White inkjet ink composition

Abstract

An aqueous white ink comprising a white metal oxide pigment having an average particle size of 100 nm to 1000 nm in an amount of 5 to 50 wt%, comprising a salt selected from the group consisting of phosphates, polyphosphates and phosphites, a water-miscible organic solvent, the ink having a kinematic viscosity of 1 to 25 mm2 / s at 32 DEG C. The aqueous white ink is suitable for ink jet printing.

Description

Technical Field

[0001] The present invention relates to an aqueous white inkjet ink comprising a white metal oxide pigment, suitable for printing white images on non-absorbent and absorptive substrates, and a method for recording by inkjet using the ink composition. Background Art

[0002] The use of inkjet printing systems has grown significantly in recent years. This growth can be attributed to significant improvements in print resolution and overall print quality combined with significant reductions in cost.

[0003] Today's inkjet printers provide acceptable print quality for many commercial, business, and home applications at a lower cost than comparable products available only a few years ago. Despite their recent success, research and development efforts continue toward improving inkjet print quality in a variety of applications, but challenges remain.

[0004] One challenge is inkjet printing white ink formulations made with metal oxide particles, such as titanium dioxide pigment, where the ink has a low viscosity and the particles have a large average particle size. These properties can lead to an increased sedimentation rate, where the ink formulation separates from the pigment and the pigment settles to form a dense deposit.

[0005] Dense deposits can be difficult to redisperse, which can lead to blockage of the printhead nozzles, pigment depletion, and / or pigment enrichment. Pigment enrichment occurs when the pigment concentration is unevenly distributed in the ink, with higher concentrations near or near the nozzles. Typical failures include the formation of solid pigment blockages (also known as decapping or encrustation) inside or on top of the printhead nozzles due to ink drying, which can cause print start-up problems.

[0006] More specifically, decapping problems can occur when jetting inks that exhibit poor redispersibility. As used herein, the terms "redispersible" and "redispersible" refer to the ability of a white inkjet ink to disperse pigment solids (including those that may have settled) substantially uniformly throughout the ink vehicle again after it has been exposed to a storage period, such as during shipping or in a printing device, with brief mixing, recirculation, or shaking.

[0007] WO22015323 discloses a white inkjet textile ink comprising a non-self-dispersible white pigment and an anionic copolymer dispersant having a weight average molecular weight (Mw) of 125,000 g / mol to about 30,000,000 g / mol. The amount of anionic copolymer dispersant should preferably be equal to 1% by weight or less relative to the weight of the white ink composition.

[0008] US2017 / 0362456 discloses a white pigment dispersion suitable for inkjet printing, comprising a white metal oxide pigment dispersed by two co-dispersants, each comprising i) a short-chain anionic dispersant having a weight-average molecular weight of 1000 Mw to 30,000 Mw and ii) a nonionic or predominantly nonionic dispersant. However, the stability of the white pigment dispersion is still insufficient for use in the industrial production of aqueous white inkjet inks.

[0009] Furthermore, the use of a combination of polymer dispersants and metal oxide particles, as in both of the aforementioned patent applications, has the disadvantage that once precipitation occurs in the white ink composition, redispersibility is nearly impossible. Therefore, avoiding precipitation is the only way to achieve reliable jetting, but this requires a suitable ink tank and a special ink supply channel design in the printing device.

[0010] US2018 / 194959A discloses an aqueous white ink comprising a white metal oxide pigment having an average particle size of 5 nm to less than 100 nm, a polymer dispersant bound to the surface of the white metal oxide pigment, and core-shell latex particles. Inks containing latex particles are known to easily form a latex film in the air / ink phase, leading to nozzle clogging, poor inkjet reliability, and decapping behavior.

[0011] There remains a need for dispersants that maintain metal oxide white pigments in a dispersed state in industrial scale white inkjet ink formulations, such that these metal oxide white pigments can be easily redispersed without the need for high shear equipment, and which result in excellent decapping behavior. SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to provide a solution to the above-mentioned problems. This object has been achieved by using a phosphate, polyphosphate or phosphite to disperse a white metal oxide pigment as defined in claim 1 in a white inkjet ink composition.

[0014] According to another aspect, the present invention comprises an aqueous inkjet ink set as defined in claim 8 .

[0015] According to another aspect, the present invention comprises a recording method as defined in claim 10 using the ink composition according to claim 1 .

[0016] 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 present invention are also defined in the dependent claims.

[0017] Implementation Plan Description

[0018] A. Water-based white inkjet ink

[0019] The aqueous white ink according to the present invention comprises a white metal oxide pigment in an amount of 5 to 50 wt %, a salt selected from phosphates, polyphosphates and phosphites and a water-miscible organic solvent, the white metal oxide pigment preferably having an average particle size of 100 nm to 1000 nm. The ink has a particle size of 1 to 25 mm at 32°C. 2 / s viscosity.

[0020] A.1. Phosphates, polyphosphates or phosphites.

[0021] The salt selected from the group consisting of phosphates, polyphosphates and phosphites present in the ink according to the present invention acts as a dispersant for the white metal oxide pigment, which is believed to adhere to the surface of the white metal oxide pigment.

[0022] Phosphate includes salts (which have the formula M3PO4, where M is an alkali or ammonium ion) or organic esters, both of which are derived from phosphoric acid. It most commonly refers to orthophosphate, a derivative of orthophosphoric acid H3PO4.

[0023] Phosphites include salts having the formula M2HPO3, where M is a base or an ammonium ion, or esters having the formula PHO(OR)2, both of which are derivatives of phosphorous acid H3PO3.

[0024] Polyphosphates are salts or esters of polymeric oxyanions formed from tetrahedral PO4 (phosphate) structural units linked together by shared oxygen atoms. Polyphosphates can adopt linear or cyclic ring structures.

[0025] Suitable polyphosphates are tetrasodium pyrophosphate, tetrapotassium pyrophosphate, sodium tripolyphosphate, potassium tripolyphosphate, polymetaphosphate [e.g., (NaPO3) x For example, sodium hexametaphosphate, where x=6, or other similar structures, where x is greater than 6.

[0026] Commercially available polyphosphates are EXA (sodium hexametaphosphate), a 2% solution from Calgon PT, PRAYPHOSS 5PP FG, sodium pentaphosphate (CASRN 18859-55-7), PRAYPHOS SHMP 458TG, sodium hexametaphosphate (CASRN 10124-56-8), PRAYPHOS STMP 118FG, sodium trimetaphosphate (CASRN 7785-84-4), and PRAYPHOS KTPP FG, potassium tripolyphosphate (CASRN 13845-36-8), all supplied by PRAYON-RUPEL NV.

[0027] Preferably, the salt selected from the group consisting of phosphates, polyphosphates and phosphites is in acid form, as an alkali metal salt or as an ammonium salt. In order to obtain a white metal oxide pigment dispersion that is stable during storage and can be easily added to an ink vehicle to form an inkjet ink in a production environment, the salt selected from the group consisting of phosphates, polyphosphates and phosphites is added during the preparation of the dispersion.

[0028] The amount of the salt selected from phosphates, polyphosphates, and phosphites is preferably 0.01 to 10% by weight, more preferably 0.01 to 1% by weight, relative to the total weight of the ink. An amount below this range may result in insufficient activity of the salt as a dispersant, leading to insufficient stability of the white metal oxide pigment in the dispersion or in the ink composition.

[0029] A.2. White metal oxide pigments

[0030] Examples of suitable white metal oxide pigments as colorants in the white ink according to the present invention include titanium dioxide (TiO2), zinc oxide (ZnO) and zirconium dioxide (ZrO2). In a preferred embodiment of the present invention, the white pigment is titanium dioxide. In a more preferred embodiment of the present invention, the titanium dioxide is in its rutile form.

[0031] In another embodiment of the present invention, the white metal oxide pigment may include white metal oxide pigment particles coated with silicon dioxide (SiO2). In yet another embodiment of the present invention, the white metal oxide pigment content to silicon dioxide content may be 100:3.5 to 5:1 by weight. In other embodiments of the present invention, the white pigment may include white metal oxide pigment particles coated with silicon dioxide (SiO2) and aluminum oxide (Al2O3). In a more preferred embodiment of the present invention, the white pigment may be metal oxide particles treated with an organic compound such as a polyol or metal oxide particles treated with an organic compound and coated with silicon dioxide.

[0032] White metal oxide pigments suitable as pigments in the aqueous white inkjet ink according to the present invention do exhibit a pH-dependent zeta potential. Particularly useful white metal oxide pigments are those that exhibit a zeta potential of less than -5 mV, more preferably less than -10 mV, and most preferably less than -20 mV at a pH of 8. Without being bound by theory, it is believed that the negatively charged surface of the metal oxide improves the adhesion of phosphate, polyphosphate, or phosphite groups, resulting in an increased dispersion effect of the white metal oxide pigment.

[0033] A suitable example of a white metal oxide pigment is Tronox CR-834 (a white pigment powder containing 97% TiO2 from TRONOX PIGMENTS BV.

[0034] Other suitable examples are Tl-PURE R960 (a TiO pigment powder having 5.5 wt% silica and 3.3 wt% alumina (based on pigment content)); Tl-PURE TS-6300 (a TiO pigment powder having 9 wt% silica and 5.8 wt% alumina (based on pigment content)); Tl-PURE R931 (a TiO pigment powder having 10.2 wt% silica and 6.4 wt% alumina (based on pigment content)); Tl-PURE R706 (a TiO pigment powder having 3.0 wt% silica and 2.5 wt% alumina (based on pigment content)); TS-6200 (a white pigment powder containing 93% TiO, 3.6% alumina and 3.3% silica); TS-4657 (a TiO pigment powder containing 93% silica and 6.4% alumina (based on pigment content); White pigment powder of TiO2, 2.2% alumina and 3.1% silica), all available from Chemours.

[0035] White metal oxide pigments can have high light scattering properties, and the average particle size of the white pigment can be selected to enhance light scattering and reduce light transmittance, thereby increasing opacity. The average particle size of non-self-dispersible white pigments can be from 10 nm to 2000 nm, more preferably from 100 nm to 1000 nm, and most preferably from 150 nm to 500 nm. A disadvantage of white metal oxide pigments having an average particle size below the stated range is insufficient opacity, resulting in a printed image with too low a color density. White metal oxide pigments having an average particle size above the stated range can cause jetting reliability issues due to nozzle clogging of the inkjet head.

[0036] The method for dispersing white metal oxide pigments comprises the steps of mixing a binder, preferably water, and a salt selected from the group consisting of phosphates, polyphosphates and phosphites to obtain a pre-dispersion.

[0037] Subsequently, high shear forces or grinding are applied to the obtained pre-dispersion, thereby obtaining a stable dispersion of the white metal oxide pigment.

[0038] The dispersion method for obtaining a stable dispersion of the white metal oxide pigment may be arbitrarily selected as long as it is a method for breaking up agglomerates of particles and uniformly dispersing the pigment in the vehicle.

[0039] Suitable equipment for dispersing white metal oxide pigments includes pressure kneaders, open kneaders, planetary mixers, dissolvers, and Dalton Universal Mixers. Other suitable grinding and dispersing equipment includes ball mills, pearl mills, colloid mills, high-speed dispersers, two-roll mills, bead mills, paint conditioners, and three-roll mills. Dispersions can also be prepared using ultrasonic energy.

[0040] Many different types of materials can be used as grinding media, such as glass, ceramic, metal, and plastic. In a preferred embodiment, the grinding media can comprise particles, preferably substantially spherical in shape, such as beads consisting essentially of a polymer resin or yttrium stabilized zirconia beads.

[0041] During mixing, grinding and dispersing, each process is preferably carried out under cooling to prevent heat accumulation.

[0042] The particle size of the white metal oxide pigment can be controlled by the high shear application or the processing time of the milling. More particularly, by increasing the processing time, the particles can be made smaller and the amount of particle residue can be reduced.

[0043] The amount of white metal oxide pigment in the dispersion may be from 10% to 70% by weight, more preferably from 20 to 65% by weight, based on the total weight of the dispersion. The white pigment dispersion may then be incorporated into an ink vehicle such that the white metal oxide pigment is present in an active amount suitable for obtaining an image by inkjet printing.

[0044] In a preferred embodiment, the white pigment dispersion is incorporated into the ink vehicle such that the white metal oxide pigment is present in an amount of 1 wt% to 50 wt% active, more preferably 3 wt% to 50 wt%, and most preferably 5 to 50 wt%, based on the total weight of the white inkjet ink.

[0045] A.3. Connecting materials

[0046] The vehicle of the aqueous white inkjet ink according to the present invention contains water and a water-miscible organic solvent.

[0047] The amount of binder and the ratio of water to water-miscible organic solvent in the inkjet ink have an impact on the viscosity of the ink.

[0048] The aqueous white ink according to the present invention has a preferable range of 1 to 25 mm at a measurement temperature of 32°C. 2 / s, more preferably 3 to 20 mm 2 When used in inkjet recording methods, the kinematic viscosity exceeds 25 mm 2 A kinematic viscosity of the ink composition of less than 0.1 / s cannot achieve stable discharge of liquid droplets of the ink composition.

[0049] Kinematic viscosity was assessed using a HERZOG HVM472 instrument (https: / / www.paclp.com / lab-instruments / brand / herzog / product / 23 / hvm-472-multirange-viscometer) at a temperature of 32°C.

[0050] Suitable water-miscible organic solvents for use in the aqueous white inkjet ink of the present invention may include glycerol, diglycerol, polyglycerol, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, 1,3-propanediol, dipropylene glycol, tripropylene glycol, polypropylene glycol, 2-pyrrolidone, N-methylpyrrolidone, dimethyl sulfoxide, and sulfolane.

[0051] From the viewpoint of wetting properties, glycerin, diethylene glycol, triethylene glycol, propylene glycol, polyethylene glycol (average molecular weight of 200 to 600), dipropylene glycol and tripropylene glycol are preferably used as the water-miscible organic solvent, and glycerin, diethylene glycol and propylene glycol are more preferred.

[0052] Other examples include propylene glycol, dipropylene glycol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 3-methyl-1,3-butanediol, 1,2-pentanediol, 1,5-pentanediol, 2-methyl-2,4-pentanediol, 3-methyl-1,5-pentanediol, 1,2-hexanediol, 1,6-hexanediol, ethanol, butanol, propanol, and pentanol.

[0053] Examples of glycol ethers include monoalkyl ethers of glycols 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 monopropyl ether are preferred. Other useful examples listed in Swiss List A and therefore compatible with food packaging include: 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; and tripropylene glycol monomethyl ether. These solvents can be used alone or in combinations of two or more.

[0054] From the viewpoint of viscosity adjustment of the ink and prevention of clogging of the inkjet print head nozzles by wetting action, these water-miscible organic solvents are preferably contained in an amount of 1 to 50 wt %, more preferably 30 to 45 wt %, and particularly preferably 30 to 40 wt % relative to the total mass of the ink.

[0055] A.4. Surfactants

[0056] In order to adjust the surface tension of the ink, a surfactant may be used, and examples thereof include anionic surfactants, amphoteric surfactants, cationic surfactants, and nonionic surfactants.

[0057] Examples of the anionic surfactant include alkyl sulfocarboxylates, α-olefin sulfonates, polyoxyethylene alkyl ether acetates, N-acylamino acids and salts thereof, N-acylmethyl taurates, alkyl sulfate polyoxyalkyl ether sulfates, alkyl sulfate polyoxyethylene alkyl ether phosphates, rosin acid soaps, sulfuric acid esters of castor oil, lauryl sulfate, alkylphenol type phosphates, alkyl type phosphates, alkylaryl sulfonates, diethyl sulfosuccinate, diethylhexyl sulfosuccinate, and dioctyl sulfosuccinate.

[0058] Examples of the amphoteric surfactant include lauryldimethylaminoacetic acid betaine, 2-alkyl-N-carboxymethyl-N-hydroxyethylimidazolinium betaine, coconut oil fatty acid amide propyldimethylaminoacetic acid betaine, polyoctylpolyaminoethylglycine, and other imidazoline derivatives thereof.

[0059] Examples of cationic surfactants include 2-vinylpyridine derivatives and poly-4-vinylpyridine derivatives. Other examples of cationic surfactants are quaternary ammonium salts of long-chain aliphatic tertiary amines, such as cetyltrimethylammonium bromide, cetyldimethylbenzylammonium chloride and hydroxyethyldimethyldodecylammonium chloride, and quaternary salts of nitrogen-containing heteroaromatic rings, such as cetylpyridinium bromide and quaternized imidazoles and benzimidazoles. Other examples are protonated long-chain aliphatic amines, such as the hydrochloride of dimethylhexadecylamine and the toluenesulfonate of diethyldodecylamine.

[0060] Examples of the nonionic surfactant include ether-based surfactants such as polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene dodecylphenyl ether, polyoxyethylene oleyl ether, polyoxyethylene lauryl ether, polyoxyethylene alkyl ether, polyether siloxane; ester-based surfactants such as polyoxyethylene oleate, polyoxyethylene distearate, sorbitan laurate, sorbitan monostearate, sorbitan monooleate, sorbitan sesquioleate, polyoxyethylene monooleate, and polyoxyethylene stearate; and acetylenediol (alcohol)-based surfactants such as 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-decyne-4-ol, and alkylene oxide adducts of 2,4-dimethyl-5-decyne-4-ol. These are available, for example, from Air Products (GB) or Nissin Chemical Industry, for example Olfine (registered trademark) such as Olfine E1010, 104 series and Surfynol (registered trademark) E series such as Surfynol 465, Surfynol 104 and Surfynol 61.

[0061] A.5. Resin

[0062] The aqueous white inkjet ink composition according to the present invention may further comprise a resin. Resins are typically added to the inkjet ink formulation to further improve adhesion of the pigment to the substrate or to improve the water and solvent resistance of the printed image. The resin is preferably a polymer, and suitable resins may be acrylic-based resins, urethane resins, urethane-modified polyester resins, carbodiimide-containing polymers, or waxes.

[0063] Suitable polyurethane resins can be selected from, for example, aliphatic polyurethane dispersions, aromatic polyurethane dispersions, anionic polyurethane dispersions, nonionic 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, or a combination of two or more thereof.

[0064] The preferred urethane resin used as the dispersant in the ink of the present invention is a polyester resin comprising a structural unit containing a urethane bond. Among such resins, water-soluble or water-dispersible urethane-modified polyester resins are preferred. Preferably, the urethane-modified polyester resin comprises at least one structural unit derived from a hydroxyl-containing polyester resin (polyester polyol) and at least one structural unit derived from an organic polyisocyanate.

[0065] Furthermore, the hydroxyl-containing polyester resin is a resin formed by an esterification reaction or an ester exchange reaction between at least one polyacid component and at least one polyol component.

[0066] Some examples of suitable polyurethane dispersions are, for example, 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 (Bayer Material Science); DAOTAN VTW 1262 / 35WA, DAOTAN VTW 1265 / 36WA, DAOTAN VTW 1267 / 36WA, DAOTAN VTW 6421 / 42WA, DAOTAN VTW 6462 / 36WA (Cytec Engineered Materials Inc., Anaheim CA); and SANCURE 2715, SANCURE 20041, SANCURE 2725 (Lubrizol Corporation), or a combination of two or more thereof.

[0067] Acrylic acid-based resins include polymers of acrylic acid monomers, polymers of methacrylic acid monomers, and copolymers of the above monomers with other monomers. These resins exist as suspensions of particles (latex) with an average diameter of about 30 nm to about 300 nm. Acrylic acid latex polymers are formed by acrylic acid monomers or methacrylic acid monomer residues. For illustration, examples of monomers of acrylic acid latex polymers include acrylic acid monomers, such as acrylates, acrylamides, and acrylic acid, and methacrylic acid monomers, such as methacrylates, methacrylamides, and methacrylic acid. Acrylic acid latex polymers can be homopolymers or copolymers of acrylic acid monomers and another monomer, such as a vinyl aromatic monomer, including but not limited to styrene, styrene-butadiene, p-chloromethylstyrene, divinylbenzene, vinylnaphthalene, and divinylnaphthalene.

[0068] Some examples of suitable acrylic latex polymer suspensions are, for example, JONCRYL 537 and JONCRYL 538 (BASF Corporation, Port Arthur TX); 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, Rancho Santa Margarita CA); NEOCRYL A-1110, NEOCRYL A-1131, NEOCRYL A-2091, NEOCRYL A-1127, NEOCRYL XK-96, and NEOCRYL XK-14 (DSM NeoResins, Sluisweg, The Netherlands); and BAYHYDROL AH XP 2754, BAYHYDROL AH XP 2741, BAYHYDROL A 2427 and BAYHYDROL A2651 (Bayer Material Science, Baytown TX), or a combination of two or more thereof.

[0069] In addition to resins and latexes, polymer capsules can be advantageously incorporated into the ink comprising the pigment dispersion of the present invention. Useful polymer capsules are the microcapsules disclosed in JP 2003-313476 and the nanocapsules disclosed in EP313198A. Preferably, nanocapsules comprising a core with a reactive chemistry can be used, the reactive chemistry being a thermo-reactive chemistry that can be directly activated by heat or indirectly activated using a photothermal conversion agent. These nanocapsules are described in [0059-0067] of EP313198A. In the latter, for example, infrared absorbing dyes convert the infrared light of an infrared laser or infrared LED into heat.

[0070] Suitable waxes are petroleum waxes, vegetable waxes and animal waxes. The example of petroleum waxes includes paraffin wax, microcrystalline wax and petrolatum (petrolatuin). The example of vegetable waxes includes carnauba wax, candelilla wax, rice wax and Japan tallow (Japan tallow). The example of animal waxes includes lanolin and beeswax. The example of synthetic waxes includes synthetic hydrocarbon wax and modified wax. The example of synthetic hydrocarbon wax includes polyethylene wax, Fischer-Tropsch wax, and the example of modified wax includes paraffin derivatives, montan wax derivatives, microcrystalline wax derivatives and fatty acid amide wax derivatives, polyethylene wax, polypropylene wax and polytetrafluoroethylene wax. In one example, the wax is selected from polypropylene wax, high-density polyethylene (HDPE) wax and combinations thereof.

[0071] The concentration of the resin in the inkjet ink according to the present invention is at least 0.4 wt %, and preferably less than 30 wt %, more preferably less than 20 wt %. An amount above this range results in unreliable ink jetting behavior, while an amount below this range results in insufficient pigment binding ability, which results in insufficient adhesion or water resistance of the image.

[0072] B. Inkjet Recording Method

[0073] In a preferred inkjet recording method according to the present invention, the method comprises the following steps:

[0074] a) jetting an aqueous white inkjet ink onto a substrate, the ink comprising a white metal oxide pigment having an average particle size of 100 nm to 1000 nm in an amount of 5 to 50 wt %, a salt selected from phosphates, polyphosphates and phosphites, a water-soluble organic solvent, the ink having a viscosity of 1 to 25 mm at 32° C. 2 / s viscosity; and

[0075] b) Drying the ejected inkjet ink by applying heat or air flow.

[0076] Prior to jetting the inkjet ink according to the present invention, an aqueous pretreatment liquid or primer may be applied to the substrate. The aqueous pretreatment liquid preferably contains a component that can aggregate the components of the aqueous inkjet ink of the present invention. Examples of such components are flocculants selected from multivalent salts, cationic surfactants, and cationic resins.

[0077] In another preferred inkjet recording method, the method comprises the following steps: a) applying an aqueous pretreatment liquid on a substrate, the pretreatment liquid comprising a component capable of aggregating the components of the aqueous inkjet ink of the present invention. b) optionally drying the applied aqueous pretreatment liquid at least partially; and c) ejecting the aqueous white ink, the aqueous white ink comprising a white metal oxide pigment having an average particle size of 100 nm to 1000 nm in an amount of 5 to 50 wt %, comprising a salt selected from phosphates, polyphosphates and phosphites, a water-soluble organic solvent, the ink having a viscosity of 1 to 25 mm at 32°C. 2 and d) drying the ejected inkjet ink. The component capable of aggregating the components of the aqueous inkjet ink of the present invention is preferably a polyvalent metal salt, a cationic polymer or an organic acid.

[0078] The substrate in the inkjet recording method may be porous, such as textiles, paper and leather, but is preferably a low-absorption substrate such as a cardboard substrate or a non-absorption substrate such as polyethylene, polypropylene, polycarbonate, polyvinyl chloride, polyester such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polylactide (PLA), polymethyl methacrylate or polyimide.

[0079] The substrate may also be a paper substrate, such as conventional paper or resin coated paper, such as polyethylene or polypropylene coated paper. There is no real restriction on the type of paper and it includes newsprint paper, magazine paper, office paper, wallpaper, but also higher grammage papers, commonly known as paperboard, such as white lined chipboard, corrugated board and packaging board.

[0080] The substrate may be transparent, translucent or opaque.

[0081] In another preferred inkjet recording method, the pretreatment liquid is applied via a technique selected from inkjet, valve jetting, and spraying. More specifically, these inkjet and valve jet technologies allow the pretreatment liquid according to the present invention to be applied image-wise, preferably to the surface on which the inkjet ink is to be printed to obtain an image. These latter means of applying the pretreatment liquid have the advantage that the amount of pretreatment liquid required is significantly lower than with other substrate-priming application methods.

[0082] Examples of heating processes for drying the pretreatment liquid or inkjet ink according to the present invention include, but are not limited to, hot pressing, atmospheric steam, high pressure steam, THERMOFIX. Any heat source can be used for the heating process; for example, an infrared source can be used.

[0083] The drying step can be carried out in air, but the heating step must be carried out by using a heat source; examples include equipment for forced air heating, radiant heating such as IR radiation (including NIR-, CIR- and SWIR radiation), conductive heating, high-frequency drying and microwave drying. The drying step is such that the temperature of the printed image is preferably obtained at less than 150°C.

[0084] The preferred inkjet head for inkjet printing system to eject inkjet ink according to the present invention is a piezoelectric inkjet head. Piezoelectric inkjet ejection is based on its movement when voltage is applied to the piezoelectric ceramic transducer. The application of voltage changes the shape of the piezoelectric ceramic transducer in the print head, creating a gap, which is then filled with ink or liquid. When the voltage is removed again, the ceramic expands to its original shape, ejecting ink droplets from the inkjet head. However, the ejection of aqueous inkjet ink or aqueous pre-treated liquid comprising a particle dispersion according to the present invention is not limited to piezoelectric inkjet printing. Other inkjet print heads can be used, and various types, such as continuous type, thermal print head type, MEM-jet head and valve spray type.

[0085] Specifically, to reliably jet metal oxide-based white ink, the inkjet printing system preferably includes a shaking, mixing, or stirring device. This device is used to redisperse the white inkjet ink, which may have settled between print jobs. In some examples, the printing system can be configured to shake the ink cartridge, ink tank, or ink buffer tank containing the white inkjet ink.

[0086] In other examples, the printing cartridge or ink tank is configured to mix or agitate the white inkjet ink prior to printing. In still other examples, the printing system can be configured to recirculate the white inkjet ink prior to printing.

[0087] One particularly useful inkjet head for printing white inkjet ink is of the type that includes ink recirculation within the head, for example, a through-flow head as disclosed in WO 2006 / 030235 A2 and WO 2006 / 064036 A1. This type of inkjet head is well suited for incorporation into a printing system comprising i) a through-flow printhead having one or more nozzles for ejecting white ink droplets onto a substrate to be printed, ii) a fluid circulation system for feeding and circulating the ink through the printhead, comprising an ink tank for containing the ink, iii) a supply buffer tank for receiving ink from a main tank and supplying the ink to the through-flow printhead, and iv) a return manifold for receiving fluid from the through-flow printhead and returning it to the main ink tank via a pump. C. Examples

[0088] C.1. Materials

[0089] Unless otherwise stated, all materials used in the following examples are readily available from standard sources such as Sigma-Aldrich (Belgium) and Acros (Belgium).The water used was demineralized water.

[0090] Tronox CR-834 is a white pigment granule containing 97% TiO2 from TRONOXPIGMENTS BV

[0091] TS-6200 is a white pigment particle containing 93% TiO2, 3.6% alumina and 3.3% silica

[0092] TIOXIDE XRFDO is an alumina-polyol surface treated rutile TiO2 pigment produced by VENATOR

[0093] TS4657 is a white TiO2 pigment supplied by Chemours

[0094] Proxelk-D3798 is a 5% solution of CASRN 127553-58-6, manufactured by PROMCHEM UK

[0095] Sokalan PA25 CIPN is a short-chain polymer anionic dispersant from BASF

[0096] ·Edaplan TM 482 is a polymer dispersant from MUNZING

[0097] Joncryl 8078 is a 32 wt% aqueous solution of poly[acrylate-c-styrene] from SC JOHNSON POLYMER BV

[0098] EXA is a 2 wt% aqueous solution of sodium hexametaphosphate supplied by Calgon PT.

[0099] PRAYPHOS S5PP FG is sodium pentaphosphate (CASRN18859-55-7) supplied by PRAYON-RUPEL NV

[0100] PRAYPHOS SHMP 458TG is sodium hexametaphosphate (CASRN10124-56-8) supplied by PRAYON-RUPEL NV

[0101] PRAYPHOS STMP 118FG is sodium trimetaphosphate (CASRN7785-84-4) supplied by PRAYON-RUPEL NV

[0102] PRAYPHOS KTPP FG is potassium tripolyphosphate, which has CASRN 13845-36-8 and is supplied by PRAYON-RUPEL NV

[0103] Surf-1 is a 75 wt% solution of 2,4,7,9-tetramethyl-5-decyne-4,7-diol in ethylene glycol from Air Products

[0104] Surf-2 is Surfynol 104PG50, a 50 wt% solution of 2,4,7,9-tetramethyl-5-decyn-4,7-diol in propylene glycol from Evonik

[0105] HD is 1,2-hexanediol

[0106] PG is propylene glycol

[0107] Aquacer 530 is a 32 wt% aqueous emulsion of oxidized HDPE from BYK CHEMIE GMBH

[0108] Hostaphat KL 340D are mono-, di- and tri-(alkyltetraethylene glycol ether)-polyphosphates produced by Clariant.

[0109] Rhodoline 3700 is a polyarylphenyl ether phosphate dispersant manufactured by Solvay

[0110] C.2. Measurement method

[0111] C.2.1. Average particle size of white metal oxide pigments in dispersion

[0112] Based on the principle of dynamic light scattering, using Zetasizer TM The average particle size of the white metal oxide pigment particles was measured using a Nano-S (Malvern Instruments, Goffin Meyvis). For reliable inkjet properties (jetting quality and printing quality), the average particle size of the dispersed particles is preferably below 300 nm.

[0113] C.2.2. Viscosity of the dispersion

[0114] The viscosity of the white metal oxide particle dispersion was evaluated using an Anton Paar MCR302 rheometer with a cone and plate setup. -1 Shear rate measurement.

[0115] As a first assessment of the storage stability of the white metal oxide particle dispersion, the viscosity increase was measured after aging the dispersion in an oven at 60°C for 1 week.

[0116] C.2.3. Stability of white metal oxide pigment dispersions

[0117] To evaluate the storage stability of the dispersion, the Zetasizer TM The average particle size of the ink was measured using dynamic light scattering using a Nano-S (Malvern Instruments, Goffin Meyvis). Samples were measured fresh and after aging in a 60°C oven for one week. The relative change in average particle size (PS) was calculated as [(PS (1 week 60°C) - PS (fresh)) / PS (fresh)] and scored according to Table 1.

[0118] Table 1: Scoring of relative changes in mean particle size (PS)

[0119]

[0120] C.2.4. Zeta potential for metal oxide pigments

[0121] The zeta potential of the pigment particles was measured using an electroacoustic spectrometer DT310 from 3P Instruments with a titration cell.

[0122] In order to measure zeta potential, suspension is prepared by adding 25g of pigment to 225g of deionized water in a 500mL container. The container is loaded with 160g of talc 5mm beads and placed on a roller bench for 24 hours. After this, 200g of suspension is poured into a 250mL glass beaker and magnetically stirred. Zeta potential probe and pH probe are placed in the suspension and measure pH. According to the pH value of the suspension, alkali (NaOH 1M) or acid (HCl 1N) is added to make pH value ≥8.5 or ≤3.5.

[0123] The titration probe is then filled with base (NaOH 1 M) or acid (HCl 1 N) for an automated stepwise titration towards the other end of the pH range.

[0124] C.2.5. Agglomeration

[0125] Aqueous white ink samples are considered clumped if the bulk composition of the ink samples differs by more than 5% after 14 days of storage. To measure this, samples are measured fresh using a turbiscan at 880 nm in transmission and backscatter, then allowed to stand in the same measuring cell at room temperature for 14 days to allow for sedimentation. After this time, the samples are manually shaken (to replicate how consumers would homogenize their samples) for approximately 10 seconds and measured again.

[0126] The "average" values ​​(in the middle cell as backscatter) were then compared between the fresh and aged samples. If the difference was <5%, the sediment was redispersed in solution; if >5%, the sediment was not redispersed and a clumping was present at the bottom of the cell.

[0127] C.2.6. Storage stability of white ink

[0128] The stability of the ink was evaluated by placing a sample tube filled 80% with ink (sealed at the top) in an oven at 60°C. The viscosity and particle size distribution of the ink were evaluated after 1 and 2 weeks of storage at this temperature. For practical purposes, the viscosity of the aqueous inkjet ink should not deviate from the original value by more than 20%. The (kinematic) viscosity at a temperature of 32°C was evaluated using a HERZOG HVM472 instrument (https: / / www.paclp.com / lab-instruments / brand / herzog / product / 23 / hvm-472-multirange-viscometer).

[0129] Particle size analysis using a Malver zetasizer (measured at a 90° angle)

[0130] C.2.7. Latency (Uncapping Time)

[0131] Latency testing was performed on a Ricoh Gen5S print head using a jetting temperature of 32°C and a firing frequency of 20 kHz. Evaluations were performed after 5-10-20 minutes of head idle time as follows.

[0132] A first control image was printed on microporous paper using an Image Expert media motion printing system at 20 kHz, and the number of malfunctioning nozzles and side-firing nozzles in this image was determined. Thereafter, a predetermined period of 5 minutes without printing, spitting, or spraying was considered, referred to as head idle time. A control image was then reprinted on microporous paper using an IX media motion printing system at 20 kHz, and the number of malfunctioning nozzles and side-firing nozzles in this image was visually determined. This process was repeated for 10 and 20 minutes of head idle time. Finally, for each ink, all malfunctioning nozzles and side-firing nozzles in the image were totaled.

[0133] C.3. Preparation of white pigment dispersion

[0134] All amounts in this section are given as wt % relative to the total weight of the dispersion.

[0135] C.3.1. Comparative white pigment dispersions were prepared using two dispersants DISP-C1 to DISP-C4.

[0136] Deionized water, a polymeric dispersant (Edaplan 482), a short-chain anionic dispersant (Sokalan PA25CIPN), 0.4 wt% Proxelk, and 50 wt% Tronox CR-834 were added to a 100 mL plastic container. The container was loaded with 160 g of 3 mm yttrium-stabilized zirconium oxide beads and the pH was adjusted to 8.0 with NaOH solution. The amount of short-chain anionic dispersant was varied, and four white metal oxide dispersions were obtained. The particle size and viscosity results are summarized in [Table 1].

[0137] Table 2: Composition and properties of comparative white metal oxide dispersions

[0138]

[0139]

[0140] It can be observed that the combination of short-chain anionic dispersants with polymeric dispersants in different ratios does not lead to stable white metal oxide pigment dispersions. Therefore, the industrial applicability of these dispersions is not possible.

[0141] C.3.2. A comparative white metal oxide pigment dispersion was prepared using the poly-styrene-acrylate based dispersant DISP-C5.

[0142] 44.6% by weight of deionized water, 5% by weight of Joncryl 8078, and 0.4% by weight of a biocide (Proxelk-D3798) were premixed in a container using a mechanical stirrer. In the next step, 50% by weight of Tronox CR-834 was sprayed into the premix, and the resulting predispersion was mixed for 30 minutes. A DynoMill-RL mill was filled to 42% with 0.4 mm yttrium-stabilized zirconium oxide beads ("High Wear Resistant Zirconia Grinding Media" from TOSOH Co.). The predispersion was pumped into the bead mill and milling was started in recirculation mode at a speed of 11.8 m / s. After a 10-minute residence time, the mill was emptied by pumping the resulting dispersion into a container.

[0143] White DISP-C5 contained particles with an average particle size of 190 nm and a viscosity of 183 mPa·s, and the dispersion appeared shear thinning.

[0144] C.3.3. Preparation of a white dispersion using polymer dispersant DISP-C6 (comparative example).

[0145] 39.6 % by weight of deionized water, 10 % by weight of Edaplan 482 and 0.4 % by weight of biocide (Proxelk-D3798) are pre-mixed in a container using a mechanical stirrer. In the next step, 50 % by weight of TS-6200 is sprayed in the pre-mixture, and the pre-dispersion obtained is mixed 30 minutes. The DynoMill-RL grinding mill is filled 42% with 0.4mm yttrium-stabilized zirconium oxide beads (" high wear-resistant zirconium oxide grinding media " from TOSOH Co.). The pre-dispersion is pumped into the bead mill, and ground with the rotating speed of 11.8m / s in a recirculation mode. After 10 minutes of residence time, the grinding mill is emptied by the dispersion pump obtained in the container.

[0146] The DISP-C6 dispersion contained particles having an average particle size of 281 nm and a viscosity of 62 mPa·s. The dispersion appeared to be slightly shear thinning.

[0147] C.3.4. Prepare white metal oxide pigment dispersions using phosphates, polyphosphates or phosphites as dispersants.

[0148] 44.6% by weight of deionized water, dispersant and 0.4% by weight of biocide (Proxelk-D3798) were premixed in a container using a mechanical stirrer in the amounts listed in Table 3. In the next step, TS6200 was sprayed into the premix in the amounts listed in Table 3 and the resulting predispersion was mixed for 30 minutes. A DynoMill-RL mill was filled to 42% with 0.4 mm yttrium-stabilized zirconium oxide beads ("High Wear Resistant Zirconia Grinding Media" from TOSOH Co.). The predispersion was pumped into the bead mill and milling was started in recirculation mode at a speed of 8 m / s. After a residence time of 25 minutes, the mill was emptied by pumping the resulting dispersion into a container.

[0149] Table 3: Composition and properties of white metal oxide dispersions

[0150]

[0151] (N) = Newtonian viscosity behavior

[0152] (S) = shear thinning behavior

[0153] With a pressure higher than 100mPa.s-1 Dispersions with a viscosity and shear-thinning behavior of 200 nm or less are more prone to structuring and flocculation and can therefore lead to inks with lower stability. If, in this case, the particle size is greater than 250 nm, the dispersion is considered unsuitable for industrial use in the preparation of white inks.

[0154] C.3.5. Preparation of white metal oxide dispersions using metal oxide particles with different zeta potentials.

[0155] A 100 mL plastic bottle was filled with 50% by weight of white metal oxide, 10% by weight of EXA, 0.4% by weight of Proxelk, and 48.85% of deionized water. The bottle was further filled with 160 g of 3 mm yttrium-stabilized zirconium oxide beads. The bottle was placed on a roller bank and rolled for 7 days, after which the dispersion was separated from the beads by decantation. The properties of the resulting dispersion of the present invention are summarized in Table 4.

[0156] Table 4: Zeta potential, particle size after aging, and viscosity increase after aging of white metal oxide dispersions of the present invention.

[0157]

[0158] *: (N) = Newtonian viscosity behavior; (S) = shear thinning behavior

[0159] As can be seen from Table 4, pigments having a zeta potential value equal to or less than -20 mV result in dispersions exhibiting Newtonian viscosity behavior and smaller average particle sizes. These dispersions are more suitable for use in inkjet inks.

[0160] C4. White ink formulation

[0161] The white pigment dispersion prepared in §C.3 was used to formulate the ink of the present invention (I-INK) and the comparative ink (C-INK) according to the compounds in Table 5. The viscosity increase of the dispersion after aging was too high to obtain an ink formulation with DISP-C8. All values ​​are weight % relative to the total weight of the ink.

[0162] Table 5: White ink composition

[0163]

[0164] Table 5 (continued)

[0165]

[0166]

[0167] The kinematic viscosity at 32° C. was measured for all comparative and inventive ink formulations as described in §C.2.6. The measured values ​​are listed in Table 6.

[0168] Table 6: Viscosity values ​​of the white ink in Table 5

[0169]

[0170] C.5. Example 1

[0171] Example 1 shows that a white ink formulation of the present invention comprising a salt selected from phosphates, polyphosphates, and phosphites exhibits no agglomeration upon standing relative to a comparative ink formulation comprising an anionic polymeric dispersant.

[0172] The inventive white ink I-INK1 and the comparative white ink C-INK1 were evaluated for agglomeration after standing according to §C.2.5. It was observed that I-INK1 comprising polyphosphate as a dispersant showed no agglomeration after 14 days of standing, whereas C-INK1 showed significant agglomeration.

[0173] C.6. Example 2

[0174] Example 2 shows that white ink formulations of the present invention containing polyphosphates, phosphates or phosphites have improved storage stability relative to comparative ink formulations

[0175] The storage stability of the white inks I-INK2 to I-INK4 according to the invention was measured according to §C.2.6. The results are summarized in Table 7.

[0176] Table 7: Storage stability of white ink

[0177]

[0178] As can be seen in Table 7, the storage stability of the white pigment ink comprising a salt selected from phosphates, polyphosphates and phosphites is more constant relative to the comparative white ink based on a phosphate-containing polymer.

[0179] D.7. Example 3

[0180] Example 3 demonstrates that the white ink formulations of the present invention exhibit improved latency (decapping time) behavior relative to the comparative ink formulation.

[0181] The latency of the comparative ink C-INK-2 and the inventive inks I-INK5 and I-INK6 was measured according to §C.2.7. The number of failures and side emissions is summarized in Table 8.

[0182] Table 8: Decapping time values ​​for white ink.

[0183]

[0184] As can be seen from Table 8, the white pigment ink of the present invention containing a polyphosphate dispersant exhibits a much better latent state, which results in a significantly increased jetting reliability. In addition, the presence of a polyphosphate dispersant allows for a higher pigment load in the inkjet ink without resulting in an unacceptable latent state. The higher pigment load results in an increased opacity of the white ink layer for a given ink load.

Claims

1. An aqueous white ink comprising a white metal oxide pigment having an average particle size of 100 nm to 1000 nm in an amount of 5 to 50 wt %, a salt selected from phosphates, polyphosphates and phosphites, a water-miscible organic solvent, the ink having a viscosity of 1 to 25 mm at 32°C. 2 / s kinematic viscosity. 2 . The aqueous white ink according to claim 1 , wherein the amount of the salt selected from the group consisting of phosphates, polyphosphates and phosphites is 0.01 to 1 wt % relative to the total weight of the ink.

3. The aqueous white ink according to any one of the preceding claims, wherein the white metal oxide pigment is selected from titanium oxide (TiO2), zinc oxide (ZnO) and zirconium dioxide (ZrO2). The aqueous white ink according to claim 3 , wherein the white metal oxide pigment comprises silicon dioxide. 5 . The aqueous white ink according to claim 1 , wherein the white metal oxide pigment has a zeta potential equal to or less than −20 mV.

6. The aqueous white ink according to any one of the preceding claims, wherein the polyphosphate comprises at least 3 phosphate groups. 7 . The aqueous white ink according to claim 1 , further comprising a binder selected from the group consisting of a polyacrylate-containing resin, a polyurethane-containing resin, a carbodiimide-containing resin, and a wax.

8. An ink set for inkjet printing, comprising the aqueous white ink as defined in claim 1 to claim 7 and an aqueous pre-treatment liquid containing a component capable of aggregating components of the aqueous inkjet ink. 9 . The ink set for inkjet printing according to claim 8 , wherein the component is a polyvalent metal salt, a cationic polymer, or an organic acid.

10. A recording method comprising the following steps: a) providing a substrate; as well as b) jetting an aqueous white inkjet ink as defined in any one of the preceding claims onto at least one surface of the substrate to form an image; as well as b) Drying the ejected inkjet ink by applying heat or air flow to the image.

11. The recording method according to claim 10, wherein: Prior to step b), applying an aqueous pre-treatment liquid comprising components capable of aggregating the components of the aqueous inkjet ink as defined in claims 1 to 7 to at least one surface of the substrate; and optionally at least partially drying the pre-treatment liquid prior to step b).

12. The recording method according to claim 11, wherein the aqueous pre-treatment liquid is applied by means of a spraying technique.

13. The recording method according to claim 11 and claim 12, wherein the ejection of the aqueous white ink is performed by means of a through-flow head.

Images