Liquid set for inkjet printing comprising an aqueous pre-treatment liquid, an aqueous inkjet ink and a varnish.

Abstract

A liquid set for inkjet printing comprising an aqueous pre-treatment liquid, an aqueous inkjet ink and a varnish, the aqueous pre-treatment liquid comprises a fixing agent, the aqueous inkjet ink comprises a pigment polymer composite particle and a water-soluble organic solvent, the varnish comprises water and resin particles, characterized in that the polymer is a vinyl-based polymer containing an acid group, and the varnish comprises a water-soluble polymer selected from the group consisting of linear PPO-PEO copolymers and arylethylphenyl polyglycol ethers. A recoding method using the liquid set is also included.

Description

DescriptionLiquid set for inkjet printing comprising an aqueous pre-treatment liquid, an aqueous inkjet ink and a varnish.Technical Field

[0001] The invention relates to liquid sets suitable for inkjet recording, more specifically inkjet printing on liners for corrugated packaging, folding card boards and corrugated card boards.Background Art

[0002] Inkjet printing is a growing area of printing of liners for corrugated packaging and of corrugated card boards. Preferably, inkjet printing is performed by means of aqueous inkjet inks. Aqueous inkjet inks are inherently safer than reactive UV inks and inks whose primary vehicle is a solvent.

[0003] Both dyes and pigments have been used as colorants for aqueous inkjet inks and both have certain advantages. Pigment inks are advantageous because they tend to provide more light-fast images than soluble dye inks.

[0004] However, since pigment inks are attached to the uppermost surface of a printed matter (hereinafter referred to as pigment ink printed matter), the printed texts and images are easily peeled off, meaning that the friction resistance of the pigment ink printed matter is inferior.

[0005] In attempts to improve the friction resistance of such a pigment ink printed matter, for example, JP-2011 -63016-A and JP-2011 -105900-A disclose methods of protecting the surface of a printed matter by applying a transparent post-processing fluid onto the recording medium after the image has been printed.

[0006] The easiest way to apply a transparent post-processing fluid onto a recording medium carrying an inkjet printed image is by over-all coating the post-processing fluid as in US 2007 / 0282037A1. Commercially available analogue post-processing fluids are available. In that case, the amount of post-processing fluid is significantly more than if the posttreatment fluid in only applied onto the inkjet printed image. Moreover,change over times that are needed to switch between different paper sizes are significant and contribute to a less economically efficient printing process.

[0007] Therefore, it is beneficial to apply the post-treatment liquid only onto the inkjet printed image. The most suitable technique for image wise applying the post-treatment liquid is inkjet and only requires an extra inkjet head or print bar into the printing equipment.

[0008] EP3099735A discloses a post-treatment fluid including a fluid vehicle and an anionic polyurethane acrylic hybrid polymer binder dispersed in the fluid vehicle to be combined with aqueous colored inks comprising pigments.

[0009] The post-treatment liquid includes ingredients, such as a resin, and wax. As the resin, or the wax form a uniform coating film on the surface of the image formed by jetting the aqueous ink, glossiness is imparted to the image. If the post-treatment liquid is applied onto a surface of an image formed by ejecting the aqueous ink in the unfixed state, however, the posttreatment liquid is mixed into the image.

[0010] Therefore, a uniform coating film cannot be formed on the surface of the image, leading to the problem that glossiness of the image is poor. To increase glossiness, LIS2016 / 0177116 A proposes an image forming set including: an aqueous ink including a dispersed pigment, a water-soluble organic solvent, and a post-treatment liquid including a water-soluble organic solvent, a urethane resin and an acrylic resin together with an isocyanate as crosslinking agent. The use of organic solvents in a posttreatment liquid together with reactive crosslinking agents is to be avoided for health and safety reasons.

[0011] Therefore, there is a strong need for obtaining durable inkjet printed images where the post-treatment liquid shows high gloss values without containing in its composition reactive crosslinking agents as isocyanates.Summary of invention

[0012] It is the objective of the present invention to provide a solution to the above stated problems. The objective has been achieved by providing a liquid set as defined in claim 1.

[0013] It is another embodiment of the invention to provide a printing method using the liquid set of Claim 1 as defined in Claim 11.

[0014] Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention. Specific embodiments of the invention are also defined in the dependent claims Description of embodimentsA. Liquid set for inkjet printing

[0015] The liquid set for inkjet printing according to the invention comprises an aqueous pre-treatment liquid, a varnish and an aqueous inkjet ink comprising a water-soluble organic solvent and a pigment polymer composite particle dispersion, the polymer being a vinyl-based polymer containing an acid group.A.1. Varnish

[0016] The varnish as part of the liquid set according to the invention is used to protect the inkjet printed image against wet & dry rubbing, scratches and solvents. The varnish therefor, comprises water, resin particles and a water soluble polymer selected from the group of linear PPO-PEO copolymers and arylethylphenyl polyglycol ethers.

[0017] Suitable linear PPO-PEO copolymers preferably have polymeric or oligomeric hydrophobic segments. They are also referred as segmented copolymers. Typical examples are diblock or triblock copolymers based on EO and PO. Typical trade names are Pluronic RPE grades or Pluronic PE grades, available from BASF and Synperonic grades from Croda. In case of triblock copolymers, the outer blocks have a different polarity than the mid-block. The most polar blocks can be located at the outside but can also be located in the middle as the mid-block. Besides diblock and triblock copolymers also multiblock copolymers can be used.

[0018] The hydrophilicity of the hydrophilic segment can be changed by variation of the number of EO units but instead of EO also other hydrophilic alkylene oxide units can be used such as based on glycidol, other oxiranes, glycerol, glucitol, glucoside, ect.. Typical trade names are Lutensol(BASF), Plurafac (BASF), Tergitol (Dow), Etocas (Croda), Pionin (NOF), Emulsogen (Clariant), Silco sperse (Keim Additec).

[0019] Other specific useful examples of the water soluble polymers useful in the varnish of the invention are arylethylphenyl polyglycol ethers such as Lucramul DA554 from LEVACO Chemicals GmbH.

[0020] Without being bound to a theory, it is thought that the above mentioned water-soluble polymers improve the film formation of the resins present in the varnish by avoiding flocculation with the fixing agent of the pretreatment liquid during the drying step of the varnish. An improved film formation leads to higher gloss values of the printed image covered with a varnish coating.

[0021] Particularly preferred the water-soluble linear copolymers based on EO and PO have an average molecular weight between 3000 g / mol and 30000 g / mol, more preferably between 4000 g / mol and 15000 g / mol and having a PPO / PEO weight ratio between 0.1 and 2.5. Varnish compositions having copolymers with these molecular weight range and PPO / PEO weight ratio do show improved gloss of the inkjet printed image and an increased resistance against wet and dry rubbing. The average Mw (average molar mass) is determined from the OH number (mg KOH / g) obtained by titration e.g., using method ISO 4326.

[0022] The water-soluble polymer in the varnish of the invention is preferably nonionic. Due to its non-ionic character, the water-soluble polymer interacts less with the cationic fixing agent in the pre-treatment liquid during the drying step of the varnish. Higher gloss values are then obtained.

[0023] The water-soluble polymer is present in the varnish composition in an amount ranging from 1 wt.% to 40 wt.%, based on the total dry solids content of the treatment composition, more preferably in an amount ranging from 2 wt.% to 35 wt.%, based on the total dry solids content of the treatment composition, most preferably in an amount ranging from 5 wt.% to 20 wt.% based on the total dry solids content of the treatment composition.Water-soluble organic solvents

[0024] One or more water-soluble organic solvents may be present in the varnish from the liquid set according to the invention for a variety of reasons. For example, it can be advantageous to add a small amount of an organic solvent to improve the dissolution of a compound in the varnish to be prepared, to obtain better penetration in porous substrates or to prevent fast drying of the varnish at the nozzle of the inkjet head. Preferable water- soluble organic solvents are polyols (e.g., ethylene glycol, glycerin, 2- ethyl-2-(hydroxymethyl)-1 ,3-propanediol, tetraethylene glycol, triethylene glycol, tripropylene glycol, 1 ,2,4-butanetriol, diethylene glycol, propylene glycol, dipropylene glycol, butyleneglycol, 1 ,6-hexanediol, 1 ,2- hexanediol, 1 ,5-pentanediol, 1 ,2-pentanediol, 2,2-dimethyl-1 ,3- prapanediol, 2-methyl-2,4-pentanediol, 3-methyl-1 ,5-pentanediol, 3- methyl-1 ,3-butanediol, and 2-methyl-1 ,3-propanediol), N-hydroxyethyl- pyrrolidon, N-butyl-pyrrolidon, amines (e.g., ethanolamine, and 2- (dimethylamino)ethanol), monohydric alcohols (e.g., methanol, ethanol, and butanol), 2,2'-thiodiethanol, amides such as N,N-dimethylformamide, heterocycles such as 2-pyrrolidone and N-methyl-2-pyrrolidone, and acetonitrile.

[0025] Examples of the glycol ether include mono alkyl ethers of glycol selected from ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol. More specifically, triethylene glycol monomethyl ether, triethylene glycol monobutyl ether, and dipropylene glycol mono propyl ether can be preferably exemplified. Other useful examples which are listed in the Swiss list A and hence compatible with food packaging are: Propylene Glycol Monomethyl Ether; Propylene Glycol Monoethyl Ether; Propylene Glycol Monopropyl Ether; Ethylene Glycol Monobutyl Ether; Propylene Glycol Monobutyl Ether; Diethylene Glycol Monoethyl Ether; Diethylene Glycol Monobutyl Ether; Dipropylene Glycol Mono n-Propyl Ether; Dipropylene Glycol Methyl Ether; Tripropylene Glycol Monomethyl Ether. These solvents may be used alone or in combination of two or more. The humectant is preferably added to the pre-coat composition formulation in an amount of 0.1 to 25 wt.% based on the total weight of the liquid.Wax

[0026] The varnish as comprised in the liquid set according to the invention may comprise a wax. The wax may improve the durability of the printed image. Generally, any suitable wax may be used in the varnish composition. As such, the wax may be polyethylene waxes, petroleum waxes, paraffin waxes, carnauba waxes, polypropylene waxes, crystalline and microcrystalline waxes, amide waxes (oleamide, stearamide, erucamide, cyclic amide, etc... ), and combinations thereof. In an aspect of the invention, the wax may be a high density polyethylene wax.

[0027] In an aspect of the invention, the wax may be a polyethylene wax or modified paraffin wax. An example of polyethylene wax includes high density polyethylene (HDPE) wax, which has a density ranging from about 0.93 g / mL to 0.97 g / mL.

[0028] An example of modified paraffin wax particles includes paraffin wax that has been modified to improve dispersability in water, e.g., via emulsification. The modified paraffin wax may be surface modified, chemically modified, etc.

[0029] Some specific examples of wax that may be used include those of the JONCRYL Wax series (such as JONCRYL Wax 22, JONCRYL Wax 26, and JONCRYL Wax 120 available from BASF Corp.), those of the AQUACER series (such as AQUACER 498, AQUACER 501 , AQUACER 505, AQUACER 513, AQUACER 530, AQUACER 531 , AQUACER 535, AQUACER 537, AQUACER 539, and AQUACER 552 available from BYK- Gardner, Columbia, Md. ) and Liquilube 404E from Lubrizol .

[0030] The wax may have i) a high melting temperature T and / or ii) a small average particle size. In an example, the wax may have a high melting T such as one that is equal to or greater than 90°C. Further, the wax may have an average particle size (in terms of effective diameter assuming that the individual wax particles are not perfectly spherical) ranging from 0.03 pm to 1.5 pm, more preferably from 0.05 pm to 1 pm (D50). If the particle size exceeds these upper limits, jetting reliability problems of the varnish are likely to occur.

[0031] The wax may be present in the varnish in an amount ranging from 3 to 25 wt. %, more preferably from 5 to 20 wt. %, relative to the total solids weight of the over-print varnish.Resin particles

[0032] Examples of the resin particles included in the varnish include well-known resins such as a urethane-based resin, an acrylic resin, a fluorene-based resin, a polyolefin-based resin, a rosin modified resin, a terpene-based resin, a poly-ester-based resin, a polyamide-based resin, an epoxy-based resin, and a vinyl chloride-based resin. The vinyl chloride-based resin includes a vinyl chloride copolymer such as a vinyl chloride-vinyl acetate copolymer. These resins may be used alone, or two or more thereof may be used in combination.

[0033] Among the aforementioned resin particles, the resin included in the varnish is preferably a urethane-based resin, an acrylic resin, a styrene- acrylic resin or a polyolefin-based resin.

[0034] As the urethane-based resin, commercially available products may be used, for example, commercially available products such as SUPER FLEX 460, 460s, 840, E-4000 (trade name, manufactured by DKS Co., Ltd.), RESAMINE D-1060, D-2020, D-4080, D-4200, D-6300, D-6455 (trade name, manufactured by Dainichiseika Color & Chemicals Mfg.Co., Ltd.), TAKELAC WS-6021 , W-512-A-6 (trade name, manufactured by Mitsui Chemicals Polyurethanes INC.,), SUNCURE 2710 (trade name, manufactured by LUBRIZOL Corporation), and PermalinUA-150 (trade name, manufactured by Sanyo Chemical Industries, Ltd.) may be used.

[0035] The acrylic resin is a general name of a polymer obtained by polymerizing at least an acrylic monomer such as (meth)acrylic acid and (meth)acrylate, and the examples thereof include a (meth)acryl resin obtained from the acrylic monomer, and a copolymer of the acrylic monomer and monomers other than the acrylic monomer (for example, a vinyl based monomer such as styrene). Acrylamide and acrylonitrile can be also used as the acrylic monomer. The acrylic resin can be non-reactive or self-cross linking. As a resin emulsion using the acrylic resin as a raw material, commerciallyavailable products may be used, and the examples thereof include FK-854 (trade name, manufactured by CH I RIKA. Co., ltd.), Mowinyl 952B, 718A (trade name, manufactured by The Nippon Synthetic Chemical Industry Co., Ltd.), Nipol LX852, and LX874 (trade name, manufactured by ZEON Corporation).

[0036] Examples of styrene-acrylic resins include poly(styrene-alkyl acrylate), poly(styrene-1 , 3-diene), poly(styrene-alkyl methacrylate), polystyrenealkyl acrylate-acrylic acid), poly(styrene-1 , 3-diene-acrylic acid), poly(styrene-alkyl methacrylate-acrylic acid), poly(styrene-alkylacrylate- acrylonitrile-acrylic acid), and poly(styrene-1 , 3-diene-acrylonitrile-acrylic acid). Other examples include poly(styrene-propyl acrylate), poly(styrene- butylacrylate), poly(styrene-butadiene-acrylic acid), poly(styrene- butadiene-methacrylic acid), poly(styrene-butadieneacrylonitrile-acrylic acid), poly(styrene-butyl acrylateacrylic acid), poly(styrene-butyl acrylatemethacrylic acid), poly(styrene-butyl acrylate-acrylononitrile), poly(styrenebutyl acrylate-acrylononitrile-acrylic acid). Commercial styrene-acrylic resins are Neocryl D2101 (Covestro) and Bonron PS001 and Bonron PS002 (Mitsui).

[0037] Other suitable resins include Esajet acrylic latices from Lamberti such as Esajet AC 20, Esajet AC22, Esajet AC 29, Esajet AC31 and Esajet AC03.

[0038] The polyolefin-based resin has olefin such as ethylene, propylene, and butylene as a structural skeleton, and the well-known resins may be appropriately selected to be used. As the olefin resin, commercially available products may be used, and the examples thereof include Arrowbase CB-1200 and CD-1200 (trade name, manufactured by UNITIKA LTD.).

[0039] The varnish contains the resin in an amount from 1 wt.% to 30 wt.% with respect to the total mass of the liquid. When the content of the resin in the varnish is within the aforementioned range, the effect of improving rub resistance of the image tends to become more excellent. More preferably the varnish contains the resin in an amount from 3 wt.% - 20 wt.% with respect to the weight of the liquid. Most preferably the varnish contains the resin in an amount from 10 wt.% to 20 wt.% with respect to the weight ofthe liquid. If the varnish contains a resin within the range from 10 to 20 wt.% with respect to the weight of the liquid, an even higher gloss of the printed image is obtained.Polyether siloxane surfactant

[0040] The varnish according to the invention may contain a polyether siloxane surfactant. A preferable surfactant it is according to formula I:Formula I

[0041] In formula I, R1 to R9 independently represent an alkyl group with 1 to 6 atoms, preferably a methyl group, or an aryl group, preferably a phenyl group, n and m independently represent an integer of 0 or more, but preferably 0 to 8. R10 to R11 independently represent a hydrogen atom or an alkyl group, preferably a methyl group, p and n preferably represent an integer of 0 or more.

[0042] In case m = 0 and n = 1 in the main structure, one defines the structure as a trisiloxane. In case n and m are larger than 0, one defines the structure as a graft copolymer. The polyether units having a degree of polymerization p and the polyether units having a degree of polymerization of q. These units could be block-wise arranged or randomly arranged. In case R10 = R11 , then the graft contains only one type of alkylene oxide unit. Preferably the graft A contains one or two different types of alkylene oxide units. In case the graft A contains one type of alkylene oxide,preferred is ethylene oxide. In case two types of alkylene oxides are present the then preferably ethylene oxide and propylene oxide are used, m in structure A is preferably 2 or 3, respectively prepared from vinyl or allyl terminated poly ethers.

[0043] The preferable content of the polyether siloxane-based surfactant is equal to 0.03 wt.% or more, more preferable equal to 0.2 wt.% or more based on the total weight of the liquid. If the content is lower, water resistance or wet rub resistance of the printed image is insufficient.

[0044] Besides the compounds of Formula I having trialkyl silyl end groups, one can also have a similar structure having alkyl end groups, substituted alkyl groups, aryl or substituted alkyl groups.

[0045] Typical examples of trisiloxane structures are Byk 3450, Byk 3451 , Tego wet 260, Tego wet 240, Tego wet KL245, Korasilon additive PS1 , Korasilon additive PS2

[0046] Typical examples of graft copolymer structures are Tego glide 410, Korasilon addtive PS5, Coatosil 7607

[0047] Another silicone polyether structure which can be used is shown in Formula II, ie. a silicone polyether block copolymer.Formula II

[0048] In formula II, R1 , R3 to R6 and R8 to R9 independently represent an alkyl group with 1 to 6 atoms, preferably a methyl group, or an aryl group, preferably a phenyl group, n and m independently represent an integer of 0 or more, but preferably 0 to 8. R10 to R11 independently represent ahydrogen atom or an alkyl group, preferably a methyl group, p and n preferably represent an integer of 0 or more.

[0049] In a typical triblock copolymer, n= 0, representing a siloxane-polyether- siloxane triblock copolymer. An example of a block copolymer silicone surfactant is Byk 3420, having an ABA structure (A=polyether, B = silicone).

[0050] Furthermore, the polyether siloxane surfactant can be a structure where both the polyether and siloxane segments are present as graft. Examples of polyether siloxanes which have siloxane grafts are Byk 3565, Byk 3566 and Byk 3568

[0051] The polyether siloxane structures can also be designed to be better hydrolytically stable, such as tradenames Silwet HS312 and Silwet HS212, available from Momentive.

[0052] Suitable surfactants which are commercially available are Olfine PD-501 (Nissin Chemical Industry Co., Ltd.) Olfine PD-570 (Nissin Chemical Industry Co., Ltd.), BYK-333 (BYK Co., Ltd.), BYK-347 (BYK Co., Ltd.), BYK-348 (BYK Co., Ltd.), Hydropalat 3120

[0053] The presence of the polyether siloxane surfactant in the varnish of the invention is increasing the rub resistance in dry and wet conditions (water resistance). Without being bound by a theory, it is thought that the polyether siloxane reduces the friction coefficient of varnish coating after drying, lowing the impact of mechanical damage of the image due to rubbing.Additives

[0054] The varnish may further contain a biocide, a thickener, a pH adjuster and a corrosion inhibitor.

[0055] The varnish may comprise a thickener to increase the viscosity in the desired range of the application method.

[0056] Suitable thickeners include urea or urea derivatives, hydroxyethylcellulose, carboxymethylcellulose, hydroxypropylcellulose, derived chitin, derived starch, carrageenan, pullulan, proteins, poly(styrenesulphonic acid), poly(styrene-co-maleic anhydride), poly(alkyl vinyl ether-co-maleicanhydride), polyacrylamid, partially hydrolyzed polyacrylamid, poly(acrylic acid), poly(vinyl alcohol), partially hydrolyzed poly(vinyl acetate), poly(hydroxyethyl acrylate), poly(methyl vinyl ether), polyvinylpyrrolidone, poly(2-vinylpyridine), poly(4-vinylpyridine) and poly(diallyldimethylammonium chloride).

[0057] The thickener is added preferably in an amount of 0.01 to 20 wt.%, more preferably 0.1 to 10 wt.% based on the liquid.

[0058] A biocide may be added to varnish to prevent unwanted microbial growth, which may occur over time. The biocide may be used either singly or in combination. Suitable biocides for the ink-jet ink of the present invention include sodium dehydroacetate, 2-phenoxyethanol, sodium benzoate, sodium pyridinethion-1 -oxide, ethyl p-hydroxybenzoate and 1 ,2- benzisothiazolin-3-one and salts thereof.

[0059] Preferred biocides are Proxel™ GXL and Proxel™ Ultra 5 available from ARCH UK BIOCIDES and Bronidox™ available from COGNIS.

[0060] A biocide is preferably added to the aqueous medium in an amount of 0.001 to 3 wt.%, more preferably 0.01 to 1.0 wt. %, each based on the total weight of the liquid.Layer thickness

[0061] The coverage of the varnish coating after drying is preferably from 0.2 g / m2to 2.5 g / m2, more preferably from 0.6 to 2.0 g / m2. If the coverage is less than 0.2 g / m2the resistance against dry and wet rubbing and the gloss of the printed image is insufficient. If the coverage is above 2.5 g / m2drying of the applied varnish is slow which limits the printing speed.A.2. Aqueous pre-treatment liquid

[0062] The aqueous pre-treatment liquid according to the invention comprises water as a vehicle and a fixing agent. The aqueous vehicle may include one or more water-soluble organic solvents.Fixing agent

[0063] The fixing agent present in the pre-treatment composition is preferably a multivalent metal salt, a cationic polymer or an organic acid. The fixing agent serves to crash, precipitate or destabilize the ink colorants and hence fix them to the substrate. This leads to an improved image quality (less bleeding, less coalescence).

[0064] The polyvalent metal salt may be present in the pre-treatment composition to improve inkjet print quality. Generally, the polyvalent metal salt may be any water-soluble polyvalent metal salt. In specific examples, the polyvalent metal salt may include calcium chloride (CaCh), magnesium chloride (MgCh), magnesium sulfate (MgSC ), aluminium chloride (AICI3), calcium nitrate (Ca(NO3)2), magnesium nitrate (Mg(NO3)2), magnesium acetate (Mg(CH3COO)2), zinc acetate (Zn(CHsCOO)2) calcium propionate (Ca(C2H5COO)2), or a combination thereof. In a particular example, the polyvalent metal salt may be calcium chloride. In further examples, the polyvalent metal salt may include a metal cation selected from calcium, copper, nickel, magnesium, zinc, barium, iron, aluminium, chromium, or another polyvalent metal.

[0065] The polyvalent metal salt may also include an anion. In some examples, the anion may be fluoride, chloride, iodide, bromide, nitrate, chlorate, sulfate, acetate, or RCOO- where R is hydrogen or any low molecular weight hydrocarbon chain, e.g., C1 to C12. In a more specific example, the anion may be a carboxylate derived from a saturated aliphatic monocarboxylic acid having 1 to 6 carbon atoms or a carbocyclic monocarboxylic acid having 7 to 11 carbon atoms. Examples of saturated aliphatic monocarboxylic acid having 1 to 6 carbon atoms may include formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, pivalic acid, and / or hexanoic acid. The cationic salt may also be a mixture of two or more different cationic salts

[0066] In some cases, the polyvalent metal salt may be present in an amount from 1 wt. % to 99 wt. % with respect to the entire weight of the pretreatment composition. In more specific examples, the polyvalent metal salt may be present in an amount from 5 wt. % to 65 wt. %, more preferably from 25 wt. % to 60 wt. %, with respect to solids content of thepre-treatment composition. If the amounts are below the lower limits, insufficient fixing of the colorants occur resulting in a reduced image quality.

[0067] Polymeric cationic polymers, suitable as fixing agent in the pre-treatment composition contain either guanidinium or fully quaternized ammonium functionalities, such as quaternized polyamine copolymers. Generally, the weight average molecular weight (Mw) of the cationic polymer allows viscosity less than 25 cP at 25°C, as measured on a Brookfield viscometer. Typical Mw are less than 500.000 g / mol, and in one aspect, less than 50.000 g / mol.

[0068] Suitable classes of cationic polymers that can be used include, but are not limited to, quaternized polyamines, dicyandiamide polycations, diallyldimethyl ammonium chloride copolymers, quaternized dimethylaminoethyl(meth)acrylate polymers, quaternized vinylimidizol polymers, alkyl guanidine polymers, alkoxylated polyethylene imines, and mixtures thereof.Resin

[0069] The pre-treatment liquid as part of the liquid set according to the invention may contain resin particles. The resin particles are preferably selected from the group consisting of poly(urethanes), poly(acrylates) or waxes. Suitable examples of resin particles are found in § A.3.3.

[0070] The pre-treatment liquid as part of the liquid set according to the invention may contain a carboxylic acid containing polymer or salt thereof or a carboxylic acid anhydride containing polymer, the polymer having an acid value equal to 950 mg KOH / g of polymer or less.

[0071] It is believed that carboxylic acid anhydrides are at least partially hydrolysed upon dissolving in the aqueous vehicle of the pre-treatment liquid to a carboxylic acid or salt thereof.

[0072] The effect of the use of carboxylic acid containing polymer or salt thereof or a carboxylic acid anhydride containing polymer in the pre-treatment liquid on image quality might be related to the interaction of these polymers with the fixing agent being multi valent metal ions or cationicpolymers present in the pre-treatment liquid. The function of these fixing agents is to interact with the dispersed colorants from the ink such as dyes or pigments that are negatively charged such as to fix the colorants and avoid image bleed.

[0073] The carboxylic acid containing polymer or salt thereof or the carboxylic acid anhydride containing polymer in the pre-treatment liquid, is not limited to a specific polymer, but should have an acid value equal to 950 mg KOH / g of polymer or less, more preferably having an acid value equal to 450 mg KOH / g of polymer or more and equal to 950 mg KOH / g of polymer or less, assuming that the carboxylic acid or salt thereof or carboxylic acid anhydride containing polymer is in its acid form. If the acid value is above this range, i.e. for maleic acid and fumaric acid based homopolymers the degree of polymerisation will be too low and consequently providing less interaction with the used multivalent salts leading to a poor ink spreading. If the acid value is below the above range, solubility of the polymer in the pre-treatment liquid may be too low to give sufficient image quality.

[0074] Preferable carboxylic acid containing polymers are homopolymers such as polyacrylic acid, polyitaconic acid, polycrotonic acid and polymethacrylic acid and, poly( N-(2-hydroxyethyl)maleamic acid), poly(3-(aminocarbonyl)- 3-butenoic acid), poly(4-amino-2-methylene-4-oxobutanoic acid) and poly(maleamic acid) and salts of the corresponding homopolymers.

[0075] Other preferable carboxylic acid or carboxylic acid anhydride containing polymers are copolymers such as poly(ethylene-co-maleic anhydride), poly(isobutylene-co-maleic anhydride), poly(isobutene-co-anhydride), poly(vinyl methyl ether-co-maleic anhydride), poly(vinyl acetate-co-crotonic acid), poly(styrene-co-maleic anhydride), poly(acrylic acid-co-maleic acid) and maleic acid-methyl vinyl ether copolymer and salts of the corresponding copolymers.

[0076] The amount of the carboxylic acid containing polymer or salt thereof or carboxylic acid anhydride containing polymer is preferably from 0.1 to 5 wt.%, more preferably from 0.2 to 4 wt.%. An amount below this range does show insufficiently an improvement of the image quality. An amount above this range can lead to (i) a decrease in jetting performance as a toohigh polymer content is present in the liquid, (ii) solubility issues leading to sedimentation, (iii) a decrease in image sharpness and an increase in bleeding and feathering.Surfactant

[0077] The pre-treatment composition may contain a surfactant. Any known surfactant may be used but preferably a glycol surfactant and / or an acetylene alcohol surfactant and / or a polysiloxane surfactant is used. The use of the acetylene glycol surfactant and / or the acetylene alcohol surfactant and / or the polysiloxane surfactant improves the drying property in printing to allow high-speed printing.

[0078] The acetylene glycol surfactant and / or the acetylene alcohol surfactant is preferably one or more selected from 2, 4, 7, 9-tetramethyl-5-decyne-4, 7- diol, alkylene oxide adducts of 2,4,7, 9-tetramethyl-5-decyne-4, 7-diol, 2,4- dimethyl-5-decyn-4-ol, and alkylene oxide adducts of 2,4-dimethyl-5- decyn-4-ol. These are available from Nissin Chemical Industry, for example, as Olfine (registered trademark) E series, such as Olfine E1010, or from Evonik (formerly Air Products (GB)) as Surfynol (registered trademark), 104, Surfynol 465, Surfynol 61. Surfynol DF110, Dynol 604 Surfynol DF110L, Surfynol DF110D, Surfynol AD01 and ethoxylated derivatives such as CAS Registry Number 91629-35-5.Additives

[0079] The pre-treatment liquid may comprise a thickener to increase the viscosity in the desired range of the application method.

[0080] Suitable thickeners include urea or urea derivatives, hydroxyethylcellulose, carboxymethylcellulose, hydroxypropylcellulose, derived chitin, derived starch, carrageenan, pullulan, proteins, poly(styrenesulphonic acid), poly(styrene-co-maleic anhydride), poly(alkyl vinyl ether-co-maleic anhydride), polyacrylamid, partially hydrolyzed polyacrylamid, poly(acrylic acid), poly(vinyl alcohol), partially hydrolyzed poly(vinyl acetate), poly(hydroxyethyl acrylate), poly(methyl vinyl ether), polyvinylpyrrolidone,poly(2-vinylpyridine), poly(4-vinylpyridine) and poly(diallyldimethylammonium chloride).

[0081] The thickener is added preferably in an amount of 0.01 to 20 wt.%, more preferably 0.1 to 10 wt.% based on the liquid.

[0082] A biocide may be added to the pre-coat composition to prevent unwanted microbial growth, which may occur over time. The biocide may be used either singly or in combination. Suitable biocides for the ink-jet ink of the present invention include sodium dehydroacetate, 2-phenoxyethanol, sodium benzoate, sodium pyridinethion-1 -oxide, ethyl p-hydroxybenzoate and 1 ,2-benzisothiazolin-3-one and salts thereof.

[0083] Preferred biocides are Proxel™ GXL and Proxel™ Ultra 5 available from ARCH UK BIOCIDES and BronidoxTM available from COGNIS.

[0084] A biocide is preferably added to the aqueous medium in an amount of 0.001 to 3 wt.%, more preferably 0.01 to 1.0 wt. %, each based on the total weight of the liquid.

[0085] In one preferred embodiment of the invention, the pre-treatment liquid may contain a water-soluble copolymer selected from the group consisting of a PPO / PEO copolymer, an arylethylphenyl polyglycol ether and fatty acid derivatives with EO / PO moieties. Suitable examples can be found in § A.1.

[0086] The pre-treatment liquid may also contain pigments. Particularly useful for printing on dark or transparent substrates, is a pre-coat composition containing a white pigment. The preferred pigment for the aqueous precoat composition ink is titanium dioxide. Titanium dioxide (TIO2) pigment useful in the present invention may be in the rutile or anastase crystalline form. Processes for making TiO2 are described in greater detail in "The Pigment Handbook", Vol. I, 2nd Ed., John Wiley & Sons, NY (1988), the relevant disclosure of which is incorporated by reference herein for all purposes as if fully set forth.

[0087] The pre-treatment liquid may contain at least one pH adjuster. Suitable pH adjusters include organic amines such as triethanolamineor NEts, NaOH, KOH, NEts, NH3, HCI, HNO3 and H2SO4. In a preferred embodiment, the pre-coat composition has a pH equal to or lower than 9.A.3. Aqueous inkjet ink

[0088] The aqueous inkjet ink as part of the liquid set according to the invention, comprises a pigment polymer composite particle, the polymer being a vinyl-based polymer containing an acid group.A.3.1. Pigment polymer composite particle dispersion The pigment polymer composite particle dispersion is characterized by containing a pigment, a vinyl-based polymer and water, in which the vinylbased polymer comprises an acid group.

[0090] Aqueous inkjet inks containing the pigment polymer composite particles are excellent in storage stability even when stored under high-temperature conditions for a long period of time.<Pigment>

[0091] The pigment in the aqueous inkjet ink according to the invention may be black, white, cyan, magenta, yellow, red, orange, violet, blue, green, brown, mixtures thereof, and the like. A colour pigment may be chosen from those disclosed by HERBST, Willy, et al. Industrial Organic Pigments, Production, Properties, Applications. 3rd edition. Wiley - VCH, 2004. ISBN 3527305769.

[0092] Suitable pigments for the invention are disclosed in paragraphs

[0128] to

[0138] of WO 2008 / 074548.

[0093] The pigment used in the present invention is not particularly limited, and may be either an organic pigment or an inorganic pigment and may also be used in the form of a mixture of these pigments.

[0094] Examples of the organic pigment include at least one pigment selected from the group consisting of condensed polycyclic pigments such as anthraquinone-based pigments, quinacridone-based pigments, indigobased pigments, dioxazine-based pigments, perylene-based pigments, perinone-based pigments, isoindolinone-based pigments, isoindolinebased pigments, phthalocyanine-based pigments, quinophthalone-based pigments and diketopyrrolopyrrole-based pigments, and azo-based pigments such as diazo-based pigments and condensed azo-based pigments. Of these organic pigments, from the viewpoint of improvingcolor reproducibility thereof, preferred is at least one pigment selected from the group consisting of quinacridone-based pigments, azo-based pigments and phthalocyanine-based pigments.

[0095] Specific examples of the quinacridone-based pigments that are capable of exhibiting a magenta color include C.l. PR (Pigment Red) 122, PR 192, PR 202, PR 207 and PR 209; and C.l. PV (Pigment Violet) 19, etc. Of these quinacridone-based pigments, preferred is at least one pigment selected from the group consisting of C.l. PR 122 and C.l. PV 19. Specific examples of the azo-based pigments that are capable of exhibiting a yellow color include C.l. PY (Pigment Yellow) 74, PY 151 , PY 154, PY 155, PY 180 and PY 213. Of these azo-based pigments, preferred is at least one pigment selected from the group consisting of C.l. PY 74 and C.l. PY 154. Specific examples of the phthalocyanine-based pigments that are capable of exhibiting a cyan color or a green color include C.l. PB (Pigment Blue) 15:1 , PB 15:2, PB 15:3, PB 15:4, PB 15:5, PB 15:6 and PB 16; and C.l. PG (Pigment Green) 7 and PG 36. Of these phthalocyanine pigments, from the viewpoint of well exhibiting a cyan color, preferred is at least one pigment selected from the group consisting of C.l. PB 15:3 and C.l. PB 15:4.

[0096] The organic pigment may also include a derivative of the organic pigment as a raw material thereof. The organic pigment derivative may be produced by conducting such a treatment in which a functional group such as a hydroxy group, a carboxy group, a carbamoyl group, a sulfo group, a sulfonamide group and a phthalimidomethyl group is bonded to the surface of the organic pigment.

[0097] Examples of the inorganic pigment include carbon blacks and metal oxides such as alumina and titanium dioxide. These inorganic pigments may be treated with a conventionally known hydrophobization agent such as a titanium coupling agent, a silane coupling agent and a higher fatty acid metal salt, etc.

[0098] The carbon blacks are preferably used for black inks. Examples of the carbon blacks include furnace blacks, thermal lamp blacks, acetyleneblacks and channel blacks, and the carbon blacks may also be surface- treated carbon blacks.

[0099] In the present invention, the pigment is preferably present in a pigment polymer composite particle dispersion in water or in the form of pigmentcontaining crosslinked polymer particles. The "pigment polymer composite particle " as used herein may mean the configuration in which the pigment is enclosed or encapsulated by the vinyl-based polymer containing an acid group, preferably crosslinked, the configuration in which the pigment is partially enclosed or encapsulated by the vinyl-based polymer and partially exposed outside from the crosslinked polymer, or the configuration in which the crosslinked polymer is adhered onto a surface of the pigment, as well as a mixture of these configurations. Among these configurations of the pigment-containing polymer particles, preferred is the configuration in which the pigment is enclosed or encapsulated in the crosslinked polymer.<Vinyl-based Polymer containing an acid group>

[0100] The vinyl-based polymer containing an acid group used in the present invention has not only a function as a pigment dispersant that is capable of exhibiting the effect of dispersing the pigment in the aqueous inkjet ink vehicle, but also a function as a fixing agent for fixing the pigment onto a substrate.

[0101] The acid group of the vinyl-based polymer can be any acid group linked to the polymer, but is preferably a carboxylic acid, a sulphonic acid or a phosphoric acid group.

[0102] In a preferred embodiment of the invention, the pigment polymer composite particle has an acid value of not more than 220 mg KOH / g polymer vinyl-based polymer containing an acid group has an acid value of not more than 220 mg KOH / g, preferably 180 KOH / g, more preferably 150 mg KOH / g most preferably 135 mg KOH / g polymer. The acid value of the vinyl-based polymer having an acid group is preferably not less than 30 mg KOH / g, more preferably not less than 10 mg KOH / g.

[0103] With respect to the above mentioned acid values of the pigment polymer composite particle, it must be mentioned that the vinyl-based polymercontaining an acid group, contributes mainly to the acid values of the pigment composite particle. Hence, in an even more preferred embodiment of the invention, the acid value of the pigment polymer composite particle is at least equal to the acid value of the vinyl-based polymer containing an acid group.

[0104] The above mentioned acid values are preferably obtained by crosslinking the acid group, preferably the carboxylic acid group with a crosslinking agent.

[0105] The vinyl-based polymer containing an acid group can be either a water- soluble polymer or a water-insoluble polymer.

[0106] Examples of suitable vinyl-based polymers containing an acid group include water-soluble acrylic polymers having a carboxy group, obtained by addition-polymerization of a vinyl monomers such as acid group- containing vinyl monomers (a) and hydrophobic vinyl monomers (b).

[0107] Among these monomers, as the component (a), preferred are acrylic acid and methacrylic acid, and as the component (b), preferred are styrene and benzyl (meth)acrylate, and more preferred is styrene.

[0108] The weight-average molecular weight of the water-soluble polymer is preferably not less than 1.000, more preferably not less than 2,000, even more preferably not less than 4.000 and further even more preferably not less than 6.000, and is also preferably not more than 100.000, more preferably not more than 50.000, even more preferably not more than 30.000 and further even more preferably not more than 20.000, from the viewpoint of improving dispersibility of the pigment and storage stability of the pigment water dispersion.

[0109] Examples of commercially available products of the water-soluble polymer include JONCRYL 52J, JONCRYL 57J, JONCRYL 60J, JONCRYL 61 J, JONCRYL 63J, JONCRYL 70J, JONCRYL 501 J, JONCRYL 683, JONCRYL JDX-6180, JONCRYL HPD-96J, JONCRYL HPD-196, JONCRYL PDX6102, JONCRYL PDX6124, JONCRYL PDX6137A, JONCRYL 6610, JONCRYL 6500, JONCRYL 8708, all available from BASF.Carboxy Group-Containing Vinyl Monomer

[0110] In a preferred embodiment of the invention, a carboxy group-containing vinyl monomer (a) in the vinyl-based polymer containing an acid group is used as a monomer component of the polymer. As the carboxy group- containing vinyl monomer, there may be used carboxylic acid monomers.

[0111] Examples of the carboxylic acid monomers include acrylic acid, methacrylic acid, cratonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, 2-methacryloyloxymethylsuccinic acid and the like. Among these carboxylic acid monomers, preferred is at least one monomer selected from the group consisting of acrylic acid and methacrylic acid, (b) Hydrophobic Vinyl Monomer

[0112] Further, in the vinyl-based polymer containing an acid group, a hydrophobic vinyl monomer (b) is used as a monomer component of the polymer. Examples of the hydrophobic vinyl monomer include an alkyl (meth)acrylate, an aromatic group-containing monomer and the like which contain an alkyl group having not less than 1 and not more than 22 carbon atoms or an aryl group having not less than 6 and not more than 22 carbon atoms.

[0113] As the alkyl (meth)acrylic acid ester, preferred are those alkyl (meth)acrylic acid esters containing an alkyl group having 1 to 22 carbon atoms and preferably 6 to 18 carbon atoms. Examples of the alkyl (meth)acrylic acid esters include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, amyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, stearyl (meth)acrylate, isopropyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, isoamyl (meth)acrylate, isooctyl (meth)acrylate, isodecyl (meth)acrylate, isododecyl (meth)acrylate and isostearyl (meth)acrylate, etc.

[0114] The aromatic group-containing monomer is preferably a vinyl monomer containing an aromatic group having 6 to 22 carbon atoms which may contain a substituent group containing a hetero atom, and more preferably a styrene-based monomer or an aromatic group-containing (meth)acrylate.

[0115] Specific examples of the preferred styrene-based monomer include styrene, a-methyl styrene, vinyl toluene and divinyl benzene. Among thesestyrene-based monomers, more preferred are styrene and a-methyl styrene.

[0116] Specific examples of the preferred aromatic group-containing (meth)acrylate include phenyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate and the like. Among these aromatic group- containing (meth)acrylates, more preferred is benzyl (meth)acrylate. Contents of the monomers

[0117] The contents of the aforementioned components (a) and (b) are preferably as follows.

[0118] The content of the component (a) is preferably not less than 8 wt.%, more preferably not less than 10 wt.% and even more preferably not less than 12 wt.%, and is also preferably not more than 60 wt.%, more preferably not more than 50 wt.% and even more preferably not more than 45 wt.%.

[0119] The content of the component (b) is preferably not less than 15 wt.%, more preferably not less than 20 wt.% and even more preferably not less than 25 wt.%, and is also preferably not more than 80 wt.%, more preferably not more than 70 wt.% and even more preferably not more than 60 wt.%.

[0120] The vinyl-based polymer may be produced by copolymerizing the monomers (a) and (b) by known polymerization methods such as a bulk polymerization method, a solution polymerization method, a suspension polymerization method and an emulsion polymerization method. Among these polymerization methods, preferred is the solution polymerization method.

[0121] In addition, the type of a polymerization chain of the monomers to be polymerized is not particularly limited, and may be any polymerization type selected from a random type, a block type and a graft type, etc.

[0122] The number-average molecular weight of the vinyl-based polymer containing an acid group is preferably not less than 2000 and more preferably not less than 5000, and is also preferably not more than 20000 and more preferably not more than 18000, from the viewpoint of improving an adsorption force of the polymer to the pigment and well exhibiting dispersion stability of the pigment. In addition, the weight-averagemolecular weight of the water-insoluble polymer used in the present invention is preferably not less than 6000 and more preferably not less than 8000, and is also preferably not more than 80000 and more preferably not more than 40000.

[0123] Examples of suitable commercially available vinyl based polymers having an acid group are: Joncryl HPD 296E, Joncryl HPD 196, Joncryl 8078, Joncryl LMV 7025, Joncryl 680, Joncryl LMV 7085, Edaplan 480, Edaplan 482, Edaplan 516, Metolat 514, Tego Dispers 757W, Tego Dispers 780W, Zetasperse 3100, Zetasperse 3700, Carbosperse K 7028 <Dispersion of Pigment>

[0124] In a preferred embodiment of the invention, after dispersing the pigment with the vinyl-based polymer containing an acid group, the polymer may be subjected to crosslinking reaction such as to achieve the above mentioned acid value range of the pigment polymer composite particle.

[0125] The pigment and the vinyl-based polymer containing an acid group after being crosslinked are preferably used in the form of pigment polymer composite particles.Production of pigment polymer composite particles>

[0126] Upon production of the pigment polymer composite particles, the vinylbased polymer containing an acid group is preferably used as the polymer before being crosslinked. For example, the pigment polymer composite particles can be efficiently produced in the form of an aqueous pigment dispersion liquid by the process including the following steps 1 to 2.

[0127] Step 1 : providing a mixture containing the vinyl-based polymer containing an acid group, the pigment and water, if required together with a neutralizing agent;

[0128] Step 2: providing the water-based pigment dispersion liquid obtained in the step 1 to a crosslinking treatment with a crosslinking agent to obtain an aqueous pigment dispersion liquid containing the pigment-polymer composite particle containing crosslinked polymer. Suitable crosslinkers include a compound having a functional group selected from the group consisting of epoxides, carbodiimides, isocyanates, N-methylols, silanes, oxazolines and mixtures thereof.

[0129] The polymeric dispersant can be neutralized prior to the start of the dispersion process. It is preferred that the acid groups contained in the vinyl-based polymer are at least partially neutralized using a neutralizing agent, preferably an alkali metal or ammonium compound. The amount of neutralization of the polymeric dispersant in the final mixture is preferably 100% neutralized or less, preferably 90% neutralized or less.

[0130] Particles in inkjet inks such as the pigment polymer composite particles should be sufficiently small to permit free flow of the ink through the inkjetprinting device, especially at the ejecting nozzles. It is also desirable to use small particles for maximum colour strength and to slow down sedimentation.

[0131] The average pigment polymer composite particle size is preferably between 0.050 and 1 pm, more preferably between 0.070 and 0.300 pm and particularly preferably between 0.080 and 0.200 pm. Most preferably, the numeric average pigment particle size is no larger than 0.150 pm. The average particle size of pigment particles is determined with a Brookhaven Instruments Particle Sizer BI90plus based upon the principle of dynamic light scattering.

[0132] The pigment polymer composite particles are preferably present in the range of 0.01 to 15 wt.%, more preferably in the range of 0.05 to 10 wt.% and most preferably in the range of 0.1 to 5 wt.%, each based on the total weight of the inkjet ink. For white inkjet inks, the white pigment polymer composite particle is preferably present in an amount of 3 wt.% to 40 wt.% by weight of the inkjet ink, and more preferably 5 wt.% to 35 wt.%. OK? A pigment polymer composite particle amount of less than 3 wt. % cannot achieve sufficient covering power in the printed solid images.A.3.2. Vehicle

[0133] The aqueous ink according to the invention comprises water as a vehicle. The aqueous vehicle may further include one or more water-soluble organic solvents.

[0134] The one or more organic solvents may be added for a variety of reasons. For example, it can be advantageous to add a small amount of an organicsolvent to improve the dissolution of a compound in the ink composition to be prepared or to prevent fast drying of the ink at the nozzle of the inkjet head. Preferable water-soluble organic solvents are listed in § A.1.

[0135] Preferably, if two or more organic solvents are present in the ink, one of the organic solvents being in the highest amount, has a boiling point of 150°C or more and 250°C or less. If the boiling point is below this range, the solvent is very volatile and may give an unwanted odour to the ink. When the boiling point of the solvent is above the mentioned range, the solvents have the tendency of not being eliminated in the drying step and remain in the printed image leading to a reduced resistance to rubbing and scratching of the printed image. This water-soluble organic solvent is preferably added to the ink composition formulation in an amount of 0.1 to 55 wt.% based on the total weight of the ink.A.3.3. Resin

[0136] The inkjet ink composition according to the invention may comprise a resin. The resin is often added to the inkjet ink formulation to achieve a good adhesion of the pigment to the recording medium. The resin is preferably a polymer and suitable resins can be an acrylic based resin, a urethane resin or a wax.

[0137] The polyurethane resin is to be incorporated in the ink formulation as a dispersion and may be selected from the group consisting of aliphatic polyurethane dispersions, aromatic polyurethane dispersions, anionic polyurethane dispersions, non-ionic polyurethane dispersions, aliphatic polyester polyurethane dispersions, aliphatic polycarbonate polyurethane dispersions, aliphatic acrylic modified polyurethane dispersions, aromatic polyester polyurethane dispersions, aromatic polycarbonate polyurethane dispersions, aromatic acrylic modified polyurethane dispersions, for example, or a combination of two or more of the above.

[0138] A preferred urethane resin to be used as dispersion in the ink of the invention is a polyester resin including a structural unit containing a urethane bond. Among such resins, a water-soluble or water-dispersible urethane-modified polyester resin is preferred. It is preferable that theurethane-modified polyester resin include at least one structural unit derived from a hydroxyl group-containing polyester resin (polyester polyol) and at least one structural unit derived from an organic polyisocyanate.

[0139] Furthermore, the hydroxyl group-containing polyester resin is a resin formed by an esterification reaction or transesterification reaction between at least one polybasic acid component and at least one polyhydric alcohol component.

[0140] A preferred polyurethane resin to be included in the ink of the invention is a polyurethane resin obtainable by reacting a polyester polyol, a polyether diol, a polyol containing an anionic group and a polyisocyanate. A particular preferred polyurethane resin is a polyurethane resin obtainable by reacting a polyester polyol, a polyether diol, a polyol containing an anionic group and a polyisocyanate, and wherein the polyester polyol is obtained by reacting an aromatic polycarboxylic acid and a polyol. Examples of suitable polyurethane resins and their preparations are disclosed in the unpublished patent application EP16196224.6.

[0141] Some examples of suitable polyurethane dispersions are NEOREZ R-989, NEOREZ R-2005, and NEOREZ R-4000 (DSM NeoResins); BAYHYDROL UH 2606, BAYHYDROL UH XP 2719, BAYHYDROL UH XP 2648, and BAYHYDROL UA XP 2631 (Covestro); DAOTAN VTW 1262 / 35WA, DAOTAN VTW 1265 / 36WA, DAOTAN VTW 1267 / 36WA, DAOTAN VTW 6421 / 42WA, DAOTAN VTW 6462 / 36WA (Allnex); and SANCURE 2715, SANCURE 20041 , SANCURE 2725 (Lubrizol Corporation), for example, or a combination of two or more of the above.

[0142] Acrylic based resins include polymers of acrylic monomers, polymers of methacrylic monomers, and copolymers of the aforementioned monomers with other monomers. These resins are present as a suspension of particles having an average diameter of about 30 nm to about 300 nm. The acrylic latex polymer is formed from acrylic monomers or methacrylic monomer residues. Examples of monomers of the acrylic latex polymer include, by way of illustration, acrylic monomers, such as, for example, acrylate esters, acrylamides, and acrylic acids, and methacrylic monomers, such as, for example, methacrylate esters, methacrylamides,and methacrylic acids. The acrylic latex polymer may be a homopolymer or copolymer of an acrylic monomer and another monomer such as, for example, a vinyl aromatic monomer including, but not limited to, styrene, styrene butadiene, p-chloromethylstyrene, divinyl benzene, vinyl naphthalene and divinylnaphthalene.

[0143] Some examples of suitable acrylic latex polymer suspensions are, JONCRYL 537 and JONCRYL 538 (BASF ); CARBOSET GA-2111 , CARBOSET CR-728, CARBOSET CR-785, CARBOSET CR-761 , CARBOSET CR-763, CARBOSET CR-765, CARBOSET CR-715, and CARBOSET GA-4028 (Lubrizol Corporation); NEOCRYL A-1110, NEOCRYL A-1131 , NEOCRYL A-2091 , NEOCRYL A-1127, NEOCRYL XK-96, and NEOCRYL XK-14 (DSM); and BAYHYDROL AH XP 2754, BAYHYDROL AH XP 2741 , BAYHYDROL A 2427, and BAYHYDROL A2651 (Bayer), for example, or a combination of two or more of the above.

[0144] The concentration of the resin in the inkjet ink according to the invention is at least 1 wt.% and preferably lower than 30 wt.%, more preferably lower than 20 wt.%.

[0145] The aqueous inkjet ink of the invention may also comprise a wax. The wax in the ink improves wet rub or wet scratch resistance of the printed layer. Suitable examples of waxes are listed in § A.1.

[0146] The inkjet ink composition according to the invention may comprise a capsule. Capsules, more preferably, nanocapsules are often incorporated in inkjet ink formulations to encapsulate colouring agents (US2009227711A, JP2004075759) or to encapsulate reactive ingredients which can cross-link. Particularly useful are the nanocapsules disclosed in WO201 5158649 [0037-0110]: The nanocapsules have a polymeric shell surrounding a core containing reactive chemistry. The shell material includes polyureas, polyurethanes, polyesters, polycarbonates, polyamides, melamine based polymers and mixtures thereof, with polyureas and polyurethanes being especially preferred. Other particularly useful nanocapsules are disclosed in WO2016165970 [0051-0138]: the nanocapsules are selfdispersable and include a dispersing group covalently coupled to the shell polymers. The core of the nanocapsules inWO201 5158649 [0037-0110] and WO2016165970 [0051-0138] comprise reactive chemistry which is able to form a reaction product upon application of heat and / or light, allowing a wide variety of substrates to be addressed. Other suitable reactive chemistry is the one which is activated upon radiation as described in WO2015158649 [0068-0110],

[0147] The resins are preferably present in the inkjet ink in an amount of no more than 30 wt.%, preferably between 5 and 25 wt.% based on the total weight of the ink.A.3.4. Additives

[0148] The ink composition may contain a surfactant. Any known surfactant may be used but preferably a glycol surfactant and / or an acetylene alcohol surfactant and / or a polysiloxane surfactant is to be used. Suitable surfactants can be found in § A.2.

[0149] A biocide may be added to the ink composition to prevent unwanted microbial growth, which may occur over time. Suitable biocides are listed in § A.2.

[0150] A biocide is preferably added to the aqueous medium in an amount of 0.001 to 3 wt.%, more preferably 0.01 to 1.0 wt. %, each based on the total weight of the ink.

[0151] The one or more optothermal converting agents are preferably present in the range of 0.1 to 10 wt.% based on the total weight of the ink.B. Recording methodB.1. Application method of the pre-treatment liquid

[0152] The pre-treatment liquid as part of the liquid set according to the present invention is suitable for treating different substrates, absorbing and nonabsorbing ones. A substrate is characterized as absorbing, when the amount of absorbed water after a contact time of 60 s is at least 10 g / m2. Absorbing substrates include paper, card board, white lined chipboard, corrugated board, packaging board, folding board, wood, ceramics, stone, leather and textile. Non-absorbing substrates include metal, glass,polypropylene, polyvinylchloride, PET, PMMA, polycarbonate, polyamide, polystyrene or co-polymers thereof.

[0153] The pre-treatment liquid is particularly suited for being coated or jetted onto papers intended for packaging applications, more preferably packaging applications including absorbing substrate such as card board, paper lines, corrugated board, packaging board, folding board and paper. The paper can be a single layer of a multilayer paper.

[0154] The paper may be brown Kraft, White Top or bleached board. The paper may be manufactured from chemical, wood, or recycled fibre. As an example, the paper may be a liner intended for printing on page wide web presses and converted into corrugated boxes. In this aspect, the liner paper may be used as a double face liner and may be converted directly in a corrugator or laminated onto a double face liner after corrugation. The paper may also be boards used for boxes and other packaging applications.

[0155] All well-known conventional methods can be used for coating or impregnating the pre-treatment liquid on a substrate. Examples of the method include air knife coating, blade coating, roll coating, gravure coating and spraying. More preferably the pre-treatment liquid is applied by means of a jetting technique.

[0156] The pre-treatment liquid is then applied using an ink jet head or valve jet head. This means of applying the pre-treatment composition, which is preferably according to an image, has the advantage that the amount of required pre-treatment liquid is substantially lower than with the other application methods. By means of a jetting head, it is possible to apply the pre-treatment liquid onto areas of the substrate where the image should be printed. Suitable inkjet head types for applying the pre-treatment liquid are piezoelectric type, continuous type, thermal print head type, Memjet-type, valve jet type or through-flow heads as described in §B.2.

[0157] After applying the pre-treatment liquid onto a substrate, the applied drops or coating is preferably at least partially dried such as to form a pre-coat layer.

[0158] Substrates to which the pre-treatment composition has been applied may be dried and optionally undergo a heat treatment, before the subsequent ink jetting step with the pigment polymer composite particles containing ink onto the formed pre-coat layer. Examples of the heating process include, but are not limited to, heat press, atmospheric steaming, high-pressure steaming, and THERMOFIX. Any heat source can be used for the heating process; for example, an infrared ray lamp is employed.

[0159] In another embodiment of the invention, the pre-treatment composition, is not substantially dried before the image is printed by means of the jetting of the aqueous ink jetting step.B.2. Ink jetting & drying

[0160] After the application of the pre-treatment liquid to the substrate, the aqueous inkjet ink as part of the liquid set according to the invention and containing the pigment polymer composite particles, the polymer being a vinyl-based polymer containing an acid group, is applied to the substrate, preferably onto the parts where the pre-treatment liquid has been applied on or where a pre-coat layer has been formed.

[0161] A preferred ink jet head for the jetting of the pre-treatment composition printing system is a piezoelectric inkjet head. Piezoelectric inkjet jetting is based on the movement of a piezoelectric ceramic transducer when a voltage is applied thereto. The application of a voltage changes the shape of the piezoelectric ceramic transducer in the print head creating a void, which is then filled with aqueous inkjet ink. When the voltage is again removed, the ceramic expands to its original shape, ejecting a drop of pretreatment composition from the inkjet head. However, the jetting of the ink according to the present invention is not restricted to piezoelectric inkjet printing. Other inkjet print heads can be used and include various types, such as a continuous type, a thermal print head type, a Memjet-type of head and a valve jet type.

[0162] In a preferred embodiment, the printing system may be configured to recirculate the aqueous ink containing the pigment polymer composite particles, the polymer being a vinyl-based polymer containing an acidgroup, prior to printing. A particularly useful inkjet head to print the aqueous inkjet inks of the liquid set of the invention, is from the type that includes a recirculation of the ink within the head such as through-flow heads as disclosed in WO 2006 / 030235 A2 and WO 2006 / 064036 A1. This type of inkjet head is very suitable to be incorporated in a printing system comprising a through-flow print head having one or more nozzle for ejecting drops of aqueous ink onto a pre-treatment liquid layer, and an ink circulation system for feeding and circulating the ink through the print head, comprising an ink tank for containing the ink, a supply buffer tank for receiving the ink from the main tank and supplying the ink to the through- flow print head, a return manifold for receiving the ink from the through- flow print head and returning ink to the main ink tank via a pump.

[0163] After having applied the aqueous inkjet ink containing the pigment polymer composite particles, the polymer being a vinyl-based polymer containing an acid group, onto at least a part of the pre-treatment liquid or pre-coat layer such that an image is formed, the image is dried.

[0164] The drying step of the image can be performed by applying an air flow and or apply heating. The heating step must be performed by using heat sources; examples include equipment for forced-air heating, radiation heating such as IR-radiation, including NIR-, CIR- and SWIR radiation, conduction heating, high-frequency drying, and microwave drying. Examples of the heating process include, but are not limited to, heat press, atmospheric steaming, high-pressure steaming, and THERMOFIX. Any heat source can be used for the heating process; for example, an infrared ray lamp is employed.B.3. Application of the varnish

[0165] After having at least partially dried the image obtained by printing the aqueous inkjet ink, the varnish which makes part of the liquid set according to the invention, is applied onto at least a part of the image.

[0166] In one of the embodiments of the invention, the varnish is applied on nonimage areas, more specifically on non-image areas which contain a pretreatment liquid or a pre-coat layer.

[0167] All well-known conventional methods can be used for coating or printing the varnish onto at least a part of the image. Examples of the method include air knife coating, blade coating, roll coating, gravure coating and spraying. The advantage of the coating or printing technique is that thick varnish layers in one pass of the recording medium can be obtained to assure sufficient rub resistance of the image.

[0168] The varnish is preferably applied via a jetting technique which allows to apply the varnish selectively onto at least part of the image. This means of applying the varnish composition, which is preferably according to an image, has the advantage that the amount of required varnish material is substantially lower than with the other application methods.

[0169] The jetting heads suitable for jetting the varnish may be the same as described in § B.1.

[0170] Finally, the applied varnish is dried according to one of the ways described above for drying the pre-treatment liquid or the aqueous inkjet ink.C. ExamplesC.1. Materials

[0171] All materials used in the following examples were readily available from standard sources such as Aldrich Chemical Co. (Belgium) and Acros (Belgium) unless otherwise specified. Where used, water is demineralised water (D.L water).• PB15:3 is Hostaperm™ B4G-KR, a C.L Pigment Blue 15:3 pigment from CLARIANT.• PY74 is Hansa Brilliant yellow 5GX 03, a C.L Pigment Yellow 74 pigment from CLARIANT.• PR122 is Inkjet Magenta E02, a C.L Pigment Red 122 pigment from CLARIANT.• Edaplan is an abbreviation used for Edaplan™ 482, a vinyl-based polymer having an acid group with a solid content of 85.5 %, from MUNZING CHEMIE GmbH.• Joncryl 8078 is a 32 wt.%, ready-made ammonia solution of a styrene-acrylic resin manufactured by BASF, having an acid value of 268 mg KOH / g polymer• Proxel is a 5 wt.% aqueous solution of 1 ,2-benzisothiazolin-3-one available as Proxel™ K from YDS CHEMICALS NV.• Sokalan CP12S is a poly(acrylic acid-co-maleic acid) having a Mw of 3000 g / mole, a calculated acid value of 826 mg KOH / g polymer from BASF• Hydropalat WE3120 is a C12-C14 ethoxylated, propoxylated alcohol from BASF• Ultralube GA 1042 PE is a PE wax from Keim additec surface GmbH, obtained via Grolman Benelux BV• Surfynol 104PG50 is a 50 wt.% solution of 2,4,7,9-Tetramethyl-5- decyne-4,7-diol in propylene glycol from Evonik• Byk 348 is a polyether modified siloxane from BYK• Synperonic PE P105 is a block copolymer of ethylene oxide and propylene oxide, with 50% EC and a molecular weight of 6500 from Croda GmbH• Synperonic PE F108 is a block copolymer of ethylene oxide and propylene oxide from Croda GmbH• Kauropal K933 is a non-ionic oxirane, mono(2-propylheptyl) ether from BASF• Tego Foamex 822 is a polyether siloxan copolymer from Evonik• Mg(NOs)2.6H2O is Magnesium nitrate hexahydrate from Merck Group• Byk 333 is a polyether siloxane surfactant from Byk• nBuOH is n-butanol• Hydropalat WE 3220 is a polyether-modified polysiloxanes from BASF• Ensocoat is a coated cardboard liner from Stora Enzo, having an absorpability of 4.4 ml / m2according to the Bristow methodNeocryl D2101 is a 42 wt.% dispersion of a styrene-acrylic resin from Covestro• D. I. water is deionized waterC.2. Evaluation methodsC.2.1. Sample preparation

[0172] The samples for the gloss measurements of images, were made by coating a pre-treatment liquid onto a coated card board liner Ensocoat (220 g / m2) from Stora Enzo, using a 4 pm spiral bar. The obtained pretreated liner was dried at 60°C in an oven for 2 minutes.

[0173] The aqueous inkjet inks were coated onto the pre-treated liner by using a 10 pm spiral bar. The coated aqueous inkjet inks were then dried in an oven during 5 minutes at 60°C.

[0174] After drying the coated aqueous inkjet inks, varnish compositions were coated onto the dried inkjet ink coatings using a spiral bar of 10 pm such as to obtain solids coverages as mentioned in Table 5. The varnish is dried in an oven Thermo Scientific™ Heratherm to obtain samples for gloss measurements. Drying was done for 5 minutes at 50°C.C.2.2. Gloss measurements

[0175] The measurement of the gloss of the dried varnish of the samples obtained in § C.2.1. is done by means of a GLOSS Zehntner at a viewing angle of 60°.C.2.3. Storage stability of the aqueous inkjet inks

[0176] An inkjet ink is considered stable upon storage, if the average particle size of the pigment particles or polymer pigment composite particles did not increase by more than 100 nm after a storage of 14 days at 60°C. If the average particle size of the pigment or polymer pigment composite particles increased more than 100 nm, jetting reliability was severely hampered because of nozzle clogging of inkjet heads.C.3. Preparation of the pre-treatment liquid PL-1

[0177] The pre-treatment liquid PL-1 was prepared by mixing the ingredients given in Table 1. The weight percentages are relative to the total weight ofthe pre-treatment liquid.Table 1 : Composition of pre-treatment liquid PL-1C.4. Preparation of the inkjet inksC.4.1 . Preparation of pigment polymer composite particle dispersions

[0178] All quantities are given in wt.% with respect to the total weight of the dispersion. The aqueous pigment polymer composite particle dispersions were prepared as follows: 61 .2 wt.% of water, 23.4 wt.% of Joncryl 8078, 15 wt.% of a pigment and 0.4 wt.% of Proxel K were premixed in a vessel using a mechanical stirrer to obtain a pre-dispersion. A DynoMill-RL mill was filled for 42% with 0.4 mm yttrium stabilized zirconia beads ("high wear resistant zirconia grinding media" from TOSOH Co.). The predispersion was pumped into the bead mill and the milling was started in recirculation mode at a rotation speed of 11 .8 m / s. After 30 min residence time, the mill was emptied by pumping the obtained dispersion in a vessel. Three pigment polymer composite particle dispersions were prepared using the pigments as listed in Table 2.Table 2: Type of pigment used in the pigment polymer composite particle dispersions.C.4.2. Crosslinking of the resin of the pigment polymer composite particle dispersions

[0179] Part of each of the three dispersions DISP-Y1 , DISP-M1 and DISP-C1 were crosslinked and the number of acid groups in the vinyl-based polymer containing an acid group, being 268 mg KOH / g polymer, was reduced by using a suitable epoxide-containing crosslinking agent. Each dispersion was diluted to a solids content of 10 wt.% with water, followed by addition of Denacol-EX321 (0.15 wt.% based on the total mass of the reaction mixture). The crosslinking reaction was performed by stirring the mixture for 5 hours at a temperature of 65 °C.

[0180] The resulting crosslinked dispersions were named XDISP-Y1 (prepared from DISP-Y1), XDISP-M1 (prepared from DISP-M1) and XDISP-C1 (prepared from DISP-C1). The acid value of the pigment polymer composite particle is found to be 204 mg KOH / g polymer.

[0181] The pigment load in each of the XDISP dispersions before ultrafiltration is 6.7 wt.%.C.4.3. Ultrafiltration of the pigment polymer composite particle dispersions

[0182] Part of each of the three dispersions XDISP-Y1 , XDISP-M1 and XDISP-C1 were purified by means of ultrafiltration using a membrane having a cut-off of 10kDa (Repligen MidiKros D04-E010-05-N, mPES membrane). The crosslinked dispersions were purified and concentrated by using an ultrafiltration membrane with 10 wash volumes per volume of crosslinked dispersion. The solid content of the crosslinked dispersion was brought to 15 wt.% with respect to the total weight of the dispersion. The resultingultrafiltered dispersions were named XUDISP-Y1 (prepared from XDISP- Y1), XUDISP-M1 (prepared from XDISP-M1) and XU DISP-C1 (prepared from XDISP-C1).C.4.4. Preparation of inks for storage stability experiments

[0183] Aqueous inks for storage stability experiments were prepared by diluting the pigment polymer composite particle dispersions from § C.4.1 . with ingredients according to Table 3. The amounts of the dispersion and ingredients are expressed in wt.% based on the total weight of the ink. Water was added to complete the ink to the desired pigment concentration.Table 3: Composition of inkjet inks for storage stability experimentsC.4.4. Preparation of inks for coating experiments

[0184] Aqueous inks were prepared by diluting the pigment dispersions from § C.4.2. and C.4.3. with other ingredients according to Table 4. The amounts of the dispersion and ingredients are expressed in wt.% based on the total weight of the ink. D.L water was added to complete the ink to the desired pigment concentration.Table 4: Composition of inkjet inks for gloss measurementsC.5. Preparation of the varnishes for gloss measurements

[0185] Comparative and inventive varnishes were prepared by mixing the ingredients given in Table 5. The weight percentages are relative to the total weight of the varnish composition.Table 5: Composition of inventive and comparative varnishesC.6. Example 1

[0186] In Example 1 it is shown that the inventive liquid sets comprising a varnish with the copolymers as claimed (=lnventive varnishes INV-V1 to INV-V4) do show an improved gloss value of the image formed by coating and drying a pre-treatment liquid and an aqueous inkjet ink from the liquid set according to the invention.

[0187] Comparative and inventive liquid sets for inkjet printing were assembled by combining the above prepared pre-treatment liquid PL-1 , with the above prepared inkjet inks INK-INV-4 to INK-INV-9 together with comparative and inventive varnishes.

[0188] Samples were prepared by coating the comparative and inventive liquid sets on a recording medium as described in § C.2.1 . Gloss was measured as described in § C.2.2. The measured values are listed in Table 6.

[0189] As can be seen from Table 6, the inventive liquid sets comprising a pretreatment liquid, an aqueous inkjet ink comprising a pigment polymer composite particle dispersion, the polymer being an vinyl-based polymerhaving an acid group and a varnish comprising water, resin particles, a polyether siloxane surfactant and a water soluble copolymer, do show significant higher gloss values than comparative liquid sets.Table 6: Gloss values of coated Liquid setsC.7. Example 2

[0190] Example 2 shows that the aqueous inkjet inks comprising a pigment polymer composite particle dispersion having an acid value of not more than 220 mg KOH / g polymer show an improved storage stability.

[0191] The storage stability of the inks COMP-INK1 to COMP-INK3 and INV-INK1 to INV-INK was determined as described in § 2.3. The results are listed in Table 7.Table 7: Average particle size increase of pigment resin composite particle dispersion after storage

[0192] From Table 7 it can be concluded that the aqueous inkjet inks containing pigment polymer composite particles having an acid value of not more than 220 mg KOH / g polymer do show an improved storage stability over the inks wherein the pigment polymer composite particles having an acid value of more than 220 mg KOH / g polymer.

Claims

ClaimsClaim 1. Liquid set for inkjet printing comprising an aqueous pre-treatment liquid, an aqueous inkjet ink and a varnish, the aqueous pre-treatment liquid comprises a fixing agent, the aqueous inkjet ink comprises a pigment polymer composite particle and a water soluble organic solvent, the varnish comprises water and resin particles selected from the group consisting of a urethane- based resin, an acrylic resin, a fluorene-based resin, a polyolefin-based resin, a rosin modified resin, a terpene-based resin, a poly-ester-based resin, a polyamide-based resin, an epoxy-based resin, a vinyl chloride-based resin and a wax, characterized in that the polymer is a vinyl-based polymer containing an acid group, and the varnish comprises a water-soluble polymer selected from the group consisting of linear PPO-PEO copolymers and arylethylphenyl polyglycol ethers.Claim 2. The liquid set according to claim 1 wherein the pigment polymer composite particle has an acid value of not more than 220 mg KOH / g polymer.Claim 3. The liquid set according to any of the preceding claims wherein the amount of pigment polymer composite particles is not less than 0.01 wt.% and not more than 15 wt.% based on the total amount of the total weight of the aqueous inkjet ink.Claim 4. The liquid set according to any of the preceding claims wherein the vinyl-based polymer is crosslinked.Claim 5. The liquid set according to claim 4 wherein the vinyl-based polymer is crosslinked by a compound having a functional group selected from the group consisting of epoxides, carbodiimides, isocyanates, N-methylols, silanes, oxazolines and mixtures thereof.Claim 6. The liquid set according to any of the preceding claims wherein the water-soluble polymer is non-ionic.Claim 7. The liquid set according to any of the preceding claims wherein the varnish comprises a polyether siloxane surfactant.Claim 8. The liquid set according to any of the preceding claims wherein the fixing agent is a multivalent salt, a cationic polymer or an organic acid.Claim 9. The liquid set according to any of the preceding claims wherein the pre-treatment liquid comprises a carboxylic acid containing polymer or saltthereof or a carboxylic acid anhydride containing polymer, the polymer having an acid value equal to 950 mg KOH / g of polymer or less.Claim 10. The liquid set according to any of the preceding claims wherein the aqueous inkjet ink comprises a resin particle selected from the group consisting of a poly(urethane), a poly(acrylate) and a wax.Claim 11. A printing method comprising the steps of: a) applying a pre-treatment liquid as defined in Claim 1 to Claim 10 onto at least a part of a substrate; and b) optionally at least partially dry the applied pre-treatment liquid such as to form a pre-coat; and c) jetting onto at least a part of the applied pre-treatment liquid or onto the precoat an aqueous inkjet ink as defined in Claim 1 to Claim 10; and c) applying heat to dry the jetted aqueous inkjet ink such as to form an image; and d) applying a varnish as defined in Claim 1 to Claim 10 onto at least a part of the image; and e) applying heat to dry the varnish.Claim 12. The printing method according to Claim 11 wherein the amount of solids of the applied varnish is equal to 0.6 g / m2or more.Claim 13. The printing method according to Claim 11 or Claim 12 wherein the pre-treatment liquid is applied via a jetting technique.Claim 14. The printing method according to Claim 11 to Claim 13 wherein the varnish is applied via a jetting technique.Claim 15. The printing method according to Claim 11 to Claim 14 wherein the substrate is an absorbing substrate.

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