nitrogen-funcfionalisation provides a negative acid number such as -0.10 or -0.15 mg.KOH/g.

The graphene particles of the present invention preferably have an oxygen content of less than 4 at%. Lower oxygen contents are believed to be even more advantageous from the perspective of improved conductivity, so preferred are graphene particles having an oxygen content of less than 2 at%, preferably less than 1.5 at%, more preferably less than 1 at% such as less than 0.5 at%.

Although it is possible to buy graphene particles having low oxygen content, commercially available graphene particles typically contain around 5 at% of oxygen even in the absence of treatments to specifically introduce oxygen. Such oxygen contents are undesirable for the present invention. Furthermore, it may be desirable to anyway reduce the oxygen content of the graphene particles starting material, to further enhance the benefits of the present invention. This can be achieved by any suitable process. In such process, it is necessary to remove moisture because the graphene particles can become oxygen functionalised in the presence of moisture.

An exemplary process that can be used to reduce oxygen content is annealing. Such may take place in argon plasma, to avoid the presence of moisture or oxygen from the air.

Effectively this constitutes a cleaning step in which high energy argon plasma is used to remove oxygen impurities from the surface of the graphene.

As the skilled person will be aware, annealing is a process of heating to a predetermined temperature for a predetermined length of time, followed by slow cooling. In the present case, annealing may be used to achieve reduced oxygen content of the graphene particles. The skilled person can determine suitable conditions, but heating to a temperature of e.g. 600-1000 °C such as 850 °C for 1-5 hours followed by cooling for 1-5 hours might be suitable. Such conditions have been found to have only a small effect on the sp2 carbon content as determined by XPS.

Preferably, the sp2 carbon content of the functionalised graphene particles is at least 65 at%, such as at least 70 at% or more.

The inventors believe that annealing before nitrogen treatment may remove oxygen and restore sp2 carbon, while heating during and after the treatment removes volafiles including any potential NOR.

It is generally preferable to nitrogen-functionalise graphene particles which already have the required low oxygen content, to maximise the available carbon for funcfionalising. The annealing can be carried out before, partway through (such as midway through), or after the plasma functionalisation and can involve the use of a furnace or the use of a heated reactor barrel as in patent application number PCT/EP2021/075691 (not yet published).

For example, annealing can be carried out first to 'clean' a sample by removing oxygen, moisture and other impurities. This is carried out under argon (or other inert gas, such as hydrogen or tetrafluoromethane). That process is followed by nitrogen functionalisation, followed by annealing again, if wanted.

Trace metals In certain embodiments, the RFID antenna according to the present invention comprises low levels of (or no) metal, these embodiments have been developed to ensure optimum microwave safety. Without being bound by any theory, it is believed that RFID antennae with low levels of metals do not cause flames and sparking in the microwave even at high powers and over extended cooking times.

In certain such embodiments, the RFID antenna is substantially metal free. This is defined as an RFID antenna comprising less than 0.1 wt.% metal based on the total weight of carbon filler, preferably less than 0.01 wt.% metal based on the total weight of carbon filler, most preferably an RFID antenna comprising only trace amounts of metal. The RFID antenna according to this embodiment is designed to be highly resistant to heating in a microwave.

In an alternative embodiment, the RFID antenna comprises low levels of metals. In these embodiments, the functionalised graphene particles are optionally decorated with metal. The RFID antenna preferably comprises functionalised graphene particles comprising less than 3 at.% metal, preferably less than 2.5 at.% metal, most preferably less than 2 at.% metal, based on the total weight of the graphene particles. The functionalised graphene particles preferably comprise from 0.1 to 2.0 at% metal based on the total weight of the graphene particles. Without being bound by any theory, it may be advantageous to include low levels of metals such as silver and copper in the graphene particles as this is believed to increase the read range of the RFID antenna. Consequently, the graphene particles may comprise from 0.1 to 2.0 at.% silver, based on the total weight of the graphene particles. Alternatively, the graphene particles may comprise copper, for example from 0.1 to 2.0 at.% copper, based on the total weight of the graphene particles. Alternatively, the graphene particles may comprise both silver and copper with the total amount of silver and copper being from 0.1 to 2.0 at.% based on the total weight of the graphene particles. Without wanting to be bound by any theory it is believed that these metals are present at the surface of the particles and are present at such low levels that they do not substantially affect the microwave safety of the RFID antenna. The graphene particles used in this embodiment may be functionalised in a glow plasma functionalisation method as described in PCT/EP2021/074727. In particular, silver and copper may be delivered into the plasma treatment vessel as a solid using a reagent dosing controller as described in PCT/EP2021/074727 and may be used to decorate the graphene particles. Alternatively, the metal can be provided as a powder and mixed into the carbon filler material. Preferably the metal particles are nanoparticles with a mean diameter measured using a light scattering analyser of from 10 to 500 nm, preferably from 10 to 200 nm, more preferably from 10 to 100 nm.

In an alternative embodiment, the RFID antenna may comprise modest amounts of metals in order to increase the read range of the RFID antenna. These RFID antennae may have read ranges of at least 6 metres, preferably at least 8 metres, preferably at least 10 metres, preferably at least 15 metres, preferably at least 18 metres, most preferably at least 20 metres. Said RFID antenna are microwave resistant but may not be able to withstand extended periods of microwaving. In these embodiments, the RFID antenna preferably comprises from 1 to 20 wt.% metal based on the total weight of carbon filler, preferably from 1 to 10 wt.% metal based on the total weight of the carbon filler. In certain such embodiments, the RFID antenna may comprise from 1 to 20 wt.% silver based on the total weight of the carbon filler, preferably from 1 to 10 wt.% silver based on the total weight of carbon filler. Alternatively, the RFID antenna may comprise from 1 to 20 wt.% copper based on the total weight of the carbon filler, preferably from 1 to 10 wt.% copper based on the total weight of carbon filler. Alternatively, the RFID antenna may comprise copper and silver with the total amount of silver and copper being from 1 to 20 wt.% based on the total weigh of carbon filler, preferably from 1 to 10 wt.% based on the total weigh of carbon filler.

The metals are preferably provided as metal powders, which means that they are printable using standard printing techniques such as screen printing. Chemical etching processes are not required. Preferably the metal particles are nanoparticles with a volume average mean diameter measured using a light scattering analyser of from 10 to 500 nm, preferably from to 200 nm, more preferably from 10 to 100 nm.

Polymer matrix material Suitably, the polymer matrix material of the RFID antenna is an elastic material. Preferably, the polymer matrix material is flexible as this means that the RFID antenna is flexible and is suitable for use with flexible packaging items, such as sachets.

Suitable materials include, for example, vinyl polymers (including polymers or copolymers of vinyl chloride, vinyl acetate and vinyl alcohol), polyester polymers, phenoxy polymers, epoxy polymers, acrylic polymers, polyamide polymers, polypropylenes, polyethylenes, silicones, elastomers such as natural and synthetic rubbers including styrene-butadiene copolymer, polychloroprene (neoprene), nitrile rubber, butyl rubber, polysulfide rubber, cis-1,4-polyisoprene, ethylene-propylene terpolymers (EPDM rubber), and polyurethane (polyurethane rubber).

The polymer matrix material may be a thermoplastic material. Alternatively, the polymer matrix material may be a thermosetting material.

The polymer matrix material may comprise or be polyurethane, for example a thermoplastic polyurethane elastomer.

Preferably, the polymer matrix material is a copolymer of vinyl chloride, vinyl acetate and/or vinyl alcohol or a polyamide, a polyvinylidene fluoride (PVDF) or a fluoropolymer resin. Most preferably, the polymer matrix material is a copolymer of vinyl chloride and vinyl acetate. Without wanting to be bound by any theory, the present inventors have found that using polyvinyl chloride polyvinyl acetate copolymers as the polymer matrix material produces antennae with good mechanical properties, in particular a good level of flexibility. These materials can also be tuned so that they have melting points above 100 °C and therefore do not melt when heated in the microwave.

Examples of copolymers of vinyl chloride and vinyl acetate include the Vinnola series manufacture by Wacker Chemie AG, for example Vinnole 40/60.

Preferably, the polymer matrix material is selected so that the mechanical properties of the matrix material (e.g. flexibility) do not degrade at temperatures below 0 °C, meaning that the RFID tags are also suitable for use on packaging for frozen products.

Substrate Preferably, the RFID antenna is coated or printed onto a substrate. The substrate used is not particularly limited, meaning that the RFID antenna can be printed onto a range of packaging 20 materials.

The substrate may be plastic, paper, cardboard, fabric, wood, glass, insulated metal, elastomers, leather, textiles or ceramic (e.g. porcelain). Preferably, the substrate is selected from plastic, paper or cardboard. Most preferably, the substrate is paper, for example a piece of paper (or cardboard) packaging.

Alternatively, the substrate may be a sticker that is a piece of paper, plastic or vinyl with a pressure sensitive adhesive on one side (the opposite side to that on which the RFID tag is printed). The pressure sensitive adhesive may be selected from the group of rubber adhesives and acrylic adhesives.

In certain embodiments, the RFID antenna is printed directly onto the substrate which forms part of the packaging article for end use. For example, the RFID may be printed directly onto paper packaging. Alternatively, the RFID antenna may be printed onto a paper substrate and then adhered to an article such as a onto plastic packaging, for example the RFID antenna may be printed onto a sticker and then adhered to the packaging material. The use of stickers is particularly preferred as they present an easy and practical way for shops and manufacturers to attach RFID tags to their products.

Multilayer structure Preferably, the RFID antenna has a multilayer structure.

By "multilayer structure" generally a multilayer stacked structure is meant.

Preferably, the RFID antenna has a multilayer structure comprising - at least one layer comprising a carbon filler dispersed in a polymer matrix material as defined above; and - a layer of electrically insulating material.

Preferably, the layer of electrically insulating material is made from a polymer selected from the group of vinyl polymers (including polymers or copolymers of vinyl chloride, vinyl acetate and vinyl alcohol), polyester polymers, phenoxy polymers, epoxy polymers, acrylic polymers, polyamide polymers, polypropylenes, polyethylenes, silicones, elastomers such as natural and synthetic rubbers including styrene-butadiene copolymer, polychloroprene (neoprene), nitrile rubber, butyl rubber, polysulfide rubber, cis-1,4-polyisoprene, ethylene-propylene terpolymers (EPDM rubber), and polyurethane (polyurethane rubber). Most preferably, the layer of electrically insulating material is made from a vinyl polymer.

Preferably the layer of electrically insulating material is at least 20 pm thick.

More preferably, the RFID antenna has a multilayer structure comprising the following layers in the following order: Layer (A): a substrate Layer (B): a first layer of electrically insulating material; Layer (C): a layer comprising a carbon filler dispersed in a polymer matrix material according to the present invention; and Layer (D): a second layer of electrically insulating material.

Preferably, layers (B) and (D) are formed from an elastic material, e.g. an elastic polymer. Preferably, the elastic polymer has a melting point of over 100 °C, most preferably over 150 °C.

The material of layers (B) and (D) may be selected independently from epoxy resin, vinyl polymers (including polymers or copolymers of vinyl chloride, vinyl acetate and vinyl alcohol), polyester polymers, phenoxy polymers, epoxy polymers, acrylic polymers, polyamide polymers, polypropylenes, polyethylenes, silicones, elastomers such as natural and synthetic rubbers including styrene-butadiene copolymer, polychloroprene (neoprene), nitrile rubber, butyl rubber, polysulfide rubber, cis-1,4-polyisoprene, ethylene-propylene terpolymers (EPDM rubber), and polyurethane (polyurethane rubber). Preferably, layers (B) and (D) are made from a copolymer of vinyl chloride, vinyl acetate and/or vinyl alcohol. Most preferably, layers (B) and (D) comprise or are the same material as the polymer matrix material described above.

Layer (B) (the first layer of electrically insulating material) may be coated (e.g. printed) onto the substrate. However, preferably layer (B) comprises an electrically insulating material and a pressure sensitive adhesive, which is adhered to the substrate. The pressure sensitive adhesive may be selected from the group consisting of rubber adhesives and acrylic adhesives. Examples of pressure sensitive adhesives include 3M fast tack water based adhesive 1000nF, 3M Fastbond Contact Adhesive 49, 3M Neoprene High performance contact adhesive 1357. Without being bound by any theory, it is preferred that when the substrate is paper the layer comprising the carbon filler is encapsulated in an electrically insulating layer, which is resistant to high temperatures. It is believed that this encapsulation helps to prevent the paper substrate from sparking or causing flames when the RFID tag is heated in a microwave.

Preferably, layer (D) comprises an electrically insulating material and a pressure sensitive adhesive, which is adhered to layer (C). The pressure sensitive adhesive may be as described above for layer (B).

The first and second layers of electrically insulating material (layers (B) and (D)) may be doped with boron nitride. Without being bound by any theory, it is believed that doping with boron nitride helps to increase the thermal conductivity of the electrically insulating layers helping the heat to be dispersed from the RFID antenna.

The RFID antenna having a multilayer structure may also have intermediate layers between layers (A) (B), (C) and (D), such as intermediate layers of adhesive.

Most preferably, the RFID antenna further comprises: Layer (E): a layer of paint.

Layer (E) can be used to disguise the RFID tag, so that it is less obvious that it is present on packaging. This may be advantageous as it can help to make the product packaging look more uniform and hence more attractive, it can also help to disguise the RFID tag from thieves who may want to remove it in order to avoid electronic article surveillance systems in shops.

In an alternative embodiment, layer (B) may be omitted from the multilayer structure described above. The RFID antenna, consequently, has a multilayer structure comprising: Layer (A): a substrate; Layer (C): a layer comprising a carbon filler dispersed in a polymer matrix material according to the present invention; and Layer (D): a layer of electrically insulating material.

This may be preferred when the substrate itself is a plastic material as it can save on manufacturing costs and means that the structure of the RFID is simplified.

In an alternative embodiment, layer (D) may be omitted from the multilayer structure described above. Therefore, the RFID antenna has a multilayer structure comprising: Layer (A): a substrate; Layer (B): a layer of electrically insulating material; and Layer (C): a layer comprising a carbon filler dispersed in a polymer matrix material according to the present invention.

Size and shape The size and shape of the RFID antenna is not particularly limited.

The RFID antenna may be a 3D RFID antenna printed or applied to a 3D substrate; however, preferably the RFID is a 20 RFID antenna printed or applied onto a flat substrate. Without being bound by any theory, 2D antennae are preferred as they can be more easily applied to the flat surfaces of food packaging.

Preferably, the RFID antenna has a surface area of less than 70 cm2, preferably less than 60 cm2, preferably less than 50 cm2, preferably less than 40 cm2, preferably less than 30 cm2, preferably less than 20 cm2, preferably less than 15 cm2. Without being bound by any theory, it is believed that by having a smaller surface area the RFID antenna absorbs less microwave radiation and consequently, is less prone to becoming damaged when heated in the microwave. Preferably the maximum thickness of the traces in the RFID antenna are less than 30 mm, preferably less than 10 mm, preferably less than 7 mm, most preferably less than 5 mm. Without being bound by any theory, it is believed that minimising the thickness of the antenna traces reduces the chance of the antenna being damaged due to excessive heat build-up during microwave cooking.

Preferably, the average thickness of the RFID antenna is 300 pm or less, more preferably 200 pm or less, most preferably 150 pm. Without being bound by any theory, it is believed that having thinner RFID antennae helps to make the RFID antennae more difficult to detect meaning that they are less likely to be discovered and removed by thieves seeking to evade electronic article surveillance systems in shops.

The shape of the antenna is not particularly limited and may be adapted for the specific applications, for example printed so that it fits neatly onto a label. Examples of possible shapes of the RFID antenna are shown in figures 6 and 7.

Alternatively, the RFID antenna may take the form of a single loop (i.e. a single rectangular shape with rounded edges) or multiple loops. The RFID antenna may also take the form of at least one serpentine track radiating from a central point. Alternatively, the RFID antenna may be a planar structure of conductive antenna components in a spiral type configuration or any other suitable configuration known in the art. The planar structure may be a rectangle, circle, square or triangle. The planar structure with a spiral type configuration creates a plurality of gaps between the conductive antenna components. Additionally, a gap in the conductive antenna component itself is where the RFID chip may be positioned such that the RFID chip is placed into the coil of the conductive antenna component. The centre of the RFID tag device and the outer edge of the planar structure of the conductive antenna components may be bridged together with a conductive trace creating an inductor (or bridge conductor) across the RFID tag and resonating at the wanted frequency. Further possible shapes for the RFID antennae are described in WO 2020/006219.

Preferably, the RFID antenna takes the form of 2 serpentine tracks radiating from a central point. Alternatively, the RFID antenna may take the form of multiple U-shaped tracks radiating out from a central line. Preferably, the traces of the antenna are narrow, for example having a width of less than 2 mm, or less than 1 mm, or less than 0.5 mm, or less than 0.25 mm. Without being bound by any theory it is believed that having traces with a width of less than 2 mm allows better operation of the RFID antenna in the microwave.

Preferably, the RFID antenna has a smooth shape. A skilled person will appreciate that most metals consist of lattice arrangements of atoms with electrons freely floating around between them. Microwave radiation may attract these electrons as it bounces around inside the microwave pulling them back and forth. Without being bound by any theory, it is believed that when metal objects with kinks or dead ends are placed in the microwave, they will cause sparks. This is because as the electrons get shuffled back and forth they will meet up with other electrons. This can create concentrated spots of negative charge. Electrons are naturally repelled from areas where there is a build-up of negative charge. If these negative spots find themselves in a place where they are near air, the electrons will jump away creating a spark and ionizing the air molecules into a plasma. Consequently, without being bound by any theory it is important to avoid RFID patterns with sharp edges in order to avoid sparking in the microwave.

RFID taq The present invention also relates to an RFID tag comprising the RFID antenna as described in the sections above. Preferably, the RFID tag comprises an RFID antenna according to the present invention and a microchip. The microchip is preferably electronically connected to the RFID antenna.

The RFID tags according to the present invention may be passive, battery assisted passive or active RFID tags. Preferably the RFID tag is a passive RFID tag. As mentioned above, generally passive RFID tags comprise two components a microchip and an antenna, which are both attached to the substrate. Generally, these RFID tags function as follows: a reader sends energy to the antenna which converts that energy into a radio frequency wave that is sent into the read-zone. Once the tag is within the read-zone, the RFID tag's internal antenna draws in energy from the radio frequency waves. This energy powers the chip which generates a signal (radio frequency wave) back to the reader. This is called backscatter. The backscatter, or change in electromagnetic of RE wave, is detected by the reader (via the antenna) which interprets the information.

The frequency range of the RFID tag is typically from 860 -960 MHz.

Preferably, the RFID tag according to the present invention has a range of at least 1 metre, preferably at least 1.5 metres, preferably at least 2 metres, preferably at least 6 metres, most preferably at least 8 metres.

The microchip typically consists of an integrated circuit which stores and processes information and modulates and demodulates radio-frequency signals. Generally, the tag information is stored in non-volatile memory on the chip. Preferably, the tag is a read only RFID tag.

The microchip used in combination with the RFID antenna is not particularly limited and in theory any commercially available microchip suitable for use with an RFID tag may be used. Examples of commercially available microchips are manufactured by Zebra, Alien Technology, Avery Dennison, Checkpoint systems, Tyco Retail solutions and Smartrac.

Generally, the microchip consists of a plurality of integrated circuits on a silicon chip. Further possible configurations for RFID tags are described in WO 2020/006219. Use

The present invention also relates to the use of a printed carbon filler dispersed in a polymer matrix material as an RFID antenna, wherein the carbon filler comprises functionalised graphene.

Any of the limitations described above for the carbon filler used in the RFID antenna also apply for the use described above.

In addition, the present invention relates to the use of an RFID tag according to the present invention as a label on a piece of packaging. For example, for use as a label on a piece of microwaveable packaging.

The RFID tag may be used to enable faster self-checkout procedures, as customers can simply stack all of the items on top of a reader at the checkout, which is able to detect all of the RFID tags on the items without them having to be in the line of sight of the reader. This avoids the added complications associated with having to manually scan a barcode.

The RFID tag may also be used as part of an electronic article surveillance system to prevent shoplifting as described above.

The RFID tag may also be used to facilitate check-out free shopping, such as Amazon Go® stores. In these systems, reader panels may be mounted at "gates" at the exit to the shop.

When a customer passes through the gate when leaving the shop, the reader panels identify the RFID tags on the items they are carrying with them and automatically take payment for these items from a registered credit card.

In addition, the RFID tags may be used to track products during production and to monitor where a particular product is in the production line. This helps to facilitate monitoring of the production date, weight, best before date, ingredients etc of the product.

The tags may also have applications for the consumer. For example, the RFID tags may be used in combination with a microwave which is able to read the RFID tag and which comprises software and hardware which can determine how long the item needs to be cooked for and at which settings. Therefore, in a certain aspect this invention also relates to the use of an RFID tag as part of a system for the automatic cooking of a food product.

Article The present invention also relates to an article comprising an RFID tag as described above.

In a certain embodiment, the article is a sticker or label.

In an alternative embodiment, the article comprising the RFID tag is a piece of packaging, for example packaging for food and drinks, packaging for toys, packaging for household products, packaging for cosmetics or toiletries, packaging for cleaning products, packaging for clothing, packaging for components of motor vehicles and accessories, packaging for sports equipment, packaging for furniture, packaging for appliances, packaging for medical supplies, packaging for scientific equipment, packaging for IT equipment and telecommunications equipment. This also includes embodiments where the RFID tag is applied to the packaging as a sticker or label.

Preferably the article refers to packaging for food. More preferably the article is microwaveable packaging for food. Within the meaning of this invention the term "food" is also considered to encompass drinks.

Preferably, the article is a food product comprising an inner plastic tray bearing the RFID tag and outer cardboard packaging surrounding the inner plastic tray. In this embodiment, it is particularly advantageous that the RFID tag should be attached to the inner plastic tray as it means that thieves are not able to avoid detection by in-shop electronic surveillance systems simply by removing the outer cardboard packaging and exiting the shop with the food item in the inner plastic tray. It also means that potential thieves are less likely to be aware of the RFID tag.

In addition, this may mean that the product can be tracked throughout the production process using the same RFID tag, which is attached to the plastic tray.

The present invention also relates to RFID tags which may be attached to the packaging of products which are designed to be microwaved directly at the checkout. This may be advantageous for example for ready meals for long distance lorry drivers, who may want to be able to buy hot food at petrol stations and who do not have facilities to heat up food in their lorries. For these applications it is desirable to be able to place the whole of the packaged product in the microwave without having to remove outer layers of packaging.

Having the RFID tag directly attached to the packaging also means that the packaging can be used to communicate automatically with a microwave as described below.

In certain embodiments, the article may be a box for packaging microwaveable food. Alternatively, the article may be a sleeve or plastic sachet for microwaveable food.

The RFID tag may be applied directly onto the article itself, for example when the article is a cardboard box, the cardboard box may be used as the substrate. Alternatively, the RFID tag may be printed onto a separate substrate and then attached to the packaging using an adhesive, for example using an adhesive tape.

Although, the present invention primarily relates to food packaging, this invention is also intended to encompass articles consisting of packaging for medical or scientific equipment which can be sterilised using microwave radiation. Therefore, the present invention also relates to an article comprising an RFID tag according to the present invention, wherein the article is a piece of microwaveable packaging for medical or scientific equipment.

Manufacturing methods In a further aspect, the present invention provides a method of manufacturing an RFID antenna according to the present invention, wherein the method comprises the following steps: Step 1: providing funcfionalised graphene particles; Step 2: combining the graphene particles with a polymer matrix material to form an ink; Step 3: depositing the ink onto a substrate to form the RFID antenna.

The step of depositing one or more layers of a conductive material over the substrate preferably involves depositing (coating) a conductive ink onto the substrate. Suitable deposition techniques include, for example, bar coating, screen printing (including rotary screen printing), flexography, rotogravure, inkjet, pad printing, and offset lithography. The conductive ink comprises the functionalised graphene particles dispersed in a solvent and polymer material.

When multiple layers of conductive ink are printed, each layer is preferably dried before a subsequent layer is added. The device may be heated after the application of each conductive ink layer to speed up drying of the ink.

When using a conductive ink, the method preferably involves a step of preparing the ink for printing. This preparation step may involve mixing or homogenising the ink to evenly distribute the graphene particles in the ink's polymer binder. Preferably, the preparation step involves homogenising the ink, since the inventors have found that this ensures a uniform distribution of carbon nanoparticles and can help to break up agglomerates of nanoparticles in the ink. Suitable homogenisation can be achieved using, for example, a three roll-mill or rotor-stator homogeniser.

Preferably, the viscosity of the ink formed in step 2 is not more than 600 mPa s-1 or not more than 500 mPa s-1 measured at 20 uC using a dynamic sheer rheometer (Kinexus DSR from Nesus analytics) Preferably the ink formed in step 2 has a viscosity value of more than 5 mPa s-1, or more than 10 mPa s-1, or more than 20 mPa The method above may also involve applying an electrically-insulating covering layer over the layer comprising graphene particles. Such an electrically-insulating covering layer may have any of the features described above in relation to the earlier aspects of the invention.

Preferably, the electrically-insulating covering layer is provided in the form of a sheet which is attached to the layer comprising graphene particles using an adhesive.

The functionalised graphene particles in step 1 of the method above may be provided according to the following method: a. providing non-functionalised graphene particles; b. plasma functionalising the graphene particles to obtain functionalised graphene particles, c. optionally mixing the functionalised graphene particles with non-functionalised graphene particles.

In certain embodiments, the present invention involves in step 1 providing two sets of graphene particles with different average particle diameters. One set of particles may have an average diameter of 5 pm (determined by light scattering) or less (A-1) and one set of particles may have an average diameter of 8 pm of more (A-2). The weight ratio of (A-1) to (A-2) may be 1:14 to 2:1, more preferably from 1:10 to 3:2, more preferably from 1:7 to 1:1.

The The volume ratio of (A-1) to (A-2) may be from 2:1 to 1:2 and is preferably around 1:1.

Alternatively, the present invention may involve in step 1 providing graphene particles and an additional conductive carbon filler. The additional conductive carbon filler may be selected from the group consisting of carbon black, acetylene black (ACB), carbon nanorods or graphite (e.g. graphitic platelets), carbon nanotubes or a combination of these materials.

Preferably the additional carbon filler is carbon black. Graphene and carbon black may be combined at a weight ratio of 1:14 to 2:1, more preferably from 1:10 to 3:2, more preferably from 1:7 to 1:1. The volume ratio of graphene to carbon black may be from 2:1 to 1:2 and is preferably about 1:1.

The present invention also relates to an RFID antenna obtainable by the given above.

In a further aspect, the present invention provides a method of manufacturing an RFID tag which comprises manufacturing an RFID antenna as described above followed by the additional steps of - providing a microchip; -mounting the microchip onto the substrate so that it is in electrical contact with the RFID antenna.

The present invention also relates to an RFID tag obtainable by the method above.

Microwave In a further aspect, the present invention relates to a microwave capable of reading an RFID tag according to the present invention.

The microwave may comprise software and/or hardware configured to be run by an external application ("app").

"Software" means a set of instructions that when installed on a computer configures that computer with the readiness to perform one or more functions. The terms "computer program," "application" and "app" are synonymous with the term software herein.

The microwave may be able to determine the type of food in the packaging from the RFID tag and to determine the required cooking program (e.g. time and temperature) required to heat up the food item.

The microwave may also be able to monitor "use by" dates on the food items and to automatically alert a user if a food is past its use by date and could therefore present a risk to the user if it is eaten. This feature is particularly useful when, for example, the printed-on information containing the product "use by" or "consume by" date is no longer readable to the human eye, or gets separated from the food product.

In addition, the software may be able to alert users to particular allergens present in food for example nuts, shellfish or gluten and warn users before they consume food products containing these allergens Preferably, the microwave is configured to be able to interact with an external computer or server such as via the internet or blue tooth connectivity. This may allow the microwave to access databases of recipes present on a server or to check product information in a database on the server. This may also allow manufacturers to communicate with end users, for example in the case of a product recall.

Preferably, the microwave is connected to the so-call internet of things. This may also allow remote control of the microwave, for example via a smartphone app.

The RFID tag according to the present invention may also comprise some form of sensor.

For example, a temperature sensor that can indicate if the RFID product is thawed, chilled or frozen, or a moisture sensor. The microwave can then select an appropriate cooking method based on the RFID data and the sensor data, for example based on whether the food item is, for example, already thawed, chilled or frozen. The output from the sensor could be used to instruct the microwave to first thaw the food product at one microwave power setting, and then cook the food product at a different power setting.

Preferred embodiments Particularly preferred embodiments include: An RFID antenna comprising a carbon filler dispersed in a polymer matrix material, the carbon filler including functionalised graphene particles according to the present invention, wherein the graphene particles are amine-functionalised, amide-functionalised, pyrrole-functionalised, pyridine-functionalised or nitrogen functionalised, wherein the graphene particles have an oxygen content of less than 4 at%; and wherein the RFID antenna comprises an electrically insulating covering layer.

Preferably, the RFID antenna has a nitrogen content of at least 3 at% based on the total weight of carbon filler.

Preferably, the polymer matrix material is a copolymer of vinyl chloride, vinyl acetate and/or vinyl alcohol or a polyamide, a polyvinylidene fluoride (PVDF) or a fluoropolymer resin.

Preferably, the electrically insulating covering layer is made from a vinyl polymer, most preferably a copolymer of vinyl chloride and vinyl acetate.

In a further particularly preferred embodiment the RFID antenna comprises -50-80 wt.% polymer matrix; and - 1 -20 wt.% carbon filler; wherein the carbon filler comprises or consists of functionalised graphene particles and wherein the functionalised graphene particles have a multimodal size distribution.

Optionally, the RFID antenna comprises less than 5 wt.%, preferably less than 3 wt.%, preferably less than 1 wt.% of graphite and carbon black based on the total weight of carbon filler and polymer matrix material.

Preferably, the polymer matrix material is a copolymer of vinyl chloride, vinyl acetate and/or vinyl alcohol or a polyamide, a polyvinylidene fluoride (PVDF) or a fluoropolymer resin.

Preferably, the functionalised graphene particles are amine-functionalised, amidefunctionalised, pyrrole-functionalised, pyridine-functionalised or nitrogen functionalised.

This embodiment may be combined with all of the preferred embodiments described above In a further preferred embodiment, the present invention relates to a method of manufacture of a RFID antenna according to the present invention comprising the steps of: Step 1: providing functionalised graphene particles; Step 2: combining the graphene particles with a polymer matrix material to form an ink; Step 3: printing the ink onto a substrate to form the RFID, wherein the functionalised particles in step 1 are provided according to the following method: a. providing non-functionalised graphene particles; b. plasma funcfionalising the graphene particles to obtain functionalised graphene particles, c. optionally mixing the functionalised graphene particles with non-functionalised graphene particles, d. providing an additional conductive carbon filler, wherein the additional conductive carbon filler is selected from the group of carbon black and/or graphite.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagram of an RFID tag.

Figure 2 is a diagram showing the layers in a 3 layer multilayered RFID antenna.

Figure 3 is a diagram showing the layers in a 4 layer multilayer RFID antenna.

Figure 4 is a diagram of a piece of food packaging with the RFID attached to the inner food tray.

Figure 5a is an RFID antenna according to the invention encapsulated with a layer of polyvinyl chloride polyvinyl acetate copolymer or epoxy with a thickness of up to 30 pm, which has been microwaved for 5 seconds.

Figure 5b is an RFID antenna according to the invention encapsulated with a layer of polyvinyl chloride polyvinyl acetate copolymer or epoxy with a thickness of up to 60 pm, 10 which has been microwaved for 5 seconds.

Figure 6 is an example of an RFID antenna according to the present invention printed onto a paper substrate.

Figure 7 is a further example of an RFID antenna according to the present invention printed onto a paper substrate.

DETAILED DESCRIPTION

Figure 1 shows a typical configuration for an RFID tag according to the present invention.

The RFID comprises an antenna 2, which is printed onto a substrate 1. The antenna 2 is attached to a microchip 3, which comprises the memory and operating circuitry necessary for the RFID tag to function.

Figure 2 is a diagram of a 3-layer RFID antenna according to the present invention. The RFID antenna comprises a first substrate layer 14, which is preferably plastic, paper or cardboard and may be part of the packaging material to which the RFID antenna is attached or alternatively a separate label or sticker. A layer 13 comprising a carbon filler dispersed in a polymer matrix material according to the present invention is deposited or printed on top of the substrate layer 14. The layer 13 comprising the carbon filler dispersed in the polymer matrix material is encapsulated by an electrically insulating layer 11. The electrically insulating layer is optionally attached to the layer 13 comprising the carbon filler dispersed in a polymer matrix material by a layer of pressure sensitive adhesive 12.

Figure 3 is a diagram of a 4-layer RFID antenna according to the present invention. As for Figure 2, the RFID antenna comprises a first substrate layer 26, which is preferably plastic, paper or cardboard and may be part of the packaging material to which the RFID antenna is attached or may alternatively be a separate label or sticker. A first electrically insulating layer, 24, is attached to the substrate. This is preferably a layer of polyvinyl material. The first electrically insulating layer, 24, is optionally attached to the substrate by a first layer of pressure sensitive adhesive, 25. A layer comprising carbon filler dispersed in a polymer matrix material according to the present invention, 23, is deposited or printed on top of the first electrically insulating layer 24. A second electrically insulating layer 21, is then attached to the layer comprising carbon filler dispersed in a polymer matrix material. The second electrically insulating layer 21, is optionally attached to the layer comprising carbon filler dispersed in a polymer matrix material by a layer of pressure sensitive adhesive 22.

Figure 4 is a diagram of an article according to the present invention. The article is specifically a piece of food packaging. The article consists of an inner plastic tray 32, bearing the RFID tag 33, and outer cardboard packaging 31, surrounding the inner plastic tray. The RFID is preferably applied to the inner plastic tray rather than the outer cardboard packaging, as this means that thieves are not able to avoid detection by in-shop electronic surveillance systems by removing the outer cardboard packaging and exiting the shop with the food item in the inner plastic tray. In addition, this may mean that the product can be tracked throughout the production process using the same RFID tag, which is attached to the plastic tray.

Figures 6 and 7 are both examples of RFID antennae according to the present invention printed onto paper substrates. These antennae are approximately 5 x 2 cm in size. Both of these antennae were able to successfully function as RFID tags and were not affected by microwaves.

EXAMPLES

Experiment 1 In the first set of experiments, the effect of encapsulation was assessed.

A composition comprising amine functionalised graphene particles according to the claims was combined with a polymer matrix material and stirred.

RFID antennae were then printed onto PET substrates.

One RFID antenna was then encapsulated with a layer of polyvinyl chloride polyvinyl acetate with a thickness of 30 pm (thin). A further RFID antenna was encapsulated with a layer of polyvinyl chloride polyvinyl acetate with a thickness of 60 pm (thick).

The samples were then microwaved for 5 seconds at a power of 800 -1000 W. Visually, it was observed that the sample with encapsulation of 30 pm began to fail after 5 seconds of microwaving, whereas the sample with encapsulation of 60 pm was more resistant to microwaving for 5 seconds. See figures 5a and 5b.

Experiment 2 In a second set of experiments, the effect of freezing a printed RFID antenna was assessed.

An RFID antenna was prepared in the same way as for experiment 1.

The sheet resistance of the RFID antenna was measuring using a resistance meter at room temperature.

The sample was then frozen at ca -18°C for 18 hours.

The RFID antenna was then allowed to return to room temperature and the sheet resistance was measured again using a resistance meter.

The results are given in table 1 Table 1: Comparison of the resistance of a freshly printed RFID antenna and a RFID antenna after freezing Sample Description Measured Sheet Resistance Dried Film Thickness Normalised Resistance @ 25pm Pre-freezing 26.6 0/sq 8 pm 8.7 0/sq Post-freezing 24.2 0/sq 8 pm 7.7 0/sq The results in table 1 demonstrate that freezing the RFID antenna does not lead to an increase in sheet resistance.

It was also determined that the frozen sample retains high flexibility. This was tested by: freezing the sample at -18 °C; whilst the sample was still frozen, manually folding the sample piece by 180 ° once; -testing to see if there is any break in electrical conductivity in the sample (no break = high flexibility).

Claims (24)

1. CLAIMS1. An RFID antenna comprising a carbon filler dispersed in a polymer matrix material, the carbon filler including functionalised graphene particles.
2. 2 The RFID antenna according to claim 1, wherein the functionalised graphene particles are amine-functionalised, amide-functionalised, pyrrole-functionalised, pyridine-functionalised, nitrogen functionalised, silane functionalised, mercaptofunctionalised, vinyl-functionalised or halogen-functionalised.
3. 3 The RFID antenna according to claim 2, wherein the functionalised graphene particles are amine-functionalised, amide-functionalised, pyrrole-functionalised, pyridine-functionalised or nitrogen functionalised.
4. 4. The RFID antenna according to any one of the preceding claims, wherein the functionalised graphene particles have a nitrogen content of at least 3 at%.
5. 5. The RFID antenna according to claim 4, wherein the functionalised graphene particles have a nitrogen content of at least 5 at% and/or wherein the functionalised graphene particles have a nitrogen content of no more than 21 at%.
6. 6. The RFID antenna according to claim 5, wherein the functionalised graphene particles have a nitrogen content of between 10 at% and 20 at%.
7. 7 The RFID antenna according to any one of the preceding claims, wherein the oxygen content is less than 4 at%, or less than 2 at%, or less than 1.5 at%, or less than 1 at%, or less than 0.5 at%.
8. 8 The RFID antenna according to any one of the preceding claims, wherein the RFID antenna has a metal content of less than 3 at% based on the total weight of the graphene particles, preferably wherein - the metal content is from 0.1 to 2.0 at% based on the total weight of the graphene particles; or - wherein the RFID antenna is substantially metal free. 35
9. 9. The RFID antenna according to any one of claims 1 to 7, wherein the RFID antenna comprise from 1 to 20 wt.% metal based on the total weight of the carbon filler.
10. 10. The RFID antenna according to any one of the preceding claims, wherein the functionalised graphene particles are graphene nanoplatelets, optionally having an average of 2 to 5 graphene layers per particle.
11. 11. The RFID antenna according to any one of the preceding claims, wherein the carbon filler has a multimodal size distribution.
12. 12. The RFID antenna according to any one of the preceding claims, wherein the carbon filler is coated or printed onto a substrate, wherein the substrate is preferably a paper substrate.
13. 13. The RFID antenna according to any one of the preceding claims, wherein the polymer matrix material comprises a polyvinyl matrix material, a polyamide matrix material, a polyvinylidene fluoride matrix material (PVDF) or a fluoropolymer resin matrix material.
14. 14. The RFID antenna according to any one of the preceding claims, wherein the RFID antenna has a multilayer structure comprising at least one layer as defined in any one of claims 1 to 13; and a layer of electrically insulating material.
15. 15. The RFID antenna according to claim 14, wherein the layer of electrically insulating material is made from a vinyl polymer.
16. 16. The RFID antenna according to claim 14 or 15, wherein the layer of electrically insulating material is at least 20 pm thick
17. 17. The RFID antenna according to any one of the preceding claims, wherein the RFID antenna has a sheet resistance of from 5 0/sq/25 pm to 0.06 0/sq/25 pm, preferably from 3 0/sq/25 pm to 0.06 0/sq/25 pm, preferably from 1 0/sq/25 pm to 0.06 0/sq/25 pm.
18. 18. The RFID antenna according to any one of the preceding claims, wherein the average thickness of the RFID antenna is 300 pm or less.
19. 19. The RFID antenna according to any one of the preceding claims, wherein the RFID antenna is microwave safe
20. 20. An RFID tag comprising an RFID antenna according to any one of the preceding claims.
21. 21. Use of a printed carbon filler dispersed in a polymer matrix material as an RFID antenna, wherein the carbon filler comprises functionalised graphene.
22. 22. An article comprising an RFID tag according to claim 20, preferably wherein the article is microwaveable packaging for a food product.
23. 23. An article according to claim 22, wherein the article is a food product comprising an inner plastic tray bearing the RFID tag and outer cardboard packaging surrounding the inner plastic tray.
24. 24 A method of manufacturing an RFID antenna according to any one of claims 1 to 19, wherein the method comprises the following steps: Step 1: providing a carbon filler comprising functionalised graphene particles; Step 2: dispersing the carbon filler in a polymer matrix material to form an ink; Step 3: printing the ink onto a substrate to form the RFID.The method of manufacture according to claim 24, wherein the functionalised graphene particles in step 1 are provided according to the following method: a. providing non-functionalised graphene particles; b. plasma functionalising the graphene particles to obtain functionalised graphene particles; and c. optionally mixing the functionalised graphene particles with non-functionalised graphene particles.