Inkjet ink, inkjet printing process and ink set for forming an image on a substrate and aqueous pigment dispersion for forming the inkjet ink

This application is a national stage filing under 35 U.S.C. 371 of pending International Application No. PCT/EP2021/077306, filed Oct. 4, 2021, which claims priority to Netherlands Patent Application No. 2026619, filed Oct. 5, 2020, the entirety of which applications are incorporated by reference herein.

The field of the invention relates to an inkjet ink, inkjet printing process and ink set of inkjet inks for forming an image on a substrate and aqueous pigment dispersion for forming the inkjet ink. The inkjet printing process for forming an image on a substrate uses an inkjet ink according to the present invention. The ink set of inkjet inks for forming an image on a substrate comprises an inkjet ink according to the present invention. The field of the invention further relates to an aqueous pigment dispersion for forming the inkjet ink according to the invention.

An inkjet printing process for forming an image on a substrate by applying a plurality of inkjet inks on the substrate is generally known. Inkjet inks for industrial printing applications primarily use pigments as colorants, except in the textile industry which often still uses reactive dye based inkjet inks. Pigments are solid materials that also remain as a solid in the inkjet ink carrier, in contrast to dyes which actually dissolve in the carrier medium. The pigment particles are finely dispersed in an ink and are of nanometer scale size. A typical inkjet printing process uses 3 or 4 colors of inkjet inks to create multicolor images. Typical colors are cyan (C), magenta (M), yellow (Y) and black (K). The CMY color inkjet inks are typically made up of organic pigments, wherein some contain a metal atom in a complex, whereas the K inks use organic pigments typically made of carbon black. The pigment powders are commercially not available as nanometer particles, but come as agglomerates and aggregates of pigment particles formed during the synthesis process. The agglomerates and aggregates of pigments need to be made smaller and will typically be brought to the nanometer scale size by grinding, milling, or other techniques to break up the agglomerates and aggregates. The particle size reduction of the pigment powder particles is often carried out in a carrier medium that will be used in a later stage in the ink. Such a process step of making pigment particles smaller is often done at higher pigment concentration than in the final ink and is then referred to as the pigment dispersion.

As discussed above, the pigments are mechanically broken down to sizes of 30-300 nm which are needed to be stabilized by adding dispersing agents (or surfactants) to physically and/or electrostatically prevent the pigments to re-agglomerate. Typically this is a physico-chemical phenomenon as there is no chemical bond between the pigment surface and the dispersing agent, which can be polymeric dispersing agents (e.g. random copolymer, block-copolymers, grafted copolymers) or surfactants.

Sometimes also covalently bound chemicals can be used for pigment particle stabilization in water based inks, but requires mostly an expensive production process.

A known approach for making pigment dispersions is based on random polymerized dispersing agents: they are used in analog inks e.g. for flexographic printing, but also in inkjet inks. Typically, the random polymers are built up of two kinds of monomers, one which is more pigmentophilic and another which is more compatible with the carrier liquid or matrixophilic so to speak. These monomers are randomly spread out in the polymer, so no polymer structure (i.e. monomer sequence) and no number of monomeric repeating units is pre-defined. Hence a number of polymers will have a favorable structure, capable of stabilizing pigment particles in the liquid, but there is a non-active population as well having a “bad” structure for stabilization (e.g. not enough pigment anchors, not close enough to each other pigment anchors, too few matrixophilic groups). These fractions often are not adsorbed or readily desorbed yielding in a non-stable dispersion. Other side effects may occur that are undesired when manufacturing dispersions for inkjet inks. Two of the main problems are that the not-absorbed high molecular weight dispersant molecules are present in the matrix and therefore contribute to a higher dispersion viscosity. Additionally visco-elastic properties of the final ink may be affected negatively. Both effects of increased viscosity and affected visco-elastic properties of the final ink may disturb the drop forming process, such as by increased mist forming.

Another known approach is block copolymer dispersants. Block copolymer dispersants containing hydrophobic and hydrophilic block segments have been disclosed in numerous inkjet ink patents. U.S. Pat. No. 5,859,113 (DU PONT) discloses an AB block copolymer dispersant with a polymeric A segment and a polymeric B segment.

A wide variety of polymeric dispersants, such as block copolymer dispersants, has been proposed, but the dispersion stability of pigments, especially in inkjet, still needs further improvement. For consistent image quality, the inkjet ink requires a dispersion stability capable of dealing with e.g. high temperatures (above 60° C.) during transport or storage of the ink to a customer and changes in the dispersion medium of the inkjet ink during use such as evaporation of water and increasing concentrations of water soluble organic solvents or the addition of functional polymers to improve the adhesion, drying, water or scratch resistance of the ink on a substrate.

In industrial inkjet processes the demands for stability of pigment dispersions usable for inkjet printing is also increasing, in particular at higher jetting frequencies, smaller droplet formation, higher jetting temperatures and/or more demanding single pass printing applications.

All the ink additives (such as surfactants, latexes, polymers, oligomers, (water soluble) organic solvents) can compete with the dispersing agents to interact with the pigment surface and thus negatively influence the pigment dispersion stability. These additives are typically organic in nature (to certain extent even incompatible with the ink matrix), and they will compete for the same organic surface of the pigments, especially pronounced in water based inks. If this occurs inside the printhead, it will lead to removal of the dispersing agent from the surface, lowering or destroying the stability and give rise to irreversibly blocked nozzles resulting in e.g. white lines and severe reduction of image quality or inks containing too large particles that negatively influence the ink flow into the head.

Due to the increasing demands, there is a need to further optimize the ink additives, such as optimization depending on the type of color pigment, to obtain a consistent image quality and a reliable inkjet process.

In addition, for some color pigments it has found to be more difficult to obtain a fine pigment dispersion in a simple way e.g. using a milling process, while enhancing the pigment stability required for an inkjet ink.

A desire remains to formulate block copolymer dispersing agents, which provide a high degree of milling performance (i.e. able to mill down very easy to the desired particle size) for different pigments without the need of additional co-solvents and keeping a high level of stability of the pigment dispersion such that the obtained pigment dispersions provide a broad versatility for formulating final inkjet inks.

As such, there is a need to be able to manufacture such stable pigmented inkjet inks, wherein a dispersion stability of the pigment in the inkjet inks may be enhanced easily for a wider diversity of inkjet inks.

According to a first aspect of the invention there is provided an inkjet ink for forming an image on a substrate, the inkjet ink comprising a pigment P, a first block copolymer dispersant D1 for dispersing the pigment P and a second block copolymer dispersant D2 for dispersing the pigment P, and an aqueous carrier; wherein said first block copolymer dispersant D1 is different from said second block copolymer dispersant D2, wherein each block copolymer dispersant D1, D2 comprises an anchoring segment A1, A2 for anchoring to the pigment P, wherein each of the block copolymer dispersant D1, D2 additionally comprises a matrix stabilization segment M for aqueous phase stabilization of the pigment P, wherein the anchoring segment A1 of the first block copolymer dispersant D1 comprises a repeating unit R1 and the anchoring segment A2 of the second block copolymer dispersant D2 comprises a repeating unit R2, wherein the repeating unit R1 is less hydrophobic than the repeating unit R2.

According to another aspect of the invention there is provided an inkjet printing process for forming an image on a substrate by applying a plurality of inkjet inks on the substrate, wherein the plurality of inkjet inks comprise an ink according to the invention.

According to another aspect of the invention there is provided an inkjet ink set comprising a plurality of colored inks, wherein at least one of the inks is an ink according to the invention.

According to another aspect of the invention there is provided an inkjet printer responsive to digital data signals, which has been equipped with an inkjet ink according to the invention or inkjet ink set according to invention.

According to another aspect of the invention there is provided an aqueous pigment dispersion for forming an inkjet ink, the pigment dispersion comprising a pigment P, a first block copolymer dispersant D1 for dispersing the pigment P and a second block copolymer dispersant D2 for dispersing the pigment P, and an aqueous carrier; wherein said first block copolymer dispersant D1 is different from said second block copolymer dispersant D2, wherein each block copolymer dispersant D1, D2 comprises an anchoring segment A1, A2 for anchoring to the pigment P, wherein each of the block copolymer dispersant D1, D2 additionally comprises a matrix stabilization segment M for aqueous phase stabilization of the pigment P, wherein the anchoring segment A1 of the first block copolymer dispersant D1 comprises a repeating unit R1 and the anchoring segment A2 of the second block copolymer dispersant D2 comprises a repeating unit R2, wherein the repeating unit R1 is less hydrophobic than the repeating unit R2; and wherein the aqueous carrier contains water.

It has been found by the inventors that an easy manufacture of a stable pigmented inkjet ink is possible using the combination of the first block copolymer dispersant D1 and the second block copolymer dispersant D2. The repeating unit R1 of the first block copolymer dispersant D1 is less hydrophobic than the repeating unit R2 of second block copolymer dispersant D2. The first block copolymer dispersant D1 has been found to improve the pigment stability required for an inkjet ink and at the same time to enhance an easy manufacturing of a fine pigment dispersion having fine pigment particles in a simple way e.g. using a milling process. The second block copolymer dispersant D2 additionally enhances the pigment stability required for an inkjet ink. The combination of first block copolymer dispersant D1 and second block copolymer dispersant D2 in the pigment dispersion or inkjet ink surprisingly combines these advantages without any adverse effects. Additionally, no or minimal water soluble organic co-solvents are needed to obtain the beneficial fine pigment particles. As such, the pigment dispersion according to the invention can be used in a versatile way to provide inkjet inks having various compositions, such as containing no or various amounts of water soluble organic co-solvents.

The use of two block copolymer dispersants may seem to increase a complexity of the pigment dispersion, but the gain in milling performance and the flexibility in mixing ratio of different block copolymer dispersants achieves the manufacturing of versatile stable pigment dispersions.

Pigments may have surfaces present with different hydrophobicity degrees meaning that one part of the pigment surface can be less hydrophobic than other parts of the pigment surface. It has been found that a very dedicated design/choice of the block copolymer dispersants is necessary to cover the entire pigment surface and thereby achieving a highly stable pigment dispersion.

In addition, the selected block copolymer dispersants D1, D2 for dispersing the pigment P in the ink may prevent that the pigment P re-agglomerate in the resulting inkjet ink, even when various ink additives, such as water soluble organic solvents, which may compete with the dispersants, are added to the inkjet ink.

Moreover use of block copolymer dispersants D1, D2 for dispersing the pigment P may support and enhance the jetting stability in industrial inkjet printing having increasing demands for ink durability and consistent image quality.

(Pigment Dispersion Stability)

A pigment dispersion stability as defined in this application may include a pigment particle grinding characteristic of the pigment dispersion and may include a pigment dispersion stability characteristic when exposed to extreme conditions, such as elevated temperature conditions (relative to room temperature or a normal operation temperature of the ink) and critical aqueous carrier conditions, such as by adding and/or increasing the amount of competing water soluble organic solvents to the carrier and/or other ink additives.

The pigment particle grinding characteristic of the pigment dispersion shows the ability to easily reduce the size of the agglomerates and aggregates of pigments down to nanometer sizes, such as 30-300 nm, which are stabilized by adding the block copolymer dispersant to physically and/or electrostatically prevent the pigments to re-agglomerate.

The following exemplary embodiments are described; however the present invention is not limited to these embodiments:

(Block Copolymer Pigment Dispersant)

The combination of block copolymer dispersants D1, D2 has been found to overall improve the pigment stability in the ink. It has been found that selecting block copolymer dispersants D1, D2 with different properties allows dedicated chemical fine-tuning of the dispersing agent in order to optimize the physical interaction of the polymeric dispersing agent to the pigment surface. The balancing act in the design of the copolymer dispersant lies in a pigmentophilic/matrixophilic balance. For water based inks important seems the relative hydrophilic/hydrophobic nature of the polymer. In this way the pigment particle P may be compatibilized with the carrier. Altering the pigmentophilic/matrixophilic balance in a block copolymer can be done in a multitude of ways. The inventors have found that only altering the number of repeating units in the segments may be insufficient, especially when facing pigments with chemically different areas on the pigment particle surface. In order to offer a solution for this problem the inventors have found that it can be highly beneficial to also use a second block copolymer dispersant having a second chemically different type of repeating unit, possible also with a different number of repeating units.

Block copolymer dispersants, unlike random copolymers, have an engineered molecular structure in chemical composition (block-wise incorporated in the polymer), narrow molecular weight distribution, and/or defined block chain lengths of the different constituting blocks or monomers. Block copolymer dispersant can be built up of two or more different monomers, arranged in blocks in the polymer.

A block of the block copolymer of the present invention has a narrow molecular weight distribution, and/or defined block chain length. A block copolymer is defined by a polymer consisting of different blocks wherein each block is substantially equal in size and composition meaning that all polymer molecules have substantially the same composition and length. The same composition means that, in case the block contains one type of repeating unit, that the repeating unit is the same, or, in case the block contains two or more different repeating units, that the number ratio between the respective repeating units is the same.

The fact that the individual blocks, and also the complete polymer, is equal in size can be expressed by the polydispersity D, defined as M/Mof the individual block or the complete polymer respectively, which is preferably lower than 1.6, more preferably lower than 1.5.

In an exemplary embodiment, the block copolymer dispersant D1, D2 has a polydispersity D lower than 1.6, more preferably lower than 1.5. In a particular embodiment, each of the blocks of the block copolymer dispersant D1, D2 has a polydispersity D lower than 1.6, more preferably lower than 1.5.

Compared to random copolymers that are used commonly as a pigment dispersing agent, the pigmentophilic monomers in block copolymer dispersants can be built as a block of appropriate size and chemistry next to one or more matrixophilic monomer-based blocks of appropriate size and chemistry. This provides options to fine-tune and maximize the interaction between a certain pigment and a block copolymer based dispersing agent. A maximized interaction will yield a strong physico-chemical connection of the dispersing agent with the pigment surface. Since dispersing agents are put on the surface of pigments to prevent re-agglomeration of the pigments, this strong fixation of the dispersing agent to the pigment surface is to inhibit desorption of the polymer from the pigment surface, may enable the creation of highly stable pigment dispersions, especially when used in the final ink, and additionally might provide the advantage of low levels of dispersing agent dissolved in the matrix.

A strong link of dispersing agent with the pigment will also withstand the competing interactions of other ink additives that in water based inks tend to be (at least partially) hydrophobic in nature and thus want to get to the organic pigment surface as well.

In an exemplary embodiment, the block copolymer dispersant D is selected from a diblock copolymer and a triblock copolymer. The matrix stabilization segment may comprise one block formed from one monomer and may further comprise another block formed from another monomer. The anchoring segment may comprise one block formed from one monomer and may further comprise another block formed from another monomer. Thus, the block copolymer dispersant may be a diblock copolymer having two blocks, may be a triblock copolymer having three blocks and may have any other suitable number of blocks larger than three.

The matrix stabilization segment and the anchoring segment of the block copolymer dispersant may be arranged in any order along the block copolymer dispersant.

In an exemplary embodiment, the block copolymer dispersant D1, D2 is a water soluble block copolymer dispersant. A block copolymer is defined as “water soluble” when the block copolymer remains in solution in water at 25° C. with at least 15 wt. % of dry polymer in water during at least a week, preferably remains in solution in water at 25° C. for at least one or more months, more preferably the weight percentage of dissolved dry polymer in water at 25° C. is at least 20 wt. % and most preferably the weight percentage of dissolved dry polymer in water at 25° C. is at least 25 wt. % or above.

A block of a block copolymer dispersant according to the invention may additionally contain an initiator moiety, a terminating moiety, an end group, and/or a linking moiety.

A repeating unit of a block may comprise a substituent. The substituent may optionally be converted to another substituent, thereby modifying the repeating unit, after polymerizing the block or after polymerizing the block copolymer.

In an exemplary embodiment, the block copolymer dispersant D1, D2 is constituted by a linear polymer chain. In another exemplary embodiment, the block copolymer dispersant D1, D2 is at least partially crosslinked, wherein the block copolymer dispersant is attached to or at least partially encapsulates the pigment.

The block dispersant D1 and D2 have each an Mw of 2000 to 20000 g/mol more preferably between 3000 to 12000 g/mol, wherein the ratio between the total number of repeating units of anchoring segment A to the total number of repeating units of matrix stabilizing segment M of D1 and D2 is in the range of 0.5 to 10 more preferably 1 to 5. When the molecular weight of the dispersant is lower than 2000 g/mol the dispersion stability is not maintained and when the molecular weight is higher than 20000 g/mol the viscosity of the dispersion is often too high or the dispersion stability may be affected by dispersant polymer chains anchoring to more than one pigment particle. Polymeric dispersants with a molecular weight above 2000 g/mol also provide the benefit of adhesion to the substrate during the drying process of the ink, even without the addition of binders. Additionally, the use of analogue or digital overprint varnishes can aid in further increasing the properties of the printed image, e.g. mechanical and chemical resistance, hot scuff resistance, and coefficient of friction.

The acid value of the mixture of block copolymer dispersants D1 and D2 is in the range of 50 to 200 mg KOH/g polymer more preferably 60-150 mg KOH/g polymer allowing a good solubility in water. The acid value of the mixture is determined by measuring/calculation the acid value (AV) of the individual block copolymer dispersants D1 and D2 and then calculating the AV of the mixture according the weight ratio of D1 and D2 in the pigment dispersion. The acid value is thus a measure of the weight ratio between the matrix stabilization segment and anchoring segment. At a too high AV, meaning that the contribution of the matrix stabilization segment is too high, the polymer dispersant might temporarily desorb from the pigment surface affecting e.g. the pigment dispersion stability. Another problem of a too high AV is that a water fastness of the ink image on a substrate could be insufficient.

On the other hand, at a too low AV, when the contribution of the anchoring segment is too high, the solubility of the polymer dispersant can be insufficient to provide an efficient milling process, especially when milling with low amounts of a co-solvent or even in absence of a co-solvent. Note that the benefit of a dispersion, which is substantially free of co-solvent is highly preferred as this leaves a free choice of ink components during ink formulation.

Another aspect of acid value is the importance of the value in terms of interaction with primers that are typically used for improving image quality on paper (i.e. uncoated kraft and recycled as well as offset coated media) and film substrates. Analogue and digitally applied primers are used to achieve lower intercolor bleeding, higher optical density, etc. However, primers typically contain cationic components (polymeric or multivalent metal cations) or acidic components, intended to reduce or ‘crash’ the stability of the dispersed pigments. Such primers work best when the acid value of the pigment dispersion is lower than 200 mg KOH/g. To create an optimal bonding between the crashed pigment and the printed substrate such primers often also contain a polymeric binder.

The combination of the proper range of molecular weight and acid value of the block dispersant D1 and D2 is supportive to prepare the pigment dispersion in an easy and cost effective way, e.g. without the use of solvents which have to be removed afterwards. The combination may also support a good interaction with a primer which may be typically used in the ink composition, e.g. for printing on uncoated corrugated board in order to enhance the color intensity on uncoated corrugated board.

In an exemplary embodiment, the anchoring segment A1 is formed using at least one monomer Mn, and the anchoring segment A2 is formed using at least one monomer Mn, wherein the monomer M, Mis selected from the group consisting of a methacrylate, an acrylate, and vinyl monomers.

The anchoring segment is an anchoring part for anchoring to the pigment, and wherein the anchoring segment A1 of the block copolymer dispersant D1 is different from the anchoring segment A2 of the block copolymer dispersant D2.

(Anchoring Segment A1)