Image recording method

Abstract

Image recording method, including the steps of i. Providing a paper substrate, ii. adhering an aqueous ink onto a first surface of the paper substrate by means of an inkjet print head, the ink comprising a pigment, a water-soluble alkane diol in an amount not less than 10 wt.% and not more than 37 wt.% with respect to the total weight of the ink, a water-soluble alkyl glycol ether in an amount not less than 5 wt.% and not more than 20 wt.% with respect to the total weight of the ink and a non-ionic surfactant, iii. Contacting the first surface of the printed substrate with a roller. A method of producing a printed corrugated cardboard is also provided.

Description

DescriptionImage recording methodTechnical Field

[0001] The present invention relates to an image recording method using high printing speeds and suitable for manufacturing printed corrugated cardboard. Suitable inkjet ink compositions to achieve this recording method are provided.Background Art

[0002] Extremely high inkjet printing speeds make the time laps between adhering aqueous inkjet inks and the contact of transport rollers very short. Typically, this time is not more than 1 second. As this time is very short, in most case too short for the ink to dry, contamination of transport rollers may occur. This contamination is also called setoff. A setoff phenomenon in which the images (partially) transfers to transport rollers or the like will cause roller stain and lead to image defects in the printed paper sheet.

[0003] Corrugated cardboard is a preferred packaging material as it is low cost and lightweight. Corrugated cardboard is formed by gluing one or more fluted sheets of paperboard (also called corrugating medium) to one or more flat sheets of linerboard (also called facings). Commonly, two types of corrugated card board are used as packaging material: single face cardboard with one fluted sheet glued to one facing (total two sheets) and single wall card board with one fluted sheet sandwiched between two facings (total three sheets).

[0004] The main part of the used corrugated board for packaging applications requires a decorative appearance or requires information and / or coding, which is typically achieved by printing an image, text or (bar or QR) codes. Most of the corrugated board for packaging applications is printed by analogue flexographic printing, either in single or in multiple colors. The manufacturing of the printing cylinder, which is required for flexographic printing, is very complex and costly and therefore can be economically justified only at high numbers of the printed parts of the same design. Assoon as any change in the printing design is required, new printing cylinders need to be manufactured. The cylinders may be stored to avoid making new ones, however this requires large storage room.

[0005] When the number of printed samples is low or variable data has to be printed such as bar codes or QR codes, only digital printing is an industrially relevant, alternative printing technique such as inkjet printing may be used for different substrates. In inkjet printing, the printhead(s) may be driven according to an electronical pattern across the substrate or may be fixed above the substrate (which is typically moved through the machine by a conveying system), thereby producing an image on the substrate when passing the printhead(s) once (single pass printing) or repeatedly (multi pass printing).

[0006] Inkjet printing is a suitable method for applying a print on corrugated board, especially when the so-called pre-print technique (the print is applied to a paper liner before the manufacturing of the corrugated board) and / or the so called post print technique (the print is applied onto the readymade corrugated board) is used.

[0007] A newer method is the integration of a digital printing unit (typically an inkjet printing unit) into a corrugated board manufacturing machine, such as a corrugator or a corrugated board production line (see e.g. DE 10 2013 202 871A1 or WO 2014 / 128115 A1), or an existing printing line such as an offset press. This method has advantages as compared to the analogue flexography printing in regard of the manufacturing costs and in production flexibility.

[0008] However, in-line digital printing in a corrugated board production line requires inks with exceptional requirements, like high frequency jet ability, good drying performance, high stability, good wetting performance on a wide range of substrates, high water resistance as well as high abrasion resistance, in particular under heat exposure.

[0009] When integrating a printing unit into a corrugator a corrugated board production line, it is not possible to incorporate a drying unit as the required space is not available. State of the art corrugators produce cardboard at conveying speeds of the paper board or liner in the order of150 m / min up to 450 m / min. This requires inkjet inks on the paper board or liner that should not contaminate the face roller as one of the transport rollers of the corrugator or corrugated board production line, downstream the paper web, even when the ink is still not dry.

[0010] EP4065652A discloses an aqueous ink comprising a film forming agent for inkjet printing on a corrugated board or on a liner for the production of corrugated board. Film forming agents may form insoluble films in ink tanks and ink tubes during stand still of the printing equipment.

[0011] EP2230281 A discloses ink compositions having a latex having a high glass transition temperature and an organic solvent having a SP value of 27.5 or lower. The inks have still to be dried before the printed images comes into contact with a face roller and setoff is avoided.

[0012] EP1520884A discloses a non-aqueous ink composition containing a charged particle containing at least a coloring material, a copolymer having a polyester structure and a polymer having a liquid crystalline structure.

[0013] EP1794245A discloses a recording ink with excellent discharge stability and storage stability, which rapidly penetrates into a recording medium and forms a coating due to the presence of resin particles of silicone modified acrylic resin. The printed inks still have to be dried before fixing properties onto a cotton swab can be evaluated.

[0014] US2014077486A discloses ink compositions comprising penetrants to shorten the drying time of the ink adhered to a recording medium. However, because penetrants also promote penetration of the colorants into the paper substrate, a polymer compound is added to avoid lower image density.

[0015] Most of the above cited prior art has in common that polymers have to be included in the aqueous inkjet ink. It is known that polymers have the tendency to form coatings and films which have a negative effect on the jetting reliability due to blocking of nozzles, film formation in the ink tanks and tubes.

[0016] There is still a need for a high-speed image recording method for paper substrates wherein ink drying can hardly occur, avoiding setoff onto facerollers in printing equipment or corrugator production line and providing excellent color depth of the printed image.Summary of invention

[0017] It is the objective of the present invention to provide a solution to the above stated problems. The objective has been achieved by providing an image recording method as defined in Claim 1.

[0018] It is another objective of the present invention to provide a method of producing a printed corrugated cardboard as defined in Claim 15.

[0019] 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. Image recording method

[0020] The image recording method according to the invention including the steps of i. Providing a paper substrate ii. Adhering an aqueous ink onto a first surface of the paper substrate by means of an inkjet print head, the ink comprising a pigment, a water- soluble alkane diol in an amount not less than 10 wt.% and not more than 37 wt.% with respect to the total weight of the ink, a water-soluble alkyl glycol ether in an amount not less than 5 wt.% and not more than 20 wt.% with respect to the total weight of the ink and a non-ionic surfactant. iii. Contacting the first surface of the printed substrate with a roller.

[0021] This printing method makes it feasible to incorporate a printing unit using the above aqueous ink into printing equipments and printing lines which must handle printed paper substrates at very high conveying speeds such as corrugators, corrugated board production lines, web-based label printers, web based digital presses, offset presses, etc.A.1 . Aqueous inkjet ink

[0022] The aqueous inkjet ink used in the recording method according to the invention comprises a pigment, a water-soluble alkane diol in an amount not less than 10 wt.% and not more than 37 wt.% with respect to the total weight of the ink, a water-soluble alkyl glycol ether in an amount not less than 5 wt.% and not more than 20 wt.% with respect to the total weight of the ink and a non-ionic surfactant.

[0023] Without being bound by a theory, it is thought that due to the specific combination of water, water-soluble alkane diol and alkyl glycol ether, the liquid vehicle of the ink (which comprises the water and the water-soluble organic solvents of the ink) penetrates very fast into the paper substrate leading to excellent setoff values, while at least a part of the pigment remains at the surface of the paper substrate leading to excellent color depths of the printed image.<Pigment>

[0024] 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 color 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.

[0025] Suitable pigments for the invention are disclosed in paragraphs

[0128] to

[0138] of WO 2008 / 074548.

[0026] 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.

[0027] 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-basedpigments. Of these organic pigments, from the viewpoint of improving color reproducibility thereof, preferred is at least one pigment selected from the group consisting of quinacridone-based pigments, azo-based pigments and phthalocyanine-based pigments.

[0028] 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, 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, C.l. PY154 and C.l. PY 155. 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.

[0029] 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.

[0030] 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.

[0031] The carbon blacks are preferably used for black inks. Examples of the carbon blacks include furnace blacks, gas black, thermal lamp blacks,acetylene blacks and channel blacks, and the carbon blacks may also be surface-treated carbon blacks.

[0032] The pigment particles are preferably dispersed in an aqueous medium using a polymeric dispersant, a surfactant or a combination thereof or encapsulated by a crosslinked polymeric shell. Self-dispersible pigments can also be used. The latter prevents interaction of the polymeric dispersant with the dispersing groups of binders or capsules which may be included in the inkjet ink (see below).

[0033] A self-dispersible pigment is a pigment having on its surface covalently bonded anionic hydrophilic groups or salt-forming groups, that allow the pigment to be dispersed in an aqueous medium without using a surfactant or a resin.

[0034] The technology for making self-dispersible pigments is well-known. For example, EP1220879A discloses pigments having attached a) at least one steric group and b) at least one organic ionic group and at least one amphiphilic counterion, wherein the amphiphilic counterion has a charge opposite to that of the organic ionic group that are suitable for inkjet inks. Also EP906371A discloses suitable surface-modified colored pigment having attached hydrophilic organic groups containing one or more ionic groups or ionizable groups. Suitable commercially available self- dispersible color pigments are, for example, the CAB-O-JET™ inkjet colorants from CABOT.

[0035] Pigment particles in inkjet inks should be sufficiently small to permit free flow of the ink through the inkjet-printing device, especially at the ejecting nozzles. It is also desirable to use small particles for maximum color strength and to slow down sedimentation.

[0036] Suitable white pigments are given by Table 2 in

[0116] of WO 2008 / 074548. The white pigment is preferably a pigment with a refractive index greater than 1.60. The white pigments may be employed singly or in combination. Preferably titanium dioxide is used as pigment with a refractive index greater than 1.60. Suitable titanium dioxide pigments are those disclosed in

[0117] and in

[0118] of WO 2008 / 074548.

[0037] The pigment particles may also be dispersed in an aqueous medium using a polymeric dispersant, a surfactant or a combination thereof.

[0038] Suitable polymeric dispersants are copolymers of two monomers but they may contain three, four, five or even more monomers. The properties of polymeric dispersants depend on both the nature of the monomers and their distribution in the polymer. Co-polymeric dispersants preferably have the following polymer compositions:• statistically polymerized monomers (e.g. monomers A and B polymerized into ABBAABAB);• alternating polymerized monomers (e.g. monomers A and B polymerized into ABABABAB);• gradient (tapered) polymerized monomers (e.g. monomers A and B polymerized into AAABAABBABBB);• block copolymers (e.g. monomers A and B polymerized into AAAAABBBBBB) wherein the block length of each of the blocks (2, 3, 4, 5 or even more) is important for the dispersion capability of the polymeric dispersant;• graft copolymers (graft copolymers consist of a polymeric backbone with polymeric side chains attached to the backbone); and• mixed forms of these polymers, e.g. blocky gradient copolymers.

[0039] Suitable dispersants are DISPERBYK™ dispersants available from BYK CHEMIE, JONCRYL™ dispersants available from BASF and SOLSPERSE™ dispersants available from Lubrizol. Other suitable dispersants are Edaplan 482 from Miinzing. A detailed list of non- polymeric as well as some polymeric dispersants is disclosed by MC CUTCHEON. Functional Materials, North American Edition. Glen Rock, N. J.: Manufacturing Confectioner Publishing Co., 1990. p.110-129.

[0040] The polymeric dispersant has preferably a number average molecular weight Mn between 500 and 30000, more preferably between 1500 and 10000.

[0041] The polymeric dispersant has preferably a weight average molecular weight Mw smaller than 100000 more preferably smaller than 50000 and most preferably smaller than 30000.

[0042] In a preferred embodiment of the invention the aqueous ink comprises a pigment which is encapsulated by means of a cross-linked polymeric shell. Encapsulated pigments provide printed images having improved physical properties such as water resistance and dry rub resistance with respect to pigments dispersed by means of un-cross-linked polymers.

[0043] Suitable encapsulated pigments are provided by Lubrizol as Diamond HSDX-dispersions and by Fujifilm as RxD pigment dispersions such as APD1000 and APD4000 premium dispersions.

[0044] The pigments are preferably present in the range of 0.05 to 15 %, more preferably in the range of 0.1 to 10 % by weight and most preferably in the range of 0.2 to 6 % by weight, each based on the total weight of the inkjet ink. For white inkjet inks, the white pigment is preferably present in an amount of 3% to 40% by weight of the inkjet ink, and more preferably 5% to 35%. An amount of less than 0.05% by weight cannot achieve sufficient covering power.

[0045] The average pigment 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 BI90 plus based upon the principle of dynamic light scattering. <Water-soluble alkane diol>

[0046] The ink composition of the invention contains at least one water-soluble alkane diol in an amount not less than 10 wt.% and not more than 37 wt.%, preferably not more than 35 wt.% with respect to the total amount of the ink.

[0047] This alkane diol is incorporated in the aqueous inkjet ink to prevent drying and clogging of ejection nozzles due to aggregates formed of adhered and dried ink.

[0048] Clogging of the ejection nozzles of a print head impairs severely the jetting reliability, especially at the start-up of the printing process and during long printing jobs.

[0049] For preventing ink drying at the ejection nozzles, the alkane diol preferably has a lower vapor pressure than that of water.

[0050] Examples of the water-soluble alkane diols are ethylene glycol, propylene glycol, 1 ,3-propane diol, diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, dipropylene glycol, 1 ,3-butanediol, 2,3- butanediol, 2-butene-1 ,4-diol, 2-ethyl-1 ,3-hexanediol, 2-methyl-2,4- pentanediol, 1 ,2-octanediol, 1 ,2-hexanediol, 1 ,2-pentanediol, 4-methyl-1 ,2- pentanediol, 2-ethyl-2-methyl-1 ,3-propanediol, 3,3-dimethyl-1 ,2- butanediol, 2, 2-diethyl-1 ,3-propanediol, 2-ethyl-1 ,3-hexanediol, 2,2,4- trimethyl-1 ,3-pentanediol, 2-methyl-2-propyl-1 ,3-propanediol, 2,4- dimethyl-2,4-pentanediol, 2,5-dimethyl-2,5-hexane diol, 5-hexene-1 ,2-diol, 2-ethyl-1 ,3-hexanediol, tripropylene glycol, polypropylene glycol having the number of propylene oxide groups of greater than or equal to 4, butylene glycol, 1 ,6-hexanediol, pentanediol, thiodiglycol.

[0051] If the aqueous inkjet ink comprises alkane diols below the above mentioned range, the effect of drying prevention is insufficient to assure continuous ink jetting reliability and sufficient good setoff at high printing speeds. If the aqueous inkjet ink comprises alkane diols above the above mentioned range, setoff onto transporting rollers prevents high printing speeds.<Alkyl glycol ether>

[0052] The aqueous inkjet ink according to the invention comprises glycol ethers in an amount not less than 5 wt.% and not more than 20 wt.% with respect to the total weight of the ink. In this amount, the alkyl glycol ethers function as a penetrant helping the penetration of the ink vehicle and keeping at least a part of the pigment at the surface of the paper substrate at the same time.

[0053] Suitable examples of alkyl glycol ethers are ethylene glycol monomethyl ether, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether, diethylene glycol mono ethyl ether, diethylene glycol mono-n-propyl ether, ethylene glycol mono-iso-propyl ether, diethylene glycol mono-iso- propyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, diethylene glycol mono-t-butyl ether, 1-methyl-1- methoxybutanol, propylene glycol monomethyl ether, propylene glycol monomethyl ether, propylene glycol mono-t-butyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-iso-propyl ether, dipropylene glycol monomethyl ether, dipropylene glycol mono ethyl ether, tripropylene glycol monomethyl ether, dipropylene glycol mono-n-propyl ether, or dipropylene glycol mono-iso-propyl ether.

[0054] If the aqueous inkjet ink comprises alkyl glycol ethers below the above mentioned range, the effect of penetration while keeping the pigments at the surface of the paper substrate is insufficient to assure sufficient color depth and good setoff at high printing speeds. Alkyl glycol ethers above the mentioned range lead to storage stability problems of the aqueous inkjet inks.<Alkyl alcohol having an alkyl group comprising 1 to 7 carbon atoms>

[0055] In a preferred embodiment of the invention, the aqueous inkjet ink comprises an alkyl alcohol having an alkyl group comprising 1 to 7 carbon atoms in an amount not less than 1 wt.% and not more than 5 wt.%. This alkyl alcohol improves the penetration of the ink vehicle while keeping the pigments at the surface of the paper substrate, leading to excellent color depths of the printed image.

[0056] Suitable examples of alkyl alcohols having an alkyl group comprising 1 to 7 carbon atoms are ethanol, methanol, butanol, propanol, isopropanol, iso butanol, pentanol and hexanol.

[0057] If the aqueous inkjet ink comprises alkyl alcohols below the above mentioned range, the effect of keeping the pigments at the surface of the paper substrate is insufficient to further improve color depth of the printed images. If the aqueous inkjet ink comprises alkyl alcohols above the above mentioned range, instabilities in the ink formulation can occur, especially upon storage of the ink.<Non-ionic surfactant>

[0058] A non-ionic surfactant is added to the aqueous inkjet ink according to the invention for the purpose of ensuring wettability of the ink onto the paper substrate.

[0059] Examples of suitable non-ionic surfactants include acetylene glycol-based surfactants, polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene alkyl ester, alkoxy polyglycol ether, fatty alcohol alkoxylates, modified ethoxylates, polysiloxanes and polyoxyethylene sorbitan fatty acid esters.

[0060] The acetylene glycol 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.

[0061] Examples of commercially available fatty alcohol alkoxylates are Hydropalat WE-series such as Hydropalat WE3120, WE3130, 3197, 3694. An example of a modified ethoxylate is Hydropalat WE3650.

[0062] The added amount of the non-ionic surfactant is determined based on the types and amounts of the water-soluble organic solvents as well as other additives, but is preferably not less than 0.01 wt.% and not more than 5 wt.% with respect to the total weight of the ink.

[0063] It is preferable that the non-ionic surfactant be contained in such an amount that the static surface tension of the ink composition may be adjusted to be within a range of from 20 mN / m to 45 mN / m, in view of performing the spreading of the ink composition satisfactorily onto the substrate, and more preferably the surfactant is contained in such an amount that the surface tension of the ink composition may be adjusted to be within a range of from 25 mN / m to 40 mN / m, and even more preferably within a range of from 25 mN / m to 35 mN / m.

[0064] When the static surface tension of the ink is within the above mentioned range, the ink vehicle penetrates quickly into the paper substrate and beading is decreased, leading to excellent setoff properties.<Viscosity>The viscosity at 32°C of the aqueous ink according to the invention is preferably not less than 1.2 mPa.s and not more than 15.0 mPa.s, more preferably not more than 10 mPa.s. If the viscosity of the aqueous inkjet ink is outside the above range, inks do not fit specifications of the targeted printheads.<Water>

[0065] The aqueous inkjet ink has water as main component making part of the ink vehicle. The water content in the ink is preferably between 30 wt.% and 70 wt.% with respect to the total weight of the ink. If the amount of water is below this range, viscosity of the ink is getting outside the preferred range leading to worse setoff properties and bad jetting reliability.<Resin>

[0066] 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 paper substrate and a good abrasion resistance. The resin is preferably a polymer and suitable resins can be an acrylic based resin, a polyester resin, a urethane resin or a wax.

[0067] 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.

[0068] 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 the urethane-modified polyester resin include at least one structural unitderived from a hydroxyl group-containing polyester resin (polyester polyol) and at least one structural unit derived from an organic polyisocyanate.

[0069] 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 patent application EP3532545A.

[0070] 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.

[0071] 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.

[0072] Some examples of suitable acrylic latex polymer suspensions are, JONCRYL 537 and JONCRYL 538 (BASF Corporation, Port ArthurTX); 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, BAYHYDROL A2651 (Bayer), Mowinyl 6899D and 6969D, 6800, and 6810 (Japan Coating Resin Co. Ltd.) or a combination of two or more of the above.

[0073] 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.%.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] An example of modified paraffin wax particles includes paraffin wax that has been modified to improve dispersibility in water, e.g., via emulsification. The modified paraffin wax may be surface modified, chemically modified, etc.

[0078] 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 theAQUACER 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 .

[0079] 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.

[0080] The wax may be present in the ink in an amount ranging from 3 to 30 wt. %, more preferably from 5 to 25 wt. %, relative to the total solids weight of the ink.

[0081] 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 WQ201 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 WQ2016165970 [0051-0138]: the nanocapsules are selfdispersable and include a dispersing group covalently coupled to the shell polymers. The core of the nanocapsules in WQ201 5158649 [0037-0110] and WQ2016165970 [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 WQ2015158649 [0068-0110],

[0082] The capsules 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. <Additives>

[0083] The aqueous inkjet ink according to the invention may comprise nonwetting coating attack inhibitors. These prevent the attack of the nonwetting coating which usually is present onto the nozzle plate of inkjet printheads. This non-wetting coating prevents accumulation of ink around the ejection nozzles and attack of this coating creates severe jetting reliability issues.In a preferred embodiment of the invention, inhibitors are selected from compounds including a six-membered aromatic ring having two hydroxyl groups in para-position and a compound according to Formula 1 :Formula 1 wherein Rs is selected from the group consisting of a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted aryl and a substituted or unsubstituted heteroaryl group; Re to Rg are independently selected from the group consisting of a hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted aryl and a substituted or unsubstituted heteroaryl group; or any of Re to Rg may represent the necessary atoms to form a five to eight membered ring.

[0084] In a preferred embodiment, Rs is selected from the group consisting of a substituted or an unsubstituted alkyl group and a substituted orunsubstituted aryl group, a substituted or unsubstituted aryl group being particularly preferred, a substituted or unsubstituted phenyl group being the most preferred.

[0085] The non-wetting coating attach inhibitors may also be a compound including a six-membered aromatic ring having two hydroxyl groups in para-position such as the ones according to Formula 2:

[0086] Formula 2 wherein Ri to R4 are independently selected from the group consisting of a hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted alkaryl group, a substituted or unsubstituted aryl or heteroaryl group, a halogen, a hydroxyl group, an ether group, an ester group, a carbonate group, an amide group, a urethane group, a carboxylic acid group or salt thereof, a nitrile group an nitro, an amine group, a thiol group, a thioether group, a sulfonic acid group or salt thereof, a sulfonamide group, a sulfonate ester group, a sulfoxide group, a sulfone group, a phosphate group or salt thereof, a phosphate ester group, a phosphonate group or salt thereof, a phosphonate ester group, an aldehyde group and a ketone group, or wherein R1 and R2 and / or R3 and R4 may represent the necessary atoms to form a five to eight membered ring.

[0087] In a preferred embodiment, R1 to R4 are independently selected from the group consisting of a hydrogen, a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group, a carboxylic acid group or salt thereof and a sulfonic acid group or salt thereof.

[0088] In an even more preferred embodiment, R1 to R4 are independently selected from the group consisting of a hydrogen, a C1 to C4 alkyl group,a substituted or unsubstituted phenyl ring, a carboxylic acid group or salt thereof and a sulfonic acid group or salt thereof.

[0089] In the compounds according to Formula 1 and Formula 2, carboxylic acid groups or salts thereof and sulfonic acid groups or salts thereof are particularly preferred substituents. The cations in the salts of the carboxylic acid and the sulfonic acid groups or salts thereof are preferably monovalent and preferably selected from Li+, K+, Na+, NH4+ and monovalent organic cations.

[0090] It has been observed that inkjet inks containing a compound according to Formula 1 or Formula 2 with a carboxylic acid group or salt thereof tends to be more efficient and effective against corrosion than other type of acid groups or salts thereof.

[0091] The non-wetting coating attack inhibitors are included in the inkjet ink according to the invention in an amount not less than 0.01 wt.% and not more than15 wt.%, preferably not less than 0.05 wt.% and not more tha10 wt.% and more preferably not less than 0.05 wt.% and not more than 2 wt. % with respect to the total weight of the aqueous inkjet ink.

[0092] A biocide may be added to the aqueous inkjet ink of the invention 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.

[0093] Preferred biocides are ProxelTM GXL and ProxelTM Ultra 5 available from ARCH UK BIOCIDES and BronidoxTM available from COGNIS.

[0094] 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.

[0095] The aqueous inkjet ink may further comprise at least one thickener for viscosity regulation in the liquid. Suitable thickeners include urea or urea derivatives, hydroxyethylcellulose, carboxymethylcellulose, hydroxypropylcellulose, derived chitin, derived starch, carrageenan, xanthan gum, 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). Particular suitable thickeners are hydrophilically modified Pll thickeners (HELIR) like Thijet 170 from Lamberti and Polyacrylic esters like BYK LP-R21675 from BYK.

[0096] The thickener is added preferably in an amount of 0.01 to 20 wt.%, more preferably 0.1 to 10 wt.% based on the total weight of the ink.A.2. Paper substrate

[0097] The paper substrate which is used in the method according to the invention is not particularly limited to a specific type, but the paper substrate should show a certain absorbability towards the ink vehicle of the aqueous inkjet ink as described in § A.1.

[0098] Absorbing paper substrates include paper, coated paper, coated paper having an ink receiving layer on at least one surface, offset paper, office paper, newspaper paper, card board, white lined chipboard, folding board.

[0099] For packaging applications absorbing paper substrates include card board, paper lines, and paper. The paper can be a single layer of a multilayer paper. The paper liner may be brown kraft liner, top kraft liner, white coated kraft liner or bleached paper board.

[0100] The absorbing paper substrate may be manufactured from chemical, wood, or recycled fibre.

[0101] In a preferred embodiment of the invention, the paper may be a liner intended for printing into a corrugator and converted into corrugated boards or 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.

[0102] In a more preferred embodiment of the invention, the paper substrate such as for example the liner intended for printing into a corrugator is a porous paper substrate such as an uncoated liner. A porous paper substrate isquite challenging as due to the porosity of the paper, the pigments of the aqueous inkjet ink tend to migrate into the paper when water-soluble organic solvents such as alkyl glycol ethers are used as penetrants to obtain good setoff properties. When the pigment of the ink penetrates into the paper substrate, low image quality of the printed image such a low color depths and contrast is obtained.

[0103] Quantitatively, a porous paper substrate indicates a "paper substrate having a porosity, measured using a Gurley air permeability meter, of equal to 100 seconds or less’, also expressed as a Gurley porosity below 100 seconds. A Gurley air permeability meter measures the airflow between the paper substrate, clamped between a flat glass plate and a circular metal head. A suitable Air Permeability Tester measuring the air permeability of a paper substrate is a Bendtsen Porosity Tester (RL- BRAPT-A) from Rycolab. The Gurley porosity is calculated from the measured values.

[0104] The method according to the invention is useful for paper substrate web width of 1.2 m, 1.6 m, 2.8 m and even up to 3.5 m. Especially, corrugators require a paper substrate web width of 2.8 m.

[0105] The paper substrate may also be separate sheets instead of web based paper. Boards used for boxes and other packaging applications are also suitable. Most preferably, the paper substrate is an uncoated paper liner. The absence of a coating further improves penetration of the ink vehicle while keeping at least a part of the pigments at the surface.A.3. Conveying speed of paper substrate

[0106] The method according to the invention can be applied at a wide range of conveying speeds of the paper substrate.

[0107] In a preferred embodiment, the conveying speed at standard printing speed of the press wherein the method of the invention is to be applied, is not less than 100 m / min, preferably not less than 250 m / min, most preferably not less than 350 m / min. These conveying speeds correspond with conveying speeds used in digital web presses and corrugators. Some corrugators operate at peak conveying speeds of 450 m / min.

[0108] Even at these extremely high conveying speeds, the aqueous inkjet ink as described in § A.1. does not show setoff onto the face rollers of the image recording method of the invention.A.4. Maximum ink coverage

[0109] To obtain high quality printed images with sufficient color depth and contrast on paper substrates, even uncoated paper substrates, the maximal aqueous inkjet ink coverage of the printed images should be not less than 3 g / m2, preferably not less than 5 g / m2, most preferably not less than 6 g / m2. Ink coverages below these values lead to solids in printed images having insufficient color depth and contrast, especially on brown kraft paper or uncoated paper liner such as commonly used for packaging applications, typically in corrugated card board manufacturing.A.5. Contacting the printed substrate with a transport roller

[0110] The recording method according to the invention contains a transporting step of the printed paper substrate following to the adhering step of the inkjet ink. This transporting step is done by means of a transport roller wherein the outer surface of the transport roller (also called face roller) contacts the first surface of printed paper.

[0111] Extremely high conveying speeds make the time laps between adhering the aqueous inkjet ink, more particularly the last adhering step if more than one inkjet printhead is used, and the contact of the face roller with the printed paper substrate very short. Typically, the time between the adhering step of the ink or the last adhering step of the ink if more than one inkjet printhead is used, and the contact of the face roller with the first surface of the printed paper substrate in the recording method according to the invention is not more than 10 seconds, preferably not more than 3 seconds, most preferably not more than 1 second.

[0112] Due to this transporting step and the absence of any setoff on the face roller contacting the printed surface of the paper substrate, the method of the invention is particularly suitable to be incorporated in existing presses such as offset presses, digital page wide web presses and corrugators.Indeed, this transporting step gives sufficient flexibility to fit in every printing equipment without contaminating the transport rollers of the existing equipment with ink components.A.6. Joining the printed paper web together with a corrugated paper medium.

[0113] In a preferred step of the method according to the invention, the printed paper substrate as obtained from the above described steps, is joined together with a corrugated paper medium such as to form a printed single faced corrugated card board. In another embodiment of the invention, the printed paper web is joined together with the corrugated paper medium which may be provided as a single faced corrugated card board web or a multifaced corrugated card board web such as to form respectively either a double faced card board web or a multi faced card board web.

[0114] The joining step is preferably performed in a corrugator or corrugated card board production line. Steps i) and ii) of the method according to the invention can be performed off-line, wherein the printed paper substrate is wounded as a web on a roll and transported to the corrugator or stacked as separate sheets. In another preferred embodiment of the invention, steps i) and ii) can be integrated in a corrugator or corrugated card board production line.

[0115] The joining of the printed paper substrate together with a corrugated paper medium is done by contacting the surface opposite to the first surface of the printed paper web with a surface of the corrugated paper medium. If the corrugated paper medium is provided as a single faced corrugated card board (also called a corrugated card board web), the surface opposite to the first surface of the printed paper web is contacted with the corrugated side of the corrugated paper medium.

[0116] Prior to the contacting of the surface opposite to the first surface of the printed paper substrate with the corrugated side the corrugated paper medium, a glue is preferably applied onto the tips of the corrugated paper medium.

[0117] In a preferred embodiment of the invention, the contacting of the surface opposite to the first surface of the printed paper web with a surface of the corrugated paper medium by pressing the printed paper web with the corrugated paper medium in a gap between a pressing roller and a corrugating roller.

[0118] In another preferred embodiment of the invention, the contacting of the surface opposite to the first surface of the printed paper web with a surface of the corrugated paper medium by pressing the printed paper web with the corrugated paper medium by means of a heating and pressing device comprising heating plates and an endlessly driven pressure belt.A.7. Application of a pre-treatment liquid

[0119] The image recoding method may also include a step of applying an aqueous pre-treatment liquid comprising a fixing agent onto the paper substrate, prior to the adhering of the aqueous ink.<Fixing agent>

[0120] 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).

[0121] 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 (AICH), 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.

[0122] 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 the pre-treatment composition. If the amounts are below the lower limits, insufficient fixing of the colorants occur resulting in a reduced image quality.

[0123] 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.

[0124] 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.

[0125] The application of the pre-treatment liquid to the paper substrate prior to the adhering of the aqueous inkjet can be done by a coating technique such as screen printing, dip coating, knife coating, doctor blade coating ir via a jetting technique.B. Printing unit for performing the image recording method.

[0126] The printing unit for performing the image recording method according to the invention comprises (i) a transport mechanism for providing a paper substrate to the (ii) means for adhering the aqueous inkjet ink onto the paper substrate and (iii) a transport roller placed downstream the conveying direction of the paper substrate and contacting the first surface of the paper substrate whereupon the ink was adhered.

[0127] As the method according to the invention is useful for paper substrate web widths of 1.2 m, 1.6 m, 2.8 m and even up to 3.5 m., the width of the printing unit is at least larger than 1.2 m, 1.6 m, 2.8 m and even up to 3.5 m respectively. Especially for being incorporated in corrugators or corrugated card board production lines, the printing unit should have a width of 2.8 m or more.(i) The transport mechanism

[0128] The transport mechanism for providing a paper substrate is not particularly limited but is preferably a conveyer belt, a vacuum conveyer belt, a composition of one or more transport rollers. Providing a paper substrate as a paper web is preferably done by means of an impression cylinder or by means of transport rollers.(ii) The means for adhering the aqueous inkjet ink

[0129] The means for adhering the aqueous inkjet ink according to the invention is not particularly limited but is preferably an inkjet printhead. The inkjet printhead may be arranged into a shuttle (multi pass system) or into a line, preferably a page wide array, also called line type inkjet printhead (single pass system). Page wide arrays having a width of at least 1.2 m, 1.6 m, 2.8 m and even up to 3.5 m make single pass printing possible.

[0130] As the recording method according to the invention is very suitable for high conveying speeds, a line type inkjet printhead (also called print bar) is particularly suitable as this guarantees high printing speeds matching these conveying speeds.

[0131] To achieve printed images having a high image quality, the inkjet ink drop resolution of the inkjet printhead should be preferably equal to 225 drops / inch or more, more preferably equal to 300 drops / inch or more, most preferably equal to 1200 drops / inch or more. Most actual commercial inkjet printheads have a nozzle density of 1200 nozzles / inch or more along a first axis corresponding with the longest dimension of the nozzle plate of the inkjet printhead. The inkjet printheads may be arranged in an array along this first axis. This array forms a printing bar which is positioned above the first surface of the paper substrate according to a direction perpendicular to the conveying direction of the paper substrate.

[0132] In the conveying direction, due to the extremely high conveying speeds, the required drop resolution is achieved by sufficient high jetting frequencies of the printhead. For example, at a conveying speed of 450m / min, set by the corrugator, corrugated board production line, digital web press or offset press, a drop resolution of only 225 dpi can be achieved at a jetting frequency of 66.4kHz per inkjet printhead. This jetting frequency corresponds to an upper limit where the jetting reliability of commercial printheads is guaranteed. In future, inkjet heads with shorter acoustic lengths will allow higher jetting frequencies and higher printing speeds. At these conveying speeds, the maximal ink coverage for a typical drop mass between 5 and 10 ng is between 2.1 and 4 g / m2(assuming a drop resolution of the inkjet printhead of 1200 drops / inch) which is insufficient to obtain solids in the printed images of sufficient color density.

[0133] As the jetting frequency of commercially available inkjet printheads is limited at which jetting reliability is guaranteed, the required drop density per ink in the conveying direction can be obtained by the use of 2 or more inkjet printheads placed such that their first axis are parallel and perpendicular to the paper conveying direction.

[0134] This configuration has the advantage that, e.g. in case of 2 print heads in parallel, due to the doubled drop density, the maximal ink coverage for a typical drop mass between 5 and 10 ng is between 4.2 and 8 g / m2(assuming a drop resolution of the inkjet printhead of 1200 drops / inch), which is within the required range as disclosed in § A.4.

[0135] Combining 2 or more inkjet printheads for each ink, parallel to each other in a flat configuration takes considerable space in the printing unit, which decreases the flexibility to incorporate the unit for the recording method of the invention into existing printing equipment such as corrugators, digital web presses, etc.

[0136] Therefore, to achieve a high compact design of the printing unit, the inkjet printhead(s) should be preferably positioned such as to wrap around a curved support or impression cylinder carrying the paper substrate during the adhering of the aqueous ink. An impression cylinder is particularly suitable for a paper substrate web which is being transported over theouter surface of the cylinder. Indeed, the dimensional stability of the paper substrate containing aqueous ink adhered onto its surface is much more guaranteed than with a conveyor belt or a combination of transport rollers in a curved configuration. As specially the width of the page wide array for a printing unit to be incorporated in a corrugator must be preferably equal to or larger than 1 .6 m, 2.8 m or even 3.5 m, this dimensional stability of the paper substrate web is very critical. A suitable example how the inkjet printheads can be positioned around an impression cylinder is disclosed in Fig. 1 of US 2002 / 0168212 A1. Another advantage of an impression cylinder for transporting and providing the paper substrate web when adhering an aqueous inkjet ink according to the invention is that the design of the printing unit is much more compact.

[0137] A preferred inkjet printhead for the jetting of the aqueous inkjet ink is a piezoelectric inkjet printhead. 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 printhead 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 liquid from the inkjet printhead. However, the jetting of the ink according to the present invention is not restricted to piezoelectric inkjet printing.

[0138] Particularly suitable piezoelectric inkjet printheads are Gen5 and Gen5S from RICOH, KJ4B from KYOCERA, Samba G3L and G5L from FUJIFILM DIMATIX and the 5601 print head from XAAR.

[0139] 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.

[0140] In a preferred embodiment, the inkjet printhead may be configured to recirculate the aqueous ink prior to printing. A particularly useful inkjet printhead to print the aqueous inkjet ink of the invention, is from the type that includes a recirculation of the ink within the printhead such as through-flow printheads as disclosed in WO 2006 / 030235 A2 and WO2006 / 064036 A1. This type of inkjet printhead is very suitable to be incorporated in a printing system comprising a through-flow print head having one or more nozzles for ejecting drops of the aqueous ink. The ink circulation system for feeding and circulating the ink through the print head, comprises 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 the ink to the main ink tank via a pump.

[0141] Commercial examples of suitable piezoelectric through-flow print heads are the print heads Samba G3L and G5L from FUJIFILM DIMATIX, Rico Gen5F heads, Epson throughflow heads and the 5601 print head from XAAR.(iii) A face roller

[0142] The printing unit according to the invention comprises a roller (also called face roller) having an outer surface contacting the first surface of the paper substrate, is present in downstream direction of the moving paper substrate. To achieve flexible incorporation of the printing unit in existing printing equipment such as corrugators, corrugated board production lines, digital web presses, offset presses, this face roller should be placed as close as possible to the means of adhering the aqueous inkjet ink of § A.1. The distance, measured along the paper substrate track after the adhering of the inkjet ink should therefor be preferably less than 10 m, more preferably less than 2 m, most preferably less than 1 m.

[0143] The outer surface of the face roller can be made of any material, but it is particularly beneficial to have transport rollers having an outer surface of metal such as stainless steel, chrome or copper. This outer surface material guarantees mechanical stability and resistance to wear and scratches occurring during the contacting with the first surface of the paper substrate.

[0144] According to a preferred embodiment of the invention, the face roller may have an outer surface which is covered with a polymer having repellent properties towards the compounds of the aqueous inkjet ink of §A.1 . Due to the repelling properties, setoff is further minimized and higher conveyingspeeds or shorter time laps between adhering the aqueous inkjet ink and contacting the face roller are possible. Suitable polymeric material which shows repelling properties are Teflon, nylon, polypropylene, Xylan, PTFE, PFA, FEP, etc...C. ExamplesC.1. Materials

[0145] 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.• Surfynol 104PG50 is a 50 wt.% solution of 2,4,7,9-Tetramethyl-5- decyne-4,7-diol in propylene glycol from Evonik• Hydropalat WE 3650 is a modified ethoxylate surfactant from BASF• HSD KX1 is a black pigment dispersion from Lubrizol Corp.• CO4107K is a self-dispersible black pigment from Cabot Corp.• D. I. water is deionized water• Paper Liner is Austroliner 3 an uncoated 135 gsm brown test liner from Hamburger Rieger having a Gurley porosity of 43.9 s.C.2. Evaluation methods C.2.1. Setoff test

[0146] Setoff was evaluated by measuring the color transferred onto a metallic (stainless steel) face roller from an ink coating adhered to a paper liner.

[0147] An aqueous ink composition was coated onto a paper liner by means of a 10 pm coating knife using a vacuum coating table from Elcometer. During the coating, at a distance of 5 cm from the coating knife, a metallic face roller was contacting the first surface of the paper liner which was covered with the ink coating. Considering the speed of the coating knife, this corresponds with a time of 1.5 s between the coating of the ink onto the paper liner and the contacting with the face roller.

[0148] The transferred ink onto the metallic face roller outer surface was collected with a cotton cloth specified in ISO 105 F09 which was impregnated with a water based flushing solution (Agora Aqueous Flush from Agfa NV). Thecotton cloth was dried at 60°C for 30 minutes and the color (= L*a*b* and optical density) of the stained cotton swab was then measured at 5 stained spots with a Gretag SPM50 spectrophotometer measuring device using a D50 illumination angle under a 2° observer.

[0149] The measured L-value was evaluated according to the criteria as listed in Table 1.Table 1 : Evaluation criteria of setoffC.2.2. Color depth

[0150] The color depth of the coated aqueous inks was measured as an L-value. L-values are according to the CIELAB color space (also referred to as L*a*b*). This color space was defined by the International Commission on Illumination (abbreviated CIE) in 1976. Therefor, a 4 pm bar coating of the ink composition was made using an Elcometer 4340 using a spiral bar of 4 pm onto the paper liner. After 24h at room temperature, the L-value is measured with a Gretag SPM 50 lab meter (D50 illumination, observer at 2°)

[0151] The measured L-value is evaluated according to the criteria as listed in Table 2.Table 2: Evaluation criteria of color depthC.2.3. Jetting reliability

[0152] The inkjet inks were filtered over a 0.7 pm Whatman filter, degassed for 30 minutes in an ultrasound bath and filled in a Samba 2.4 pL cartridge supplied by Dimatix, which was mounted in a Dimatix TM DMP 2831 system.

[0153] The inks were jetted using a head temperature of 32°C, a firing voltage of 20 V, aiming at a drop speed of 7 m / s. Visual evaluations were done of the jetting behaviour of 5 nozzles and by counting for each test the number of nozzles failing in jetting and number of nozzles showing side shooting (= deviating nozzles causing inaccurate landing of drops) onto an Austroliner 3 substrate with respectively a 1 drop per drop per dot and 2 drops per dot waveform.

[0154] For the 30 s reliability during 30 seconds, inks were continuously jetted for 30 s at 50 kHz jetting frequency, after which the number of side shooting nozzles was counted.

[0155] For the 60 kHz reliability test, inks were jetted continuously at 60kHz for the time indicated in the table, and the number of side shooting nozzles was counted after respectively 30 seconds and 2 minutes of jetting.

[0156] For the open head time test, the jetting was interrupted for 5 minutes. Upon restarting the jetting, the number of failing nozzles was counted.C.2.4. Non-wetting coating attack

[0157] Non-wetting coating (NWC) Si wafer substrates were available as nonwetting coating test samples from Fujifilm Dimatix, as used for the Samba G3L and G5L printheads with the ‘generation 1 non wetting coating’. The coating attack was evaluated as follows.

[0158] The substrates were cut in two samples and immersed in a plastic bottle filled with the relevant inkjet ink, after which they were placed in an oven at 35°C.

[0159] Every x days, the samples were removed from the ink bottle, flushed with water, isopropanol and blown dry. Next the sample was left at room temperature for about 30 minutes before measuring the contact angle with a Kruss DSA 100 (using Ellips Image analysis software), operated at room temperature, using a 3 pl drop size, framing every 10 seconds. Water drops were deposited on the glossy side of the sample, three measurements per sample were done, varying the measurements position over the sample surface.

[0160] After each measurement the sample is re-immersed in the bottle filled with the inkjet ink. If the contact angle after 36 days is higher than 90°, the sample passes the test, if its below 90°, the test fails.C.3. Ink preparations

[0161] Comparative and inventive aqueous inks were prepared by diluting the pigment dispersions with the ingredients according to Table 3 and Table 4. The amounts of the dispersion and ingredients are expressed in wt.% based on the total weight of the ink. D. I. -water was added to complete the ink to the desired pigment concentration.Table 3: Comparative ink compositions for setoff and color depth test.

[0162] Table 4: Inventive ink compositions for setoff and color depth tests.

[0163] Table 5: Inventive ink compositions for non-wetting coating attack tests.C.4. Evaluation of setoff and color depth of printed images.

[0164] The color depth of the inks coated onto a paper liner as described in § C.2. together with the setoff of the coated inks onto a metallic roller (as described in § C.2.) are listed in Table 6.Table 6: Results of color depth (L-value) and setoff onto metallic transport roller

[0165] From Table 6 it can be observed that the coatings on paper liner with the inventive inks show an excellent color depth (L-value) and at the same time do not transfer ink components to the metallic transport roller when the coatings have not been dried.C.5. Evaluation of the jetting reliability experiments

[0166] The jetting reliability of three inks, INV-INK 2, INV-INK 4 and COMP-INK 5 was evaluated according to § C.2.3. The results of the tests are summarized in Table 7.Table 7: Jetting reliability results of aqueous inkjet inks

[0167] From Table 7 it can be observed that the recording method according to the invention including inventive inks show a better ink jetting reliability and open head time than the comparative ink.C6. Attack of the non-wetting coating of the print head nozzle plate

[0168] The attack of the non-wetting coating Si wafer substrates was evaluated according to § C.2.4. for INV-INK 8 and INV-INK 9 inks. The results are listed in Table 8.Table 8: Contact angle of water on non-wetting coating Si wafer substrates after immersion in inkjet inks.

[0169] From Table 8 it can be observed that the inkjet ink containing a nonwetting coating attack inhibitor does show less attack than the ink not having this compound.

Claims

ClaimsClaim 1. Image recording method, including the steps of i. Providing a paper substrate; and ii. Adhering an aqueous ink onto a first surface of the paper substrate by means of an inkjet print head to obtain a printed paper substrate, the aqueous ink comprising a pigment, a water-soluble alkane diol in an amount not less than 10 wt.% and not more than 37 wt.% with respect to the total weight of the ink, a water-soluble alkyl glycol ether in an amount not less than 5 wt.% and not more than 20 wt.% with respect to the total weight of the ink and a nonionic surfactant; and iii. Contacting the first surface of the printed paper substrate with a roller.Claim 2. Image recording method according to claim 1 wherein the aqueous ink comprises an alkyl alcohol having an alkyl group comprising 1 to 7 carbon atoms in an amount not less than 1 wt.% and not more than 5 wt.% with respect to the total weight of the ink.Claim 3. Image recording method according to any of the preceding claims wherein the pigment is selected from a group consisting of a self-dispersible pigment and a pigment encapsulated by means of a crosslinked polymer.Claim 4. Image recording method according to any of the preceding claims wherein the contacting is within 3 seconds after the adhering the aqueous ink.Claim 5. Image recording method according to any of the preceding claims wherein the paper substrate has a Gurley porosity equal to 100 seconds or less.Claim 6. Image recording method according to any of the preceding claims wherein the paper substrate is a paper substrate web and the adhering of the aqueous ink onto the paper web is done by means of at least two inkjet printheads having a first axis and a nozzle resolution of not less than 300 nozzles / inch in the direction of the first axis, both first axis being parallel to each other and perpendicular to the moving direction of the paper web.Claim 7. Image recording method according to Claim 6 wherein the inkjet printheads are positioned such as to at least double the drop density in the direction of the moving paper web.Claim 8. Image recording method according to any of the preceding claims wherein the non-ionic surfactants are selected from a group consisting of acetylene glycol-based surfactants, polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene alkyl ester, alkoxy polyglycol ether, fatty alcohol alkoxylates, modified ethoxylates, polysiloxanes and polyoxyethylene sorbitan fatty acid esters.Claim 9. Image recording method according to any of the preceding claims wherein the inkjet printhead is a single pass system.Claim 10. Image recording method according to any of the preceding claims wherein the outer surface of the roller is metallic.Claim 11. Image recording method according to any of the preceding claims wherein the adhering amount of aqueous ink in the image area is not less than 3 g / m2.Claim 12. Image recording method according to any of the preceding claims wherein the paper substrate in step (i) is provided onto an impression cylinder.Claim 13. Image recording method according to any of the preceding claims wherein no forced drying step by applying heat or airflow to the printed paper substrate is done.Claim 14. Image recording method according to any of the preceding claims wherein the aqueous ink comprises a non-wetting coating attack inhibitor being a compound including a six-membered aromatic ring having two hydroxyl groups in para-position.Claim 15. A method of producing a printed corrugated cardboard including the steps of i. Providing a paper substrate; and ii. Adhering an aqueous ink onto a first surface of the paper substrate by means of an inkjet print head to obtain a printed paper substrate, the aqueous ink comprising a pigment, a water-soluble alkane diol in an amount not less than 10 wt.% and not more than 37 wt.% with respect to the total weight of the ink, a water-soluble alkyl glycol ether in an amount not less than 5 wt.% and not more than 20 wt.% with respect to the total weight of the ink and a nonionic surfactant; and iii. Contacting the first surface of the printed paper substrate with a roller; andiv. Joining the printed paper substrate together with a corrugated paper medium by contacting the surface opposite to the first surface of the printed paper substrate with a surface of the corrugated paper medium.

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