Method of sorting plastics

Abstract

A method of sorting plastics substrates uses infrared spectroscopy, wherein at least one substrate has a black inkjet ink layer comprising a radiation-curable monomer, <2 wt.% carbon black, and two or

Description

This invention relates to a method of sorting plastics, and in particular to a method of sorting black plastic substrates. Waste management is a continuous challenge. Plastic waste in particular has been increasingly produced and there is growing pressure to recycle plastic waste for environmental reasons. Waste sorting is a key step in waste recycling. Waste sorting is the process of separating the waste into different types of waste. There is a wide range of plastic substrates and in order to successfully sort plastic substrates for recycling, the plastic substrates must be accurately identified and then sorted according to the type of plastic substrate. In plastic waste recycling, plastics are often identified and then sorted using infrared spectroscopy, typically near-infrared (NIR) spectroscopy. In this regard, plastic substrates are irradiated with infrared radiation, the IR absorption of each plastic is measured and the plastics are then sorted according to their IR absorption. This technology has many advantages over other sorting techniques, including the ability of sorting plastics with a high degree of accuracy, at high speed and is non-polluting. However, it is challenging to identify and sort black plastics using such a method of sorting. In this regard, black plastics have a high absorption in the IR spectral region, particularly NIR. This means that it is not possible to identify the type of plastic substrate and hence black plastics cannot be sorted using such a method. In order to overcome this issue, the use of different wavelengths of IR have been explored, although these have been met with limited success. It is undesirable to compromise on the colour of the plastic in order to improve the sorting capabilities. And black plastic substrates need to maintain the required black colour of the plastic substrate. Therefore, there remains a need in the art to provide a method of sorting black plastic substrates using infrared spectroscopy, with a high degree of accuracy, whilst maintaining high speed and the required black colour of the plastic substrate. Accordingly, the present invention provides a method of sorting one or more plastic substrates using infrared spectroscopy, wherein one or more of the plastic substrates comprises a layer of black inkjet ink, wherein the black inkjet ink comprises a radiation-curable monomer and less than 2.0% by weight of carbon black, based on the total weight of the ink, wherein the ink further comprises two or more non-black pigments which in combination provide a black colour, wherein the ink has an L* value of 0 to 40, an a* value of -20 to +20, and a b* value of -20 to +20, and wherein the ink has a viscosity of 1 to 40 mPas at 25°C. The present invention will now be described with reference to the accompanying drawings, in which Fig. 1 shows an IR spectrum of percentage transmittance of NIR radiation over a wavelength range for inks useful in the method of the present invention when compared to comparative inks. The present invention provides a method of sorting one or more plastic substrates using infrared spectroscopy, wherein one or more of the plastic substrates comprises a layer of black inkjet ink, wherein the black inkjet ink comprises a radiation-curable monomer and less than 2.0% by weight of carbon black, based on the total weight of the ink, wherein the ink further comprises two or more non-black pigments which in combination provide a black colour, wherein the ink has an L* value of 0 to 40, an a* value of-20 to +20, and a b* value of-20 to +20, and wherein the ink has a viscosity of 1 to 40 mPas at 25°C. The inventors have surprisingly found that the method of sorting one or more plastic substrates of the present invention allows black plastic substrates to be sorted with a high degree of accuracy using infrared spectroscopy, whilst maintaining high speed and the required black colour of the plastic substrate. Specifically, the inventors have found that a plastic material comprising a layer of black inkjet ink, wherein the black inkjet ink comprises the specific blend of components as claimed maintains the required black colour and can be sorted with a high degree of accuracy and at high speed using infrared spectroscopy. Previously, in order to achieve the required black colour of a plastic substrate, carbon black has been used as the colouring agent. However, carbon black absorbs light in the IR region, particularly the NIR region, which means that black plastic substrates using carbon black as the colouring agent cannot be identified and sorted using infrared spectroscopy, particularly NIR spectroscopy. In contrast, in the method of the present invention, black plastic substrates can be successfully identified and sorted using infrared spectroscopy, including NIR spectroscopy. The sorting of plastic substrates using infrared spectroscopy is well-known in the art and a detailed description is not required. The sorting of plastic substrates using IR spectroscopy is conducted using an infrared spectrometer, which produces an infrared spectrum. An infrared spectrometer measures the interaction between the infrared radiation and the plastic substrate as a function of wavelength, to generate an infrared spectrum. In this regard, when exposed to infrared radiation, a molecule absorbs the electromagnetic photons and undergoes vibrational transition. As such, IR spectroscopy is often referred to as vibrational absorption spectroscopy. The IR absorption and molecular vibration differs depending on the type of molecule and the resultant signature can be used to identify the plastic composition. Each plastic composition therefore has characteristic absorbance peaks in the IR spectrum. The sorting of plastic substrates using IR spectroscopy may be conducted using an IR spectrometer using IR light having a wavelength of from 700 nm to 1,000 pm. A FT-IR spectrometer may used. For example, a Thermo Nicolet iS50 FTIR spectrometer may be used, having an IR source with a scan range of 4000 to 7800 cm1, a DTGS KBR detector, a KBr beam splitter, a NaCI FTIR cell window, in transmission mode of collection. Preferably, the sorting of plastic substrates uses NIR spectroscopy and may be conducted using a NIR spectrometer using NIR light having a wavelength of from 0.70 to 1.40 pm. For example, TrinamiX PAL One may be used, which has a spectral range of 1450 nm to 2450 nm with a spectral resolution of 1% of wavelength and a signal to noise ration of >5000 per spectral resolution element. The method of sorting of plastic substrates using infrared spectroscopy generally comprises: (a) irradiating one or more plastic substrates with infrared radiation; (b) measuring absorption of infrared radiation of one or more of the irradiated plastic substrates; and (c) sorting the one or more plastic substrates according to the absorption of infrared radiation by the one or more plastic substrates. As a result, plastic substrates may be identified and sorted by polymer-type based on their absorption of Infrared radiation. The method of sorting one or more plastic substrates of the present invention comprises, (a) irradiating the one or more plastic substrates with infrared radiation. The one or more plastic substrates can be any plastic substrate. By plastic is meant a polymer that can be moulded or formed into a desired shape or configuration. The plastic substrate may be a thermoplastic or a thermosetting polymer. Thermoplastics are deformable in that they soften when heated to a sufficient temperature and harden when cooled to below said temperature. In contrast, once formed and cured into a shape, thermosetting polymers will not soften to allow remoulding or reuse of the material. Thermplastic polymers are preferred for recycling purposes and therefore, the plastic substrate is preferably a thermoplastic polymer substrate. Preferably, the plastic substrate is selected from polyvinyl chloride (PVC), polystyrene (PS), polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), polylactic acid (PLA), polyolefin (e.g. polyethylene, polypropylene, or mixtures or copolymers thereof), polyethylene terephthalate glycol modified (PETG), or copolymers or mixtures thereof. In a preferred embodiment, the plastic substrate is selected from polyvinyl chloride (PVC), polystyrene (PS), polyethylene terephthalate (PET), polyethylene (PE) (including high density polyethylene (HDPE) and low density polyethylene (LDPE)), polypropylene (PP), or copolymers or mixtures thereof. When discussing the substrate, it is the surface which is most important, since it is the surface which is wetted by the ink. Thus, at least the surface of substrate is composed of the abovediscussed material. The one or more plastic substrates are irradiated with infrared (IR) radiation. Infrared radiation is electromagnetic radiation in the spectral band having a wavelength of from 700 nm to 1,000 pm. As is well-known in the art, infrared radiation can be subdivided into smaller spectral bands, including near-infrared (NIR), short-wavelength infrared (SWIR), mid-wavelength infrared (MWIR), long-wavelength infrared (LWIR) and far-infrared (FIR). By near-infrared (NIR), it is meant electromagnetic radiation having a wavelength of from 0.70 to 1.40 pm. In preferred embodiment, the infrared radiation is near-infrared (NIR) radiation. Accordingly, in a preferred embodiment, the method of sorting one or more plastic substrates of the present invention uses near-infrared (NIR) spectroscopy. Preferably, the infrared radiation has a wavelength of from 0.70 to 1.40 pm. Surprisingly, the present inventors have found that the method of the present invention allows for black plastic substrates to be successfully identified and sorted using infrared spectroscopy, Including NIR spectroscopy. The infrared radiation may be generated by any source of infrared radiation, including halogen light bulbs and LEDs. Preferably, the source of infrared radiation is capable of emitting infrared radiation having a wavelength of from 0.70 to 3.0 pm, preferably from 0.80 to 2.5 pm. Preferably the source of infrared radiation is capable of emitting near-infrared radiation, preferably having a wavelength of 0.70 to 1.4 pm. The method of sorting one or more plastic substrates of the present invention comprises, (b) measuring absorption of infrared radiation of one or more of the irradiated plastic substrates. After irradiation of one or more plastic substrates with infrared radiation, the absorption of infrared radiation of one or more of the irradiated plastic substrates is measured. The absorption of infrared radiation is measured using an IR spectrometer, preferably a NIR spectrometer, which can measure the reflection, absorption and / or transmittance of wavelengths of infrared radiation of the plastic substrate, which generates an IR spectrum. Each type of plastic absorbs IR radiation in differing amounts at each wavelength. The amount of IR radiation that the plastic absorbs at each wavelength can be assessed by directly measuring the absorption, transmission or sometimes reflection of wavelengths of IR radiation using an IR spectrometer, preferably a near-IR spectrometer. In a preferred embodiment, the sorting of one or more plastic substrates of the present invention uses Fourier-transform infrared (FTIR) spectroscopy. Preferably therefore, the absorption of infrared radiation of one or more of the irradiated plastic substrates is measured using a Fourier-transform infrared (FTIR) spectrometer. In a preferred embodiment, the sorting of one or more plastic substrates of the present invention uses NIR spectroscopy. Preferably therefore, the absorption of infrared radiation of one or more of the irradiated plastic substrates is measured using a NIR spectrometer. The method of sorting one or more plastic substrates of the present invention comprises, (c) sorting the one or more plastic substrates according to its absorption of infrared radiation. In this regard, each type of plastic has a distinct absorption of infrared radiation and hence generates a distinct IR spectrum. Thus, on reviewing the absorption of radiation of each irradiated plastic by reviewing the IR spectrum of each irradiated plastic, which is typically done with a series of detectors capable of recognising characteristic absorbance peaks in each polymer’s spectrum, it is possible to sort each plastic substrate according to its polymer-type. Such a method of sorting plastics by IR spectroscopy is a well- known method in the art. The sorting step can be done by hand or is preferably conducted using air jets that are angled to blow each piece of plastic into the correct processing stream. This has not previously been possible however for black plastic substrates owing to the presence of carbon black, which absorbs the IR radiation but is required to provide the necessary black colour. However, the method of the present invention allows for black plastic substrates to be successfully sorted with speed and accuracy, whilst maintaining the required black colour. In the method of sorting one or more plastic substrates of the present invention, one or more of the plastic substrates comprises a layer of black inkjet ink. In this regard, one or more of the plastic substrates has a black inkjet ink applied onto a surface of the substrate in order to provide the necessary black colour of the plastic substrate. In a preferred embodiment, the one or more plastic substrates comprises a single layer of black inkjet ink. A single layer of ink has its standard meaning. Each layer of ink is distinguished by a difference in the physical nature of the ink of each layer. This difference in physical nature may be imparted by an inherent difference in the ink used for each layer (for example, inks comprising different components) or by modifying the ink of each layer through curing (for example, by exposing the ink to a dose of radiation or by exposing the ink to ambient radiation over time). As such, each layer of ink has a different surface energy. A single layer is preferred over multiple layers as multiple layers has an increased film weight, which can negatively affect the image quality and film properties. Accordingly, the method using a black inkjet ink of the invention is preferred to a method using multiple layers of different coloured ink to provide an overall black colour. In a preferred embodiment, the inkjet ink is inkjet printed onto the substrate to form the layer of black inkjet ink on a surface of the one or more plastic substrates. Printing is preferably performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto a substrate, on a roll-to-roll printer or a flat-bed printer. Inkjet printing is well-known in the art and a detailed description is not required. The ink is therefore preferably jetted from one or more reservoirs or printing heads through narrow nozzles on to a substrate to form a printed image. The black inkjet ink used in the method of the present invention is a black inkjet ink having a black colour. By black is meant that the ink has an L* value of 0 to 40, an a* value of -20 to +20 and a b* value of -20 to +20. The black colour is provided by blending the two or more non-black pigments. Preferably, the inkjet ink used in the method of the present invention has an L* value of 20 to 40, more preferably 20 to 35. Preferably, the inkjet ink used in the method of the present invention has an a* value of-10 to +10. Preferably, the inkjet ink used in the method of the present invention has a b* value of-10 to +10. The CIELAB (L*a*b*) colour space system can be used to determine the colour of an inkjet ink. The CIELAB (L*a*b*) colour space system is a well-known colour space system in the art defined by the International Commission on Illumination and a detailed description is not required. The CIELAB (L*a*b*) colour space system expresses colour as three values: L*, a* and b*. L* defines the lightness from black (0) to white (100) and is the key value when assessing a black ink. L* represents the darkest black at L* = 0 and the brightest white at L* = 100. The a* is the axis for the green-red component, with green in the negative direction (-) and red in the positive direction (+). The b* is the axis for the blue-yellow component, with blue in the negative direction blue (-) and yellow in the positive direction (+). The L*, a* and b* values of the inkjet ink used in the method may be measured by applying a 12 pm drawdown of the inkjet ink onto a PVC substrate using an automated K101 control coater, and measuring the L*, a* and b* values using an eXact Advanced Spectrophotometer with a viewing angle of 2°, an illuminate source of D50 and no filter. The inkjet ink used in the method of the present invention comprises a radiation-curable monomer. As is known in the art, monomers may possess different degrees of functionality, which include mono, di, tri and higher functionality monomers. Monomers typically have a molecular weight of less than 600, preferably more than 200 and less than 450. Monomers are typically added to inkjet inks to reduce the viscosity of the inkjet ink. They therefore preferably have a viscosity of less than 150 mPas at 25°C, more preferably less than 100 mPas at 25°C and most preferably less than 20 mPas at 25°C. Monomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 25°C with a shear rate of 25 s 1. The radiation-curable monomer is not particularly limited and the formulator is free to include any radiation-curable monomer in the inkjet ink used in the method to improve the properties or performance of the ink. This radiation-curable monomer can include any radiation-curable monomer readily available and known in the art in inkjet inks. By “radiation-curable” is meant a monomer that polymerises and / or crosslinks upon irradiation, for example, when exposed to actinic radiation, in the presence of a photoinitiator. The amount of radiation-curable monomer is not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. In a preferred embodiment, the inkjet ink used in the method comprises 20 to 90% by weight, more preferably 35 to 80% by weight and most preferably 50 to 75% by weight, of a radiation-curable monomer, based on the total weight of the ink. In a preferred embodiment, the radiation-curable monomer comprises a di- and / or multifunctional monomer. For the avoidance of doubt, mono and difunctional are intended to have their standard meanings, i.e. one or two groups, respectively, which take part in the polymerisation reaction on curing. Multifunctional (which does not include difunctional) is intended to have its standard meaning, i.e. three or more groups, respectively, which take part in the polymerisation reaction on curing. In a preferred embodiment, the di- and / or multifunctional monomer is a di-, tri-, tetra-, penta- or hexa- functional monomer, i.e. the radiation-curable monomer has two, three, four, five or six functional groups. In a particularly preferred embodiment, the inkjet ink comprises at least two di-and / or multifunctional radiation-curable monomers. The di- and / or multifunctional radiation-curable monomer may possess different degrees of functionality, and a mixture including combinations of di, tri and higher functionality monomers may be used. The functional group of the di- and / or multifunctional radiation-curable monomer, which is utilised in the ink used in the method of the present invention may be the same or different but must take part in the polymerisation reaction on curing. Examples of such functional groups include any groups that are capable of polymerising upon exposure to radiation and are preferably selected from a (meth)acrylate group and a vinyl ether group. Preferably the radiation-curable monomer of is a (meth)acrylate monomer and / or a vinyl ether monomer, and most preferably a (meth)acrylate monomer. The substituents of the di- and / or multifunctional radiation-curable monomer are not limited other than by the constraints imposed by the use in an ink-jet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, Ce-io aryl and combinations thereof, such as Ce-io aryl- or C3-18 cycloalkyl-substituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure. In a preferred embodiment, the inkjet ink comprises 40 to 85% by weight of a di- and / or multifunctional radiation-curable monomer, more preferably 45 to 80% by weight and most preferably 50 to 75% by weight, based on the total weight of the ink. In a particularly preferred embodiment, the inkjet ink comprises a difunctional monomer. In a particularly preferred embodiment, the inkjet ink comprises 40 to 85% by weight of a difunctional radiation-curable monomer, more preferably 45 to 80% by weight and most preferably 50 to 75% by weight, based on the total weight of the ink. In a preferred embodiment, the inkjet ink comprises a multifunctional monomer. The amount of the multifunctional monomer, when present, is preferably 1-45% by weight, more preferably 1 to 40% by weight, most preferably 1 to 35% by weight, based on the total weight of the ink. Examples of the di- and / or multifunctional radiation-curable monomer include difunctional (meth)acrylate monomers, multifunctional (meth)acrylate monomers, divinyl ether monomers, multifunctional vinyl ether monomers and di- and / or multifunctional vinyl ether (meth)acrylate monomers. Mixtures of di- and / or multifunctional radiation-curable monomer may also be used. In a preferred embodiment, the radiation-curable material comprises a (meth)acrylate monomer, more preferably a di- and / or multifunctional (meth)acrylate monomer. In a particularly preferred embodiment, the inkjet ink comprises a difunctional (meth)acrylate monomer. Difunctional (meth)acrylate monomers are well-known in the art and a detailed description is therefore not required. Examples include hexanediol diacrylate (HDDA), 1,8-octanediol diacrylate, 1,9-nonanediol diacrylate, 1,10-decanediol diacrylate (DDDA), 1,11-undecanediol diacrylate and 1,12-dodecanediol diacrylate, polyethylene glycol diacrylate (for example tetraethylene glycol diacrylate, PEG200DA, PEG300DA, PEG400DA, PEG600DA), dipropylene glycol diacrylate (DPGDA), tripropylene glycol diacrylate (TPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), neopentylglycol diacrylate, 3-methyl-1,5-pentanediol diacrylate (3-MPDDA), and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, propoxylated neopentylglycol diacrylate (NPGPODA), and mixtures thereof. Also included are esters of methacrylic acid (i.e. methacrylates), such as hexanediol dimethacrylate, 1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, 1,11-undecanediol dimethacrylate and 1,12-dodecanediol dimethacrylate, triethyleneglycol dimethacrylate, diethyleneglycol dimethacrylate, ethyleneglycol dimethacrylate, 1,4-butanediol dimethacrylate and mixtures thereof. 3-MPDDA is particularly preferred. Preferably, the inkjet ink used in the method comprises 30 to 75% by weight, more preferably 35 to 70% by weight and most preferably 40 to 65% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink. However, for some applications of the present invention, the amount present may be higher and in such a preferred embodiment, the ink used in the method comprises up to 80% by weight of a difunctional (meth)acrylate monomer, based on the total weight of the ink. Examples of multifunctional (meth)acrylate monomers (which do not include difunctional (meth)acrylate monomers) include tri-, tetra-, penta-, hexa-, hepta- and octa-functional monomers. Examples of the multifunctional acrylate monomers that may be included in the inkjet ink include trimethylolpropane triacrylate, di pentaerythritol triacrylate, tri(propylene glycol) triacrylate, bis(pentaerythritol) hexaacrylate, and the acrylate esters of ethoxylated or propoxylated glycols and polyols, for example, ethoxylated trimethylolpropane triacrylate and ethoxylated pentaerythritol tetraacrylate (EOPETTA, also known as PPTTA), and mixtures thereof. Multifunctional (meth)acrylate monomers also include esters of methacrylic acid (i.e. methacrylates), such as trimethylolpropane trimethacrylate. Mixtures of (meth)acrylates may also be used. The di- and / or multifunctional radiation-curable monomer may have at least one vinyl ether functional group. In a preferred embodiment, the inkjet ink used in the method comprises a divinyl ether monomer, a multifunctional vinyl ether monomer, a divinyl ether (meth)acrylate monomer and / or a multifunctional vinyl ether (meth)acrylate monomer. In a particularly preferred embodiment, the inkjet ink used in the method comprises a divinyl ether monomer. Examples of a divinyl ether monomer include triethylene glycol divinyl ether (DVE-3), diethylene glycol divinyl ether, 1,4-cyclohexanedimethanol divinyl ether, bis[4-(vinyloxy)butyl] 1,6-hexanediylbiscarbamate, bis[4-(vinyloxy)butyl] isophthalate, bis[4-(vinyloxy)butyl] (methylenedi-4,1-phenylene)biscarbamate, bis[4-(vinyloxy)butyl] succinate, bis[4-(vinyloxy)butyl]terephthalate, bis[4-(vinyloxymethyl)cyclohexylmethyl] glutarate, 1,4-butanediol divinyl ether and mixtures thereof. Triethylene glycol divinyl ether (DVE-3) is particularly preferred. DVE-3 is preferred because of its low viscosity. It has a lower viscosity than the equivalent acrylate monomer because the vinyl ether groups have fewer polar interactions than acrylates. An example of a multifunctional vinyl ether monomer is tris[4-(vinyloxy)butyl] trimellitate. Examples of a vinyl ether (meth)acrylate monomer include 2-(2-vinyloxy ethoxy)ethyl acrylate (VEEA), 2-(2-vinyloxy ethoxy)ethyl methacrylate (VEEM) and mixtures thereof. In a preferred embodiment, the di- and / or multifunctional radiation-curable monomer is selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), polyethylene glycol diacrylate, tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5- pentanediol diacrylate (3-MPDDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), trimethylolpropane triacrylate (TMPTA), di-trimethylolpropane tetraacrylate (DiTMPTA), di-pentaerythritol hexaacrylate (DPHA), ethoxylated trimethylolpropane triacrylate (EOTMPTA), ethoxylated pentaerythritol tetraacrylate (EOPETTA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof. In a preferred embodiment, the difunctional monomer, when present, is selected from 1,10-decanediol diacrylate (DDDA), hexanediol diacrylate (HDDA), polyethylene glycol diacrylate, tripropylene glycol diacrylate (TPGDA), 3-methyl 1,5-pentanediol diacrylate (3-MPDDA), dipropylene glycol diacrylate (DPGDA), tricyclodecane dimethanol diacrylate (TCDDMDA), propoxylated neopentyl glycol diacrylate (NPGPODA), triethylene glycol divinyl ether (DVE-3) and mixtures thereof. In a preferred embodiment, the inkjet ink used in the method comprises a monofunctional monomer, such as a monofunctional (meth)acrylate monomer. Monofunctional monomers are well-known in the art. A radiation-curable monofunctional monomer has one functional group, which takes part in the polymerisation reaction on curing. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation and is preferably selected from a (meth)acrylate group and a vinyl ether group. The substituents of the monofunctional monomer are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The substituents are typically alkyl, cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms. Non-limiting examples of substituents commonly used in the art include C1-18 alkyl, C3-18 cycloalkyl, Ce-io aryl and combinations thereof, such as Ce-w aryl- or C3-18 cycloalkylsubstituted C1-18 alkyl, any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure. In a preferred embodiment, the inkjet ink used in the method comprises a monofunctional monomer present in 1 to 80% by weight, more preferably 1 to 50% by weight and most preferably 0 to 25% by weight, based on the total weight of the ink. In a preferred embodiment, the inkjet ink used in the method comprises a monofunctional (meth)acrylate monomer, which are well-known in the art and are preferably the esters of acrylic acid. A detailed description is therefore not required. Mixtures of (meth)acrylates may also be used. For the avoidance of doubt, (meth)acrylate is intended herein to have its standard meaning, i.e. acrylate and / or methacrylate. The substituents of the monofunctional (meth)acrylate monomer are not limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The monofunctional (meth)acrylate monomer may be a cyclic monofunctional (meth)acrylate monomer and / or an acyclic-hydrocarbon monofunctional (meth)acrylate monomer. The substituents of the cyclic monofunctional (meth)acrylate monomer are typically cycloalkyl, aryl and combinations thereof, any of which may be interrupted by heteroatoms and / or substituted by alkyl. Non-limiting examples of substituents commonly used in the art include C3 is cycloalkyl, Ce 10 aryl and combinations thereof, any of which may be substituted with alkyl (such as C1-18 alkyl) and / or any of which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents. The substituents may together also form a cyclic structure. The cyclic monofunctional (meth)acrylate monomer may be selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA / Medol-10), 4-tert-butylcyclohexyl acrylate (TBCHA), benzyl acrylate (BA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA) and mixtures thereof. The substituents of the acyclic-hydrocarbon monofunctional (meth)acrylate monomer are typically alkyl, which may be interrupted by heteroatoms. A non-limiting example of a substituent commonly used in the art is C1-18 alkyl, which may be interrupted by 1-10 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted. The acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear or branched C6-C20 group. It may be selected from octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof. In a preferred embodiment, the acyclic-hydrocarbon monofunctional (meth)acrylate monomer contains a linear C6-C20 group. In a preferred embodiment, the monofunctional (meth)acrylate monomer is selected from isobornyl acrylate (IBOA), phenoxyethyl acrylate (PEA), cyclic TMP formal acrylate (CTFA), tetrahydrofurfuryl acrylate (THFA), (2-methyl-2-ethyl-1,3-dioxolane-4-yl)methyl acrylate (MEDA / Medol-10), 4-fert-butylcyclohexyl acrylate (TBCHA), benzyl acrylate (BA), 3,3,5-trimethylcyclohexyl acrylate (TMCHA), octadecyl acrylate (ODA), 2-(2-ethoxyethoxy)ethyl acrylate, tridecyl acrylate (TDA), isodecyl acrylate (IDA), lauryl acrylate and mixtures thereof. Lauryl acrylate is particularly preferred. Lauryl acrylate is preferred because it has a long straight chain that introduces flexibility into the cured ink film. In a preferred embodiment, the inkjet ink used in the method comprises a monofunctional (meth)acrylate monomer present in 1 to 70% by weight, more preferably 1 to 50% by weight and most preferably 1 to 25% by weight, based on the total weight of the ink. In a preferred embodiment, the inkjet ink used in the method comprises at least one N-vinyl amide monomer and / or N-(meth)acryloyl amine monomer. N-Vinyl amide monomers are well-known monomers in the art. N-Vinyl amide monomers have a vinyl group attached to the nitrogen atom of an amide which may be further substituted in an analogous manner to the (meth)acrylate monomers. Preferred examples are N-vinyl caprolactam (NVC), N-vinyl pyrrolidone (NVP), N-vinyl piperidone, N-vinyl formamide and N-vinyl acetamide. Similarly, N-acryloyl amine monomers are also well-known in the art. N-Acryloyl amine monomers also have a vinyl group attached to an amide but via the carbonyl carbon atom and again may be further substituted in an analogous manner to the (meth)acrylate monomers. A preferred example is N-acryloylmorpholine (ACMO). In a preferred embodiment, the inkjet ink used in the method comprises 10-30% by weight, more preferably 15-25% by weight, of an N-vinyl amide monomer, an N-acryloyl amine monomer or mixtures thereof, based on the total weight of the ink. In a preferred embodiment, the inkjet ink used in the method comprises at least one of NVC and / or ACMO. N-Vinyl amide monomers are particularly preferred, and most preferably NVC. The inkjet ink used in the method may also comprise an N-vinyl monomer other than an N-vinyl amide monomer and / or N-(meth)acryloyl amine monomer. Examples include N-vinyl carbazole, N-vinyl indole and N-vinyl imidazole. In a preferred embodiment, the inkjet ink used in the method comprises 10-30% by weight, more preferably 15-25% by weight, of an N-vinyl monomer other than an N-vinyl amide monomer and / or N-(meth)acryloyl amine monomer, based on the total weight of the ink. The inkjet ink used in the method of the present invention may further comprise a radiation-curable (i.e. polymerisable) oligomer, such as a (meth)acrylate oligomer. Any radiation-curable oligomer that is compatible with the other ink components is suitable for use in the ink. The term “curable oligomer” has its standard meaning in the art, namely that the component is partially reacted to form a pre-polymer having a plurality of repeating monomer units, which is capable of further polymerisation. The oligomer preferably has a molecular weight of at least 600. The molecular weight is preferably 4,000 or less. Molecular weights (number average) can be calculated if the structure of the oligomer is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards. The oligomers may possess different degrees of functionality, and a mixture including combinations of mono, di, tri and higher functionality oligomers may be used. The degree of functionality of the oligomer determines the degree of crosslinking and hence the properties of the cured ink. The oligomer is preferably multifunctional meaning that it contains on average more than one reactive functional group per molecule. The average degree of functionality is preferably from 2 to 6. Oligomers are typically added to inkjet inks to increase the viscosity of the inkjet ink or to provide film-forming properties such as hardness or cure speed. They therefore preferably have a viscosity of 150 mPas or above at 25°C. Preferred oligomers for inclusion in the ink used in the method of the invention have a viscosity of 0.5 to 10 Pas at 50°C. Oligomer viscosities can be measured using an ARG2 rheometer manufactured by T.A. Instruments, which uses a 40 mm oblique / 2° steel cone at 60°C with a shear rate of 25 s ’1. Radiation-curable oligomers comprise a backbone, for example a polyester, urethane, epoxy or polyether backbone, and one or more radiation-curable groups. The polymerisable group can be any group that is capable of polymerising upon exposure to radiation. In a preferred embodiment, the radiation-curable oligomer is a (meth)acrylate oligomer. The radiation-curable oligomer may include amine functionality, as the amine acts as an activator without the drawback of migration associated with low-molecular weight amines. In a preferred embodiment, the radiation-curable oligomer is amine modified. In a particularly preferred embodiment, the radiation-curable oligomer is an amine-modified (meth)acrylate oligomer. Particularly preferred radiation-curable oligomers are di-, tri-, tetra-, penta- or hexa-functional acrylates. More preferably, the radiation-curable oligomer is an amine-modified acrylate oligomer. A suitable amine-modified polyester acrylate oligomer is commercially available as UVP6600. A suitable amine-modified polyether acrylate oligomer is commercially available as CN3715LM. Other suitable examples of radiation-curable oligomers include epoxy based materials such as bisphenol A epoxy acrylates and epoxy novolac acrylates, which have fast cure speeds and provide cured films with good solvent resistance. The amount of radiation-curable oligomer, when present, is preferably 0.1 to 10% by weight, based on the total weight of the ink. The ink used in the method may also contain a resin. The resin preferably has a weight-average molecular weight (Mw) of 20-200 KDa, and most preferably 20-60 KDa. The Mw may be measured by known techniques in the art, such as gel permeation chromatography (GPC), using a polystyrene standard. The resin is preferably solid at 25°C. It is preferably soluble in the liquid medium of the ink (the radiation-curable diluent and, when present, additionally the solvent). The resin is a passive (i.e. inert) resin, in the sense that it is not radiation curable and hence does not undergo cross-linking under the curing conditions to which the ink is subjected. The resin may improve adhesion of the ink to the substrate. It is preferably soluble in the ink. The resin, when present, is preferably present at 0.1 to 5% by weight, based on the total weight of the ink. The inkjet ink used in the method of the present invention comprises two or more non-black pigments which in combination provide a black colour. That is, the two or more non-black pigments, when present in an inkjet ink without the presence of black pigment, provide a black inkjet ink. However, when used alone, they do not provide a black ink. By black ink, it is meant that the ink has an L* value of 0 to 40, an a* value of-20 to +20 and a b* value of-20 to +20. Preferably, the two or more non-black pigments provide a black inkjet ink having an L* value of 20 to 40, an a* value of-10 to +10 and a b* value of-10 to +10. More preferably, the two or more non black pigments provide a black inkjet ink having an L* value of 20 to 35, an a* value of-10 to +10 and a b* value of-10 to +10. By non-black pigment, it is meant any coloured pigment, other than black pigment. A non-black pigment includes any pigment that when present as the sole pigment in an inkjet ink, does not provide a black ink. By black ink, it is meant that the ink has an L* value of 0 to 40, an a* value of -20 to +20 and a b* value of-20 to +20. Examples of combinations of two or more non-black pigments that in combination can provide a black colour include blue and orange, red and green, yellow and purple, and blue and brown. Examples of combinations of three or more non-black pigments that in combination can provide a black colour include cyan, magenta and yellow, yellow, cyan and violet, yellow, green and violet, yellow, blue and orange, green, orange and violet, red, cyan and yellow, and red, green and blue. A preferred combination of three or more non-black pigments that in combination can provide a black colour include cyan, magenta and yellow. Preferred non-black pigments include cyan, magenta, yellow, blue, brown, green, orange, red, violet and white. The pigments are dispersed in the liquid medium of the ink. The non-black pigments can be selected from a wide range of suitable pigments that would be known to the person skilled in the art. Non-limiting examples of suitable non-black pigment include zinc oxide, titanium oxide, zinc sulphide, calcium carbonate, phthalocyanine, anthraquinones, arylides, arylamides, diarylides, benzimidazolones, toluidines, napthols, dioxanzines, diazos, perylenes, carbazoles, monoazo and disazobenzimidazoles, rhodamines, indigoids, quinacridones, diazopyranthrones, dinitranilines, pyrazoles, diazopryanthrones, dinityanilines, pyrazoles, dianisidines, pyranthrones, tetracholoroisoindolines, dioxazines, monoazoacrylides and anthrapyrimidines. The following commercially-available pigments classified according to Colour Index International according to the following tradenames are preferred non-limiting examples of non-black pigments that can be used in the ink used in the method of the present invention: Blue pigments: PB1, PB1:2, PB9, PB15, PB15:1, PB15:2, PB15:3, PB15:4, PB15:6, PB16, PB17, PB17:1, PB22, PB24, PB25, PB27, PB27:1, PB28, PB29, PB30, PB31, PB32, PB33, PB34, PB35, PB36, PB36:1, PB50, PB60, PB61, PB61:1, PB62, PB63, PB66, PB68, PB71, PB72, PB73, PB74, PB75, PB76, PB79, PB80, PB81, PB82, PB84, PB86 and PB128. Brown pigments: PBr1, PBr6, PBr7, PBr7, PBr8, PBr9, PBr10, PBr11, PBr12, PBr22, PBr23, PBr24, PBr25, PBr27, PBr29, PBr30, PBr31, PBr33, PBr34, PBr35, PBr37, PBr39, PBr40, PBr41, PBr42, PBr43, PBr44, PBr45, PBr46 PB5, PB23 and PB265. Green pigments: PG1, PG2, PG4 PG7, PG10, PG13, PG14, PG15, PG16, PG17, PG18, PG19, PG20, PG21, PG22, PG23, PG24, PG26.PG36, PG38, PG39, PG41, PG42, PG45, PG48, PG50, PG51, PG55 and PG56. Yellow pigments: PY1, PY1:1, PY2, PY3, PY4, PY5, PY6, PY9, PY10, PY12, PY13, PY14, PY16, PY17, PY21 PY24, PY30, PY31, PY32, PY33, PY34, PY34:1, PY35, PY35:1, PY36, PY36:1, PY37, PY37:1, PY38, PY39, PY40, PY41, PY42, PY43, PY44, PY45, PY46, PY47, PY48, PY55, PY61, PY62, PY62:1, PY63 PY65, PY73, PY74, PY75, PY77, PY81 PY83, PY87, PY93, PY94, PY95, PY97, PY98, PY100, PY101, PY104, PY105, PY108, PY109, PY110, PY111, PY112, PY113, PY115, PY116, PY117, PY118, PY119 PY120, PY126, PY127, PY127:1 PY128, PY129, PY130, PY133, PY134, PY136, PY137, PY138, PY139, PY147, PY 148.PY150, PY151, PY152, PY153 PY154, PY155, PY156, PY172, PY173, PY174 PY175, PY176, PY179 PY180, PY181, PY182, PY183, PY184 PY185, PY1888, PY189, PY190, PY191, PY191:1, PY192, PY193, PY194, PY200, PY203, PY204, PY207, PY213, PY216, PY219, PY223, PY224, PY226 and PY227. Orange pigments: PO1, PO2, PO 3, PO 5, PO 13, PO15, PO16, PO 17, PO 17:1, PO 20, PO 20:1, PO 21, PO 21:1, PO 22, PO 23, PO 23:1, PO 31, PO 34, PO 36, PO 38, PO 40, PO 41, PO 43, PO 45, PO 46, PO 47, PO 48, PO 49, PO 51, PO 52, PO 53, PO 59, PO 60, PO 61, PO 62, PO 64, PO 65, PO 66, PO 67, PO 68, PO 69, PO 71, PO 72, PO 73, PO 74, PO 75, PO 77, PO 78, PO 79, PO 80, PO 81, PO 82, PO 85, PO 84 and PO 86. Red pigments: PR1, PR2, PR3, PR4, PR5, PR6, PR7, PR8, PR9, PR12, PR13, PR14, PR15, PR17, PR19, PR21, PR22, PR23, PR31, PR32, PR38, PR39, PR47, PR48, PR48:1, PR48:2, PR48:3, PR48:4, PR49, PR49:1, PR49:2, PR52:1, PR52:2, PR53, PR53:1, PR57, PR57:1, PR57:2, PR58:4, PR60, PR60:1, PR61, PR62, PR63, PR63:1, PR69, PR81, PR81:1, PR81:2, PR81:3, PR81:4, PR83, PR83:1, PR83:3, PR85, PR88, PR89, PR90, PR90:1, PR101, PR101:1, PR102, PR103, PR104, PR105, PR107, PR108, PR108:1, PR109, PR112, PR113, PR113:1, PR114, PR119, PR120, PR121, PR122, PR123, PR139, PR144, PR146, PR147, PR148, PR149, PR150, PR160, PR166, PR168, PR169, PR170, PR170:1, PR171, PR172, PR173, PR174, PR175, PR176, PR177, PR178, PR179, PR1880, PR181, PR13, PR184, PR185, PR1887, PR188, PR190, PR192, PR193, PR194, PR197, PR200, PR202, PR204, PR206, PR207, PR208, PR209, PR210, PR211, PR212, PR213, PR214, PR216, PR220, PR221, PR223, PR224, PR226, PR230, PR231, PR232, PR233, PR235, PR236, PR238, PR239, PR242, PR243, PR245, PR251, PR252, PR253, PR254, PR255, PR256, PR257, PR258, PR259, PR260, PR262, PR264, PR265, PR266, PR268, PR269, PR270, PR271, PR272, PR273, PR274, PR275, PR276, PR279, PR282, PR286, PR287 and PR288. Violet pigments: PV1, PV1:1, PV1:2, PV2, PV2:2, PV3, PV3:1, PV3:3, PV5, PV5:1, PV7, PV13, PV14, PV15, PV16, PV18, PV19, PV23, PV25, PV27, PV29, PV31, PV32, PV36, PV37, PV39, PV42, PV44, PV47, PV48, PV49, PV50, PV55, PV58 and PV171. White pigments: PW1, PW2, PW3, PW4, PW5, PW6, PW6:1, PW7, PW8, PW10, PW11, PW12, PW13, PW14, PW15, PW16, PW17, PW1, PW18:1, PW19, PW20, PW21, PW22, PW23, PW24, PW25, PW26, PW27, PW28, PW30, PW32 and PW33. The two or more non-black pigments which in combination provide a black colour include any combination of two or more non-black pigments, that when present in an inkjet ink without the presence of black pigment, provide a black inkjet ink. By black ink, it is meant that the ink has an L* value of 0 to 40, an a* value of -20 to +20 and a b* value of -20 to +20. Preferably, the two or more non-black pigments provide a black inkjet ink having an L* value of 20 to 40, an a* value of-10 to +10 and a b* value of-10 to +10. More preferably, the two or more non-black pigments provide a black inkjet ink having an L* value of 20 to 35, an a* value of-10 to +10 and a b* value of-10 to +10. In a preferred embodiment, the inkjet ink used in the method comprises three or more non-black pigments which in combination provide a black colour. The three or more non-black pigments which in combination provide a black colour include any combination of three or more non-black pigments, that when present in an inkjet ink without the presence of black pigment, provide a black inkjet ink. By black ink, it is meant that the ink has an L* value of 0 to 40, an a* value of -20 to +20 and a b* value of -20 to +20. Preferably, the three or more non-black pigments provide a black inkjet ink having an L* value of 20 to 40, an a* value of-10 to +10 and a b* value of-10 to +10. More preferably, the three or more non-black pigments provide a black inkjet ink having an L* value of 20 to 35, an a* value of-10 to + 10 and a b‘ value of -10 to +10. In a preferred embodiment, each non-black pigment is present in the inkjet ink used in the method at 0.5 to 6% by weight, preferably 1 to 5% by weight, based on the total weight of the ink. The inkjet ink used in the method of the present invention comprises less than 2.0% by weight of carbon black, based on the total weight of the ink. It has been surprisingly found that a plastic substrate having a layer of black inkjet ink, wherein the black inkjet ink comprises two or more non-black pigments and wherein the inkjet ink has less than 2.0% by weight of carbon black, based on the total weight of the ink, allows for the black plastic to be sorted with a high degree of accuracy and at high speed using infrared spectroscopy, whilst maintaining the required black colour. Carbon black is a paracrystalline form of carbon that is obtainable by thermal decomposition or incomplete combustion of hydrocarbons. Carbon black is manufactured under controlled processes to produce carbon particles that can vary in particle size, aggregate size, shape, porosity and surface chemistry. The characteristics of carbon black depend on the manufacturing process and therefore, carbon black is generally classified by manufacturing process. Manufacturing processes include Furnace Black process, Degussa Gas Black process, Lamp Black process, Thermal Black process and Acetylene Black process. Carbon black typically contains more than 95% carbon and carbon black particles usually range from 10 to 500 nm in size, more preferably 20 to 200 nm. It has a lower surface area than activated carbon but a higher surface area than soot. A typical surface area is 1 to 500 m2 / g and more typically 20 to 200 m2 / g. Various carbon black grades can be produced all of which are restricted in the ink used in the method of the present invention. The following commercially-available pigments classified according to Colour Index International according to the following tradenames are examples of carbon black pigments: PBk6 and PBk7. Examples of commercially-available carbon black pigments include: XPB091, XPB151, XPB233, XPB280, XPB293, XPB312, XPB319, XPB323, XPB325, XPB348, XPB366, XPB403, XPB406, XPB409, XPB423, XPB441, XPB459, XPB509, XPB519, XPB533, XPB538, XPB541, XPB545, XPB547, XPB552, XPB553, XPB559, XPB565, XPB567, XPB568, XPB569, XPB570, XPB571, XPB577, XPB579, XPB581, XPB582, XPB583, XPB584, XPB585, XPB590, XPB591, XPB595, XPB596, XPB600, XPB604, BLACK PEARLS, ELFTEX, MOGUL, MONARCH, REGAL, SPHERON, STERLING, VULCAN, CSX, CRX, IRX, FCXO, SHOBLACKO, DLO, PROPELO, LITXO, PBXO carbon black, BLACK PEARLS / MOGUL L, BLACK PEARLS / MOGUL E, MOGUL H, Special Black, Printex, Nerox and REGAL 400 / 400R carbon black. The amount of carbon black present in the inkjet ink is restricted as carbon black absorbs IR radiation, particularly NIR radiation, which means that it is not possible to sort black plastic which contains carbon black. However, a small amount of carbon black can be tolerated and as such, the inkjet ink comprises less than 2.0% by weight of carbon black, based on the weight of the ink. In a preferred embodiment, the inkjet ink comprises less than 1.5% by weight, more preferably less than 0.8% by weight and most preferably the inkjet ink used in the method of the present invention is substantially free of carbon black, where the amounts are based on the total weight of the ink. By substantially free is meant that only small amounts will be present. However, minor amounts of carbon black, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight, more preferably less than 0.1% by weight and most preferably less than 0.05% by weight of carbon black, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of carbon black. As the amount of carbon black present in the ink is further restricted, i.e. from the ink comprising less than 1.5% by weight of carbon black, based on the total weight of the ink, to the ink being free of carbon black, the degree of accuracy of plastic sorting increases, as the identification of the type of plastic by IR absorption is not negatively impacted by the presence of carbon black. In a preferred embodiment, the black inkjet ink used in the method of the invention comprises a radiation-curable monomer and less than 0.1 % by weight of carbon black, based on the total weight of the ink, wherein the ink further comprises two or more non-black pigments which in combination provide a black colour, wherein the ink has an L* value of 0 to 40, an a* value of-20 to +20, and a b* value of-20 to +20, and wherein the ink has a viscosity of 1 to 40 mPas at 25°C. More preferably, the black inkjet ink used in the method of the invention comprises a radiation-curable monomer and less than 0.05% by weight of carbon black, based on the total weight of the ink, wherein the ink further comprises two or more non-black pigments which in combination provide a black colour, wherein the ink has an L* value of 0 to 40, an a* value of-20 to +20, and a b* value of-20 to +20, and wherein the ink has a viscosity of 1 to 40 mPas at 25°C. In a particularly preferred embodiment, the black inkjet ink used in the method of the invention comprises a radiation-curable monomer, and two or more non-black pigments which in combination provide a black colour, wherein the ink is free of carbon black, wherein the ink has an L* value of 0 to 40, an a* value of -20 to +20, and a b* value of -20 to +20, and wherein the ink has a viscosity of 1 to 40 mPas at 25°C. It is surprising that the required black colour, and particularly the L* value of Oto 40, can be provided without recourse to carbon black. The inkjet ink used in the method of the present invention may optionally include black pigment, other than carbon black. By black pigment, it is meant any pigment that when present as the sole pigment in an inkjet ink, provides a black ink. By black ink, it is meant that the ink has an L* value of 0 to 40, an a* value of-20 to +20 and a b* value of-20 to +20. The presence of black pigment, other than carbon black, has not been found to adversely affect the accuracy of the plastic sorting as black pigments, other than carbon black, have not been found to absorb IR radiation and thus do not negatively affect the IR spectrum. And so, the presence of black pigment, other than carbon black, is not restricted. It is not however possible to achieve the required black colour and formulation requirements in an inkjet ink that only has a black pigment present, other than carbon black, in the inkjet ink. In a preferred embodiment, the inkjet ink used in the method comprises from 0.5 to 6% by weight in total of black pigment, other than carbon black, based on the total weight of the ink. The following commercially-available pigments classified according to Colour Index International according to the following tradenames are non-limiting examples of black pigments, other than carbon black: PBk1, PBk11, PBk12, PBk13, PBk14, PBk22, PBk23, PBk 24, PBk25, PBk26, PBk27, PBk28, PBk29, PBk30, PBk31, PBk32, PBk33, PBk34 and PBk35. Preferably, the inkjet ink used in the method comprises less than 2.0% by weight in total of black pigment, other than carbon black, based on the total weight of the ink. In a preferred embodiment, the inkjet ink used in the method comprises less than 1.5% by weight in total, more preferably less than 0.8% by weight in total and most preferably the inkjet ink used in the method of the present invention is substantially free of black pigment, other than carbon black, where the amounts are based on the total weight of the ink. By substantially free is meant that only small amounts will be present. However, minor amounts of black pigment, other than carbon black, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight in total, more preferably less than 0.1% by weight in total and most preferably less than 0.05% by weight in total of black pigment, other than carbon black, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of black pigment. In a preferred embodiment, the inkjet ink used in the method comprises less than 2.0% by weight in total of black pigment, based on the total weight of the ink. In a preferred embodiment, the inkjet ink used in the method comprises less than 1.5% by weight in total, more preferably less than 0.8% by weight in total and most preferably the inkjet ink used in the method of the present invention is substantially free of black pigment, where the amounts are based on the total weight of the ink. By substantially free is meant that only small amounts will be present. However, minor amounts of black pigment, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink may comprise less than 0.5% by weight in total, more preferably less than 0.1% by weight in total and most preferably less than 0.05% by weight in total of black pigment, based on the total weight of the ink. In a preferred embodiment, the inkjet ink used in the method is free of black pigment. Pigment particles dispersed in the ink should be sufficiently small to allow the ink to pass through an inkjet nozzle, typically having a particle size less than 8 pm, preferably less than 5 pm, more preferably less than 1 pm and particularly preferably less than 0.5 pm. By providing a black inkjet ink based on two or more non-black pigments, wherein the inkjet ink comprises less than 2.0% by weight of carbon black, based on the total weight of the ink, the degree of accuracy of plastic sorting can be improved, speed of sorting and colour of plastic can be maintained. As the amount of carbon black present in the ink is further restricted, i.e. from the ink comprising less than 1.5% by weight of carbon black, based on the total weight of the ink, to the ink being free of carbon black, the degree of accuracy of plastic sorting is further improved whilst maintaining colour. Other colouring agents, which include colouring agents other than pigments, may optionally be included in the black inkjet ink used in the method of the present invention, including dyes. The dyes include but are not limited to azo dyes, anthraquinone dyes, xanthene dyes, azine dyes, and combinations thereof. In a preferred embodiment, the inkjet ink used in the method is substantially free of other colouring agents. By substantially free is meant that only minor amounts will be present and can be tolerated. For example, the ink used in the method may comprise less than 0.5% by weight, more preferably less than 0.1% by weight and most preferably less than 0.05% by weight of other colouring agents, based on the total weight of the ink. In a preferred embodiment, the inkjet ink used in the method is free of other colouring agents. In other words, preferably, pigments are the sole colouring agents present in the ink used in the method. If the ink used in the method is cured by exposure to a source of actinic radiation without an inert environment, one or more photoinitiators will be required. If the ink used in the method is cured by exposure to a source of low-energy electron beam radiation or a source of actinic radiation in an inert environment, the ink may still contain a photoinitiator, although photo initiators are not required. In a preferred embodiment, the ink used in the method of the present invention further comprises one or more photoinitiators. Preferred are photoinitiators which produce free radicals on irradiation (free radical photoinitiators) such as, for example, benzophenone, 1-hydroxycyclohexyl phenyl ketone, 2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzil dimethylketal, phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide or mixtures thereof. Such photoinitiators are known and commercially available such as, for example, under the trade names Omnirad (from IGM) and Esacure (from Lamberti). Mixtures of free radical photoinitiators can be used and preferably, the ink used in the method comprises a plurality of free radical photoinitiators. The total number of free radical photoinitiators present is preferably from one to five, and more preferably, two or more free radical photoinitiators are present in the ink. Polymeric photoinitiators are preferred. Examples include Omnipol TP®, Omnipol 910® and Speedcure 7010®. Omnipol TP® is commercially available from IGM. It is a polymeric phosphine oxide photoinitiator, and is known by the chemical name polymeric ethyl (2,4,6-trimethylbenzoyl)-phenyl phosphinate or polymeric TPO-L, It has the following structure: The total value of a, b and c of the chemical formula for polymeric TPO-L is equal to 1-20. Omnipol 910® is also commercially available from IGM. It is a piparazino-based aminoalkylphenone having the following structure: The value of n of the chemical formula for Omnipol 910® is equal to 1-10. Speedcure 7010L® is a particularly preferred photoinitiator for inclusion in the ink. Speedcure 7010L® is commercially available from Lambson®. Speedcure 7010L® is a liquid at 20°C and is a solution of 1,3-di({a-[1-chloro-9-oxo-9H-thioxanthen-4-yloxy]acetylpoly[oxy(1-methylethylene)]} oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yloxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl) propane in trimethylolpropane ethoxylate triacrylate. 1,3-Di({a-[1 -chloro-9-oxo-9H-thioxanthen-4-yloxy]acetylpoly[oxy(1-methylethylene)]}oxy)-2,2-bis({a-[1-chloro-9-oxo-9H-thioxanthen-4-yloxy]acetylpoly[oxy(1-methylethylene)]}oxymethyl) propane has the following structure: The total value of a, b, c and d of the chemical formula for Speedcure 7010 is equal to 1-20. In a preferred embodiment, the value of a+b+c+d of the chemical formula for Speedcure 7010 is equal to 1-15. Preferably, the photoinitiator if present, is present from 1 to 20% by weight, preferably from 5 to 15% by weight, of the ink. The presence of a photoinitiator is optional as it is not necessary to include a photoinitiator in the inkjet ink used in the method in order to achieve a thorough cure of the ink. This is because the ink can cure without the presence of a photoinitiator by curing with a low-energy electron beam or curing by actinic radiation in an inert environment. Therefore, in a preferred embodiment, the photoinitiator is present in an amount of less than 20% by weight, preferably less than 5% by weight, more preferably less than 3%, more preferably less than 1%, based on the total weight of the ink. As such, the inkjet ink used in the method may be substantially free of photoinitiator. By “substantially free” is meant that no photoinitiator is intentionally added to the ink. However, minor amounts of photoinitiator, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink used in the method may comprise less than 0.5% by weight of photoinitiator, more preferably less than 0.1% by weight of photoinitiator, most preferably less than 0.05% by weight of photoinitiator, based on the total weight of the ink. The inkjet ink used in the method may also be free of photoinitiator. An inkjet ink that is substantially free of photoinitiator is advantageous for various applications as there will be no unreacted photoinitiator or unreacted photoinitiator fragments present in the cured inkjet ink film. Photoinitiators create free radicals when exposed to radiation. These radicals react with reactive components of the ink (such as reactive monomers and oligomers). However, some photoinitiator and photoinitiator fragments will remain unreacted in the cured ink film and this is problematic for certain applications, such as food packaging, as such unreacted components can migrate into the substrate. However, an inkjet ink that is cured with a low-energy electron beam or actinic radiation in an inert environment may still contain less than 5% by weight of a photoinitiator, based on the total weight of the ink. This is required if the ink is first pinned with actinic radiation. By pinning is meant arresting the flow of the ink by treating the ink droplets quickly after they have impacted onto the substrate surface. Pinning provides a partial cure of the ink and thereby maximises image quality by controlling bleed and feathering between image areas. Pinning does not achieve full cure of the ink. By curing is meant fully curing the ink. Pinning leads to a marked increase in viscosity, whereas curing converts the inkjet ink from a liquid ink to a solid film. The dose of radiation used for pinning is generally lower than the dose required to cure the radiation-curable material fully. The inkjet ink used in the method of the present invention preferably dries primarily by curing, i.e. by the polymerisation of the monomers present, as discussed hereinabove, and hence is a curable ink. The ink does not, therefore, require the presence of water or a volatile organic solvent to effect drying of the ink. Accordingly, the inkjet ink used in the method preferably comprises less than 5% by weight of water and volatile organic solvents combined, based on the total weight of the ink. Preferably, the inkjet ink used in the method comprises less than 3% by weight of water and volatile organic solvent combined, more preferably less than 2 % by weight combined, more preferably less than 1% by weight combined, and most preferably the inkjet ink used in the method is substantially free of water and volatile organic solvents, where the amounts are based on the total weight of the ink. By substantially free is meant that only small amounts will be present, for example some water will typically be absorbed by the ink from the air and solvents may be present as impurities in the components of the inks, but such low levels are tolerated. In other words, no water or a volatile organic solvent is intentionally added to the ink. However, minor amounts of water or a volatile organic solvent, which may be present as impurities in commercially available inkjet ink components, are tolerated. For example, the ink used in the method may comprise less than 0.5% by weight of water or a volatile organic solvent, more preferably less than 0.1% by weight of water or a volatile organic solvent, most preferably less than 0.05% by weight of water ora volatile organic solvent, based on the total weight of the ink. In a preferred embodiment, the inkjet ink used in the method is free of water or a volatile organic solvent. In a preferred embodiment, the ink used in the method of the present invention comprises a surfactant. The surfactant controls the surface tension of the ink. Surfactants are well-known in the art and a detailed description is not required. An example of a suitable surfactant is BYK307. Adjustment of the surface tension of the inks allows control of the surface wetting of the inks on various substrates, for example, plastic substrates. Too high a surface tension can lead to ink pooling and / or a mottled appearance in high coverage areas of the print. Too low a surface tension can lead to excessive ink bleed between different coloured inks. Surface tension is also critical to ensuring stable jetting (nozzle plate wetting and sustainability). The surface tension is preferably in the range of 18-40 mNm1, more preferably 20-35 mNm1 and most preferably 20-30 mNm1. Other components of types known in the art may be present in the ink used in the method of the present invention to improve the properties or performance. These components may be, for example, additional surfactants, defoamers, dispersants, synergists, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, biocides and identifying tracers. The inks used in the method of the invention may be prepared by known methods such as, for example, stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill. The ink used in the method of the present invention has a viscosity of 1 to 40 mPas at 25°C. The ink preferably has a viscosity of 1 to 20 mPas. Ink viscosity may be measured using a Brookfield viscometer fitted with a thermostatically controlled cup and spindle arrangement, such as a DV1 low-viscosity viscometer running at 20 rpm at 25°C with spindle 00. Preferably, the black inkjet ink used in the method of the present invention has a reflective density of 0 to 3. By reflective density, it is meant the percentage of light reflected from the substrate and the ink. This can be calculated by the following formula: Reflective density = Iog10 1 / reflective factor. The reflective factor is measured using a Reflection Densitometer Viptronic Vipdens C5 drawn down onto PVC using an automated K101 control coater to provide 12 pm drawdowns. Preferably, the black inkjet ink used in the method of the present invention has a surface tension of 18 to 40 dynes / cm at 25°C. Surface tension may be measured using a Static Surface Tensiometer Sigma 702. The liquid density of an ink is measured at 25°C. This density reading is then used to measure the average of three surface tension readings at 25°C using a platinum Du Nouy ring made to fulfil the requirements of at least the following standards: ISO 301, ISO 4311, ISO 6889, ASTM D1331 and ASTM D 971. The Huh Mason value is recorded, giving an average of three measurements. In a preferred embodiment, the layer of black inkjet ink is cured by any means known in the art, such as exposure to actinic radiation and low-energy electron beam radiation. It should be noted that the terms “dry” and “cure” are often used interchangeably in the art when referring to radiation-curable inkjet inks to mean the conversion of the inkjet ink from a liquid to solid by polymerisation and / or crosslinking of the radiation-curable material. Herein, however, by “drying” is meant the removal of the water by evaporation and by “curing” is meant the polymerisation and / or crosslinking of the radiation-curable material. Further details of the printing, drying and curing process are provided in WO 2011 / 021052. In a preferred embodiment, the layer of ink is cured by exposing the printed ink to a source of actinic radiation. The source of actinic radiation can be any source of actinic radiation that is suitable for curing radiation-curable inks but is preferably a UV source. Suitable UV sources are well-known in the art and a detailed description is not required. These include mercury discharge lamps, fluorescent tubes, light emitting diodes (LEDs), flash lamps and combinations thereof. One or more mercury discharge lamps, fluorescent tubes, or flash lamps may be used as the radiation source. Preferably, the source of actinic radiation is a mercury discharge lamp and / or LEDs. When LEDs are used, these are preferably provided as an array of multiple LEDs. The most common UV light source used to cure inkjet inks is a mercury discharge lamp. These lamps operate by creating a plasma between two electrodes in a high pressure mercury gas contained in a quartz envelope. Although these lamps have some drawbacks in terms of their operational characteristics, no other UV light source has yet managed to challenge their position in terms of UV output performance. LEDs are increasingly used to cure inkjet inks. UV light is emitted from a UV LED light source. UV LED light sources comprise one or more LEDs and are well-known in the art. Thus, a detailed description is not required. It will be understood that UV LED light sources emit radiation having a spread of wavelengths. The emission of UV LED light sources is identified by the wavelength which corresponds to the peak in the wavelength distribution. Compared to conventional mercury lamp UV sources, UV LED light sources emit UV radiation over a narrow range of wavelengths on the wavelength distribution. The width of the range of wavelengths on the wavelength distribution is called a wavelength band. LEDs therefore have a narrow wavelength output when compared to other sources of UV radiation. By a narrow wavelength band, it is meant that at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength within a wavelength band having a width of 50 nm or less, preferably, 30 nm or less, most preferably 15 nm or less. In a preferred embodiment, at least 90%, preferably at least 95%, of the radiation emitted from the UV LED light source has a wavelength in a band having a width of 50 nm or less, preferably 30 nm or less, most preferably 15 nm or less. LEDs have a longer lifetime and exhibit no change in the power / wavelength output overtime. LEDs also have the advantage of switching on instantaneously with no thermal stabilisation time and their use results in minimal heating of the substrate. In a preferred embodiment, the layer of ink is cured by exposing the printed ink to low-energy electron beam (ebeam). The source of low-energy electron beam (ebeam) can be any source of low-energy electron beam that is suitable for curing radiation-curable inks. Suitable low-energy electron beam radiation sources include commercially available ebeam curing units, such as the EB Lab from ebeam Technologies with energy of 80-300 keV and capable of delivering a typical dose of 30-50 kGy at line speeds of up to 30 m / min. By “low-energy” for the ebeam, it is meant that it delivers an electron beam having a dose at the substrate of 100 kGy or less, preferably 70 kGy or less. Ebeam curing is characterised by dose (energy per unit mass, measured in kilograys (kGy)) deposited in the substrate via electrons. Electron beam surface penetration depends upon the mass, density and thickness of the material being cured. Compared with UV penetration, electrons penetrate deeply through both lower and higher density materials. Unlike UV curing, photoinitiators are not required for ebeam curing to take place. Ebeam curing is well-known in the art and therefore a detailed explanation of the curing method is not required. In order to cure the printed ink, the ink used in the method of the invention is preferably exposed to the ebeam, which produces sufficient energy to instantaneously break chemical bonds and enable polymerisation or crosslinking. There is no restriction on the ebeam dose that is used to cure the inkjet inks of the present invention other than that the dose is sufficient to fully cure the ink. Preferably, the dose is more than 10 kGy, more preferably more than 20 kGy, more preferably more than 30 kGy and most preferably more than 40 kGy. Preferably, the dose is less than 100 kGy, more preferably less than 90 kGy, more preferably less than 80 kGy and most preferably less than 70 kGy. Preferably, the dose is more than 30 kGy but less than 70 kGy, more preferably more than 30 kGy but less than 60 kGy and most preferably, more than 30 kGy but 50 kGy or less. Doses above 50 kGy may cause damage to the substrate and so doses of 50 kGy or less are preferred. The energy associated with these doses is 80-300 keV, more preferably 70-200 keV and most preferably 100 keV. The ink cures to form a relatively thin polymerised film. The ink used in the method of the present invention typically produces a printed film having a thickness of 1 to 20 pm, preferably 1 to 10 pm, for example 2 to 5 pm. Film thicknesses can be measured using a confocal laser scanning microscope. The present invention also provides a method of recycling one or more plastic substrates, wherein the method comprises the method of sorting one or more plastic substrates of the present invention. The recycling of one or more plastic substrates is a well-known process in the art and a detailed description is not required. This usually involves collecting the one or more plastic substrates, sorting the one or more plastic substrates, washing the one or more plastic substrates, shredding the one or more plastic substrates, and extruding the one or more plastic substrates. In the present invention, the method of recycling one or more plastic substrates comprises the method of sorting one or more plastic substrates of the present invention and may further comprise any of the well-known recycling steps, which are known in the art. In a preferred embodiment, the method of recycling one or more plastic substrates comprises collecting the one or more plastic substrates. In a preferred embodiment, the method of recycling one or more plastic substrates comprises washing the one or more plastic substrates. This washing step is well- known in the art and may include removing waste, adhesives, foods and labels from the one or more plastic substrates. In a preferred embodiment, the method of recycling one or more plastic substrates comprises shredding the one or more plastic substrates. In a preferred embodiment, the method of recycling one or more plastic substrates comprises extruding the one or more plastic substrates. Preferably, the method of recycling one or more plastic substrates comprises the following steps: (i) collecting the one or more plastic substrates; (ii) sorting the one or more plastic substrates according to the present invention; 5 (iii) washing the one or more plastic substrates; (iv) shredding the one or more plastic substrates; and (v) extruding the one or more plastic substrates. The present invention may also provide the use of a black inkjet ink in a method of sorting one or 10 more plastic substrates using infrared spectroscopy, wherein the black inkjet ink comprises a radiation-curable monomer and less than 2.0% by weight of carbon black, based on the total weight of the ink, wherein the ink further comprises two or more non-black pigments which in combination provide a black colour, wherein the ink has an L* value of 0 to 40, an a* value of-20 to +20, and a b* value of-20 to +20, and wherein the ink has a viscosity of 1 to 40 mPas at 25°C. 15 The invention will now be described with reference to the following examples, which are not intended to be limiting. Examples Example 1 5 Inkjet inks were prepared according to the formulations set out in Table 1. The inkjet ink formulations were prepared by mixing the components in the given amounts. Amounts are given as weight percentages based on the total weight of the ink. Table 1. Component Comparative ink 1 Ink 2 Ink 3 Ink 4 Ink 5 Ink 6 Ink 7 Comparative ink 8 3-MPDDA 68.3 61.1 63.2 64.0 64.2 - 47.0 - DVE-3 6.9 6.9 7.0 7.0 7.0 - 6.0 - HDDA - - - - - 26.5 - 31.2 VEE A - - - - - 34.0 - 28.0 TMPTA - - - - - 4.0 - 8.3 DPHA - - - - - - - 5.0 UV22 0.5 0.5 0.5 0.5 0.5 - 0.5 - UV1 - - - - - 0.1 - 0.1 Bykjet 9151 - - - - - 0.3 - 0.3 UVP6600 3.0 - - - - - - 2.0 Omnirad 819 4.5 4.5 4.5 4.5 4.5 3.0 4.5 3.0 Omnirad 2959 3.0 3.0 3.0 3.0 3.0 - 3.0 - KIP 160 5.3 5.3 5.3 5.3 5.3 - 5.3 - TPO - - - - - 10.0 - 10.0 ITX - - - - - 6.0 - 6.0 Black pigment dispersion 1 7.5 - - - - - - 5.5 Magenta pigment dispersion 1 - 6.0 - - 7.0 - 15.8 - Yellow pigment dispersion 1 - 8.7 - - - - - - Cyan pigment dispersion 1 - - 3.0 - - 3.0 - - Cyan pigment dispersion 2 - - - 3.0 - - - - Cyan pigment dispersion 3 - 3.0 - - 3.0 - 6.8 - Magenta pigment dispersion 2 - - 7.5 - 7.5 - - Magenta pigment dispersion 3 - - - 7.2 - - - - Yellow pigment dispersion 2 - - 5.0 4.5 4.5 5.0 10.1 - Byk 307 1.0 1.0 1.0 1.0 1.0 0.6 1.0 0.6 Total 100 100 100 100 100 100 100 100 Viscosity / mPa.s 13.0 14.6 14.0 14.2 12.4 14.3 22.6 15.0 3-MPDDA, DVE-3, HDDA, VEEA, TMPTA, DPHA, PEA, CTFA, IBOA, NVC and DPGDA are radiation-curable monomers as defined above. UV22 and UV1 are stabilisers. Bykjet 9151 is a wetting and dispersing additive. UVP6600 and CN964 A85 are radiation-curable oligomers as discussed above. Omnirad 819 (BAPO), Omnirad 2959, KIP 160, TPO and ITX are photoinitiators. Byk 307 is a surfactant. Black pigment dispersion 1 contains 30% dispersant, 0.9% synergist, 29.1% DVE-3 and 40% carbon black. Magenta pigment dispersion 1 contains 42% dispersant, 28% DVE-3 and 30% magenta pigment. Yellow pigment dispersion 1 contains 22.5% dispersant, 47.5% DVE-3 and 30% yellow pigment Cyan pigment dispersion 1 contains 10% dispersant, 1 % stabiliser, 59% PEA and 30% cyan pigment. Cyan pigment dispersion 2 contains 10% dispersant, 1% stabiliser, 59% NPGPODA and 30% cyan pigment. Cyan pigment dispersion 3 contains 20% dispersant, 50% DVE-3 and 30% cyan pigment. Magenta pigment dispersion 2 contains 12% dispersant, 1.5% stabiliser, 56.5% PEA and 30% magenta pigment. Magenta pigment dispersion 3 contains 20% dispersant, 1% stabiliser, 49% PEA and 30% magenta pigment. Yellow pigment dispersion 2 contains 10% dispersant, 1% stabiliser, 69% 3-MPDDA and 20% yellow pigment. The viscosity of the inks was measured using a Brookfield DV1 low-viscosity viscometer, which is fitted with a thermostatically controlled cup and spindle arrangement, running at 20 rpm at 25°C with spindle 00. All of the inkjet inks have a viscosity of less than 30 mPa.s and so have an ink-jettable viscosity. The inks of Table 1 were drawn down onto 220 micron Gloss PVC using an automated K101 control coater to provide 12 pm drawdowns. The inks were then cured as shown in Table 2. Table 2. Ink Cure settings Per pass Dose / mJ / cm2 Intensity / mW / cm2 Comparative Ink 1 Medium pressure mercury lamp -80 m / min 165 2810 Ink 2 Medium pressure mercury lamp -80 m / min 165 2810 Ink 3 Medium pressure mercury lamp -80 m / min 165 2810 Ink 4 Medium pressure mercury lamp -80 m / min 165 2810 Ink 5 Medium pressure mercury lamp -80 m / min 165 2810 Ink 6 395nm 20W LED lamp - 90 m / min 275 14579 Ink 7 Medium pressure mercury lamp -80 m / min N / A N / A Comparative Ink 8 395nm 20W LED lamp - 90 m / min 275 14579 The properties of the cured ink films were assessed as shown in Table 3 and in Figure 1. Table 3. Component Comparative ink 1 Ink 2 Ink 3 Ink 4 Ink 5 Ink 6 Ink 7 Comparative ink 8 L* 10.21 31.17 28.26 29.79 28.28 27.09 9.73 14.26 a* 1.31 -2.23 0.99 2.51 0.66 2.23 0.87 1.84 b* 3.25 -1.15 2.94 3.92 1.32 7.15 -2.42 5.16 The L*, a* and b* values of the inks were measured using an eXact Advanced Spectrophotometer with a viewing angle of 2°, an illuminate source of d50 and no filter. As can be seen from the results, the inks for use in the method of the invention are black inkjet inks, having the required a*, b* and L* values, without recourse to a black dispersion. It is surprising that an L* value of 0 to 40 and in particular, an L* value of less than 30 for inks 3-7 of the invention, can be achieved without recourse to a carbon black pigment. The cured ink films of comparative ink 1 and ink 2 were then irradiated with infrared radiation and the absorption of infrared radiation for each was measured using a Fourier-transform infrared (FTIR) spectrometer. Specifically, a Thermo Nicolet iS50 FTIR spectrometer was used having an IR source with a scan range of 4000 to 7800 cm1, a DTGS KBR detector, a KBr beam splitter, a NaCI FTIR cell window, in transmission mode of collection, with six scans and four resolution. In this case, the absorption of infrared radiation for each ink sample was measured as a percentage transmittance by measuring the infrared radiation after travelling through the ink sample. Loss in light transmittance is due to absorbance by the ink sample. As can be seen from Figure 1, the percentage light transmittance of the black inkjet ink used in the method of the present invention is much higher across the NIR wavelength range when compared to the comparative ink, which comprises carbon black. As such, the black inkjet ink used in the method of the present invention does not absorb IR radiation to the same extent as an inkjet ink containing carbon black. This means that it will be possible to sort one or more plastic substrates, wherein one or more of the plastic substrates has a black inkjet ink as used in the method of the present invention, using infrared spectroscopy. In contrast, it will not be possible to sort one or more plastics using carbon black to provide the black colour as the carbon black absorbs the IR radiation.

Claims

1. A method of sorting one or more plastic substrates using infrared spectroscopy, wherein one or more of the plastic substrates comprises a layer of black inkjet ink, wherein the black inkjet ink comprises a radiation-curable monomer and less than 2.0% by weight of carbon black, based on the total weight of the ink, wherein the ink further comprises two or more non-black pigments which in combination provide a black colour, wherein the ink has an L* value of 0 to 40, an a* value of -20 to +20, and a b* value of -20 to +20, and wherein the ink has a viscosity of 1 to 40 mPas at 25°C.

2. A method of sorting one or more plastic substrates as claimed in claim 1, wherein the infrared spectroscopy is near-infrared spectroscopy.

3. A method of sorting one or more plastic substrates as claimed in claims 1 or 2, comprising: (a) irradiating one or more plastic substrates with infrared radiation;(b) measuring absorption of infrared radiation of one or more of the irradiated plastic substrates; and(c) sorting the one or more plastic substrates according to the absorption of infrared radiation by the one or more plastic substrates.

4. A method of sorting one or more plastic substrates as claimed in claim 3, wherein the infrared radiation has a wavelength of from 0.70 to 3.0 pm.

5. A method of sorting one or more plastic substrates as claimed in claims 3 or 4, wherein the infrared radiation is near-infrared radiation.

6. A method of sorting one or more plastic substrates as claimed in any preceding claim, wherein the ink comprises two or more non-black pigments selected from cyan, magenta, yellow, blue, brown, green, orange, red, violet and white.

7. A method of sorting one or more plastic substrates as claimed in any preceding claim, wherein the ink comprises three or more non-black pigments which in combination provide the black colour.

8. A method of sorting one or more plastic substrates as claimed in claim 7, wherein the three or more non-black pigments are selected from: cyan, magenta and yellow; yellow, cyan and violet; yellow, green and violet; yellow, blue and orange; green, orange and violet; red, cyan and yellow; and red, green and blue.

9. A method of sorting one or more plastic substrates as claimed in any preceding claim, wherein each non-black pigment of the ink is present in the ink at 0.5 to 6% by weight, based on the total weight of the ink.

10. A method of sorting one or more plastic substrates as claimed in any preceding claim, wherein the ink comprises less than 0.8% by weight of carbon black, based on the total weight of the ink, and is preferably substantially free of carbon black.

11. A method of sorting one or more plastic substrates as claimed in any preceding claim, wherein the ink comprises less than 2.0% by weight in total, preferably less than 0.8% by weight in total, of black pigment, based on the total weight of the ink.

12. A method of sorting one or more plastic substrates as claimed in any preceding claim, wherein the ink comprises less than 5% by weight of water and volatile organic solvents combined, based on the total weight of the ink13. A method of sorting one or more plastic substrates as claimed in any preceding claim, wherein the ink has a reflective density of 0 to 3 and / or a surface tension of 18 to 40 dynes / cm at 25°C.

14. A method of sorting one or more plastic substrates as claimed in any preceding claim, wherein the radiation-curable monomer of the ink comprises a di- and / or multifunctional monomer.

15. A method of sorting one or more plastic substrates as claimed in any preceding claim, wherein the radiation-curable monomer of the ink is a (meth)acrylate monomer.

16. A method of recycling one or more plastic substrates, wherein the method comprises the method of sorting one or more plastic substrates as claimed in any preceding claim.