Printing ink

Abstract

The present invention provides an inkjet ink comprising: a continuous aqueous phase; a pigment having one or more first reactive groups, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected second reactive groups; and less than 3% by weight of a binder resin, based on the total weight of the ink; wherein the two or more protected second reactive groups, when deprotected, are reactive to the one or more first reactive groups. The present invention further provides a method of inkjet printing comprising the following steps in order: (i) providing an inkjet ink as claimed in any preceding claim; (ii) inkjet printing the inkjet ink onto a substrate to provide a printed substrate; (iii) drying the printed substrate to remove water; and (iv) deprotecting the two or more protected second reactive groups. The present invention further provides printed substrate having the inkjet ink of the invention printed thereon.

Description

[0001] Printing ink

[0002] The present invention relates to a printing ink and, in particular, an inkjet ink for printing onto textiles. The present invention also relates to a method of printing said ink.

[0003] Digital inkjet printing is commonly used in the textile industry as it offers many advantages over analogue printing such as a rapid and facile print process, print flexibility and much shorter preparation times. Digital inkjet printing is also more environmentally friendly than analogue printing as it offers substantial reductions in energy consumption and chemical, water and carbon dioxide waste.

[0004] There are two inkjet inks typically used in the textile industry, dye-based inkjet inks and pigmented inkjet inks.

[0005] Dye-based inkjet inks provide printed textiles with brilliant colours, good handle and wash- and rubfastness. However, the textile substrate dictates the particular dye used. Reactive dyes are used for cellulosic and protein fibres, acid dyes are limited to protein fibres and dispersed and sublimation dyes can only be applied to polyester substrates.

[0006] Printing processes involving dye-based inkjet inks also tend to be multi-step and complex. For example, reactive dyes necessitate pre-treating the textile followed by printing, steaming and then washing the textile. All of these steps are energy and water intensive. Sublimation dyes require printing onto a transfer medium and applying heat and pressure to transfer the print to the substrate.

[0007] Pigmented inkjet inks offer a significant advantage in terms of universal applicability to textile substrates. Printing using pigmented inkjet inks is also simpler, quicker and more environmentally friendly than using dye-based inkjet inks as the ink can just be printed on the textile and heat treated.

[0008] Pigmented inkjet inks typically contain a binder resin to enable the pigment to bind to the textile substrate. However, the binder resin tends to provide printed substrates with a poor handle that feel very stiff to the touch, and sub-optimal optical density, print wash, rub and perspiration fastness.

[0009] There is therefore a need in the art for a pigmented inkjet ink for printing onto textiles that provides a printed substrate with a good handle and optical density, without compromising the wash-fastness and adhesion.

[0010] Accordingly, the present invention provides an inkjet ink comprising: a continuous aqueous phase; a pigment having one or more first reactive groups, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected second reactive groups; and less than 3% by weight of a binder resin, based on the total weight of the ink; wherein the two or more protected second reactive groups, when deprotected, are reactive to the one or more first reactive groups.

[0011] The present invention also provides a method of inkjet printing comprising the following steps in order:

[0012] (i) providing an inkjet ink of the present invention;

[0013] (ii) inkjet printing the inkjet ink onto a substrate to provide a printed substrate;

[0014] (iii) drying the printed substrate to remove water; and

[0015] (iv) deprotecting the two or more protected second reactive groups.

[0016] The present invention will now be described with reference to the accompanying drawings, in which:

[0017] Fig. 1 shows photographs of substrates, to which comparative Ink 1 has been drawn down upon, after no washes (left-hand side) and two washes (right-hand side);

[0018] Fig. 2 shows photographs of substrates, to which comparative Ink 2 has been drawn down upon, after no washes (top) and two washes (bottom); and

[0019] Fig. 3 shows photographs of substrates, to which Ink 3 of the invention has been drawn down upon, after no washes (left-hand side) and two washes (right-hand side).

[0020] The inventors have surprisingly found that the inclusion of a cross-linking agent of the present invention in an aqueous inkjet ink containing a pigment of the present invention provides a pigmented inkjet ink for printing onto textiles, which provides a printed substrate with a good handle and optical density, without compromising the wash-fastness and adhesion.

[0021] The two or more protected second reactive groups of the cross-linking agent, when deprotected, can react with the one or more first reactive groups of the pigment and any free reactive groups on the substrate, thus binding pigment particles together and tethering the pigment to the substrate. This improves the adhesion between the pigment and the substrate leading to wash-fast and robust printed substrates. It is surprising that such a formulation can provide such good adhesion without requiring a binder resin and therefore the printed substrates also benefit from a good handle and optical density.

[0022] The inkjet ink of the present invention comprises a continuous aqueous phase and is hence an aqueous ink. Water and, when present, an organic solvent define the continuous aqueous phase. The continuous aqueous phase acts as a carrier for the components of the ink and ensures that the ink has the appropriate viscosity for printing. As with known aqueous inks, water is required to evaporate from the printed ink, typically on heating, in order to allow the ink to dry.

[0023] In a preferred embodiment, water is present in a total amount of 30 to 80% by weight, more preferably 30 to 70% by weight and most preferably 35 to 65% by weight, based on the total weight of the ink. The total amount of water includes water that is added as a separate component and any water that may be present in other components such as the pigment and the cross-linking agent.

[0024] The continuous aqueous phase preferably comprises, in addition to water, an organic solvent. The organic solvent is in the form of a liquid at ambient temperature and is miscible with water. As with known solvent-based inkjet inks, the organic solvent is required to evaporate from the printed ink, typically on heating, in order to allow the ink to dry.

[0025] The organic solvent may be a single solvent or a mixture of two or more solvents. The solvent can be selected from any solvent commonly used in the printing industry, such as glycol ethers, glycol ether esters, alcohols, glycols, ketones and esters. In a preferred embodiment, the inkjet ink further comprises triethylene glycol and / or glycerol.

[0026] The organic solvent is preferably present in the inkjet ink in a total amount of 5 to 55% by weight, more preferably 20 to 50% by weight and most preferably 30 to 45% by weight, based on the total weight of the ink.

[0027] The inkjet ink of the present invention further comprises a pigment. The pigment is dispersed in the continuous aqueous phase of the inkjet ink.

[0028] The pigment has one or more first reactive groups. The one or more first reactive groups are on the surface of the pigment. They are thus available to react with the two or more protected second reactive groups of the cross-linking agent present in the inkjet ink of the present invention, when deprotected. The pigments have been functionalised with the one or more first reactive groups, such as by chemical attachment of the one or more first reactive groups to the surface of the pigment. The one or more first reactive groups on the surface of the pigment ensures that the pigment is uniformly and stably dispersed in the continuous aqueous phase of the ink.

[0029] The one or more first reactive groups are not particularly limited as long as they are reactive to the two or more protected second reactive groups of the cross-linking agent, when deprotected. Thus, the one or more first reactive groups react with the two or more protected second reactive groups of the cross-linking agent, when deprotected, leading to enhanced adhesion between the pigment and the substrate and / or enhanced cross-linking between the pigment particles on the substrate. For the avoidance of doubt, the one or more first reactive groups of the pigment and the two or more protected second reactive groups of the cross-linking agent are both reactive groups but are different reactive groups to each other as they react with each other.

[0030] In a preferred embodiment, the pigment has one or more first reactive groups which are reactive to isocyanate groups. In this regard, the two or more protected second reactive groups of the crosslinking agent preferably comprise protected isocyanate groups.

[0031] In a preferred embodiment, the pigment has one or more first reactive groups selected from carboxyl, hydroxy, amino, sulfonate, phosphoryl, sulfinate, amide and mixtures or salts thereof. Acid groups may be in the free acid or salt form.

[0032] Sodium, potassium and lithium salts are preferred.

[0033] In a preferred embodiment, the pigment has one or more first reactive groups selected from carboxyl, hydroxy, sulfonate, sulfinate, phosphonate, and mixtures or salts thereof.

[0034] In a preferred embodiment, the one or more first reactive groups are charged. For example, the reactive groups can be selected from -COO-, -SOs-, -POs’, -SO2-, and mixtures thereof. This creates a repulsive nature between pigment particles that reduces agglomeration of pigment particles within the continuous aqueous phase.

[0035] In a preferred embodiment, the pigment particles has two or more first reactive groups. This provides improved binding between pigment particles and improved tethering of the pigment to the substrate, leading to better adhesion between the pigment and the substrate, and improved washfastness and robustness.

[0036] The one or more first reactive groups may be ionic or non-ionic, preferably ionic. Any ionic groups may be cationic or anionic but anionic groups are preferred. Examples of anionic groups include phenoxy, carboxyl, sulfonic acid, sulfuric acid, sulfinic acid, phosphonic acid, polyphosphoric acid and phosphoric acid groups. Carboxyl, sulfonic acid, sulfuric acid, sulfinic acid, phosphonic acid, polyphosphoric and phosphoric acid groups may be in the free acid or salt form.

[0037] In a preferred embodiment, the one or more first reactive group are hydrophilic, preferably selected from hydroxyl, carboxyl, amino, and mixtures or salts thereof. This provides the advantage that the pigment is more hydrophilic. A more hydrophilic pigment allows for deeper penetration of the pigment in hydrophilic substrates such as textile substrates having free hydroxyl groups at the surface of the substrate, leading to enhanced reaction and adhesion between the pigment and the substrate. Such a printed substrate therefore has improved wash-fastness and robustness. By hydrophilic, it is meant that they are attracted to water molecules. Hydroxyl, carboxyl, sulfonate and sulfinate groups are particularly preferred as they are good at reacting with the two or more protected second reactive groups of the cross-linking agent, when deprotected. This leads to improved binding between pigment particles and improved tethering of the pigment to the substrate, leading to better adhesion between the pigment and the substrate. The printed substrates are hence more wash-fast and robust than would otherwise be obtained with less reactive groups. Carboxyl groups are also good at stabilising the pigment in the continuous aqueous phase. Thus, in a preferred embodiment, the pigment has one or more first reactive groups selected from carboxyl, hydroxy, sulfonate, sulfinate and mixtures or salts thereof.

[0038] The one or more first reactive groups may be primary and / or secondary reactive groups but are preferably primary reactive groups. Primary reactive groups react faster than secondary reactive groups, leading to an improved reaction between the pigment and the cross-linking agent, when deprotected.

[0039] The one or more first reactive groups can be directly attached to the surface of the pigment or can be attached via one or more other chemical groups, preferably organic groups, or via a polymer chain. In a preferred embodiment, the one or more reactive groups are attached via one or more organic groups. The organic groups may be aliphatic, cyclic, or an organic compound having aliphatic and cyclic portions. The organic groups may be substituted, unsubstituted, branched or unbranched.

[0040] Pigments having one or more first reactive groups for inclusion in the inkjet ink are known and include Cab-o-Jet® dispersion 4107K, 4507C, 4707Y, 4607M, 200K, 300K, 325K, 400K, 480V and 740Y pigment dispersions commercially available from Cabot®.

[0041] The preparation of a pigment for inclusion in the inkjet ink of the present invention is also known and is described in at least US 5,554,739, US 5,922,118, US 5,571 ,311 , US 7,794,902, WO 97 / 48769 and US 10,954,402.

[0042] Typically, the pigment having one or more first reactive groups is prepared through the diazonium treatment of a raw pigment. That is, the pigment having one or more first reactive groups can be prepared by reacting at least one diazonium salt with the pigment. A diazonium salt is an organic compound having one or more diazonium groups. Preferably, the at least one diazonium salt has been functionalised with the one or more first reactive groups. Alternatively, the one or more first reactive groups can be added in a subsequent reaction. Mixtures of diazonium salts may be used. The process is well known in the art.

[0043] As is known in the art, diazonium salts may be formed by the reaction of primary amines with an aqueous solution of nitrous acid, or by the reaction of primary amines with a nitrite, optionally in the presence of an acid. This is a well-known process in the art and is often referred to as diazotization. The nitrite may be any metal nitrite (including lithium nitrite, sodium nitrite, potassium nitrite or zinc nitrite) or organic nitrite (including isoamylnitrite or ethylnitrite). The acid may be any acid, including inorganic or organic acid, which is effective in the generation of a diazonium salt. Preferred examples include nitric acid, hydrochloric acid and sulfuric acid. The primary amine may be any primary amine, which has preferably been functionalised by the one or more first reactive groups.

[0044] The diazonium salt may be prepared prior to the reaction with the pigment particles or generated in situ, using techniques known in the art.

[0045] Preferably, the diazonium salt is prepared in situ. For example, pigment particles are mixed with a primary amine (often termed the treating agent), such as a functionalised aniline, and a nitrite in a liquid medium. In this process, the treating agent attaches onto a surface of the pigment particles through a stable covalent bond, resulting in a surface-modified pigment.

[0046] For example, to prepare Cab-o-Jet® dispersion 200K, aniline that is functionalised with a sulfonate group is used as the treating agent. This results in a pigment that has one or more sulfonate groups on the surface of the pigment. Likewise, in order to prepare Cab-o-Jet® dispersion 300K, aniline that is functionalised with one or more carboxylate groups is used as the treating agent. This results in a pigment that has one or more carboxylate groups on the surface of the pigment. The treating agent can be a small molecule or a polymer.

[0047] Examples of specific methods for preparing a pigment having one or more first reactive groups for inclusion in the inkjet ink of the invention can be found in for example, Example 4 of US 5,571 ,311 , Example 1 of WO 97 / 48769 and Example 8 of US 7,794,902.

[0048] Pigment particles include any of the classes of pigment particles described in the Third Edition of the Colour Index (1971) and subsequent revisions of, and supplements thereto, under the chapter headed “Pigments”. Examples of organic pigment particles include those from the azo (including disazo and condensed azo), thioindigo, indanthrone, isoindanthrone, anthanthrone, anthraquinone, isodibenzanthrone, triphendioxazine, quinacridone, diketopyrrolopyrrole and phthalocyanine series, such as copper phthalocyanine and its nuclear halogenated derivatives, and also lakes of acid, basic and mordant dyes. Examples of inorganic pigment particles include carbon black, titanium dioxide, aluminium oxide, iron oxide and silicon dioxide.

[0049] Preferred pigment particles are phthalocyanines, azo, indanthrone, anthanthrone and quinacridone, diketopyrrolopyrrole, carbon black pigment particles or any combination thereof.

[0050] Preferably, the pigment particles are cyan, magenta, yellow, black, violet, red, orange, green and white pigment particles or any combination thereof. In a preferred embodiment, the pigment is a black pigment and the pigment has one or more first reactive groups selected from carboxyl, sulfonate, phosphonate, sulfinate and mixtures or salts thereof.

[0051] In a preferred embodiment, the pigment is a non-black pigment and the pigment has one or more first reactive groups selected from sulfonate, sulfinate and mixtures or salts thereof.

[0052] In a preferred embodiment, the pigment is added to the ink in the form of a pigment dispersion. Preferably, the pigment dispersion is added to the ink in an amount of 10 to 40% by weight, more preferably 15 to 35% by weight and most preferably 20 to 30% by weight, based on the total weight of the ink. A higher concentration of encapsulated pigment dispersion may be required for white inks, for example up to and including 70% by weight, based on the total weight of the ink. For the avoidance of doubt, the amount of pigment dispersion includes any water or additional solvent added as part of the pigment dispersion.

[0053] Preferably, the pigment is present in an amount of 0.2 to 20% by weight, preferably 0.5 to 15% by weight, more preferably 2 to 8% by weight, and most preferably 3 to 7% by weight, based on the total weight of the ink. A higher concentration of the pigment may be required for white inks, for example up to and including 30% by weight, or 25% by weight, based on the total weight of the ink. For the avoidance of doubt, these amounts correspond to the pigment per se.

[0054] 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, more preferably less than 0.5 pm and particularly preferably less than 0.2 pm.

[0055] The inkjet ink of the present invention further comprises a cross-linking agent having two or more protected second reactive groups, wherein the two or more protected second reactive groups, when deprotected, are reactive to the one or more first reactive groups of the pigment.

[0056] The cross-linking agent is not particularly limited as long as the two or more protected second reactive groups can be deprotected and are then reactive to the one or more first reactive groups of the pigment.

[0057] In a preferred embodiment, the cross-linking agent has two or more protected second reactive groups, which when deprotected, are reactive to carboxyl, hydroxy, amino, sulfonate, sulfinate, amide and / or phosphonate groups. In a preferred embodiment, the cross-linking agent has two or more protected second reactive groups, which when deprotected, are reactive to hydroxy, sulfonate, sulfinate, phosphonate and / or carboxy groups. In this regard, the one or more first reactive groups of the pigment preferably comprise hydroxy, sulfonate, sulfinate, phosphonate and / or carboxy groups.

[0058] In a preferred embodiment, the cross-linking agent is a blocked di- and / or poly-isocyanate where the two or more isocyanate groups are protected. That is, the cross-linking agent preferably is a blocked diisocyanate, a blocked polyisocyanate or a mixture of a blocked diisocyanate and a blocked polyisocyanate. The blocked di- and / or poly-isocyanate has two or more protected isocyanate groups.

[0059] In a preferred embodiment, the cross-linking agent is a blocked diisocyanate. In another preferred embodiment, the cross-linking agent is a blocked polyisocyanate. In a further preferred embodiment, the cross-linking agent is a mixture of a blocked diisocyanate and a blocked polyisocyanate.

[0060] As used herein, a blocked diisocyanate comprises two blocked isocyanate groups and a blocked polyisocyanate comprises three or more blocked isocyanate groups. When a blocked polyisocyanate is present, the blocked polyisocyanate preferably comprises three to six blocked isocyanate groups, more preferably three or four blocked isocyanate groups. A blocked polyisocyanate having three blocked isocyanate groups is particularly preferred.

[0061] When the cross-linking agent is a blocked di- and / or poly-isocyanate, the two or more protected second reactive groups are therefore two or more blocked isocyanate groups and the two or more second reactive groups are two or more isocyanate groups. The isocyanate groups are reactive to the first reactive groups of the pigment, including for example, hydroxy groups forming a urethane linkage.

[0062] Synthesis of blocked isocyanates is well-known to the skilled person and has been reviewed by D.A. Wicks and Z.W. Wicks Jr., Progress in Organic Coatings, 1999, 36, 148-172 and E. Delebecq et al., Chem; Rev., 2013, 113, 80-118. Classic blocked isocyanates are defined as chemical components that are capable of forming isocyanates from a precursor upon thermal activation. In general, the reaction proceeds as shown below, where X is any suitable leaving group:

[0063] O

[0064] R 11 heat + H— X

[0065] XN^X H X

[0066] H

[0067] The activation temperature, also called the deblocking temperature, is dependent on the leaving group. Suitable isocyanate precursors are shown below having a variable deblocking temperature between 100°C and 170°C: wherein R is one or more blocked isocyanate groups connected by a linking group. The linking group is not particularly limited other than by the constraints imposed by the use in an inkjet ink, such as viscosity, stability, toxicity etc. The linking group is typically alkylene, cycloalkylene, arylene or combinations with at least one of alkyl, cycloalkyl and / or aryl, any of which may be interrupted by heteroatoms. Non-limiting examples of linking groups commonly used in the art include C1-18 alkylene, C3-18 cycloalkylene, CB- arylene and combinations with at least one of C1-18 alkyl, C3-18 cycloalkyl and / or Cs- aryl, such as Cs- aryl- or C3-18 cycloalkyl-substituted C1-18 alkylene, any of which may be interrupted by 1-16 heteroatoms, such as oxygen or nitrogen, with nitrogen further substituted by any of the above described substituents.

[0068] Active methylene compounds as blocking agents are widely used as alternatives for classic blocked isocyanates, operating via an alternative reaction pathway, not yielding an intermediate isocyanate but crosslinking the system via ester formation as disclosed in Progress in Organic Coatings, 36, 148-172 (1999), paragraph 3.8. Suitable examples of active methylene group blocked isocyanates are shown below: wherein R is as described above.

[0069] Preferred blocked di- and / or poly-isocyanates are selected from blocked hexamethylene diisocyanate, isophorone diisocyanate, tolyl diisocyanate, xylylene diisocyanate, a hexamethylene diisocyanate trimer, trimethylhexylene diisocyanate, diphenylmethane diisocyanate, dicyclohexylmethane diisocyanate and mixtures thereof. Preferably, the blocked di- and / or polyisocyanate is a blocked hexamethylene diisocyanate, more preferably a blocked hexamethylene diisocyanate trimer, most preferably a blocked hexamethylene diisocyanate isocyanurate trimer.

[0070] The leaving group of the blocked di- and / or poly-isocyanate is not particularly limited and can be any group capable of leaving at a temperature between 100°C and 170°C and one that is not hazardous. The leaving groups of the blocked di- and / or poly-isocyanate may the same or different but are preferably the same. In a preferred embodiment, the leaving group of the blocked di- and / or poly-isocyanate is 3,5-dimethylpyrazyl (DMP). DMP is favoured from a health and safety perspective and because of its relatively low deblocking temperature of 115°C.

[0071] When the blocked di- and / or poly-isocyanate is a blocked hexamethylene diisocyanate, the blocked hexamethylene diisocyanate may comprise additional functionality such as ethylene oxide groups or a residue of a polyhydroxy carboxylic acid as described in US 6,063,860.

[0072] A particularly preferred blocked di- and / or poly-isocyanate is a hexamethylene diisocyanate isocyanurate trimer blocked with 3,5-dimethylpyrazyl leaving groups. Such a preferred blocked di- and / or poly-isocyanate has the following structure:

[0073] Suitable blocked di- and / or poly-isocyanates include Imprafix® products commercially available from Covestro AG, Trixene® products commercially available from LANXESS, Hydrosin products commercially available from Maflon S.p.A and Roflex® products commercially available from Lamberti, including Roflex® BK18. A particularly preferred blocked polyisocyanate is Imprafix® 2794, which is a water-based aliphatic blocked polyisocyanate of 38% active strength.

[0074] In a preferred embodiment, the cross-linking agent, when deprotected, has a molecular weight of 600 to 2000, preferably 600 to 1900. In other words, when the two or more protected second reactive groups of the cross-linking agent are deprotected, the cross-linking agent preferably has a molecular weight of 600 to 2000, preferably 600 to 1900. Molecular weights (number average) can be calculated if the structure of the cross-linking agent is known or molecular weights can be measured using gel permeation chromatography using polystyrene standards.

[0075] In a preferred embodiment, the inkjet ink comprises: a continuous aqueous phase; a pigment having one or more reactive groups that are reactive to isocyanate groups, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected isocyanate groups; and less than 3% by weight of a binder resin, based on the total weight of the ink.

[0076] In a preferred embodiment, the inkjet ink comprises: a continuous aqueous phase; a pigment having one or more reactive groups selected from carboxyl, hydroxy, amino, sulfonate, phosphoryl, sulfinate, amide and mixtures or salts thereof, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected isocyanate groups; and less than 3% by weight of a binder resin, based on the total weight of the ink.

[0077] In a preferred embodiment, the inkjet ink comprises: a continuous aqueous phase; a pigment having one or more reactive groups selected from carboxyl, hydroxy, sulfinate, sulfonate, phosphonate and mixtures and salts thereof, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected isocyanate groups; and less than 3% by weight of a binder resin, based on the total weight of the ink.

[0078] The cross-linking agent is preferably added to the ink in the form of an aqueous dispersion and is thus preferably a water-based dispersion. It is preferably soluble in the continuous aqueous phase of the ink.

[0079] In a preferred embodiment, the cross-linking agent dispersion is added to the ink in an amount of 0.2 to 15.0%, preferably 0.2 to 10.5%, more preferably 0.5 to 10%, more preferably 0.5 to 5.5% by weight, based on the total weight of the ink. In another preferred embodiment, the cross-linking agent dispersion is added to the ink in an amount of 0.5 to 15.0%, more preferably 2.0 to 10.0% by weight, based on the total weight of the ink. For the avoidance of doubt, the amount of cross-linking agent dispersion includes any water added as part of the cross-linking agent dispersion.

[0080] Preferably, the two or more protected second reactive groups, when deprotected, are present in the ink in an amount of 0.01 to 1 .00%, preferably 0.02 to 0.80%, more preferably 0.03 to 0.60% by weight, based on the total weight of the ink. When the cross-linking agent is a blocked di- and / or poly-isocyanate, these amounts correspond to the isocyanate loading in the ink.

[0081] The two or more protected second reactive groups are only reactive to the one or more first reactive groups of the pigment when they are deprotected, i.e. it is the two or more deprotected second reactive groups that are reactive to the first reactive groups of the pigment. Deprotection typically happens after the inkjet ink is printed and dried to remove water.

[0082] The two or more protected second reactive groups are preferably deprotected using heat, i.e. the cross-linking agent is preferably a cross-linking agent having two or more protected second reactive groups, wherein the two or more protected second reactive groups, when thermally deprotected, are reactive to the one or more first reactive groups of the pigment. Preferably, the two or more protected second reactive groups are deprotected by heating to a temperature of 100°C or higher, preferably 120°C or higher, more preferably 135°C or higher. The maximum temperature is dictated by cost but preferably, the temperature sufficient to deprotect the two or more protected second reactive groups is up to 170°C. Therefore, in a preferred embodiment, the two or more protected second reactive groups are deprotected by heating to a temperature between 100°C and 170°C, preferably between 120°C and 170°C, more preferably between 135°C and 170°C. This temperature is higher than the temperature used to initially remove water, and optionally an organic solvent, from the printed ink. This allows the ink to be dried and then cured (cross-linked) in subsequent steps by applying the lower and then higher temperature, respectively.

[0083] Preferably, the printed ink is dried by heating to a temperature of less than 100 °C. In this embodiment, the printed ink needs to be heated to a temperature sufficient to dry the printed ink but preferably, the printed ink is dried by heating to a temperature of 60°C or higher. Therefore, in a preferred embodiment, the printed ink is dried by heating to a temperature between 60°C and less than 100°C.

[0084] The two or more protected second reactive groups, when deprotected, are reactive to the one or more first reactive groups of the pigment. The two or more second reactive groups can react with the one or more first reactive groups of the pigment, and / or any free reactive groups on the substrate that are capable of reacting with the two or more second reactive groups of the crosslinking agent, typically hydroxy or amino groups. In this way, the cross-linking agent can act as a bridge between pigment particles embedded in the substrate and between the pigment and the substrate. Binding pigment particles together and tethering the pigment to the substrate in this way provides good adhesion between the pigment and the substrate, resulting in the printed image being both wash-fast and robust. The printed substrate also has a good handle and optical density.

[0085] By optical density is meant OD = - log ( / / / 0), where l0is the intensity of incident light on a print, and I is the intensity of light reflected form a print. It is the measure of darkness of a print. The higher the optical density, the darker the print.

[0086] Binder resins are typically added to pigmented inkjet inks to facilitate adhesion between the ink and substrate. However, the inclusion of a binder resin in an inkjet ink results in a printed substrate with a poor handle and a sub-optimal optical density.

[0087] Therefore, the ink contains less than 3% by weight, more preferably less than 2% by weight, more preferably less than 1 % by weight and most preferably is substantially free of a binder resin, 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 as impurities in the components present. In otherwords, no binder resin is intentionally added to the ink. However, minor amounts of a binder resin, 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, most preferably less than 0.05% by weight of a binder resin, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of a binder resin.

[0088] By binder resin is meant a polymer capable of binding pigment to a substrate without formation of covalent bonds between the polymer and pigment. The binder resin may include reactive groups that may allow self-cross-linking of the polymer as the ink dries. The polymer typically contains polymerised ethylenically unsaturated monomers optionally with reactive groups remaining for cross-linking. Suitable monomers include vinyl monomers such as (meth)acrylates, styrenes, acrylamides, vinyl ethers and halogenated vinyl compounds. Examples of binder resins include Rovene® 4170 (a carboxylated self-cross-linking styrene butadiene copolymer available from Mallard Creek Polymers) and Lubrijet® T340 (an acrylic emulsion copolymer binder available from Lubrizol).

[0089] Binder resins are typically prepared by emulsion polymerisation, as is known in the art. In this case, the binder resin may be provided in the form of a latex. The latex includes polymer particles dispersed in a liquid carrier. The liquid carrier is typically water, which may contain small amounts of other solvents. The liquid carrier may include additives such as agents to control the pH, for example amines. The polymer particles can be self-dispersing or the particles can be dispersed with the aid of a surfactant. Suitable surfactants include conventional surfactants such as sulfates, sulfonates, and ethylene oxide / propylene oxide copolymers. Commercially available latexes typically comprise 40 to 60% by weight of polymer, based on the total weight of the latex. For the avoidance of doubt, the restrictions on the amount of binder resin above includes any water, additives and surfactants added as part of the binder resin component.

[0090] The binder resin typically has a weight-average molecular weight of more than 2000 to 500,000, preferably 4000 to 100,000, as determined by gel permeation chromatography with polystyrene standards.

[0091] In a preferred embodiment, the ink contains less than 2% by weight, more preferably less than 1 % by weight and most preferably is substantially free of polymer capable of binding pigment to a substrate without formation of covalent bonds between the polymer and pigment, where the amounts are based on the total weight of the ink.

[0092] By substantially free is meant that only small amounts will be present, for example as impurities in the components present. In other words, no polymer capable of binding pigment to a substrate without formation of covalent bonds between the polymer and pigment is intentionally added to the ink. However, minor amounts of such a polymer, 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, most preferably less than 0.05% by weight of polymer capable of binding pigment to a substrate without formation of covalent bonds between the polymer and pigment, based on the total weight of the ink. In a preferred embodiment, the inkjet ink is free of polymer capable of binding pigment to a substrate without formation of covalent bonds between the polymer and pigment.

[0093] In the present invention, it has been found that the saturation of optical density is achieved more rapidly at lower print densities as the pigment can penetrate the substrate more readily instead of being obscured by any binder resin. In this regard, a printed ink film containing a binder resin is confined to the surface of the substrate by the binder resin, which typically produces a rough topography and the pigment particles are enveloped in binder resin. These film properties scatter the light and have detrimental effects on the optical density. An ink containing minimal binder resin has improved wetting properties on the substrate owing to the lower amount of binder resin present in the ink, leading to deeper penetration and greater drop spread, which produces a smoother ink film topography with less scattering of light. The printed substrate therefore has good colour strength, even when using a small amount of ink.

[0094] The printed substrate prepared using the inkjet ink of the present invention also has a better handle and is softer than a comparative ink containing a binder resin instead of a cross-linking agent of the present invention.

[0095] Minimising the amount of binder resin present in the inkjet ink of the present invention also improves the breathability of the printed substrate, which is particularly important in sportswear where the inclusion of a binder resin is thought to block sweat. In this regard, the inclusion of a binder resin in an ink limits the ink film to the surface of the substrate. Moreover, when such an ink is printed onto a textile substrate, the individual fibres in the substrate can be matted together by the binder resin. As discussed above in relation to optical density, an ink containing minimal binder resin will wet the substrate more readily and penetrate more deeply into the substrate. As such, a printed textile substrate is more breathable.

[0096] In a preferred embodiment, the inkjet ink of the present invention further comprises a wax. The inclusion of a wax in the ink improves dry crock and provides a conditioning effect. Examples of suitable waxes include Hytec Wax E-6314 and Hytec E-9015 commercially available from Toho Chemical Industry Co., Ltd., Adiwax H 606 F commercially available from Lamberti S.p.A and Nopcote PEM17 commercially available from San Nopco. Preferably, the wax is an aqueous emulsion, more preferably an aqueous polyethylene and / or polypropylene emulsion, most preferably an aqueous oxidised polyethylene and / or polypropylene emulsion. Hytec Wax E-6314 is an oxidised polyethylene wax emulsion. Hytec E-9015 is an aqueous emulsion of polyethylene wax. Adiwax H 606 F is an aqueous emulsion of oxidised polyethylene and paraffines wax, which is free of nonylphenols. Nopcote PEM17 is a polyethylene wax emulsion.

[0097] Additional examples of suitable waxes include: synthetic waxes such as polyolefin wax and stearic acid amide; natural waxes such as carnauba, animal and beeswax; petroleum waxes such as paraffin and Vaseline®; and mineral waxes such as montan wax.

[0098] In a preferred embodiment, the surface tension of the ink is controlled by the addition of one or more surface active materials such as commercially available surfactants. Therefore, the inkjet ink of the present invention preferably further comprises a surfactant.

[0099] Surfactants are well-known in the art and a detailed description is not required. A particularly preferred surfactant is Surfynol 440. Adjustment of the surface tension of the inks allows control of the surface wetting of the inks on various 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. The surface tension is preferably in the range of 20-40 mNnr1and more preferably 30-35 mNnr1.

[0100] Preferably, the surfactant is present in the inkjet ink in an amount of 0.01 to 3% by weight, based on the total weight of the ink.

[0101] In a preferred embodiment, the inkjet ink of the present invention preferably further comprises a biocide.

[0102] Biocides are well-known in the art and a detailed description is not required. Biocides prevent microbial growth in the ink. An example of a suitable biocide is Proxel GXL commercially available from Arxanda. Proxel GXL is a 20% aqueous dipropylene glycol solution of 1 ,2-benzisothiazolin- 3-one.

[0103] Preferably, the biocide is present in the inkjet ink in an amount of 0.01 to 0.075%, more preferably 0.02 to 0.075%, most preferably 0.05 to 0.075% by weight, based on the total weight of the ink. Forthe avoidance of doubt, the amount of biocide includes any solvent added as part of the biocide component.

[0104] Other components of types known in the art may be present in the ink of the present invention to improve the properties or performance. These components may be, for example, pH buffers, humectants, defoamers, dispersants, synergists, stabilisers against deterioration by heat or light, reodorants, flow or slip aids, identifying tracers and viscosity modifiers. In a preferred embodiment, the present invention provides an inkjet ink comprising: a continuous aqueous phase; a pigment having one or more reactive groups, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected reactive groups; and less than 3% by weight of a binder resin, based on the total weight of the ink; wherein the two or more protected reactive groups of the cross-linking agent, when deprotected, are reactive to the one or more reactive groups of the pigment.

[0105] In a preferred embodiment, the present invention provides an inkjet ink consisting of: a continuous aqueous phase including water and optionally, an organic solvent; a pigment having one or more first reactive groups, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected second reactive groups, wherein the two or more protected second reactive groups, when deprotected, are reactive to the one or more first reactive groups; and optionally a wax, a surfactant, a biocide, a pH buffer, a humectant, a defoamer, a dispersant, a synergist, a stabiliser against deterioration by heat or light, a reodorant, a flow or slip aid, an identifying tracer, a viscosity modifier and combinations thereof.

[0106] More preferably, the present invention provides an inkjet ink consisting of: a continuous aqueous phase including water and optionally, an organic solvent; a pigment having one or more first reactive groups, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected second reactive groups, wherein the two or more protected second reactive groups, when deprotected, are reactive to the one or more first reactive groups; and optionally a wax, a surfactant, a biocide, a viscosity modifier and combinations thereof.

[0107] In each of these embodiments, the preferred features of the inkjet ink are as discussed above.

[0108] The amounts by weight provided herein are based on the total weight of the ink.

[0109] The inkjet ink may be prepared by known methods such as combining all of the components and stirring with a high-speed water-cooled stirrer, or milling on a horizontal bead-mill. The preparation of the inkjet ink of the present invention is remarkably simple. For example, one may simply use water, a commercially available pigment dispersion, a commercially available cross-linking agent dispersion and any other additional components. The cross-linking agent dispersion may be added to the pigment dispersion first or the cross-linking agent dispersion may be added to the pigment dispersed in the continuous aqueous phase of the ink. The inkjet ink preferably exhibits a desirable low viscosity (100 mPas or less, more preferably 50 mPas or less and most preferably 35 mPas or less at 25°C). The ink most preferably has a viscosity of less than 20 mPas at 25°C. Viscosity may be measured using a rotational viscometer fitted with a thermostatically controlled cup and spindle arrangement, running at 20 rpm at 25°C.

[0110] The present invention may also provide an inkjet ink set, wherein the inkjet ink set of the invention has at least one ink that falls within the scope of the inkjet ink according to the present invention. Preferably, all of the inks in the set fall within the scope of the inkjet ink according to the present invention.

[0111] Usually, the inkjet ink set of the present invention is in the form of a multi-chromatic inkjet ink set, which typically comprises a cyan ink, a magenta ink, a yellow ink and a black ink (a so-called trichromatic set). This set is often termed CMYK. The inks in a trichromatic set can be used to produce a wide range of colours and tones.

[0112] The present invention also provides a method of inkjet printing comprising the following steps in order:

[0113] (i) providing an inkjet ink of the present invention;

[0114] (ii) inkjet printing the inkjet ink onto a substrate to provide a printed substrate;

[0115] (iii) drying the printed substrate to remove water; and

[0116] (iv) deprotecting the two or more protected second reactive groups.

[0117] The method of the present invention is a method of inkjet printing.

[0118] In inkjet printing, minute droplets of black, white or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate, which is moving relative to the reservoirs. The ejected ink forms an image on the substrate.

[0119] For high-speed printing, the inks must flow rapidly from the printing heads, and, to ensure that this happens, they must have in use a low viscosity, typically 200 mPas or less at 25°C, although in most applications the viscosity should be 50 mPas or less, and often 25 mPas or less. Typically, when ejected through the nozzles, the ink has a viscosity of less than 25 mPas, preferably 3-15 mPas and most preferably between 4-1 1 mPas at the jetting temperature, which is often elevated to, but not limited to 30-50°C (the ink might have a much higher viscosity at ambient temperature). The inks must also be resistant to drying or crusting in the reservoirs or nozzles. For these reasons, inkjet inks for application at or near ambient temperatures are commonly formulated to contain a large proportion of a mobile liquid vehicle or solvent such as water or another solvent or mixture of solvents. The method of the present invention comprises (i) providing an inkjet ink of the present invention.

[0120] The inkjet ink used in the method of the present invention is the inkjet ink of the present invention as described above. The preferred inkjet ink used in the method of the present invention is as described above for the inkjet ink of the present invention.

[0121] The method of the present invention further comprises (ii) inkjet printing the inkjet ink onto a substrate to provide a printed substrate.

[0122] The printing is performed by inkjet printing, e.g. on a single-pass inkjet printer, for example for printing (directly) onto a substrate, or a multiple-pass printer where the image is built up in print swathes. Inkjet printing is well-known in the art.

[0123] The ink is jetted from one or more reservoirs or printing heads through narrow nozzles onto a substrate to provide a printed substrate.

[0124] In order to produce a high quality printed image a small jetted drop size is desirable. Preferably the inkjet ink is jetted at drop sizes below 90 picolitres, preferably below 35 picolitres and most preferably below 10 picolitres.

[0125] The inkjet ink of the present invention is compatible with print heads that are capable of jetting drop sizes of 90 picolitres or less because of its low viscosity.

[0126] Print heads account for a significant portion of the cost of an entry level printer and it is therefore desirable to keep the number of print heads (and therefore the number of inks in the ink set) low. Reducing the number of print heads can reduce print quality and productivity. It is therefore desirable to balance the number of print heads in order to minimise cost without compromising print quality and productivity.

[0127] The substrate can be any substrate suitable for printing. Textile substrates are particularly preferred including treated and untreated textile substrates. Suitable textile substrates include cotton, jersey, silk, rayon, wool, polyester and nylon.

[0128] Jersey is a stretchable, close-knit fabric having a latticework of twisted vertical yarns connected by untwisted horizontal yarns. Jersey is composed of natural fibres such as wool (animal-derived) or cotton (plant-derived), both of which are optionally blended with synthetic fibres such as polyester or rayon.

[0129] The ink of the present invention is suitable for printing onto a wider range of textile substrates than typical inks containing a binder resin. In particular, the ink of the present invention is suitable for printing onto stretchable textile substrates such as jersey. Inks containing a binder resin typically adhere poorly to stretchable textile substrates as the ink film is confined to the surface and only physically embedded in the substrate. In contrast, the strong covalent bonds formed between the pigment and the substrate in the present invention and the deep penetration of the ink film into the substrate means that stretchable textile substrates can be printed onto successfully.

[0130] In a preferred embodiment, the substrate is a textile substrate, wherein the textile substrate is composed of a polymeric material having one or more third reactive groups which are available to react with the two or more protected second reactive groups of the cross-linking agent, when deprotected, preferably wherein the textile substrate comprises a cellulosic material. More preferably, the substrate is a textile substrate, wherein the textile substrate is composed of a polymeric material having free hydroxyl groups at the surface of the substrate. Cellulosic textile substrates are particularly preferred. Examples of cellulosic textile materials include cotton and jersey. Printing onto such textile substrates provides a printed substrate with particularly advantageous properties.

[0131] Preferably, once the two or more protected second reactive groups are deprotected, the crosslinking agent is able to react with the one or more first reactive groups on the pigment and the one or more third reactive groups of the textile substrate, including any free hydroxy groups at the surface of the substrate. The one or more third reactive groups of the textile substrate can be the same or different to the one or more first reactive groups of the pigment, but when present, are different to the two or more protected second reactive groups of the cross-linking agent. In this regard, preferably, the textile substrate comprises one or more third reactive groups which can react with the two or more protected second reactive groups of the cross-linking agent. The crosslinking agent therefore tethers the pigment to the textile substrate, providing good adhesion between the pigment and the textile substrate. The pigment particles also embed in the substrate and the cross-linking agent cross-links the pigment particles together, increasing the density of the ink film. These cross-linking reactions results in the printed image being both washfast and robust.

[0132] Suitable substrates include silk, cotton, rayon, wool, polyester and / or nylon. Preferred substrates comprise silk and / or cotton. Silk may contain amino groups which can react with the two or more reactive groups of the cross-linking agent. A particularly preferred substrate comprises cotton and more preferably is jersey. Jersey composed of natural fibres is preferred. Natural fibres have the ability to decompose meaning jersey composed of natural fibres is more environmentally friendly. A particularly preferred jersey substrate is composed of 95% cotton and 5% Lycra®. The ink of the present invention adheres particularly well to this stretch jersey leading to wash-fast printed substrates.

[0133] If there are no free reactive groups on the substrate, the two or more protected second reactive groups of the cross-linking agent, when deprotected, can still react with the one or more first reactive groups of the pigment and cross-link the pigment, so that a cross-linked film of the ink is formed on the substrate. Such a printed substrate would still have a good handle and optical density, even if the adhesion between the pigment and the substrate is not optimal. However, substrates with no free reactive groups may need to be pre-treated prior to printing so that sufficient bonding to the substrate is achieved.

[0134] It is standard practice to use a pre-treat (also known as a primer) on a textile substrate to enhance wash- and crock-fastness of the printed ink. Pre- and post-treats are also commonly applied to enhance the handle, image and colour quality of the prints. Post-treats typically involve steaming, washing, drying and / or heating steps. Preferably, the method of the present invention does not use pre- or post-treats. It is surprising that the method of the present invention can provide printed images with a high-quality handle, image and colour without resorting to pre- or post-treats. The fact that the method of the present invention does not require any additional pre- or post-treatment steps means that it is also simpler, quicker, cheaper and more environmentally friendly than other methods using dye-based inkjet inks.

[0135] 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.

[0136] The method of the present invention further comprises (iii) drying the printed substrate to remove water.

[0137] By drying, it is meant the removal of the water (and optional organic solvent) by evaporation. Evaporation of the water can occur simply by exposure of the inks to the atmosphere, but the ink may also be heated to accelerate evaporation. If the ink is heated to accelerate evaporation, the ink is preferably heated to a temperature of less than 100 °C. The minimum temperature is less critical but if the ink is heated to accelerate evaporation, the printed ink is preferably heated to a temperature of 60°C or higher. Therefore, in a preferred embodiment, the printed ink is heated to a temperature between 60°C and less than 100°C.

[0138] Drying the printed substrate to remove water occurs before the two or more second reactive groups are deprotected in order to minimise the two or more second reactive groups reacting with water.

[0139] The method of the present invention further comprises (iv) deprotecting the two or more protected second reactive groups. Deprotecting the two or more protected second reactive groups can be done by any means. The means of deprotection depends on the nature and reactivity of the two or more protected second reactive groups. The two or more protected second reactive groups are reactive to the one or more first reactive groups of the pigment when thermally deprotected, and preferably, deprotecting the two or more protected second reactive groups is by heating the printed substrate to a temperature sufficient to deprotect the two or more protected second reactive groups of the cross-linking agent.

[0140] Therefore, the present invention preferably provides a method of inkjet printing comprising the following steps in order:

[0141] (i) providing an inkjet ink comprising: a continuous aqueous phase; a pigment having one or more first reactive groups, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected second reactive groups, wherein the two or more protected second reactive groups, when thermally deprotected, are reactive to the one or more first reactive groups; and less than 3% by weight of a binder resin, based on the total weight of the ink;

[0142] (ii) inkjet printing the inkjet ink onto a substrate to provide a printed substrate;

[0143] (iii) drying the printed substrate to remove water; and

[0144] (iv) heating the printed substrate to a temperature sufficient to deprotect the two or more protected second reactive groups.

[0145] The temperature is selected based on the cross-linking agent used and what temperature is required to deprotect the two or more protected second reactive groups. For example, the blocked isocyanate groups of Imprafix® 2794 are deprotected at a temperature of 135°C. Preferably, the temperature sufficient to deprotect the two or more protected second reactive groups is 100°C or higher, more preferably 120°C or higher and most preferably 135°C or higher. The maximum temperature is dictated by cost but preferably, the temperature sufficient to deprotect the two or more protected second reactive groups is up to 170°C.

[0146] The printed substrate is preferably heated to a temperature sufficient to deprotect the two or more protected second reactive groups from 30 seconds to 10 minutes, more preferably from 1 to 7 minutes.

[0147] In this step, the two or more protected second reactive groups of the cross-linking agent are deprotected. The two or more second reactive groups can then react with the one or more first reactive groups of the pigment and any free third reactive groups on the substrate which are capable of reacting with the two or more second reactive groups of the cross-linking agent, thereby binding pigment particles together and tethering the pigment to the substrate. These cross-linking reactions provide good adhesion between the pigment and the substrate, resulting in the printed image being both wash-fast and robust.

[0148] The present invention also provides a printed substrate having the inkjet ink as defined herein printed thereon. The present invention further provides a printed substrate obtainable by the method of the present invention. Preferred substrates are those given above. As discussed above, the printed substrate has a good handle and optical density, without compromising the washfastness and robustness of the printed substrate. The printed substrate therefore has superior properties than achieved using other pigmented inkjet inks. It is surprising that these properties can be achieved without requiring a binder resin.

[0149] The invention will now be described with reference to the following examples, which are not intended to be limiting.

[0150] Examples

[0151] Example 1

[0152] 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 inks.

[0153] Table 1

[0154] Cab-o-jet® 300 Black is a pigment dispersion as claimed, which is commercially available from Cabot®.

[0155] Pro-Jet® APD 1000 black is a comparative pigment dispersion as it does not have one or more reactive groups on the surface of the pigment. It is an encapsulated black pigment, which is commercially available from FUJIFILM Imaging Colorants Limited.

[0156] Triethylene glycol and glycerol are organic solvents. Surfynol® 440 is a surfactant commercially available from Evonik. Proxel® GXL is a biocide commercially available from Arxanda. Rovene® 4170 is a carboxylated self-cross-linking styrene butadiene copolymer binder resin commercially available from Mallard Creek. Imprafix® 2794 is a water-based aliphatic blocked polyisocyanate cross-linking agent commercially available from Covestro.

[0157] Inks 1 and 4 contain a binder resin instead of a cross-linking agent as in the ink of the invention and so are comparative inks. Ink 2 contains no binder resin or cross-linking agent and so is also a comparative ink. Ink 3 contains a cross-linking agent and pigment as claimed and is an ink of the invention. Test i

[0158] Inks 1 , 3 and 4 were assessed for handle / softness on untreated cotton.

[0159] Inks 1 , 3 and 4 were drawn down in 40 pm films using 40 pm wires wound K-bars onto transfer paper (cold-peel PET film suitable for water-based systems). The wetted transfer paper was pressed onto an untreated (Premier 6978) cotton substrate. The inks were heated by placing the printed ink films in a pre-heated oven for 15 minutes at a temperature of 70°C.

[0160] The inks were then heated (cured) in a hot-press for two minutes at 180°C. The conditions used were sufficient to fully dry / cure the ink.

[0161] For Ink 3, the conditions used were sufficient to deprotect the blocked polyisocyanate cross-linking agent to provide a polyisocyanate, which was then able to react with the carboxylic acid groups of the pigment and the hydroxyl groups of the cotton substrate. In this way, the cross-linking agent bound the pigment particles together and tethered the pigment to the substrate. These crosslinking reactions do not happen for Inks 1 and 4, in which a binder resin is present instead of a cross-linking agent as in the invention.

[0162] Handle was assessed by performing a survey on a group of thirty people with basic textile handle training, referred to as the handle panel.

[0163] 22 / 30 people said that the cured ink film of Ink 3 of the invention was softer than the cured ink film of comparative Ink 4.

[0164] 23 / 30 people said that the cured ink film of Ink 3 of the invention was softer than the cured ink film of comparative Ink 1.

[0165] Accordingly, the handle test supports that the ink of the invention, which contains a cross-linking agent and a pigment as claimed, and does not contain a binder resin, produces softer cured ink films compared to inks, which contain a binder resin and do not contain a cross-linking agent as claimed.

[0166] Test 2

[0167] Inks 1-3 were assessed for wash-fastness. The inks were transferred to untreated cotton using the same transfer method, heating and curing conditions given in Test 1 . All of these inks were applied to the substrate using a single transfer layer.

[0168] The cured ink films were then photographed. The cured ink films were subsequently loaded in a washing machine with 1 kg of polyester sheets and washed for 140 minutes at a temperature of 60°C with 50 g of Persil® non-biological washing powder. This process was repeated a second time.

[0169] The cured ink films were then photographed again.

[0170] The results are set out in Figs 1 , 2 and 3.

[0171] Figure 1 shows the photographs taken for the cured ink films for comparative Ink 1 . Figure 2 shows the photographs taken for the cured ink films for comparative Ink 2. And Figure 3 shows the photographs taken for the cured ink films for Ink 3 of the invention.

[0172] For Inks 1 and 3, the large piece of cloth on the left-hand side corresponds to the unwashed ink, and the large piece of cloth on the right-hand side corresponds to the ink after two washes. For Ink 2, the large piece of cloth at the top of the image corresponds to the unwashed ink, and the large piece of cloth at the bottom of the image corresponds to the ink after two washes.

[0173] As can be seen in Figs. 1 -3, comparative Ink 1 (containing a binder resin and no cross-linking agent) and Ink 3 of the invention both have superiorwash-fastness compared with comparative Ink 2 (containing no binder or cross-linking agent).

Claims

Claims1 . An inkjet ink comprising: a continuous aqueous phase; a pigment having one or more first reactive groups, which is dispersed in the continuous aqueous phase; a cross-linking agent having two or more protected second reactive groups; and less than 3% by weight of a binder resin, based on the total weight of the ink; wherein the two or more protected second reactive groups, when deprotected, are reactive to the one or more first reactive groups.

2. An inkjet ink as claimed in claim 1 , wherein the cross-linking agent has two or more protected second reactive groups, which when deprotected, are reactive to hydroxy groups.

3. An inkjet ink as claimed in claims 1 or 2, wherein the cross-linking agent having two or more protected second reactive groups is a blocked di- and / or poly-isocyanate where the two or more isocyanate groups are protected.

4. An inkjet ink as claimed in any preceding claims, wherein the pigment has one or more first reactive groups which are reactive to isocyanate groups.

5. An inkjet ink as claimed in any preceding claim, wherein the pigment has one or more first reactive groups selected from carboxyl, hydroxy, amino, sulfonate, phosphoryl, sulfinate, amide and mixtures or salts thereof.

6. An inkjet ink as claimed in any preceding claim, wherein the pigment is present in an amount of 0.2 to 20% by weight, based on the total weight of the ink.

7. An inkjet ink as claimed in any preceding claim, wherein the two or more protected second reactive groups of the cross-linking agent, when deprotected, are present in the ink in an amount of 0.01 to 1 .00%, preferably 0.02 to 0.80%, more preferably 0.03 to 0.60% by weight, based on the total weight of the ink.

8. An inkjet ink as claimed in any preceding claim, wherein water is present in a total amount of 30 to 80% by weight, based on the total weight of the ink.

9. An inkjet ink as claimed in any preceding claim, wherein the ink contains less than 1 % by weight of a binder resin, based on the total weight of the ink, preferably wherein the ink is free of a binder resin.

10. An inkjet ink as claimed in any preceding claim, wherein the pigment is a black pigment and the pigment has one or more first reactive groups selected from carboxyl, sulfonate, phosphonate, sulfinate and mixtures or salts thereof.

11. An inkjet ink as claimed in any preceding claim, wherein the pigment is a non-black pigment and the pigment has one or more first reactive groups selected from sulfonate, sulfinate and mixtures or salts thereof.

12. A method of inkjet printing comprising the following steps in order:(i) providing an inkjet ink as claimed in any preceding claim;(ii) inkjet printing the inkjet ink onto a substrate to provide a printed substrate;(iii) drying the printed substrate to remove water; and(iv) deprotecting the two or more protected second reactive groups.

13. A method of inkjet printing as claimed in claim 12, wherein the two or more protected second reactive groups are reactive to the one or more first reactive groups when thermally deprotected, and wherein deprotecting the two or more protected second reactive groups is by heating the printed substrate to a temperature sufficient to deprotect the two or more protected second reactive groups.

14. A method of inkjet printing as claimed in claims 12 or 13, wherein the substrate is a textile substrate, and wherein the textile substrate is composed of a polymeric material having one or more third reactive groups which are available to react with the two or more protected second reactive groups, when deprotected, preferably wherein the textile substrate comprises a cellulosic material.

15. A method of inkjet printing as claimed in any one of claims 12 to 14, wherein drying the printed substrate to remove water comprises heating the printed substrate to a temperature of less than 100°C.

16. A method of inkjet printing as claimed in any one of claims 12 to 15, wherein the temperature sufficient to deprotect the two or more protected second reactive groups is 100°C or higher, preferably 120°C or higher, more preferably 135°C or higher.

17. A printed substrate having the inkjet ink as claimed in any one of claims 1 to 11 printed thereon.

18. A printed substrate obtainable by the method of any one of claims 12 to 16.

Images