WO2008065085A1

WO2008065085A1 - Fluorescent liquid ink composition for inkjet printing

Info

Publication number: WO2008065085A1
Application number: PCT/EP2007/062827
Authority: WO
Inventors: Vincent Millot; Grégory GUILLOT-PATRIQUE; Jean-Yves Sabys
Original assignee: Imaje S.A.
Priority date: 2006-11-28
Filing date: 2007-11-26
Publication date: 2008-06-05
Also published as: JP2010511069A; FR2909095A1; EP2087051A1; FR2909095B1; JP5363331B2

Abstract

Ink composition which is invisible to the naked eye, which is fluorescent, for inkjet printing, and which is liquid at ambient temperature, comprising: a) a binder, comprising at least 0.5% by weight, with respect to the total weight of the ink, of at least one thermoplastic polyamide resin, soluble in ethanol; b) a solvent comprising at least 10% by weight, with respect to the total weight of the ink, of one or more organic solvent compound (s); c) at least 0.05% by weight, with respect to the total weight of the ink, of one or more dye(s) and/or pigment (s) which is (are) invisible to the naked eye and which is (are) fluorescent in the visible region only when they are exposed to ultraviolet light and which are chosen from rare earth metal chelates and rare earth metal complexes; d) less than 10% by weight of water, with respect to the total weight of the ink. Marking process, substrate and method for identifying items.

Description

FLUORESCENT LIQUID INK COMPOSITION FOR INKJET PRINTING

DESCRIPTION

TECHNICAL FIELD

The invention relates to fluorescent ink compositions for the marking of supports, substrates and items of all kinds, the properties of which are particularly well suited to the inkjet marking or printing, both by the "continuous inkjet" technique and by the "drop-on-demand" technique, of a very wide variety of supports, substrates and items, whether porous or nonporous.

More specifically, the ink compositions according to the invention are ink compositions which are colourless when illuminated by visible light, invisible to the naked eye, and fluorescent under UV light, that is to say emitting visible light when they are illuminated by ultraviolet light.

For this reason, the ink compositions according to the invention are very particularly suitable for the marking of all porous or nonporous supports in a way invisible to the naked eye, whether for security codes, bar codes or codes which can be read by automatic reading systems, such as postal index code readers .

InkJet printing is a well known technique which makes possible the printing, the marking or the decorating of all kinds of items, at high speed and without contact of these items with the printing device, with messages which can be varied at will, such as bar codes, sell-by dates, and the like, even on nonplanar supports .

The inkjet printing systems are divided into two main types: the "drop-on-demand" (DOD) type and the "continuous inkjet" (CIJ) type.

Deflected continuous inkjet projection consists in conveying ink under pressure into a cavity comprising a piezoelectric crystal, from where the ink escapes via an orifice (nozzle) in the form of a jet. The piezoelectric crystal, vibrating at a predetermined frequency, causes disturbances in pressure in the inkjet, which oscillates and gradually breaks up into spherical drops or droplets. An electrode, placed on the path of the jet, at the point where it breaks up into drops, makes it possible to give an electrostatic charge to these drops, if the ink is conducting. The drops, thus charged, are deflected in an electric field and make possible the printing. The uncharged and thus undeflected drops are recovered in a gutter, where the ink is sucked up and then recycled to the ink circuit.

This type of continuous inkjet ink projection provides contact-free marking at a high rate of forward progression on items which are not necessarily flat and with the possibility of changing the message at will.

The technique is particularly suited to the marking and the identification (expiry dates, serial numbers, batch numbers, bar codes, and the like) of industrial products on production lines or to the postal indexing of postal letters and packets. Postal indexing requires inks, both for porous supports, such as the paper of letters, and for nonporous or only slightly porous supports, such as the plastic wrapping films for magazines and other papers distributed by post.

"Drop-on-demand" jet projection can be carried out by a "bubble" jet or by a "piezoelectric" jet. In the first case, the ink is vaporized in the vicinity of the nozzle and this vaporization brings about the ejection of the small amount of ink situated between the resistance which vaporizes the ink and the nozzle. In the second case, a sudden variation in pressure, brought about by an actuator set in motion by the electrical excitation of a piezoelectric crystal or ceramic situated in the vicinity of the nozzle, brings about the ejection of a drop of ink.

The diameter of the nozzles for the DOD jet is of the order of 10 to a few tens of microns. For the continuous inkjet, the diameter of the nozzles is greater, of the order of 30 to 100 microns.

The ink compositions suitable for jet projection have to satisfy a certain number of criteria inherent to this technique relating, inter alia, to the viscosity, the solubility in a solvent for the cleaning, the compatibility of the ingredients, the correct wetting of the supports to be marked, and the like, and the electrical conductivity in the case of the deflected continuous inkjet.

Furthermore, these inks have to rapidly dry and be capable of flowing or remaining immobile in the vicinity of the nozzle without blocking it, with great stability of orientation of the jet, while making possible ready cleaning of the printhead.

These inks must be carefully formulated and filtered in order not to comprise particles with a size capable of blocking the nozzles. Filtrations at thresholds between 0.2 and 1 μm are generally applied.

The ingredients of which current inks for inkjets of deflected continuous inkjet type are composed are organic or inorganic products; they are colouring materials, such as dyes or pigments, resins or binders, in one or more, more or less volatile solvent (s) or in water, and optionally one or more salts (s) which introduce (s) conductivity, as well as various additives.

The ingredients of which current inks for inkjets of drop-on-demand (DOD) type are composed are also organic or inorganic products; dyes or pigments, resins or binders, in one or more more or less volatile solvent (s) or in water, in proportions different from those of the inks for deflected continuous inkjets, but without the need for electrical conductivity.

The colouring materials are referred to as dyes or pigments according to whether they are respectively soluble or insoluble in the solvent used.

Pigments, by nature insoluble, are thus dispersed and can be opaque or nonopaque. They contribute, to the ink, its colour, its opacity or specific optical properties, such as fluorescence (cf. Patents or Patent Applications US-A-4 153 593, US-A-4 756 758, US-A-4 880 465, EP-A-O 289 141, US-A-5 395 432, GB-A-2 298 713) . In some cases, the dyes themselves also contribute enough conductivity to the ink for there to be no need to add a conductivity salt. The dyes known under the names C.I. Solvent Black 27, 29, 35 and 45 come within this case.

The inks intended for postal indexing are generally fluorescent when illuminated under visible light or by UV rays in order to be able to be read by automatic systems which carry out their identification and their automatic sorting with very high rates of reading. They are visible to the naked eye and generally orange in colour in order to be easily spotted by the operators of postal services. When the surfaces of postal items, such as letters or packets, have an excessively strong colour, this interferes with the fluorescent dye of the ink and the fluorescence is reduced to the point where the marking is no longer fluorescent enough to be automatically detected and read.

Some invisible marking systems intended for combating forgery are also based on colourless inks which are invisible to the naked eye but which become visible under UV illumination.

Some types of ink comprise dyes of optical brightener type which absorb UV light and which emit visible light bluish in colour. When such inks are deposited on white paper, competition occurs between the optical brighteners of the paper and those of the ink, making the ink difficult to spot.

Some other systems are based on inks comprising dyes which do not absorb very much in the visible spectrum but which absorb and fluoresce in infrared light. These systems require special equipment in order to be detected as the emission of infrared light is not visible to the naked eye.

There thus exists a specific need, for postal indexing, as for security applications, for fluorescent inks which are less sensitive to the colour of the support in order for the level of reading to be better than with coloured inks; more particularly, there exists a need for inks which are visible to the naked eye under UV illumination, thus emitting visible light under UV illumination, but which emit visible light with a wavelength different from that of the optical brighteners generally used in paper.

The dyes which have the property of being colourless and invisible, which absorb UV radiation and which emit, under UV illumination, visible light at greater wavelengths than those of optical brighteners are rare earth metal chelates. These well known dyes are available commercially, for example from Honeywell. These dyes absorb UV radiation in order to re-emit visible light, for example at approximately 610-620 nm for europium chelates or at approximately 490 and 580 nm for dysprosium chelates.

However, due to their high absorption of ultraviolet light, these dyes can be decomposed by the light and the stability of the fluorescence can be greatly affected without a visible change in colour and thus insidiously.

In addition, due to the recirculation of the ink in printers of deflected continuous inkjet type, the ink is continually in intimate contact with the surrounding air. The stability of the complexes of these rare earth metals can be affected by this contact with the air. The consequence can be the complete loss of the fluorescence.

There thus exists a specific need for inks, the stability over time and during continuous operation in a printer of which is sufficient for the ink to retain its essential fluorescence properties.

The binder (s) or resin (s) is (are) generally for the most part (a) solid and polymeric component (s) and their choice is dictated by their solubility in the solvents selected and by their compatibility with the dyes and the other additives but also and in particular for the properties which they contribute to the ink film once dry (see Patents or Patent Applications US-A-4 834 799, GB-A-2 286 402, US-A-5 594 044, US-A-5 316 575, WO-A-96/23844, WO-A-95/29 287) .

Their first function is to contribute, to the ink, the adhesion to the maximum number of supports or to specific supports, for example nonporous supports. They also make it possible to provide the ink with a viscosity appropriate for the formation of the drops starting from the jet and they contribute, to the ink or rather to the marking obtained, the main part of its properties of resistance to physical and/or chemical attacks.

The additives comprise: plasticizers, which render the dry ink film flexible, which can improve the adhesion and the cohesion of the ink on the marked support, dispersants, which make possible the dispersion of the pigments. Such dispersants stabilize the pigments by a steric effect and/or by an electrostatic effect, according to whether or not they are ionizable and according to the polarity of the solvent, agents which inhibit the corrosion brought about by certain salts, such as chlorides, which contribute conductivity (see documents EP-A-O 510 752 and US-A-5 102 458), additives which protect the ink against the growth of bacteria and other microorganisms: they are biocides, bactericides, fungicides and others, which are of particular use in inks which comprise water, pH-regulating buffers (see EP- A 0 735 120) , antifoaming agents.

The inks for inkjets can also comprise surfactants or surface-active agents which modify the wetting or penetrating power of the ink (cf . Patent

US-5 395 431), in particular those which modify or regulate the static or dynamic surface tension, such as

Fluorad FC 430 from 3M. Such products regulate the size of the impacts of the drops. By virtue of them, the impacts of the drops all have the same diameter, whatever the nature, the cleanliness or the evenness of the support.

The additives also optionally comprise, in the case of inks for deflected continuous inkjet printing, one or more conductivity salts. The optional conductivity salt(s) contribute (s) , to the ink, the conductivity necessary for the electrostatic deviation. Reference may be made, on this subject, to the document US-A-4 465 800. However, it may be noted that, in some cases too, the dyes already contribute sufficient conductivity to the ink for there to be no need to add a conductivity salt.

Use is made, among salts which contribute conductivity, of any type of ionizable entity which can dissolve and dissociate in the solvent medium of the ink.

For inks where the predominant solvent is water, the contribution of conductivity to the ink does not generally present a problem as the majority of water-soluble products are ionizable entities.

However, the aqueous medium does not allow a great variety of formulations as the solvating power of water is limited and the rate of evaporation of water is too slow to be able to take advantage of the possibilities of high-speed printing allowed by inkjets. Moreover, the variety of organic solvents with a strong dissolving power for polymers and dyes makes it possible to formulate inks which dry rapidly and which adhere very well to all kinds of substrates.

On the other hand, in these organic media, conductivity is possible only when the solvents are sufficiently dissociating, that is to say when their dielectric permittivity and their dipole moment are sufficiently high. When this is the case, it is necessary to find a salt having a cation and an anion sufficiently soluble in the medium. The solvents which can be used for the inks are, on the one hand, generally in a predominant amount, volatile solvents of low viscosity, in order to make possible the very rapid drying of the markings and to adjust the viscosity to the desired value, for example from 2 to 10 mPa.s, such as acetone or ethanol; and, on the other hand, more viscous and less volatile solvents which dry more slowly, in a lower amount, in order to prevent drying of the ink in the nozzle during the phases in which the printing device is shut down.

There thus exists a need for a rapid-drying ink which exhibits a good start-up quality after lengthy shutdowns.

In more detail, the volatile solvents generally used are alcohols, ketones and esters of low molecular weight, such as is indicated in Patents US-A-4 567 213 and US-A-5 637 139. Mention may essentially be made, among these solvents, of methanol, ethanol, 1-propanol, 2-propanol, acetone, methyl ethyl ketone (MEK) and methyl isobutyl ketone.

The less volatile solvents having in particular a role of delaying the drying are generally ketones, such as cyclohexanone, glycol ethers, mentioned in the documents US-A-4 024 096 and US-A-4 567 213, ethers and acetals, such as furan or dioxane, mentioned in the document US-A-4 155 767, dimethylformamide or dimethyl sulphoxide

(US-A-4 155 895), lactones (EP-A-O 034 881),

N-methylpyrrolidone (EP-A-O 735 120), glycols (WO-A-96

23 844) and even aliphatic hydrocarbons (US-A-4 166 044) or also water, alone or in combination with other solvents mentioned above; reference will be made, in this respect, to the documents US-A-4 153 593, GB-A-2 277 094 and FR-A-2 460 982.

Generally, the main or predominant solvents of inks for inkjet printing have to meet a certain number of criteria, in particular: their volatility has to be sufficient for the ink to dry rapidly on the support to be marked, but not excessively great, in order not to evaporate too quickly in the printer, in particular during shutdown phases; their solvating power, with regard to the binders of the ink, dyes or pigment dispersions and with regard to the supports to be printed, must make it possible to confer good adhesion on the dry ink; the predominant or main solvents (and also the minor solvents, to a lesser extent) must, in the case where the ink is an ink for deflected continuous inkjet printing, have the ability to keep ionic entities dissociated, such as the salts which confer, on the ink, its electrical conductivity.

There also exists a need for an ink suitable in particular for deflected continuous inkjet printing which makes possible the rapid marking of all types of items with different surface characteristics, both porous and nonporous .

In addition, the ink must give a marking which exhibits good adhesion and good resistance to chemical attacks; in particular, it must give a water- resistant marking and, generally, it must exhibit all the properties usually required for inks for inkjet printers, in particular for inks for printers using the continuous inkjet technique: viscosity, resistivity, and the like. The ink must have, at the same time, the property of rapid drying and of not blocking the nozzles and of thus making possible rapid startup even after a lengthy shutdown.

Furthermore, the ink must make possible the high-speed marking of items of all kinds, even weakly porous items, while always giving printing or marking of excellent quality and consistency.

The aim of the invention is thus to provide an ink composition suitable for inkjet printing, in particular but not solely for deflected continuous inkjet printing, which meets, inter alia, all the needs indicated above, which satisfies the criteria and requirements mentioned above, which does not exhibit the disadvantages, limitations, failings and inconveniences of the ink compositions of the prior art and which overcomes the problems of the compositions of the prior art.

The aim of the invention is more particularly to provide an ink composition for inkjet printing which is colourless, invisible to the naked eye and fluorescent when illuminated by ultraviolet light or more specifically which gives a marking which is colourless, invisible to the naked eye and fluorescent when illuminated by ultraviolet light, the said ink composition comprising a complex of rare earth metals, which meets the specific needs for such fluorescent ink compositions, in particular as regards stability of the fluorescence, which were listed above.

This aim and yet others are achieved, in accordance with the invention, by an ink composition which is invisible to the naked eye, which is fluorescent, for inkjet printing, and which is liquid at ambient temperature, comprising: a) a binder, comprising at least 0.5% by weight, with respect to the total weight of the ink, of at least one thermoplastic polyamide resin, soluble in ethanol; b) a solvent comprising at least 10% by weight, with respect to the total weight of the ink, of one or more organic solvent compound (s) ; c) at least 0.05% by weight, with respect to the total weight of the ink, of one or more dye(s) and/or pigment (s) which is (are) invisible to the naked eye and which is (are) fluorescent in the visible region only when it is (they are) exposed to ultraviolet light and which is (are) chosen from rare earth metal chelates and rare earth metal complexes; d) less than 10% by weight of water, with respect to the total weight of the ink.

The fluorescent dyes or pigments of the ink composition according to the invention are colourless and invisible to the naked eye; that is to say that they do not exhibit significant absorption in the wavelength range of visible light from approximately 400 to approximately 700 nm.

In addition, these fluorescent dyes or pigments, when they are exposed to ultraviolet light radiation generally in the wavelength range from 275 to 400 nm and only when they are exposed to this radiation, emit fluorescent radiation in the visible region . More specifically, it may be said that it is more precisely the marking or the printing produced by the ink composition according to the invention which is invisible to the naked eye or colourless and which is fluorescent under the conditions described in the present document.

The wavelength for fluorescence of the dye included in the composition according to the invention is generally less than or equal to approximately

700 nm, preferably less than or equal to approximately

650 nm.

Preferably, when it is desired to carry out printing on a white substrate, the ink composition according to the invention comprises a dye which is fluorescent within a wavelength range lying above the wavelength for fluorescence of optical brighteners, which is generally from 380 to 500 nm.

Advantageously, the wavelength for fluorescence of the dyes and/or pigments of the compositions according to the invention is thus from approximately 550 or 575 nm to approximately 700 nm and more preferably from 600 to 650 nm.

The above fluorescence wavelength conditions are themselves de facto fulfilled owing to the fact that rare earth metal complexes are involved.

The term "rare earth metals" is understood to mean all the elements also known as lanthanides, that is to say the elements with an atomic number of between 57 and 71 inclusive. The preferred elements are samarium, europium, gadolinium, neodymium, terbium, erbium and dysprosium, and the fluorescent dye(s) and/or pigment (s) is (are) thus chosen from samarium, europium, gadolinium, neodymium, terbium or dysprosium complexes or chelates, and their mixtures.

Examples of rare earth metal complex (es) or chelate (s) of the compositions according to the invention are those formed by the chelation or complexing of ions of rare earth metals or lanthanides, such as neodymium, europium, samarium, dysprosium, gadolinium, erbium and terbium ions, with organic ligands, such as acetylacetone, benzoylacetone, dibenzoylmethane and salicylic acid.

Examples of such complexes include europium acetylacetonate, samarium acetylacetonate, neodymium benzoylacetonate, terbium silicylate and dysprosium benzoylacetonate.

The complexes or chelates can be prepared by any process known to a person skilled in the art. For example, a ligand, such as acetylacetone, can be reacted under appropriate conditions with a rare earth metal halide, such as europium trichloride, to produce the rare earth metal chelate. Reference may be made in this regard to the document US-A-4 736 425.

The specific chelates mentioned above absorb ultraviolet radiation and are fluorescent in the visible region.

Europium acetylacetonate is fluorescent with an emission line in the red region which is particularly suitable for printing on white substrates or substrates possessing light colours.

Examples of these rare earth metal chelates or complexes which can be included in the ink compositions according to the invention are the compounds commercially available under the name Lumilux, more particularly Lumilux C, available from Honeywell (formerly Hoechst Celanese, Riedel-de Haen) .

The rare earth metal organic chelates Lumilux C have a melting point from approximately 130⁰C to approximately 160⁰C and a bulk density from approximately 500 kg/m³ to approximately 1100 kg/m³. Examples of Lumilux C organic pigments comprise Red CD 316, Red CD 331, Red CD 332, Red CD 335 and Red CD 339, which are yellowish (it should be specified that this yellowish colour relates only to the pure solid product. As soon as this product is dissolved and diluted, this colouring no longer appears) in the nonexcited state and which are fluorescent in the orange-red region when they are excited by ultraviolet radiation .

These pigments are soluble in organic solvents .

Red CD 331 is a europium chelate; it is provided in the form of a yellowish powder having an emission peak at 612 nm, a melting point in the range from 153 to 155°C and a density of 600 kg/m³.

Red CD 331 is soluble in acetone, ethyl acetate, ethanol, xylene, dichloromethane, dimethylformamide, n-hexane and dibutyl phthalate. Red CD 316 is a rare earth metal acetylacetonate .

Red CD 332 is a rare earth metal diketonate and has a melting point of 135 to 138°C and a density of 500 kg/m³.

Red CD 335, which is a europium chelate, has a melting point of 133°C and a density of 1030 kg/m³.

The term "ambient temperature" is generally understood to mean a temperature from 5 to 30⁰C, preferably from 10 to 25°C, more preferably from 15 to 24°C, better still from 20 to 23°C. It is clearly understood that the ink is liquid at atmospheric pressure .

Advantageously, the ink composition according to the invention comprises from 0.05 to 25% by weight, preferably from 0.1 to 25% by weight, more preferably from 0.2 to 10% by weight, better still from 0.5 to 5% by weight, with respect to the total weight of the ink, of the pigment (s) and/or dye(s) chosen from rare earth metal chelates and complexes.

According to the invention, the binder of the ink composition comprises at least 0.5% by weight, with respect to the total weight of the ink, of at least one polyamide resin.

The term "polyamides" is also understood to mean the copolymers derived from these, also known as copolyamides . Polyamide resins are often defined according to their solubility in the solvents. According to the invention, polyamide resins which are soluble in ethanol are used as these are more easily dissolved and do not reguire other more unharmful solvents such as hydrocarbons.

The terms "which are soluble in ethanol" defining polyamide resins are well known to a person skilled in the art and are commonly used in this technical field. The term "polyamide resin which is soluble in ethanol" is generally understood to mean that this resin is soluble in ethanol in all proportions.

Examples of polyamide resins which can be used in the binder of the ink compositions according to the invention are the thermoplastic polyamides available from Cray Valley under the name Crayamid.

The Crayamid polyamides of the "series 700" are thermoplastic polyamides which are soluble in alcohol (ethanol) . They have excellent adhesion to various substrates, including all types of plastic films .

Some characteristics of the Crayamid" polyamides are given below.

(1) 35% N. V. (nonvolatiles) 1:1 toluene : n-butanol

Preferred polyamides among the Crayamid polyamides are those of the "series 700" and in particular Crayamid^® 791.

Other polyamides are the polyamides (and copolyamides) available from Huntsman or Schering under the name Eurelon^®, such as Eurelon^® 962, 965, 966, 969 and 975, which are soluble in ethanol.

Some characteristics of the Eurelon polyamides are given below:

Reference No. Nature Softenrng Melt vrscosrty Amrne number Acrd number Colour pornt (⁰C) (Pa-s) (mg KOH/g) (mg KOH/g) (Gardner)

Eurelon 962 Copolyamrde 105-115 0.3-0.4 at 160 °C < 7 < 7 < 10

Eurelon 965 Copolyamrde 109-119 0.4-1.0 at 160 ⁰C < 5 < 6 < 10

Eurelon 966 Copolyamrde 122-128 0.2-0.3 at 160 ⁰C < 5 < 5 < 10

Eurelon 969 Copolyamrde 97-107 0.4-0.7 at 160 ⁰C < 8 < 5 < 15

Eurelon 975 Copolyamrde 171-181 9.0-13.0 at < 5 < 6 < 8 230⁰C

K*

O

A preferred Eurelon polyamide is Eurelon 975.

Other polyamides which can be used in the ink compositions according to the invention are described in the document US-A-5 998 502.

Advantageously, the ink composition comprises from 0.5 to 25% by weight, preferably from 1 to 20% by weight, more preferably from 4 to 20% by weight, better still from 6 to 10% by weight, with respect to the total weight of the ink, of at least one polyamide resin.

The amount of polyamide resin is adjusted in order to regulate the viscosity while retaining the effect of stability of the fluorescence.

The ink composition according to the invention is fundamentally distinguished from the compositions of the prior art in that it comprises, as binder, at least one polyamide resin, in that this polyamide resin is, in addition, a very specific thermoplastic resin which is soluble in ethanol, in that it comprises, as dye or pigment, at least one rare earth metal complex or chelate and, finally, in that it comprises an amount of water of less than 10%.

No document of the prior art describes an ink exhibiting the combination of the specific characteristics of the composition according to the invention .

In particular, no document of the prior art describes an ink composition comprising at the same time the specific binder according to the invention, which comprises a specific or particular thermoplastic polyamide resin which is soluble in ethanol, and the specific dye according to the invention; the said ink additionally comprising a low or zero amount of water.

Thus, the document US-B2-6 905 538 describes a highly penetrating water-based ink comprising fluorophor dyes of rare earth metal complex types. The predominant presence of water reguires that all the ingredients be soluble in water, including the "fluorophor". Such an ink cannot be applied to nonporous supports, such as plastic wrapping.

In addition, this ink can comprise polar solvents, surface-active agents and humectants, the presence of binders is not mentioned and even less that of polyamide resins, a fortiori of thermoplastic polyamide resins which are soluble in ethanol.

The document US-A-5 093 147 relates to invisible inks which absorb infrared light in order to emit, by fluorescence, infrared light with a greater wavelength. These inks comprise a fluorescent organic dye and not a rare earth metal complex or chelate. In the examples, use is made of a resin which is either a rosin ester or a poly (vinyl alcohol) PVA. The incorporation of a polyamide binder is neither mentioned nor suggested.

These inks require special equipment in order to be detected as emission of infrared light is not visible to the eye.

The paper by R. Chandrasekhar published in the "Journal of Imaging Technology, Volume 16, Number 4, pages 158 - 161" in August 1990 provides for the use, in preparing inks for inkjets, of rare earth metal chelates (europium, dysprosium and terbium) complexed by dibenzoylmethane, in combination with ethylcellulose and polyvinyl butyral binders. There exists no mention or suggestion in this document indicating that such chelates can be advantageously combined with resins of polyamide type and even less with specific thermoplastic polyamide resins which are soluble in ethanol .

The document WO-A-97/10307 (EP-A-O 850 281) describes inks based on dyes of rare earth metal complex type in combination with binding resins which can, for example, be chosen from at least 38 resins or families of resins. Among these resins, polyamides are not mentioned and even less thermoplastic polyamides which are soluble in ethanol. The preferred resins are, with regard to the European patent granted on the basis of the abovementioned WO application, polyurethanes, polyvinylpyrrolidone and ethylcellulose.

There exists no mention in this document indicating that the inks which are described therein can comprise resins of polyamide type and a fortiori resins of specific thermoplastic polyamide which is soluble in ethanol type, and the unexpected effects described above, related to the use in the composition of the invention of specific polyamide resins, are neither mentioned nor touched on in this document.

The ink composition according to the invention meets all the needs mentioned above, satisfies all the criteria and requirements listed above and solves the problems which were encountered with inks of the prior art, in particular fluorescent inks comprising rare earth metal complexes.

It has been found that the composition according to the invention, due to the presence of polyamide resin, essentially as binder, makes it possible to formulate stable ink compositions which rapidly dry while not bringing about blocking of the nozzles, which provide ready startup and which generally satisfy all the criteria, requirements and needs of inkjet technology, whether deflected continuous inkjet technology or drop-on-demand technology.

In addition, it has been demonstrated, surprisingly, that the polyamide resin confers, on these inks, stability of all their fluorescence properties over time, even for a long period of time which can reach, for example, from 6 months to 1 year in a pot, on a shelf, as during continuous operation in a printer.

In addition, this stabilizing effect is obtained whatever the rare earth metal complex employed and even for low or high concentrations of this complex, but this stabilizing effect is particularly shown for europium acetylacetonate, samarium acetylacetonate, neodymium benozylacetonate, terbium salicylate and dysprosium benzoylacetonate and for the compounds Lumilux CD 331 and CD 316.

Such a stabilizing effect of the polyamide resin on the stability of the fluorescence of the rare earth metal complexes and chelates is neither described nor suggested in the prior art and is completely surprising .

This stabilizing effect is not obvious and is extremely surprising as other binders, such as nitrocellulose, known for its excellent behaviour from the viewpoint of its stability, of the operation of the inks in printers and of its rate of drying, do not introduce stability of the fluorescence of the rare earth metal complexes. Likewise, phenolic resins of novolak type, styrene/acrylic resins, acrylic resins and shellac, all widely used to formulate inks for inkjets, do not provide stability of fluorescence to formulations with rare earth metal chelates.

The compositions of the invention give markings of high quality and consistency, wherever the support .

In other words, it may be said that there exists a true synergy between, on the one hand, the rare earth metal complexes or chelates and, on the other hand, the specific polyamide resin of the binder of the ink according to the invention, which results in an unexpected stabilization of the fluorescence properties related to these complexes, while the ink and the marking prepared from the ink also exhibit all the excellent properties required of the inks for inkjet printing and the markings obtained with them.

The markings obtained are resistant to water.

This is because, in contrast to the compositions of the prior art, the ink composition according to the invention comprises a very low amount of water, generally less than 10% by weight, preferably less than 5% by weight, more preferably less than 1% by weight, with respect to the total weight of the ink.

The ink composition according to the invention can even be regarded as being essentially devoid, free, of water (0% of water) .

In fact, the water present is only the water introduced which is found as impurity in the various components of the ink. The greater the degree of purity of the components chosen, the lower the water content .

The low content or absence of water in the ink composition according to the invention promotes the formation of the ink film when the binders and other dyes of the composition are insoluble in water, thus improving the properties of resistance and of adhesion of the ink.

In the composition according to the invention, the solvent comprises one or more organic solvent compound (s) which generally represent (s) at least 10% by weight of the total weight of the ink composition; preferably, the solvent (s) represent (s) from 30 to 90% by weight, more preferably from 60 to 80% by weight, of the total weight of the ink composition.

The solvent comprises one or more organic solvent compound (s) and optionally water, provided that the amount of water observes the conditions indicated above. The said organic solvent compound (s) forming part of the solvent is (are) chosen, for example, from alcohols, in particular alcohols of low molecular weight, for example aliphatic alcohols; ketones, preferably of low molecular weight; alkylene glycol ethers; alkylene glycol esters and alkylene glycol ether esters, such as the acetates; dimethylformamide; N-methylpyrrolidone; acetals; esters; linear or cyclic ethers; cyclic or noncyclic, for example linear or branched, aliphatic hydrocarbons; aromatic or nonaromatic hydrocarbons; and carbonates, such as propylene carbonate, ethylene carbonate, dimethyl carbonate and diethyl carbonate; and their mixtures .

Preferably, this or these solvent (s) has (have) the property of dissolving the other ingredients of the ink, in particular the binder, the colouring materials, the additives, and the like.

The alcohols will preferably be chosen from linear or branched aliphatic alcohols having from 1 to

8 carbon atoms, such as methanol, denatured or nondenatured ethanol, propan-1-ol, propan-2-ol, n- butanol, butan-2-ol, tert-butanol, and the like.

The ketones will preferably be chosen from ketones having from 3 to 10 carbon atoms, such as acetone, butanone (methyl ethyl ketone) , pentan-2-one (methyl propyl ketone) , 3-methylbutan-2-one (methyl isopropyl ketone) and 4-methylpentan-2-one (methyl isobutyl ketone) .

The alkylene glycol ethers are preferably chosen from the monoalkyl (Cl to C6 alkyl group) or dialkyl (Cl to C6 alkyl groups) ethers of alkylene glycols comprising 1 to 10 carbon atoms in the alkylene chain; preferably, they are ethylene or propylene glycol ethers, such as methoxypropanol .

The glycol esters and the glycol ether esters are preferably chosen from the esters of glycol and of glycol ethers with saturated aliphatic carboxylic acids having from 1 to 6 carbon atoms, such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid and caproic acid.

Mention may be made, for example, of methoxypropyl acetate, butyl diglycol acetate, and the like.

The esters are preferably chosen from esters of low molecular weight, such as the formates, the acetates, the propionates or the butyrates of alcohols having from 1 to 10 carbons.

The acetals are preferably chosen from acetals of low molecular weight, such as ethylal and methylal.

The ethers are preferably chosen from ethers of low molecular weight, such as dioxolane or tetrahydrofuran .

The ink composition according to the invention comprises one (or more) dye(s) and/or pigment (s) which is (are) chosen from rare earth metal chelates as described above. The binder of the ink composition according to the invention can comprise, in addition to one or more polyamide resins as described above, one or more other resins and/or polymers different from polyamide resins which are generally chosen from resins compatible with the said polyamide resin (s).

Generally, the polyamide resin (s) (which represent (s) , by weight, at least 0.5% by weight of the total weight of the ink composition) preferably represent (s) at least 50% by weight of the total weight of the binder, preferably at least 75% by weight of the total weight of the binder, more preferably 100% by weight of the binder; in other words, the binder is then composed solely of polyamide resin (s) and the stabilizing effect is obtained optimally.

The proportion of binder (total) in the ink composition according to the invention is generally from 0.5 to 40% by weight, preferably from 1 to 25% by weight, more preferably from 3 to 20% by weight.

The ink composition can additionally comprise one or more plasticizer (s) (of the resin (s) or polymer (s) of the binder) chosen, for example, from the plasticizers known to a person skilled in the art and chosen according to the binder used, which comprises one or more polymer (s) and/or resin (s); mention may be made, as plasticizer, for example, of thermoplastic polyurethanes .

The said plasticizer (s) generally represent (s) from 0.1 to 20% by weight of the total weight of the ink composition. The ink composition according to the invention, if it has to be able to be projected by a deflected continuous inkjet, additionally generally comprises at least one conductivity salt, unless another ingredient is itself a salt which can introduce conductivity when it is dissociated and provides the ink with sufficient conductivity for there to be no need to add conductivity salt proper.

This is because the ink according to the invention, when it has to be able to be applied by a deflected continuous inkjet, has to have sufficient electrical conductivity in the liquid state, generally greater than 300 μS/cm at 20⁰C, preferably greater than 500 μS/cm, for example of the order of 500 to 2000 μS/cm (at 20⁰C) .

This or these conductivity salt(s) is (are) generally chosen from salts of alkali metals, such as lithium, sodium or potassium, salts of alkaline earth metals, such as magnesium and calcium, and salts of simple or quaternary ammoniums; in the form of halides

(chlorides, bromides, iodides or fluorides) , perchlorates, nitrates, thiocyanates, formates, acetates, sulphates, propionates, trifluoroacetates, triflates (trifluoromethanesulphonates) , hexafluorophosphates, hexafluoroantimonates, tetrafluoroborates, picrates, carboxylates and sulphonates .

These conductivity salts will thus be present, if necessary, in the ink composition so as to bestow the above conductivity on the ink: preferably, their amount is from 0.1 to 20% by weight, more preferably from 0.1 to 10% by weight and better still from 0.1 to 5% by weight, of the total weight of the ink composition.

The composition according to the invention can additionally comprise one or more additives chosen from compounds which improve the solubility of some of these components, the printing quality, the adhesion or the control of the wetting of the ink on different supports .

The additive (s) can be chosen, for example, from antifoaming agents, chemical stabilizers, UV stabilizers, surface-active agents, agents which inhibit corrosion by salts, bactericides, fungicides and biocides, pH-regulating buffers, and the like.

The additive (s) is (are) used at very low doses, generally of less than or equal to 5% and sometimes as low as 0.01%, depending on whether antifoaming agents, stabilizers or surface-active agents are involved.

Another subject-matter of the invention is a process for marking substrates or supports, for example porous or nonporous substrates or supports, by projection onto these substrates or supports, by an inkjet printing technique, of an ink composition as has been described above.

Marking is carried out either by the deflected continuous inkjet technique or by the drop- on-demand technique.

Another subject-matter of the invention is a substrate or support, for example a porous or nonporous substrate or support, provided with a marking obtained by the drying and/or the absorption of the ink composition as described above.

The said marking essentially comprises the said chelate dye or pigment and also the binder and it is obtained by evaporation and/or absorption into the substrate of essentially all of the other constituents of the ink, such as the solvents.

This substrate can be made of metal, for example of aluminium or of steel (drink cans) , of glass

(glass bottles) , of ceramic, of a cellulose-comprising material, such as paper, which is optionally coated or glazed, board or wood, of synthetic polymer ("plastic")/ in particular in the form of films, such as PVCs, PETs or polyolefins, such as polyethylenes (PEs) or polypropylenes (PPs), of "Plexiglas", of cloth or of any other nonporous or porous substance or of a composite of several of the preceding materials.

Preferably, the substrate is made of paper, such as ordinary white paper, kraft paper or bond paper.

This substrate can be chosen in particular from postal items, such as letters, envelopes, parcels, packets and stamps; identity documents, such as identity cards and passports, banknotes, cheques, shares, bonds, tickets, including entrance tickets, plastic wrapping films, for example for magazines and other newspapers distributed by post, and the like.

Markings or printings of excellent quality are obtained on all the substrates, even on substrates of very low porosity and even on substrates exhibiting a highly uneven surface. Likewise, the stability of the fluorescence is obtained whatever the substrate, for example porous or nonporous . The fluorescence can be detected in a reliable way on all types of substrates, whether white, light or highly coloured and whatever their colour.

The invention additionally relates to a method for identifying items which comprises the following successive stages: safety markings are produced on the items (for example one marking for each item) using the ink according to the invention as described above, the said markings being invisible to the naked eye and visible only when they are illuminated by ultraviolet radiation; the items are subjected to excitation by ultraviolet radiation; - the said markings are read by detecting the fluorescent radiation emitted by these markings; the said items are identified according to the said reading.

The invention will be better understood on reading the following description of embodiments of the invention, which are given as illustrative and nonlimiting examples.

EXAMPLES

The following ink compositions according to the invention were prepared by mixing the products mentioned in Table I below in the proportions indicated. The viscosities and the conductivities of the inks obtained are also given below in Table I.

TABLE I

rji

1: Alnovol PN320 from Vianova 7: Elvacite 2810 from ICI

2: Joncryl 67 from Johnson Polymer 8: CA4 AlO from Bergerac NC

3: Crayamid 791 from Cray Valley 9: Europium chelate, sold by Honeywell

4: Eurelon 975 from Schering 10: Europium chelate, sold by Honeywell

5: Sepifilm SN from Seppic 11: Silicone

6: Pharmacoat 603 from Shin Etsu

The inks of Examples 1 to 10 were prepared by simple mixing of the various ingredients with stirring .

After filtering through a filter with an absolute porosity of 2 μm, each of Examples 1 to 10 gives an ink which can be projected by inkjets in Imaje printers of Series 8 and Series 9020, 9030 and 9040 type and makes it possible to obtain printings of excellent quality. Markings were produced on postal plastic packaging films made of polyethylene.

These markings were also compared with those produced on the same supports with an ink of the same type for Videojet printers with a reference

165480Q, which comprises a europium chelate and which has apparently the same fluorescence properties.

The stability of the fluorescence is assessed by comparing the fluorescence under UV illumination at 365 nm of markings produced with each of the inks, immediately after marking and, on the one hand, after 3 weeks with the exclusion of light and, on the other hand, after exposure to sunlight together and under the same conditions.

The fluorescence was evaluated visually and graded from 0 to 5. The grade 0 means complete loss of fluorescence and the grade 5 means "no visible difference from the original".

The stability of the fluorescence of the markings produced were the best inks were also tested after continuous operation of the printer without printing for 1 and 3 weeks. Under these conditions, the influence of the recirculation of the ink and of bringing it into contact with the ambient air is at a maximum and represents an extremely severe case far beyond that which is encountered in industrial applications.

The results of the tests are summarized in the following Table II:

NT = Not tested

It is found that the stability of the fluorescence according to these various tests is obtained, among the tested compositions which comprise different resins, only with the compositions in accordance with the invention (7, 8, 9, 10), which comprise resins of polyamide type, this being the case whatever the level or the type of fluorescent complex.

Claims

1. Ink composition which is invisible to the naked eye, which is fluorescent, for inkjet printing, and which is liquid at ambient temperature, comprising : a) a binder, comprising at least 0.5% by weight, with respect to the total weight of the ink, of at least one thermoplastic polyamide resin, soluble in ethanol; b) a solvent comprising at least 10% by weight, with respect to the total weight of the ink, of one or more organic solvent compound (s) ; c) at least 0.05% by weight, with respect to the total weight of the ink, of one or more dye(s) and/or pigment (s) which is (are) invisible to the naked eye and which is (are) fluorescent in the visible region only when they are exposed to ultraviolet light and which are chosen from rare earth metal chelates and rare earth metal complexes; d) less than 10% by weight of water, with respect to the total weight of the ink.

2. Composition according to Claim 1, in which the fluorescent dye(s) and/or pigment (s) emit(s) fluorescent radiation in the visible region when they are exposed to ultraviolet light in the wavelength range from 275 to 400 nm.

3. Composition according to Claim 1 or 2, in which the fluorescent dye (s) and/or pigment (s) emit(s) fluorescent radiation having a wavelength of less than or equal to approximately 700 nm, preferably of less than or equal to approximately 650 nm.

4. Composition according to Claim 3, in which the fluorescent dye(s) and/or pigment (s) emit(s) fluorescent radiation having a wavelength from approximately 550 or 575 nm to approximately 700 nm, preferably from 600 to 650 nm.

5. Composition according to any one of the preceding claims, in which the fluorescent dye (s) and/or pigment (s) is (are) chosen from samarium, europium, gadolinium, neodynium, terbium, dysprosium, complexes, or chelates, and their mixtures.

6. Composition according to any one of the preceding claims, in which the complexes or chelates are formed by the chelation or complexing of rare earth metal ions with organic ligands, such as acetylacetone, benzoylacetone, dibenzoylmethane and salicylic acid.

7. Composition according to Claim 6, in which the complexes or chelates are chosen from europium acetylacetonate, samarium acetylacetonate, neodyme benzoylacetonate, terbium salicylate and dysprosium benzoylacetonate.

8. Composition according to any one of the preceding claims, which comprises from 0.05 to 25% by weight, preferably from 0.1 to 25% by weight, more preferably from 0.2 to 10% by weight, better still from 0.5 to 5% by weight, with respect to the total weight of the ink, of the pigment (s) and/or dye(s) chosen from rare earth metal chelates and complexes.

9. Composition according to any one of the preceding claims, comprising from 0.5 to 25% by weight, preferably from 1 to 20% by weight, more preferably from 4 to 20% by weight, better still from 6 to 10% by weight, with respect to the total weight of the ink, of said at least one polyamide resin.

10. Composition according to any one of the preceding claims, in which the binder additionally comprises one or more other resins and/or polymers different from polyamide resins.

11. Composition according to Claim 10, in which the polyamide resin or resins represent at least 50% by weight of the total weight of the binder, preferably at least 75% by weight of the total weight of the binder, preferably 100% by weight of the binder.

12. Ink composition according to any one of the preceding claims, comprising from 0.5 to 40% by weight, preferably from 1 to 25% by weight, more preferably from 3 to 20% by weight, of binder.

13. Composition according to any one of the preceding claims, which comprises less than 5%, preferably less than 1% by weight, better still 0% by weight, of water, with respect to the total weight of the ink.

14. Composition according to any one of the preceding claims, in which the solvent represents from 30 to 90% by weight, preferably from 60 to 80% by weight, of the total weight of the ink composition.

15. Composition according to any one of the preceding claims, in which the solvent comprises one or more organic solvent compound (s) and optionally water.

16. Ink composition according to Claim 15, in which the said organic solvent compound (s) is (are) chosen from alcohols; ketones; alkylene glycol ethers; alkylene glycol esters and alkylene glycol ethers esters, such as the acetates; dimethylformamide; N- methylpyrrolidone; ethers; acetals; esters; cyclic or noncyclic aliphatic hydrocarbons; aromatic or nonaromatic hydrocarbons; and carbonates, such as propylene carbonate, ethylene carbonate, dimethyl carbonate and diethyl carbonate; and their mixtures.

17. Composition according to Claim 16, in which the said organic solvent compound (s) is (are) chosen from linear or branched aliphatic alcohols having from 1 to 8 carbon atoms; ketones having from 3 to 10 carbon atoms; monoalkyl (Cl to C6 alkyl group) or dialkyl (Cl to C6 alkyl groups) ethers of alkylene glycols comprising from 1 to 10 carbon atoms in the alkylene chain, such as ethylene glycol and propylene glycol; or esters of alkylene glycols and esters of alkylene glycol ethers with saturated aliphatic carboxylic acids having from 1 to 6 carbon atoms.

18. Composition according to Claim 17, in which the solvent is composed of an organic solvent compound chosen from denatured or nondenatured ethanol, ketones having from 3 to 10 carbon atoms, and their mixtures .

19. Ink composition according to any one of the preceding claims, additionally comprising one or more plasticizer (s) in a proportion of 0.1 to 20% by weight .

20. Composition according to any one of the preceding claims, which additionally comprises at least one conductivity salt.

21. Composition according to Claim 20, in which the conductivity salt represents from 0.1 to 20% by weight, preferably from 0.1 to 10% by weight, more preferably from 0.1 to 5% by weight, of the total weight of the ink composition.

22. Ink composition according to Claim 21, in which the said conductivity salt is chosen from salts of alkali metals, alkaline earth metals and simple or quaternary ammoniums, in the form of halides, perchlorates, nitrates, thiocyanates, formates, acetates, sulphates, propionates, trifluoroacetates, triflates, hexafluorophosphates, hexafluoroantimonates, tetrafluoroborates, picrates, carboxylates and sulphonates .

23. Ink composition according to any one of the preceding claims, which has a conductivity of greater than 300 μS/cm at 20⁰C, preferably from 500 μS/cm to 2000 μS/cm.

24. Ink composition according to any one of the preceding claims, additionally comprising one or more additive (s) chosen from antifoaming agents, chemical stabilizers, UV stabilizers, surface-active agents, agents which inhibit corrosion by salts, bactericides, fungicides and biocides, and pH- regulating buffers.

25. Process for marking substrates by projection onto these substrates of an ink composition by an inkjet printing technique, in which the ink composition projected is an ink composition according to any one of Claims 1 to 24.

26. Marking process according to Claim 25, in which the inkjet printing technique is the deflected continuous inkjet technique or the drop-on-demand technique .

27. Substrate, characterized in that it is provided with a marking obtained by drying and/or absorption of the ink composition according to any one of Claims 1 to 24.

28. Substrate according to Claim 27, characterized in that the substrate is made of metal, for example of aluminium or of steel; of glass, of ceramic; of a cellulose-comprising material, such as paper, which is optionally coated or glazed, for example ordinary white paper, kraft paper or bond paper, board or wood; of a synthetic polymer

("plastic"), such as PVCs, PETs or polyolefins, such as polyethylenes (PEs) or polypropylenes (PPs) ; of

"Plexiglas"; of cloth; or of any other nonporous or porous substance or of a composite of several of the preceding materials.

29. Substrate according to Claim 28, which is chosen from postal items, such as letters, envelopes, parcels, packets and stamps; identity documents; banknotes; cheques; shares; bonds; tickets, including entrance tickets; postal plastic wrapping films .

30. Method for identifying items which comprises the following successive stages: safety markings are produced on the items using the ink according to any one of Claims 1 to 24, the said markings being invisible to the naked eye and visible only when they are illuminated by ultraviolet radiation; the items are subjected to excitation by ultraviolet radiation; the said markings are read by detecting the fluorescent radiation emitted by these markings; - the said items are identified according to the said reading.