Printing method

Abstract

A printing method is provided in which a primer composition comprising at least one pigment aggregating agent is applied to the coated substrate (120) prior to an inkjet printing step, the inkjet printing step (150) being performed using inkjet droplets having a drop diameter of x μm, the inkjet droplets being printed on the coated substrate with a printing resolution corresponding to a rectangular unit cell having cell dimensions of y μm by z μm, wherein the ratio of x / y is greater than 0.60.

Description

Detailed Description of the Invention

[0001] [Field of the Invention] The field of the invention relates to printing methods.

[0002] [background] In the inkjet printing method, droplets of ink are ejected directly onto a printing substrate from a very fine nozzle, and are deposited on the printing substrate to obtain a printed matter on which characters or images are printed. The inkjet printing method has various advantages, such as ease of full coloration, low cost, versatility, the ability to use plain paper as a printing medium, and non-contact with the characters or images to be printed, and is therefore widely used today not only for general consumer printing applications, but also for commercial and industrial printing applications in recent years.

[0003] The ink droplets may be printed using pigmented water-based inks. Typically, more than one type of colored pigmented water-based ink, such as cyan, magenta, yellow and black, is used to print a multicolor image. The colors can be formed by printing one, two or more different colored pigmented water-based inks at the same printing location on the substrate.

[0004] Some commercial and industrial inkjet printers utilize a fixed printhead and a moving substrate, such as a web, to achieve high speed printing, also referred to as single pass printing, since the substrate is transported along the fixed printhead only once while the full color image is formed on the substrate.

[0005] Corrugated board is a material that contains at least one fluted corrugated sheet and one or two flat liners that cover the corrugated sheet. Several of these packages can be combined into one stack to make one corrugated board. It is made on a flute lamination machine or corrugator and is used to make displays, (art) items, shipping containers and corrugated boxes. Corrugated boxes are used to ship a variety of items due to their strength, durability, light weight, recyclability and cost-effectiveness. Corrugated boxes used as shipping containers may require printing and labeling to identify the contents, provide regulatory information and provide bar codes for delivery. Corrugated packaging boxes and other objects used for marketing, merchandising and point of sale often have advanced graphics that help communicate the contents or messages related to the contents or brand.

[0006] When pigmented water-based inks are printed onto a coated substrate, the ink droplets may be absorbed slowly by the coated substrate. Many coated substrates that can be used as corrugated board liners are tailored to provide durability (e.g., water resistance), gloss, and a luxurious appearance to the corrugated board, and are not designed to absorb pigmented water-based inks. If the ink droplets remain on the surface of the coated substrate for a long period of time, the ink will dry more slowly, which can result in image defects such as coalescence or bleeding between colors.

[0007] Typically, primers are used in (inkjet) printing to improve the color strength (optical density), adhesion and / or image sharpness of inkjet inks on fast absorbing (uncoated) substrates. One type of primer contains polyvalent metal salts or acidic compounds. Their mechanism of action is mainly based on destabilization and aggregation of pigment particles. The result is increased optical density, color strength and reduced transparency because the pigment aggregates penetrate less deeply into the paper pores. Bleeding / mottling is usually not an issue with these types of substrates because the ink liquid medium penetrates the paper more easily and the color image is fixed almost instantly.

[0008] For less porous coating substrates, such primers would not be useful because the pigment from the inkjet ink is already held on the surface of the coating substrate instead of being lost in the depths of the pores. Furthermore, it has been found that printing on various coating substrates can result in uneven color layers with white streaks in areas with lower ink drop volumes and at the same time intercolor bleeding in areas with higher ink drop volumes. This primer composition is known to inhibit or reduce ink drop spreading on the substrate. This effect on ink spreading is inconsistent with solving both problems, including white streaks.

[0009] The second type of primer uses a binder that seals the pores of the absorbent substrate material. By blocking the pores, the pigments from the inkjet ink are kept on the surface instead of being lost in the depths of the paper (board), the latter causing a loss of color intensity. However, due to the loss of porosity, the ink particles can move freely on the substrate, which can cause dewetting effects or on the other hand severe bleeding, feathering and / or mottling, edge effects with increased color density. Furthermore, a significant amount of binder is required to block the pores, which greatly contributes to the total cost of the imaging process. Finally, the binder layer changes the gloss of the substrate, which is undesirable in most cases.

[0010] A third type of primer or precoat for substrate paper (such as corrugated board) uses porous inorganic particles (e.g. silica) with high specific surface area, typically embedded in a polymer matrix (see the second type of primer above). This paper precoat with porous inorganic particles with controlled pore size and uniform distribution over the paper surface allows for reduced ink bleeding and sharper images with enhanced color intensity (optical density). The drawback of this technology is the high complexity and high cost of such coatings. To have a practical amount of void space to absorb the ink fluid, the wet coating layers are very thick and, if applied in-line, they need to be thoroughly dried, requiring very high drying capacity especially for high speed printing processes. Moreover, with this type of coating, it is very difficult to find a balance between binder content and pigment concentration to achieve high capillarity while still achieving sufficient adhesion to the substrate and avoiding dusting effects.

[0011] Application of these known primer types to some slow absorbing coating substrates has been found to have a marginal effect on image quality.

[0012] To solve the image defects, selecting a suitable coating substrate and / or adapting the coating of the coating substrate is often used, but this approach is very time-consuming, expensive and not versatile. Another solution that has been attempted to solve the problem of inter-color bleeding uses software-based image adjustment (U.S. Patent No. 6,361,144).

[0013] In view of all these observations, there is a need to provide a printing method that results in good image quality using inkjet printing on coated substrates, especially on slowly absorbing coated substrates.

[0014] [overview] According to a first aspect of the present invention, there is provided a printing method comprising applying a primer composition comprising at least one pigment aggregating agent to a coating substrate prior to an inkjet printing step, said inkjet printing step being carried out with ink droplets having a drop diameter of x μm, said ink droplets being printed on the coating substrate with a printing resolution corresponding to a rectangular unit cell having cell dimensions of y μm×z μm, the ratio of x / y being greater than 0.60.

[0015] Surprisingly, it has been found that by first applying a primer composition to the coating substrate and selecting an x / y ratio higher than 0.60 for the ink droplets for the inkjet printing step, both image defects of white streaks in areas with lower ink droplet volume and intercolor bleeding in areas with higher ink droplet volume are simultaneously solved. It is known that applying a primer composition to the coating substrate reduces the ink droplet spreading on the substrate, and it was not expected to solve both problems simultaneously. Increasing the droplet size x μm and / or decreasing the cell dimension y can result in an x / y ratio higher than 0.60.

[0016] The inkjet printing step is performed by depositing ink droplets on the coating substrate with a printing resolution corresponding to a unit cell that is rectangular. The unit cell has cell dimensions of y μm×z μm, with the y dimension corresponding to the printing resolution of the ink droplets in a first direction and the z dimension corresponding to the printing resolution of the ink droplets in a second direction. The y and Z dimensions are arranged perpendicular to each other, and the unit cell has a rectangular form, preferably a square form.

[0017] A unit cell according to the present invention is illustrated diagrammatically in Figure 3. Figure 3 shows a unit cell 2 that is rectangular. The unit cell has a y dimension along a first direction A of the surface of the substrate, and a z dimension along a second direction B of the surface of the substrate. The y and z dimensions may be substantially equal to one another or may be unequal in length.

[0018] In inkjet printing, the unit cell 2 is repeated on the surface of the substrate in other unit cells 2' along both the first direction A and the second direction B. Although only three other unit cells 2' are illustrated in FIG. 3, the number of other unit cells 2' is not limited. Typically, the other unit cells 2' may extend across the entire printing area of ​​the substrate. The other unit cells 2' have the same dimensions as the unit cell 2, y in the first direction A and z in the second direction B, respectively.

[0019] The unit cell y dimension may be selected in any suitable direction on the substrate, in particular the unit cell y dimension may be selected perpendicular to the substrate transport direction, which is the transport direction of the substrate during the inkjet printing step.

[0020] It has been found that, especially in high speed printing using single pass inkjet printing, the selection of the print resolution in the direction perpendicular to the substrate transport direction may be most relevant to simultaneously solving the problems of white streaks and bleeding. In this example, the y dimension is selected perpendicular to the substrate transport direction.

[0021] The unit cell z dimension of the unit cell may be any suitable distance (μm). Preferably, the ratio of the y dimension to the z dimension is between 1:2 and 2:1. The z dimension may be suitably selected such that one inkjet droplet may be placed on the coating substrate within the unit cell to substantially fill the area of ​​the unit cell on the coating substrate.

[0022] The droplets have a droplet diameter of x μm. The droplet diameter can be determined as the average droplet diameter for a number of droplets. The droplet diameter according to the present invention can be calculated by gravimetrically determining the average weight of the ink droplets by collecting a number of ink droplets, and calculating the droplet diameter by assuming an ideal spherical shape of the ink droplets using the known density of the ink in the ink droplets. The number of ink droplets to be collected is determined by the firing frequency and the firing duration. The size of the ink droplets can be controlled by printhead settings such as waveform and firing frequency.

[0023] In certain embodiments, the ratio of x / y is greater than 0.70. It has been found that a ratio of x / y greater than 0.70 can further improve the low streak level without bleeding even for coating substrates with low ink absorption rates.

[0024] In certain embodiments, the ratio of x / y is greater than 0.90. It has been found that a ratio of x / y greater than 0.90 can further improve the low streak levels without bleeding, even for inks with relatively small static contact angles with the coating substrate.

[0025] In one embodiment, the ratio of x / y is greater than 0.60 and less than 2.0. Preferably, the ratio of x / y is less than 1.5. Even more preferably, the ratio of x / y is less than 1.0.

[0026] The primer composition includes at least one pigment agglomerating agent. The pigment agglomerating agent is not particularly limited as long as it is capable of agglomerating the pigment contained in the ink-jet ink.

[0027] In one embodiment, the at least one pigment flocculant is selected from the group consisting of ionic polymers, polyvalent metal salts, acids and mixtures thereof. Primer compositions comprising pigment flocculants are known from the prior art and can be used as primer compositions according to the present invention. In an example, the ionic polymer may be an anionic polymer or a cationic polymer. In an example, the acid may be an organic acid or an inorganic acid.

[0028] The primer composition may use only one type of pigment flocculant or may use two or more types of pigment flocculants. Examples of polyvalent metal salts include calcium chloride, magnesium nitrate, and aluminum chloride. Examples of acids include malonic acid, malic acid, citric acid, phosphoric acid, and succinic acid.

[0029] The primer composition can be applied to the coating substrate in any suitable manner. The primer composition may be applied using a technique that completely covers the coating substrate with a uniform layer of primer composition, or it may be applied using digital printing techniques, such as inkjet printing, to apply the primer composition image-wise.

[0030] The primer composition may be a water-based primer composition, in which a suitable amount of water, including optional co-solvents, is used as a carrier liquid for at least one pigment aggregating agent, which may be dissolved, emulsified or dispersed in the water-based primer composition.

[0031] The primer composition on the coated substrate may be heat treated after application onto the coated substrate to at least partially or completely dry the primer composition before depositing the ink droplets. Alternatively, the ink droplets may be printed without actively thermally drying the primer composition on the coated substrate. It has been found that not actively drying the primer composition on the coated substrate may have the advantage of better limiting bleeding of the ink on the coated substrate.

[0032] The primer composition may be applied once before printing ink droplets of several colored inks onto the coated substrate, for example, once before printing ink droplets of cyan, magenta, yellow and black inks onto the coated substrate. Alternatively, the primer composition may be applied to the coated substrate at any time prior to printing inkjet droplets of each colored ink onto the coated substrate.

[0033] In one embodiment, the inkjet printing process uses pigmented water-based inks. Pigmented water-based inks are primarily used to provide durable color images, for example for outdoor use. Pigmented water-based inks can be dried by actively applying thermal heat to the coated substrate before, after, or during printing. Pigmented water-based inks can have the disadvantage of being slowly absorbed by the coated substrate.

[0034] In one embodiment, the coated substrate has an absorption rate defined by a drop migration distance greater than 10.0 cm, preferably between 10.0 cm and 25.0 cm, more preferably between 12.0 cm and 20.0 cm, the drop migration distance being defined as the distance traveled by a 2 ml drop of the reference liquid in 50 seconds after being placed on the substrate, the substrate being placed at an angle of 60° to the horizontal. A coated substrate having a drop migration distance greater than 10.0 cm has a slow absorption rate. This reference liquid composition is similar to an ink without pigments and has a very low viscosity. The absorption rate test according to the present invention and its test conditions are further described in the detailed description.

[0035] The coated substrate may have such slow absorption rates due to low surface porosity of the coated substrate and / or high hydrophobicity of the coated substrate. Slow absorption rates may allow the ink droplets to spread further on the surface, resulting in inter-color bleeding. Another drawback of slow absorption rates is increased drying time of the ink.

[0036] In one embodiment, the ink of the inkjet droplets has a static contact angle on the coated substrate of less than 26°, preferably less than 23°, more preferably less than 20°, especially between 26° and 2°, more preferably greater than or equal to 5°. A lower static contact angle of the ink on the coated substrate correlates to improved spreading of the ink on the coated substrate and lower levels of white streaks in areas of low ink density.

[0037] In one embodiment, the ink of the inkjet droplets has a static contact angle on the various coating substrates of less than 26°, preferably less than 23°, more preferably less than 20°. The various coating substrates may have different ink absorption rates and / or static contact angles. If the ink has a lower static contact angle on the various coating substrates, good image quality can be obtained using the same ink on each coating substrate. The respective static contact angles of the various coating substrates are 2° or more, more preferably 5° or more. Even when the static contact angle of the ink is very low and close to the lower limit, the printing method of the present invention can solve bleeding.

[0038] In one embodiment, the ink of the inkjet droplets has a static surface tension of less than 32 mN / m, preferably between 30 and 18 mN / m. The static surface tension is preferably between 30 and 18 mN / m. Surfactants and / or wetting agents can be selected to reduce the static surface tension.

[0039] In one embodiment, the random copolymer is used as a dispersant for the stabilization of the pigment in the pigment dispersion.

[0040] In one embodiment, a graft copolymer is used to disperse the pigment particles and create the pigment dispersion.

[0041] In one embodiment, the pigments used in the inkjet inks are dispersed using encapsulated dispersion technology. Known methods can be used to disperse the pigments, followed by further processing of the resulting dispersion to crosslink the resin around the pigment surface, thereby encapsulating the pigment. Detailed descriptions are provided with examples from patent literature by Fujifilm Imaging Colorants, Kao Corporation, DuPont, Riso Kagaku Corporation, Xerox, and Seiko Epson Corporation.

[0042] In one embodiment, the pigments used in the inkjet are dispersed using block copolymer dispersants. The properties of the polymeric dispersant depend on both the nature of the monomers and the distribution of the monomers in the polymer. The use of block copolymer dispersants provides improved dispersion stability. Block copolymer dispersants can include hydrophobic and hydrophilic blocks.

[0043] In one embodiment, the ratio of z / y is between 0.5 and 2.0, with z being oriented orthogonal to y. This ratio, in which the z and y dimensions of the unit cell are substantially similar to one another, is preferred to obtain good filling of the unit cell area by the inkjet droplets.

[0044] In certain embodiments, the ratio of z / y is between 0.6 and 1.5, more preferably between 0.8 and 1.2, and even more preferably the ratio of z / y is between 0.9 and 1.1.

[0045] In one embodiment, the coated substrate is a liner suitable for producing corrugated board. The coated substrate can be combined with a corrugated board to form a corrugated board, either before or after a printing process is performed to form an image on the coated substrate. The corrugated board can have one coated substrate on one side of the corrugated board or two coated liners bonded to the corrugated board with the corrugated board disposed between the coated liners.

[0046] In one embodiment, the printing method is performed on a coated liner that is not attached to a fluted liner (which may be a corrugated board or another corrugated media). This coated liner may be in sheets or rolls (webs) and is then further used to produce corrugated board. The printing method used is often referred to as a pre-print technique.

[0047] In one embodiment, the ink droplets are printed onto the liner surface after the board is manufactured, which has the advantage that the board has already been manufactured and the printed image is not damaged by the manufacturing steps that make the board.

[0048] In one embodiment, the ink droplet jetting distance is more than 2 mm. Jetting distances greater than 2 mm have already been used to assemble larger products such as corrugated board, and are suitable for printing on coating substrates that are not very flat while the coating substrate is transported along the inkjet printhead. It has been found that when the ink droplet jetting distance is greater than 2 mm, white streaks and bleeding problems may occur more frequently or occur on more coating substrates. The printing method according to the present invention can solve this problem even when the ink droplet jetting distance is greater than 2 mm.

[0049] In one embodiment, the inkjet printing step is carried out at a substrate transport speed of at least 0.5 m / s. This substrate transport speed is particularly suitable for printing on a substrate in a single pass printing process, where the substrate passes once through a print head station to form an image on the substrate. The substrate may be transported using a belt, a web or any other transport means.

[0050] In one embodiment, the inkjet printing step is followed by a thermal drying step that may ensure that the coated substrate and the ink on the coated substrate are dry before further processing the coated substrate and / or before inkjet printing ink droplets of additional ink colors onto the coated substrate.

[0051] In one embodiment, the inkjet printing method uses a piezoelectrically actuated printhead. Piezoelectrically actuated printheads are preferred due to their durability, control of ink drop size (grayscale capability) and flexibility to print a variety of inks, including more viscous inks. Printhead technology is known to be used in corrugated printing. It should be noted that UV inkjet is only used in combination with piezoelectric printheads, as thermal printheads cannot handle the reactive monomers present in UV inks. In this embodiment, the method uses a drop-on-demand (DOD) piezo head for water-based inks. Thermal heads may be less preferred due to lack of grayscale printing (different drop sizes depending on waveform, as available with piezo printheads), scorching issues, short life span and low commercial availability in industrial print quality (small office and home office printing mainly uses thermal heads, but not industrial quality). Additionally, commercial industrial thermal heads are available from Memjet, but the use of pigmented inks remains limited. For piezo DOD printheads, there are numerous variations that exist in this field, including MEMS-based, bulk piezo, resolution, (non-)distribution technology, variations in digital or binary waveforms, variations in jetting frequency, etc. In this embodiment of the invention, there is no limitation to the sub-category of piezo. Fujifilm, Konica Minolta, Xaar, Kyocera, Xerox, Seiko Holdings, and Ricoh are all manufacturers offering this type of printhead.

[0052] In one embodiment, the inkjet printing step includes printing at least four inkjet printing colors including cyan, magenta, yellow, and black.

[0053] In one embodiment, the method includes a step of thermally drying the coated substrate before, during or after printing the ink droplets onto the coated substrate, which may be preferred when the ink has a slow drying time, such as when the ink is absorbed slowly into the substrate.

[0054] The accompanying drawings are used to illustrate presently preferred, non-limiting, exemplary embodiments of the device of the present invention. The above and other advantages of the features and objects of the present invention will become more apparent and the invention will be better understood from the following detailed description, read in conjunction with the accompanying drawings, in which: [Brief description of the drawings]

[0055] [Figure 1] FIG. 1 is a schematic diagram of a test chart for testing image quality. [Figure 2A] 2A-2E are schematic diagrams of an embodiment of a printing system for carrying out a printing method according to the present invention. [Figure 2B] 2A-2E are schematic diagrams of an embodiment of a printing system for carrying out a printing method according to the present invention. [Figure 2C] 2A-2E are schematic diagrams of an embodiment of a printing system for carrying out a printing method according to the present invention. [Figure 2D] 2A-2E are schematic diagrams of an embodiment of a printing system for carrying out a printing method according to the present invention. [Figure 2E] 2A-2E are schematic diagrams of an embodiment of a printing system for carrying out a printing method according to the present invention. [Diagram 3] FIG. 3 is a schematic diagram of a unit cell according to the present invention.

[0056] [Detailed Description] As used herein, the term "dispersion" refers to a two-phase system in which one phase consists of finely divided particles (often in the colloidal size range) distributed throughout a bulk substance, the particles being the dispersed or internal phase, and the bulk substance being the continuous or external phase.

[0057] As used herein, the term "dispersant" refers to a surfactant added to a suspending medium to promote uniform and maximum separation of very fine solid particles, often of colloidal size. In the case of pigments, the dispersant is most often a polymeric dispersant, and the dispersant and pigment are usually combined using a dispersing device.

[0058] As used herein, the term "aqueous" refers to water or a mixture of water and at least one water-soluble or partially water-soluble organic solvent (co-solvent).

[0059] As used herein, the term "water-based primer composition" refers to a primer composition having a carrier of water or a mixture of water and at least one water-soluble or partially water-soluble organic solvent (co-solvent, wetting agent and / or penetrant).

[0060] As used herein, the term "substantially" means to a large extent, almost entirely.

[0061] The materials, methods, and examples provided herein are illustrative only and, unless expressly stated, are not intended to be limiting.

[0062] (Primer composition) The pigment flocculant is not particularly limited as long as it can flocculate the pigment contained in the inkjet ink.Some suitable pigment flocculants include ionic polymers and copolymers, such as anionic polymers, anionic copolymers, cationic polymers and cationic copolymers, polyvalent metal salts, and acids, such as organic acids and inorganic acids.Only one type of pigment flocculant can be used, or two or more types of pigment flocculants can be combined.

[0063] A way to characterize a pigment flocculant is to add a drop containing the pigment to a test tube containing the primer, causing it to precipitate / flocculate / solidify rather than spreading throughout the rest of the liquid (appearing dissolved) via Brownian motion.

[0064] Cationic polymers and copolymers in primer compositions attract and fix the oppositely charged anionic pigment dispersions and anionic binder molecules to the substrate. Such cationic resins incorporate charged groups as side groups on the main polymer backbone or polymer chain, but generally contain quaternary ammonium groups, so that a formal positive charge exists regardless of the pH level. Cationic polymers containing quaternary ammonium, sulfonium or phosphonium groups are also synthesized. Weak electrolyte versions have been used that acquire cationic properties in acidic media and are based on polyamines or mixtures thereof containing primary, secondary or tertiary amino groups. Preparation techniques often include polymerization by chain growth and step-growth mechanisms in simple aqueous solutions, but also as water-in-oil emulsions, and modification of existing polymers.

[0065] Cationic polymers for use in the primer coating may include, but are not limited to, polymers and copolymers of diallyldialkammonium monomers, such as diallyldimethylammonium chloride, e.g., polydiallyldimethyl-ammonium chloride (PDADMAC), polymers and copolymers of cationic acrylates and acrylamides, such as polyacryloxyethyldimethylammonium chloride or polyacrylamidoethyldimethylammonium chloride, polymers and copolymers of quartemized vinylpyridines, such as polymethylvinylpyridine chloride, polymers and copolymers of polyalkylamines and quaternary ammonium, linear and branched polyethyleneimines, polyvinylamines, and epichlorohydrin-derived polymers and copolymers, such as epihalohydrin-amine polymers; "Multivalent" indicates two or more oxidation states, typically for the element "Z". 2+ , Z 3+ , Z 4+ For simplicity, multivalent cations are referred to herein as Z x The multivalent cation is substantially soluble in the aqueous primer solution and preferably exists (in solution) in a substantially ionized state.

[0066] Z x The primer composition includes, but is not limited to, at least one multivalent cation of the following elements: Mg, Ca, Sr, Ba, Sc, Y, La, Ti, Zr, V, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Cu, Au, Zn, Al, Ga, In, Sb, Bi, Ge, Sn, Pb. Preferably, the multivalent cation is calcium. The primer composition is preferably based on acceptable regulatory and toxicological guidelines.

[0067] Z x can be incorporated into the primer solution by addition in the form of a salt or an alkaline form and used as a base for adjusting the pH of the primer solution.

[0068] The relevant anionic material can be selected from any common anionic material, especially halides, nitrates and sulfates. The anionic form is selected so that the polyvalent cation is soluble in the aqueous primer solution. The polyvalent cation salt can be used in its hydrated form. One or more polyvalent cation salts can be used in the primer solution. In the case of calcium, the preferred polyvalent cation salts are calcium chloride, calcium nitrate, calcium nitrate hydrate and mixtures thereof.

[0069] The acid as the flocculating agent lowers the pH of the ink and coagulates the pigment dispersion and other ink components, thereby causing the ink droplets to precipitate. Specific examples of the acid include polyacrylic acid, acetic acid, glycolic acid, malonic acid, maleic acid, ascorbic acid, succinic acid, glutaric acid, fumaric acid, citric acid, tartaric acid, lactic acid, phosphorous acid, sulfonic acid, orthophosphoric acid, pyrrolidone carboxylic acid, pyrone carboxylic acid, pyrrole carboxylic acid, furan carboxylic acid, pyridine carboxylic acid, coumaric acid, thiophene carboxylic acid, nicotinic acid, and derivatives of these compounds. Polyacrylic acid, citric acid, and acetic acid are particularly preferred.

[0070] Alternatively, if the dye is cationic or the pigment is primarily stabilized by cationic functionality, the primer may be comprised of an anionic flocculant. The anionic flocculant may be an inorganic base or a polymer with anionic functionality. Examples of anionic flocculants include Zetag 4145 (BASF), Magnafloc LT30 (BASF), and SETAQUA® 6407 (Allnex). The primer composition may also include colorants, organic solvents, wetting agents, binders, surfactants, biocides, corrosion inhibitors, viscosity modifiers, and other ingredients as described below for the inkjet ink.

[0071] Also, commercially available primer compositions can be used as long as they agglomerate the pigment particles. Commercially available primers are available, for example, as 171236PX from Michelmann, TP unicoat UCB and TP WB unicoat LC, both from Siegwerk.

[0072] The viscosity of the primer solution is adjusted according to the application technique, but is preferably between 1 mPa.s and 200.0 mPa.s, even more preferably between 3.0 mPa.s and 50 mPa.s, determined at room temperature.

[0073] (Water-based ink) Water-based inks can be broadly classified into pigment dispersion inks and dye inks. In recent years, there has been an increasing demand for pigment dispersion inks that have excellent color development and exhibit solvent resistance, gas resistance, and (UV-) light resistance. On the other hand, in the case of pigment water-based dispersion inks, satisfactory pigment dispersibility is often not obtained because the pigment is insoluble in water. Therefore, in order to maintain favorable pigment dispersibility in water-based inks, pigment dispersing resins are used to achieve better dispersion stability of the pigment in water. These pigments are also thought to perform better against migration into food.

[0074] The use of colorants in the ink as described above is the most important form of water-based ink. However, to prevent the ink from drying out at the nozzle, water-based inks used in inkjet printing processes typically also contain a water-soluble solvent with a high boiling point and favorable solubility in water. This type of solvent is considered a humectant in water-based inks.

[0075] Additionally, aqueous inks used in inkjet printing processes typically also contain one or more surfactants to enable minimal wetting and spreading of the aqueous ink in the printhead, on the substrate, etc.

[0076] Finally, the aqueous ink composition may also contain various types of additives, such as defoamers, thickeners, binders, and preservatives, if necessary, which make the aqueous ink composition more suitable for use as an inkjet ink.

[0077] (Coloring agent) (dye) The colorant in the ink may be a pigment, a dye, or a combination thereof. The colorant may be present in the ink in an amount of from 0.5% to 20% by weight.

[0078] The colorant may be a dye. The dye may be a mixture of nonionic, cationic, anionic, or nonionic, cationic and / or anionic dyes. Specific examples of dyes that may be used include, but are not limited to, sulforhodamine B, acid blue 113, acid blue 29, acid red 4, rose bengal, acid yellow 17, acid yellow 29, acid yellow 42, acridine yellow G, acid yellow 23, acid blue 9, nitro blue tetrazolium chloride monohydrate or nitro BT, rhodamine 6G, rhodamine 123, rhodamine B, rhodamine B isocyanate, safranine O, azure B, and azure B eosinate, all available from Sigma-Aldrich Chemical Company (St. Louis, Missouri). Examples of anionic water-soluble dyes include, but are not limited to, Direct Yellow 132, Direct Blue 199, Magenta 377 (available from Ilford AG, Switzerland) alone or in combination with Acid Red 52. Examples of non-water-soluble dyes include azo, xanthene, methine, polymethine and anthraquinone dyes. Specific examples of non-water-soluble dyes include Orasol® Blue GN, Orasol® Pink, Orasol® Yellow dyes available from Ciba-Geigy Corp. Black dyes include, but are not limited to, Direct Black 154, Direct Black 168, Fast Black 2, Direct Black 171, Direct Black 19, Acid Black 1, Acid Black 191, Mobay Black SP, Acid Black 2.

[0079] (Pigments) The pigment is preferably used from the viewpoint of providing excellent water resistance, light resistance, weather resistance, gas resistance, etc. Examples of pigments that can be used in the present invention include conventional organic pigments and inorganic pigments.

[0080] The pigments may be selected from those disclosed by HERBST, W. et al., Industrial Organic Pigments, Production, Properties, Applications. 2nd edition. vch, 1997.

[0081] The pigment particles in pigmented inkjet inks should be small enough to allow the ink to flow freely through the inkjet printing device, especially at the ejecting nozzles. It is also desirable to use small particles to maximize color strength and retard settling.

[0082] The average pigment particle size in the pigment inkjet ink should be between 0.005 μm and 15 μm. Preferably, the average pigment particle size is between 0.005 μm and 5 μm, more preferably between 0.005 μm and 1 μm, particularly preferably between 0.005 μm and 0.3 μm, and most preferably between 0.040 μm and 0.150 μm. Larger pigment particle sizes may be used as long as the objectives of the present invention are achieved.

[0083] The pigment is used in the pigment inkjet ink in an amount of 0.1 to 20% by weight, preferably 1 to 10% by weight, based on the total weight of the pigment inkjet ink.

[0084] Examples of cyan pigments that can be used in the present invention include CI Pigment Blue 1, 2, 3, 15:3, 15:4, 15:6, 16 and 22, and CI Vat Blue 4 and 6. Of these, CI Pigment 15:3 and 15:4 are preferred. These cyan pigments may be used individually or in combination of two or more pigments.

[0085] Examples of magenta pigments that can be used in the present invention include C.1. Pigment Red 5, 7, 12, 22, 23, 31, 48(Ca), 48(Mn), 49, 52, 53, 57(Ca), 57:1, 112 and 122, Quinacridone Solid Solution 146, 147, 150, 185, 238, 242, 254, 255, 266 and 269, and Cl Pigment Violet 19, 23, 29, 30, 37, 40, 43 and 50. Among these, it is preferable to use one or more pigments selected from the group consisting of CI Pigment Red 122, 150, 155, 185, 266 and 269, and CI Pigment Violet 19.

[0086] Examples of yellow pigments that can be used in the present invention include CI Pigment Yellow 10, 11, 12, 13, 14, 17, 20, 24, 74, 83, 86, 93, 94, 95, 109, 110, 117, 120, 125, 128, 137, 138, 139, 147, 148, 150, 151, 154, 166, 168, 180, 185 and 213. Among these, the use of one or more pigments selected from the group consisting of CI Pigment Yellow 13, 14, 74, 150, 155 and 185 is preferred.

[0087] Examples of black pigments that can be used in the present invention include organic pigments such as aniline black, lumogen black, and azomethine black, and inorganic pigments such as carbon black and iron oxide. Furthermore, a plurality of color pigments such as the above-mentioned yellow pigment, magenta pigment, and cyan pigment may be mixed and used as a black pigment.

[0088] There is no particular limitation on the inorganic pigment that can be used in the present invention. An example of an inorganic pigment other than the above-mentioned carbon black and iron oxide is titanium oxide.

[0089] Examples of carbon black pigments that can be used in the present invention include carbon blacks produced using the furnace process or the channel process.

[0090] Examples of commercially available products are listed below, and any of these products can be preferably used.

[0091] Specific examples include No. 33, 40, 45, 52, 900, 2200B, 2300, MA7, MA8 and MCF88 (all manufactured by Mitsubishi Chemical Corporation), RAVEN 1255 (manufactured by Columbian Chemicals Co., Inc.), REGAL 330R, 400R and 660R and MOGUL L (all manufactured by Cabot Corporation), and Nipex 1601Q, Nipex 1701Q, Nipex 75, Printex 85, Printex 95, Printex 90, Printex 35 and Printex U (all manufactured by Orion Engineered Carbons LLC).

[0092] In this embodiment of the present invention, the pigment is not limited to the above pigments, and other special colors such as orange pigments and green pigments can also be used. Furthermore, multiple pigments may be combined. Furthermore, in another embodiment, the aqueous ink composition of this embodiment of the present invention may be used as an ink set in combination with a clear ink that does not contain a pigment.

[0093] Any other pigments and / or dyes that are useful for modifying the color of the ink can be used. Additionally, colorants can include white pigments such as titanium dioxide or other inorganic pigments such as zinc oxide and iron oxide.

[0094] (Pigment dispersion resin) In order to be able to use the above pigments in an aqueous ink composition, the pigment must be able to be stably dispersed in water and then stably retained within the dispersion. Known methods can be used to disperse the pigment. A first example includes a method in which dispersion is achieved using a dispersing resin. The resin is crosslinked around the pigment surface, thereby resulting in an encapsulated pigment (EP 2896666 (Riso Kagaku Corporation), EP 1838427 (FFIC), EP 3275949 (Kao Corporation), US 20140011941 (DuPont), US 9267044 (FFIC), (Seiko Epson Corporation), EP 2252664 (Xerox)). Dispersing resins or dispersants are substances that promote the formation and stabilization of a dispersion of pigment particles in a dispersion medium. Dispersants are generally surface-active materials with an anionic, cationic or non-ionic structure. The presence of a dispersant substantially reduces the required dispersion energy. Dispersed pigment particles may have a tendency to re-agglomerate after the dispersion operation due to mutual attractive forces. The use of a dispersant also counteracts this tendency of the pigment particles to re-agglomerate.

[0095] Polymeric dispersants contain so-called anchor groups in the part of the molecule that adsorb to the pigment to be dispersed. In a spatially separated part of the molecule, the polymeric dispersant has a polymer chain protruding, which adapts, i.e. stabilizes, the pigment particle to the dispersion medium.

[0096] Examples of polymer dispersants include block copolymers, random copolymers, and salts thereof. The polymer dispersant may have a structure derived from two or more monomers, styrene, styrene derivatives, vinylnaphthalene derivatives, acrylic acid, acrylic acid derivatives, maleic acid, maleic acid derivatives, itaconic acid, itaconic acid derivatives, fumaric acid, and fumaric acid derivatives. Only one type of polymer dispersant may be used, or two or more types of polymer dispersants may be used.

[0097] The properties of polymeric dispersants depend on both the nature of the monomers and their distribution in the polymer. Polymeric dispersants obtained by polymerizing monomers randomly (e.g. monomers A and B polymerized ABBAABAB) or by polymerizing alternating monomers (e.g. monomers A and B polymerized ABABABAB) generally result in poor dispersion stability. Improved dispersion stability has been obtained by using graft copolymer and block copolymer dispersants. Graft copolymer dispersants consist of a polymer backbone with side chains attached to the backbone. CA2157361 (DU PONT) discloses pigment dispersions made with graft copolymer dispersants having a hydrophobic polymer backbone and hydrophilic side chains.

[0098] Other graft copolymer dispersants are disclosed in US Pat. No. 6,652,634 (LEXMARK), US Pat. No. 6,521,715 (DU PONT) and US Patent Publication No. 2004102541 (LEXMARK).

[0099] Block copolymer dispersants containing hydrophobic and hydrophilic blocks have been disclosed in many ink-jet ink patents.

[0100] U.S. Pat. No. 5,859,113 (DU PONT) discloses AB block copolymer dispersants having a polymeric A segment of polymerized glycidyl (meth)acrylate monomer reacted with an aromatic or aliphatic carboxylic acid and a polymeric B segment of polymerized alkyl (meth)acrylate monomer, hydroxyalkyl (meth)acrylate monomer having 1 to 12 carbon atoms in the alkyl group.

[0101] U.S. Pat. No. 6,413,306 (DU PONT) discloses ABC block copolymer dispersants having a polymeric A segment of polymerized alkyl (meth)acrylate monomers, aryl (meth)acrylate monomers, cycloalkyl (meth)acrylate monomers having 1 to 12 carbon atoms in the alkyl group, a polymeric B segment of polymerized alkylaminoalkyl (meth)acrylate monomers having a quaternary alkyl group, and a polymeric C segment of polymerized hydroxyalkyl (meth)acrylate monomers.

[0102] The design of polymeric dispersants for inkjet inks is discussed in SPINELLI, Harry J.. Polymeric Dispersants in Ink Jet Technology. Advanced Materials. 1998, vol. 10, no. 15, p. 1215-1218.

[0103] Alternatively, dispersion can be achieved using surfactants, such as water-soluble surfactants and / or water-dispersible surfactants. Other methods include chemically and / or physically introducing hydrophilic functional groups onto the surface of pigment particles, allowing the pigment to be dispersed and / or dissolved in water without the use of dispersants or surfactants. In an embodiment of the present invention, certain pigment dispersing resins, described below, are used as a method of dispersing pigments using pigment dispersing resins.

[0104] When using a pigment dispersing resin, the pigment and the pigment dispersing resin are preferably used in the form of a pigment dispersion obtained by dispersing the pigment and the pigment dispersing resin. There is no particular restriction on the method used to produce the pigment dispersion, but one exemplary method is described below. First, the pigment is added to an aqueous medium prepared by mixing the pigment dispersing resin and water, and then mixing and stirring are performed. The resulting mixture is then dispersed using a dispersing device to obtain a pigment dispersion. Thereafter, a centrifugation treatment or a filtration treatment may be performed as necessary. Any wet dispersing device can be used as the dispersing device, and among various devices, it is preferable to use a bead mill.

[0105] (Surfactant) The inkjet ink according to the present invention may comprise at least one surfactant. The surfactant(s) may be anionic, cationic, non-ionic or zwitterionic and are typically added in a total amount of less than 10 wt.-% based on the total weight of the pigmented inkjet ink, in particular less than 5 wt.-% based on the total weight of the pigmented inkjet ink.

[0106] Surfactants suitable for the inkjet ink according to the present invention include silicone, acrylic and fluorosurfactants, fatty acid salts, ester salts of higher alcohols, alkylbenzenesulfonates, sulfosuccinates and phosphates of higher alcohols, ethylene oxide adducts of higher alcohols, ethylene oxide adducts of alkylphenols, ethylene oxide adducts of polyhydric alcohol fatty acid esters and acetylene glycols and their ethylene oxide adducts. Commercially available examples include Byk-348, Byk-347, Byk 3450, Dynwet 800 (Byk Chemie Gmbh), Surfynol 104, Surfynol 465, Metolat 364, Dynol 800, Dynol 960, (Evonik Industries), KF-640, KF-642 (Shin-Etsu Chemical Co., Ltd.), ID-40, ID-70 (Sanyo Chemical Industries, Ltd.), etc.

[0107] (Wetting agent) As long as the effects of the present invention can be obtained, the type of organic solvent is not particularly limited. The organic solvent is preferably water-soluble from the viewpoint of increasing compatibility with water. Examples of water-soluble organic solvents include alcohols, polyhydric alcohols, amines, amides, glycol ethers, 1,2-alkanediols, etc. Only one type of organic solvent may be used, or two or more types of organic solvents may be used. ...It should be noted that some wetting agents, such as diethylene glycol monobutyl ether, ethylene glycol monobutyl ether, ethylene glycol isopropyl ether, and 1,2-hexanediol, also have surface tension active properties, thereby reducing the surface tension of the ink.

[0108] Examples of the alcohols include methanol, ethanol, propanol, isopropanol, butanol, isobutanol, secondary butanol, and tertiary butanol.

[0109] Examples of the polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol having five or more ethylene oxide groups, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol having four or more propylene oxide groups, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, and thiodiglycol.

[0110] Examples of the above amines include ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, and tetramethylpropylenediamine.

[0111] Examples of the amides include formamide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0112] Examples of the glycol ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether.

[0113] Examples of 1,2-alkanediols include 1,2-propanediol, 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, and 1,2-heptanediol.

[0114] Among these, when the organic solvent is a polyhydric alcohol, bleeding during high speed printing can be preferably suppressed. Preferred examples of the polyhydric alcohol include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol.

[0115] The content of the organic solvent in the printing ink can be, for example, in the range of 5% by weight to 60% by weight.

[0116] (binder resin) In one embodiment, the aqueous ink composition of the present invention preferably also includes a binder resin. Known binder resins for aqueous ink compositions include water-soluble resins and resin microparticles (emulsion / latex). Examples of types of resins that can be used as resin microparticles include acrylic, styrene / acrylic, urethane, styrene / butadiene, vinyl chloride, and polyolefin resins.

[0117] (Biocide) Suitable biocides for the pigmented inkjet ink of the present invention include sodium dehydroacetate, 2-phenoxyethanol, sodium benzoate, sodium pyridinethione-1-oxide, ethyl p-hydroxybenzoate, 2-methyl-1,2-thiazol-3-one and 1,2-benzisothiazolin-3-one and salts thereof. The biocides are preferably added in an amount of 0.001 to 3% by weight, more preferably 0.01 to 1.00% by weight, based on the total weight of the pigment inkjet ink.

[0118] (Other ingredients) In the ink for inkjet printing, various known additives such as polysaccharides, viscosity modifiers, resistivity modifiers, film-forming agents, ultraviolet absorbers, antioxidants, color-fading inhibitors, fungicides, rust inhibitors, and pH adjusters may be suitably selected and used in addition to the above-mentioned components, depending on the purpose of improving all performances such as ejection stability, suitability of the print head or ink cartridge, storage stability, and image storage stability. Examples of known additives include oil droplet fine particles such as liquid paraffin, dioctyl phthalate, tricresyl phosphate, and silicone oil, as disclosed in JP-A-57-74193, JP-A-57-87988, and JP-A-62-26. Examples of such agents include ultraviolet absorbers described in JP-A-1476, anti-fading agents described in JP-A-57-74192, JP-A-57-87989, JP-A-60-72785, JP-A-61-146591, JP-A-1-95091, and JP-A-3-13376, and fluorescent brighteners described in JP-A-59-42993, JP-A-59-52689, JP-A-62-280069, JP-A-61-242871, and JP-A-4-219266, such as fluorescent agents, tracer molecules for security and counterfit, and IR absorbing molecules for further enhancing drying or security purposes.

[0119] (Preparation of Pigment Inkjet Inks) Pigmented inkjet inks according to the present invention can be prepared by precipitating or grinding the pigment in a dispersion medium in the presence of a dispersant, or simply by mixing self-dispersed color pigments into the ink (similar to the preparation of dye-based inkjet inks).

[0120] The mixing equipment can include a pressure kneader, an open kneader, a planetary mixer, a dissolver and a Dalton Universal Mixer. Suitable grinding and dispersion equipment is a ball mill, a pearl mill, a colloid mill, a high-speed disperser, a two-roller, a bead mill, a paint conditioner and a three-roller. The dispersion can also be prepared using ultrasonic energy.

[0121] Methods for preparing very fine dispersions of pigments are disclosed, for example, in U.S. Pat. No. 5,679,138 (KODAK), U.S. Pat. No. 5,538,548 (BROTHER INDUSTRIES, LTD.), U.S. Pat. No. 5,443,628 (VIDEOJET SYSTEMS), U.S. Pat. No. 4,836,852 (OLIVETTI), U.S. Pat. No. 5,285,064 (EXTREL), U.S. Pat. No. 5,184,148 (CANON INC.), and U.S. Pat. No. 5,223,026 (XEROX).

[0122] After milling is complete, the milling media is separated from the milled particulate product (in dry or liquid dispersion form) using conventional separation techniques such as filtration, screening through a mesh screen, etc. Often, for example in bead mills, a screen is built into the mill. The milled pigment concentrate is preferably separated from the milling media by filtration.

[0123] Generally, it is desirable to produce color inks in the form of concentrated mill grinds and then dilute them to the appropriate concentration for use in inkjet printing systems. This technique allows for a larger amount of pigmented ink to be prepared from the device. If the mill grind is made with a solvent, it is diluted to the appropriate concentration with water and optionally other solvents. If made with water, it is diluted with additional water or water-miscible solvents to produce a mill grind of the desired concentration. Dilution adjusts the ink to the desired viscosity, color, hue, saturation density and print area coverage for a particular application.

[0124] Inkjet inks are prepared by mixing the dispersion and the components using a conventional mixing device. The method of stirring and mixing is not particularly limited, and can be appropriately selected as needed, for example, using a homogenizer, a paint shaker, an ultrasonic disperser, a stirrer using a conventional stirring blade, a magnetic stirrer, and a high-speed disperser. The ink is finally filtered before use. In many cases, when the drop size is less than 10 pl and the printhead nozzle is less than 20 microns, the filtration is performed at less than 1 / 10 of the nozzle size (2 microns), and significantly less for dye-based inks. It is important to prevent fine particles from reaching the nozzle, since a single failure can lead to a very costly replacement of the entire printhead.

[0125] A wide variety of inkjet ink chemistries are used, and several different filtration techniques must be used to obtain optimal results. For a typical dye-based ink, multiple stages of filtration are used after mixing of the colorant and carrier, and after the addition and dilution of additives. The purpose is to remove any insoluble elements, particles, environmental contaminants, and biological materials. The choice of filter is critical, especially when multiple stages of filtration are used. For pigmented inks, multiple stages of filtration are typically used after making the dispersion and again after the addition and dilution of additives. The main purpose here is the removal of any oversized or agglomerated pigment from the dispersion, as well as oversized particles and contaminants from other processes.

[0126] Available filter technologies have different applications, advantages and disadvantages. Examples of filters include membrane, depth and hybrid filter types. Common suppliers of filters for inkjet inks are Nippon Pall Corporation, Porvair and Membrane Solutions.

[0127] (Preparation of primer solution) The primer is produced by dissolving all water-soluble components in an aqueous medium, adding other components, and further stirring and mixing as necessary. The method of stirring and mixing is not particularly limited, and can be appropriately selected as necessary, for example, using a homogenizer, a paint shaker, an ultrasonic disperser, a stirrer using a normal stirring blade, a magnetic stirrer, or a high-speed disperser.

[0128] (Coating substrate) Coated substrates are generally known to be less receptive to aqueous inkjet inks than uncoated papers. These coated substrates or coated papers may have low surface porosity due to calendaring and / or application of one or more hydrophobic coating layers. The resulting low porosity means fewer flow paths for the ink vehicle to access, which leads to greater reliance on evaporation for ink drying. Furthermore, the hydrophobicity of the coating layer reduces the wetting and spreading of aqueous inks during printing, which in turn can cause ink droplets to puddle on the media surface. The combined effect of reduced dot spreading and slower drying results in more image defects when printing aqueous inks directly onto coated substrates.

[0129] (Printing process / printing method) The printing process uses a primer application step followed by an inkjet printing step. The primer application can be done in-line or off-line in an analog manner using rollers, flexographic plates, offset plates, curtain coating, etc., but also digitally using inkjet printheads, either piezo or inkjet, and possibly piston-based inkjet, as commercially available from Valvejet. The inkjet step can be carried out using any type of printhead, such as thermal, piezo, continuous inkjet, etc. With a printhead like Valvejet, the image generation is less likely. Between these (at least two) steps (primer application step, followed by inkjet printing step) one or more drying steps can be incorporated. The primer step and the inkjet printing step can be immediately followed by another inkjet printing step, but also new primer and inkjet printing steps can be used. The invention is not limited to a specific set-up or to a specific drying means, as long as the primer application step is followed by an inkjet printing step. The drying means can be of any type related to the inkjet ink technology or primer technology used. The drying means can be heat-based (hot air, infrared, near infrared or a combination thereof) or actinic radiation-based, and the drying intensity or volume need not be the same for the primer composition and the inkjet ink. The print can also be dried using an additional final drying step of any type and intensity.

[0130] 2A-2E show some options regarding printing methods, however the invention is not limited to these options.

[0131] FIG. 2A is a schematic diagram of a printing apparatus according to a first embodiment, comprising a transport belt 100 for moving a coating substrate 200, a primer application station 120, a first print head station 150, a first drying station 160, a second print head station 250 and a second drying station 260. The transport belt 100 is configured to continuously move the coating substrate 200 along the primer application station 120, the first print head station 150, the first drying station 160, the second print head station 250 and the second drying station 260. The primer application station 120 applies a primer composition to the coating substrate 200. The primer composition may be dry or still wet when the coating substrate 200 arrives at the first print head station 150. The first print head station 150 deposits inkjet droplets of a first ink on the substrate at a printing resolution and droplet size according to the present invention. The first drying station 160 dries the coating substrate with the first ink. A second printhead station 250 deposits inkjet droplets of the second ink onto the substrate at the printing resolution and droplet size according to the present invention. A second drying station 260 dries the coated substrate including the first and second ink images.

[0132] Optionally, the coating substrate is preheated prior to the primer application step, either in-line or by offline methods.

[0133] Optionally, the first printhead station 150 can deposit inkjet droplets of a first ink and a second ink, such as cyan and magenta, to a substrate at a printing resolution and drop size in accordance with the present invention, and the second printhead station 250 can deposit inkjet droplets of a third ink and a fourth ink, such as yellow and black, to a substrate at a printing resolution and drop size in accordance with the present invention.

[0134] Furthermore, the printing apparatus may include additional printhead stations for printing additional inks onto the coating substrate 200 and drying stations for drying the additional inks on the coating substrate 200. In a preferred example, the setup as shown in FIG. 2A further holds a third printhead station and drying station and a fourth printhead station and drying station. In a more preferred example, each printhead station holds multiple printheads printing the same color (eventually arranged in a staggered fashion to allow seamless stitching). In a most preferred example, the color printing order of the inkjet inks after priming is yellow, then cyan, then magenta, then black.

[0135] 2B is a schematic diagram of a printing apparatus according to a second embodiment, comprising a conveyor belt 100 for moving a coating substrate 200, a first primer application station 120, a first primer drying station 130, a first print head station 150, a first drying station 160, a second primer application station 220, a second primer drying station 230, a second print head station 250 and a second drying station 260. The conveyor belt 100 is configured to continuously move the coating substrate 200 along the first primer application station 120, the first primer drying station 130, the first print head station 150, the first drying station 160, the second primer application station 220, the second primer drying station 230, the second print head station 250 and the second drying station 260. The first primer application station 120 and the second primer application station 220 are in the form of roller assemblies. In this embodiment, the first primer application station 120 applies a primer composition to the coating substrate upstream of the first print head station 150. In a next step, the primer composition is dried on the coating substrate by the primer drying station 130. The second primer application station 220 applies a new primer composition to the coating substrate upstream of the second print head station 250. In a next step, the primer composition is dried on the coating substrate by the second primer drying station 230. The primer compositions applied by the primer application stations 120, 220 may be the same or different.

[0136] FIG. 2C is a schematic diagram of a printing apparatus according to a third embodiment, comprising a transport belt 100 for moving a coating substrate 200, a first primer application station 120, a first primer drying station 130, a first print head station 150, a first drying station 160, a second primer application station 220, a second primer drying station 230, a second print head station 250 and a second drying station 260. The transport belt 100 is configured to move the coating substrate 200 continuously along the first primer application station 120, the first primer drying station 130, the first print head station 150, the first drying station 160, the second primer application station 220, the second primer drying station 230, the second print head station 250 and the second drying station 260. The first primer application station 120 is in the form of a print head, such as a piston-based inkjet print head. The second primer application station 220 is in the form of a roller assembly. The primer compositions applied by the primer application stations 120, 220 may be the same or different. In one example, the primer composition applied by the first application station 120 may have a lower viscosity than the primer composition applied by the second application station 120. Additionally, the print head 120 may apply the primer composition uniformly to the coating substrate or may apply the primer composition locally to the substrate, for example according to a digital image.

[0137] 2D is a schematic diagram of a printing apparatus according to a fourth embodiment, comprising a belt 100 for moving a coating substrate 200, a first primer application station 120, a first primer drying station 130, a first print head station 150, a first drying station 160, a second primer application station 220, a second primer drying station 230, a second print head station 250 and a second drying station 260. The transport belt 100 is configured to move the coating substrate 200 continuously along the first primer application station 120, the first primer drying station 130, the first print head station 150, the first drying station 160, the second primer application station 220, the second primer drying station 230, the second print head station 250 and the second drying station 260. The first primer application station 120 and the second primer application station 220 are each in the form of a print head, such as a piston-based inkjet print head. The print head 120, 220 may apply the primer composition uniformly to the coated substrate or may apply the primer composition locally to the substrate, for example according to a digital image, in this embodiment, active drying of the primer composition on the coated substrate is performed between the steps of applying the primer composition and depositing the inkjet droplets.

[0138] FIG. 2E is a schematic diagram of a printing apparatus according to a fifth embodiment, comprising a belt 100 for moving a coating substrate 200, a first primer application station 120, a first printhead station 150, a first drying station 160, a second primer application station 220, a second printhead station 250 and a second drying station 260. The transport belt 100 is configured to move the coating substrate 200 continuously along the first primer application station 120, the first printhead station 150, the first drying station 160, the second primer application station 220, the second printhead station 250 and the second drying station 260. The first primer application station 120 and the second primer application station 220 are each in the form of a printhead, such as a piston-based inkjet printhead. In this embodiment, there is no active drying of the primer composition on the coating substrate between the steps of applying the primer composition and depositing the inkjet droplets.

[0139] The print station may have a "printhead resolution" perpendicular to the substrate transport direction. The print station may comprise a single printhead having a length of at least the width of the desired print area. The print station may also be constructed by combining two or more inkjet heads such that the combined length of the individual inkjet heads covers the entire width of the print area. Additionally or alternatively, the printheads may be arranged in a staggered arrangement with a first row of printheads and a second row of printheads staggered relative to the first row of printheads. Such staggered arrangements of printheads are generally known in the art. The staggered arrangement provides a page-width array of nozzles that are substantially equidistant along the length of the printhead station (perpendicular to the substrate transport direction T). The staggered arrangement may provide nozzle redundancy in the areas where the first and second rows of inkjet heads overlap. Further, staggering may be used to reduce the nozzle pitch along the length of the print station (and thus increase the "printhead resolution"), for example by positioning the inkjet heads in the second row such that the positions of the nozzles of the inkjet heads in the second row are offset along the length of the inkjet marking device by half the nozzle pitch (nozzle pitch is the distance between adjacent nozzles in an inkjet head), and the "printhead resolution" of the print station can be further increased by using more rows of inkjet heads arranged such that the positions of the nozzles in each row are offset longitudinally relative to the positions of the nozzles in every other row.

[0140] (Measurement method) (Liquid absorption) The absorption rate of the coating substrate is measured by placing a 2 ml drop of the reference liquid on the substrate placed at an angle of 60° to the horizontal. The absorption rate measurement is performed at temperatures between 20°C and 25°C and at a relative humidity of 30-50%. This reference liquid composition is similar to a pigment-free ink and has a very low viscosity (20% by weight glycerol / 10% by weight 1,2-propanediol / 3% by weight diethylene glycol monobutyl ether / 0.13% by weight Surfynol 104 H, the remainder is water). Under gravity, the drop flows down the paper while the liquid is absorbed, and the time it takes to travel a certain distance is recorded. The longer the travel distance, the less the ink is absorbed into the substrate. The distance traveled by the drop is determined between the position where the drop was placed and the position after a certain time, and is measured in cm. The measurement time is typically 50 seconds after the drop is placed.

[0141] (Contact angle measurement) Contact angle measurements allow the measurement of the wetting of the ink on the substrate. Contact angle measurements were performed at temperatures between 20°C and 25°C and at relative humidity between 30% and 50%. The contact angle was measured by placing a 3 μl droplet of liquid ink on the substrate. The droplet is placed on the substrate using a microsyringe and a high-speed resolution camera records the outline of the spreading droplet (OCA 25, DataPhysics Instruments GmbH, Filderstadt GE). The contact angle reaches a steady value 2 seconds after the droplet is released from the syringe and first contacts the substrate. The average contact angle was calculated from the three measurements.

[0142] (surface tension) The surface tension was measured by the bubble pressure method using a surface tensiometer SITA Pro Line T15 (SITA Messtechnik Co., Dresden, Germany) at temperatures between 23.0 °C and 26.0 °C. The bubble lifetime used is 10 seconds, which is the time from the creation of a new air-liquid interface (the tip of the capillary immersed in the ink liquid) until the maximum bubble pressure is reached. The measured maximum pressure is automatically recalculated to the surface tension value of the liquid (mN / m) after calibration of the device with distilled water.

[0143] (Image quality evaluation) 1 is a schematic diagram of a test chart for testing image quality. The test chart comprises two yellow areas 110 and 130. The yellow areas 100, 110 are rectangular and have their longest dimension along the substrate transport direction T. The test chart comprises three cyan areas 210, 220, 230. The cyan areas 210, 220, 230 are rectangular and have their longest dimension perpendicular to the substrate transport direction T. In an overlay area 300 the yellow and cyan areas overlap.

[0144] After printing the image of FIG. 1, the cyan and yellow overlay areas 300 were evaluated for bleeding by visual inspection. 1: Little to no visible breathing 2: Acceptable breathing 3: Unacceptable breathing

[0145] (Muscle evaluation) After printing the image of FIG. 1, the cyan areas 210, 220, 230 (outside of the overlay area 300) were evaluated for streaks by visual inspection. 1. No or few visible streaks 2: Acceptable muscle tone 3: Unacceptable levels of streaks 4: Very disturbing muscle

[0146] [Example] Water-based inks C1-C5 were prepared by mixing the ingredients listed in Table 1 for 30 minutes. The amounts in Table 1 are the weight percent of the total weight of the water-based ink. An amount of water was added to bring the total to 100 weight percent. The inks were then filtered through a 1 μm filter. The viscosity of the inks was approximately 6 mPa.s, which is within the specifications for Kyocera's KJ4BYH 600 dpi and Fuji's Samba G3L 1200 dpi heads.

[0147] [Table 1]

[0148] APD1000: A pigment dispersion with a pigment concentration of 14%, available from Fujifilm Corporation. Surfynol 104 (acetylenic diol surfactant) available from Evonik Industries; Surfynol 465, available from Evonik Industries; Byk 348 (a polyether modified polysiloxane surfactant) available from Byk additives; Metoloat 364 (non-ionic surfactant) available from Munzing; Dynol 960, available from Evonik Industries; Joncryl J8050E, available from BASF; Yellow inks Y1, Y2, Y3, Y4, Y5 were prepared with the same composition as the corresponding C1, C2, C3, C4 and C5 inks, but using APD1000 yellow dispersion from Fujifilm Corporation.

[0149] The physical properties of the ink are summarized in Table 2.

[0150] [Table 2]

[0151] Substrate 1: Metsaboard pro WKL 135gsm coated substrate from Metsaboard Substrate 2: Sappi Fusion 135gsm coated substrate from Sappi

[0152] From the table, it is clear that the contact angles on both substrates 1 and 2 are improved for C1-C3. At the same time, the static surface tension does not show a clear trend correlating with the contact angle of the substrate. Ink C5 has a small contact angle on both substrates 1 and 2, much smaller than inks C1-C4. This demonstrates that ink C5 has a high wetting behavior.

[0153] The absorbency properties of both substrates were tested by the drop migration distance test and are listed in Table 3.

[0154] [Table 3]

[0155] Substrate 3; Metsaboard bright 135gsm uncoated substrate from Metsaboard

[0156] From Table 3 it is clear that the uncoated substrate absorbs the reference solution much faster.

[0157] The inks were evaluated on an inkjet test rig equipped with a belt carrying the substrate.

[0158] The belt, and therefore the print speed, was set at 1 m / sec. The ink was loaded into a 600 dpi Kyocera KJ4B YH printhead available from Kyocera Corporation, purged and wiped until there were no bad nozzles. Figure 1 was printed at a print distance of 2.5 mm and evaluated for bleeding and streaking. Base material 1 Base material 2 Substrate 1 with primer Substrate 2 with primer

[0159] The primer solutions are prepared according to Table 4, where for primer solution P1, citric acid is the active coagulant, together with the other ingredients listed in the table below. For primer solution P2, the polyvalent metal ion compound dialuminium chloride pentahydroxide is the active coagulant, together with the other ingredients listed in the table below. For primer solution P3, the cationic polymer poly(diallyldimethylammonium chloride) having a Mw of 100.000 g / mol is the active coagulant, together with the other ingredients listed in the table below.

[0160] Further, distilled water is added to reach 100% by weight and finally the solution is stirred for at least 30 minutes at room temperature conditions.

[0161] [Table 4]

[0162] Top liner paper sheets, typically used in the manufacture of corrugated board, were treated with the primer solution using an automated wire bar applicator. The wire bar type (Elcometer 4360 / 4361 models) and coating speed were selected to achieve a wet coating weight of 10μm. The coated sheets were then dried in a vented oven at 100°C for 2 minutes to ensure that all water had evaporated. Finally, the treated liner was allowed to condition for a minimum of 10 minutes before inkjet printing.

[0163] This procedure was repeated with a 1200 dpi Samba G3L printhead available from Fujifilm Dimatix.

[0164] The printing resolution perpendicular to the substrate transport direction is determined by the distance between the nozzles of the printhead in this direction and is selected to be equal to the printing resolution of the printhead in the direction perpendicular to the substrate transport direction.

[0165] The test conditions and results are summarized in Table 5.

[0166] [Table 5] TIFF0007637125000006.tif28149

[0167] In Experiments 1 to 13 and the Comparative Experiments (Comparative Experiments 1 to 6), the primer composition used in the primer experiments was P1 (the composition shown in Table 4).

[0168] In Experiments 14-15 and 18, the primer composition used in the primer experiments was P2 (composition shown in Table 4).

[0169] In Experiments 16-17, the primer composition used in the primer experiments was P3 (composition shown in Table 4).

[0170] The ratio R in this table is defined as drop size (microns) (x) / printhead resolution (microns) (y).

[0171] From the table it is clear that streaks occur when the ratio R is too low (Comparative Experiments 1 to 4). Streaks occur even when the ink is formulated to enhance wetting properties (see Comparative Experiments 1 and 2 with C3 / Y3) or when a substrate with a smaller contact angle is chosen (Comparative Experiments 3 and 2).

[0172] From comparative experiment 5 it is clear that when an ink with poor wetting properties is used, the application of a primer further increases the streak level.

[0173] Experiments 1 to 6 show that increasing the ratio R improves the streak level, but at the same time causes bleeding. In particular, Experiments 1, 7, and 9 show that white streaks and bleeding occur at the same time.

[0174] Runs 1 to 6 further illustrate that a high ratio of R combined with applying a primer (P1) to the substrate can reduce bleeding without the white streaking problem reoccurring.

[0175] Experiments 7 to 11 show that for inks with poor wetting properties, the streak level is acceptable if some bleeding is already observed at ratios R>0.6. The bleeding can be overcome by applying a primer (P1).

[0176] Experiments 12 and 13 demonstrate that even with the highly wetting ink C5, a higher ratio R (>0.60) in combination with applying a primer (P1) to the substrate allows the use of a primer (P1) to reduce bleeding without the white streaking problem reoccurring. At the same time, comparative example 6 shows that a ratio R below 0.60 leads to streaks, even when a primer is used (ink C5 is designed with high wetting properties and a substrate 2 with a low contact angle is chosen).

[0177] Thus, the beneficial results of a ratio of x / y>0.60 are shown by Experiments 1-4, 7-9 and 12-13 to be substantially independent of the ink composition and wetting behavior of the ink.

[0178] Runs 1 to 4, compared to Runs 7 to 9, show that for the ink with improved wetting properties (C3 / Y3), the streak level is further improved at ratios above 0.7.

[0179] Experiments 14 and 15 demonstrate that the use of primer composition P2 (having a polyvalent metal ion compound as the solidifying agent) also provides the beneficial results shown for primer composition P1 in experiments 12 and 13. Furthermore, experiments 16 and 17 demonstrate that the use of primer composition P3 (having a cationic polymer compound as the solidifying agent) also provides beneficial results.

[0180] Moreover, experiment 18 demonstrates that the beneficial results shown in experiment 2 are also obtained when primer composition P2 (having a polyvalent ionic compound as a coagulant) is used instead of primer composition P1.

[0181] Thus, these experiments 12-18 show that the beneficial results of a ratio of x / y>0.60 are independent of the primer composition and in particular of the coagulant.

[0182] Those skilled in the art should understand that any block diagrams herein represent conceptual views of illustrative units or modules embodying the principles of the invention.

[0183] While the principles of the present invention have been described above with reference to specific embodiments, it should be understood that this description is made merely as an example and not as a limitation on the scope of protection determined by the appended claims.

Claims

1. applying a primer composition comprising at least one pigment aggregating agent to a coating substrate prior to an inkjet printing step of depositing ink using inkjet droplets; and performing the inkjet printing step, The inkjet printing step further comprises: Inkjet printing color cyan, Inkjet printing color magenta, Inkjet printing color yellow and Inkjet printing includes printing the color black; said inkjet printing step being performed using inkjet droplets having a droplet size of x μm; inkjet droplets are printed onto the coated substrate with a printing resolution corresponding to a rectangular unit cell having cell dimensions of y μm×z μm; the ratio of x / y is greater than 0.60; The droplet size, x μm, is determined as the average droplet size for a number of droplets, calculated by gravimetrically determining the average weight of the inkjet droplets by collecting a number of inkjet droplets, and calculating the droplet size by assuming an ideal spherical shape of the inkjet droplets using a known concentration of ink in the inkjet droplets; the y dimension is perpendicular to the substrate transport direction; wherein the inkjet printing step is carried out using a single pass of the substrate at a substrate transport speed of at least 0.5 m / s; The printing method, wherein the pigment flocculant is selected from the group consisting of ionic polymers, polyvalent metal salts, and mixtures thereof.

2. The printing method according to claim 1 , wherein the primer composition is a water-based primer composition.

3. A printing method as described in claim 1 or 2, wherein the ionic polymer is a cationic polymer.

4. The printing method according to any one of claims 1 to 3, wherein the inkjet printing step uses a pigmented water-based inkjet ink.

5. 5. The method of printing according to claim 1, wherein the coated substrate has an absorption rate defined by a drop migration distance of greater than 10.0 cm, the drop migration distance being measured by the distance traveled 50 seconds after a 2 ml drop of reference liquid is placed on the coated substrate at an angle of 60° to the horizontal.

6. The method of printing according to any one of claims 1 to 5, wherein the ink of the inkjet droplets has a static contact angle with the coated substrate of less than 26°.

7. 7. The method of printing of claim 6, wherein the ink of the inkjet droplets has a static contact angle of less than 26 degrees on various coated substrates.

8. The printing method according to any one of claims 1 to 7, wherein the ink of the inkjet droplets has a static surface tension of less than 32 mN / m.

9. 5. The printing method of claim 4, wherein the pigment used in the pigmented water-based inkjet ink is dispersed using an encapsulation dispersion technique and / or the pigment used in the pigmented water-based inkjet ink is dispersed using a block copolymer dispersant.

10. The printing method according to any one of claims 1 to 9, wherein the ratio of x / y is greater than 0.60 and less than 2.

0.

11. A printing method according to any one of claims 1 to 10, wherein the ratio of x / y is greater than 0.70 and less than 1.

5.

12. A method of printing according to any one of the preceding claims, wherein the ratio z / y is between 0.5 and 2.

0.

13. The printing method according to any one of claims 1 to 12, wherein the coated substrate is a liner suitable for producing corrugated board.

14. A printing method according to any one of the preceding claims, wherein the ratio x / y is greater than 0.

70.

15. The method of printing according to any one of the preceding claims, wherein the inkjet droplets are applied from a jetting distance of more than 2 mm.

16. A printing method according to any one of claims 1 to 15, wherein the ratio z / y is between 0.6 and 1.

5.

17. A printing method according to any one of claims 1 to 16, wherein the ratio z / y is between 0.9 and 1.

1.

18. A printing method according to any one of the preceding claims, comprising a thermal drying step of the coated substrate before, during or after the inkjet printing step.

19. A method of printing according to any preceding claim, wherein said inkjet printing step uses a piezoelectrically actuated printhead.

20. The printing method of claim 1, wherein the inkjet droplets are printed without actively thermally drying the primer composition on the coating substrate.

21. The method of printing of claim 1 , further comprising a thermal drying step after each inkjet printing color is applied by printing.

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