REPLACEMENT OF VOCS WITH VAPOR PRESSURE-FREE SOLVENTS

Description

Field of the invention

[0001] The invention relates to an (ink) composition and the use thereof as ink in the inkjet printing process, Background of the invention

[0002] For printing on plastic films, digital printing processes have increasingly become established alongside plate-based printing methods (especially flexographic printing, gravure printing, and offset printing). In these processes, the printing ink is applied to the film without a printing plate – for example, using an inkjet process. Water-soluble printing inks can also be used in all printing processes, which is particularly advantageous for printing food packaging.

[0003] Plastic films, such as those made of polypropylene (PP) or polyethylene (PE), are generally non-polar, which makes printing with water-soluble and therefore polar inks difficult. If necessary, the adhesion and drying of water-soluble inks can be improved by applying a primer to the film surface prior to printing. However, even with a primer, a rather complex subsequent final drying of the water-based ink is often necessary to remove the volatile components from the ink by evaporation or convection.

[0004] State-of-the-art water-based inks contain, in addition to water, significant amounts of low-boiling organic solvents (VOCs, or volatile organic compounds), typically in amounts of approximately 20-50% by weight. These VOCs are released during printing or the subsequent drying of the ink. This results in problems such as the following.

[0005] Firstly, VOCs are often harmful to health. Especially in mass production, this can lead to problems due to high VOC emissions in the workplace. Furthermore, the release of VOCs can lead to the formation of explosive and / or flammable mixtures. Likewise, known ink systems containing VOCs often exhibit adverse drying properties, especially when using radiation drying systems, and often result in optical properties of the printed product that require improvement, particularly with regard to gloss and color gamut. Finally, there is a need for improvements in the adhesion and chemical resistance of printed products.

[0006] It is therefore the object of the present invention to provide an ink composition that overcomes the disadvantages of the prior art, in particular to provide an ink composition for inkjet printing in which the amount of VOCs is reduced or which is VOC-free. EP1676894 can be cited as relevant prior art. Overview of the invention

[0007] This object is achieved by the subject matter of the independent claims. Preferred embodiments are set forth in the subclaims.

[0008] The object underlying the invention is achieved in particular by a composition comprising: a) 50-98 wt.% water; b) 1-49 wt.% of an ionic liquid; and c) 1-30 wt.% of a pigment, a dye or a mixture thereof; in each case based on the total weight of the composition, the composition further comprises a polymer, a precursor thereof or a mixture of the polymer and the precursor; the polymer is a polyacrylate and the composition further comprises a base.

[0009] Surprisingly, the inventors discovered that low-boiling organic solvents (VOCs) in ink compositions for digital printing can be partially or completely replaced by ionic liquids. It was found that the use of ionic liquids instead of VOCs not only solves problems associated with VOC emissions, but also offers additional advantages. In particular, it was found that the composition according to the invention allows the production of printed products with improved ink adhesion and chemical resistance. Advantages were also achieved with regard to the solubility of additives, the dispersibility of pigments and polymers, and improved optical properties of the printed product.

[0010] The composition according to the invention is an aqueous ink composition. The ink composition can be used in inkjet printing. The aqueous ink composition comprises at least 50% by weight of water, based on the total weight of the composition. Alternatively, the aqueous ink composition may contain at least 60% by weight of water, alternatively at least 70% by weight, or alternatively at least 80% by weight of water, in each case based on the total weight of the composition. The composition may contain water in an amount of at most 95% by weight, alternatively at most 90% by weight, or alternatively at most 85% by weight, in each case based on the total weight of the composition.

[0011] It can be provided that the ionic liquid is present in the composition in an amount of at least 2 wt.%, alternatively at least 3 wt.%, alternatively at least 4 wt.%, alternatively at least 5 wt.%, alternatively more than 5 wt.%, alternatively at least 5.5 wt.%. It can also be provided that the ionic liquid is present in the composition in an amount of at most 45 wt.%, alternatively at most 40 wt.%, alternatively at most 30 wt.%, alternatively at most 20 wt.%, alternatively at most 10 wt.%, alternatively at most 8 wt.%. The percentages in each case refer to the total weight of the composition.

[0012] It can also be provided that the pigment / dye is present in the composition in an amount of at least 2 wt.%, alternatively at least 3 wt.%, alternatively at least 4 wt.%, alternatively at least 5 wt.%. It can also be provided that the pigment / dye is present in the composition in an amount of at most 25 wt.%, alternatively at most 20 wt.%, alternatively at most 15 wt.%, alternatively at most 10 wt.%, alternatively at most 8 wt.%. The percentages in each case refer to the total weight of the composition.

[0013] An ionic liquid is an ionic compound (salt) comprising one or more anions and one or more cations, preferably one anion and one cation, which is liquid at a temperature of 200°C or less, preferably 150°C or less, and more preferably 100°C or less, without the salt being dissolved in a solvent such as water. According to the invention, the ionic liquid can have a melting point above 60°C. Ionic liquids are completely miscible with water. Ionic liquids have no (measurable) vapor pressure, meaning they are not released into the gas phase even at high temperatures.

[0014] Examples of cations that may be present in the ionic liquid according to the invention are, in particular, imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium, guanidinium, uronium, or thiouronium. These cations are preferably alkylated cations.

[0015] Suitable anions according to the invention include, for example, halides and more complex ions, such as tetrafluoroborates, trifluoroacetate, triflate, hexafluorophosphate, phosphinate, and tosylate. Organic anions, such as imides or amides, may also be used. Preferred anions are I -< , Cl -< , Br, SCN -< , BF 4 -< , PF 6 -< , ClO 4 -< , SbF 6 -< (CF 3 SO 2 ) 2 N -< , (CF 3 CF 2 SO 2 ) 2 N -< , Ph 4 B -< , (C 2 H 4 O 2 ) 2 B -< , (CF 3 SO 2 ) 3 C -< , CF 3 COO -< , CF 3 SO 3 -< , C 6 F 3 SO 3 -< and MeO(EtO) 2 SO 3 -< ,

[0016] In one embodiment, the ionic liquid is an imidazonium salt. The imidazolium may be a dialkylimidazolium of the following formula: wherein R 1< and R 2< are independently selected from the group consisting of C 1 -C 8 alkyl, preferably C 1 -C 6 alkyl, preferably C 1 -C 4 alkyl, particularly preferably C 1 -C 3 alkyl, most preferably C 1 -C 2 alkyl.

[0017] In one embodiment, the cation of the ionic liquid is 1-ethyl-3-methylimidazolium (R 1< = methyl and R 2< = ethyl).

[0018] In a further embodiment, the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate.

[0019] The pigment according to the invention can be an inorganic pigment or an organic pigment. Common corresponding pigments are known to those skilled in the art. Suitable inorganic pigments include, in particular, titanium dioxide, red iron oxide, antimony(V) sulfide, cadmium yellow, cobalt blue, ultramarine blue, Prussian blue, carbon black, graphite, or other colored pigments, as well as calcium carbonate, kaolin, clay, barium sulfite, aluminum hydroxide, talc, etc.

[0020] Suitable organic pigments are, for example, azo pigments, copper phthalocyanine pigments, condensed polycyclic pigments, etc. In principle, all dyes that have sufficient solubility in the aqueous system according to the invention are suitable as dyes.

[0021] In a further embodiment, the composition may comprise one or more additives independently selected from the group consisting of binders, fillers, rheology additives, stabilizers, surfactants, preservatives and humectants.

[0022] It can be provided that the total amount of additive is from 0.1 to 10 wt.%, based on the total weight of the composition.

[0023] The composition further comprises a polymer, a precursor thereof, or a mixture of the polymer and the precursor.

[0024] The polymer that can be present in the composition according to the invention or that can be created by polymerizing the precursor / by reacting the precursor with the polymer serves as a fixing resin. It serves to fix the pigment / dye to the product to be printed. In this context, the polymer (fixing resin) and / or the precursor can be present in the composition in a total amount of 0.05 wt.% to 15 wt.%, preferably 1 wt.% to 10 wt.%, particularly preferably 5 wt.% to 7 wt.%, based on the total weight of the composition.

[0025] The polymer is a polyacrylate. In this embodiment, the precursor is an acrylate monomer or oligomers of the acrylate.

[0026] Furthermore, the invention may provide that in the embodiment in which the composition comprises a polymer and / or a precursor thereof, the composition further comprises a crosslinking agent suitable for crosslinking the polymer.

[0027] The composition further comprises a base. This can be particularly advantageous when a fixing resin containing carboxyl groups is used to neutralize the corresponding acid groups. Suitable basic compounds include inorganic bases such as sodium hydroxide, potassium hydroxide, or ammonia, or organic bases such as methylamine, ethylamine, ethanolamine, dimethylethanolamine, diethylethanolamine, dibutylethanolamine, diethylanolamine, triethanolamine, morpholine, etc.

[0028] It can be provided that the amount of base is 0.1 to 5 wt.%, based on the total weight of the composition.

[0029] In a further embodiment, it can be provided that the composition does not comprise an organic solvent. Organic solvents within the meaning of the invention are, in particular, volatile organic compounds (VOCs) and very volatile organic compounds (VVOCs). Organic solvents within the meaning of the present invention are, in particular, polar organic solvents, i.e., solvents that are soluble in water. Organic solvents within the meaning of the present invention (i.e., solvents that are preferably not present in the composition) include, in particular, monoalcohols, polyalcohols, polyalcohols of lower alkyl ethers, ketones, ethers, esters, nitrogen-containing solvents, etc.

[0030] In a further embodiment, it is provided that the composition according to the invention does not comprise an organic solvent and the ionic liquid is contained in an amount of more than 5% by weight, based on the total weight of the composition.

[0031] In a further embodiment, it is provided that the composition does not comprise an organic solvent and the ionic liquid is contained in an amount of 5.5 wt.% to 20 wt.%, based on the total weight of the composition.

[0032] The object is further achieved by a method for printing a carrier using a digital printing process, comprising: a) providing the carrier; and b) printing the carrier using a digital printing process using the composition according to the invention as ink.

[0033] In particular, a plastic film can be provided as the carrier according to the invention. In one embodiment, the plastic film is a film made of polyethylene, polypropylene, or mixtures thereof.

[0034] The digital printing process used in the process according to the invention, as well as the process in which the composition according to the invention is used as ink, can be any digital printing process known in the art, such as an inkjet printing process.

[0035] Finally, the problem is solved by using a composition as ink in the digital printing process. Examples of implementation

[0036] In the following, the invention will be described using specific embodiments, whereby the scope of protection is of course not limited to the specific embodiments. Comparison example

[0037] A prior art ink composition was prepared by mixing the following ingredients. Pigment Black 7 5.0 mass% Water-soluble acrylic resin 4.5% by mass Isopropyl alcohol (4.3 kPa / 20 °C) 5.5% by mass Water 82.0% by mass Triethanolamine (Johncryl 7001) 1.0 mass% Acrylic emulsion (Johneryl 7001) 1.5% by mass Wetting agent (Byk 349) 0.8% by mass 1,2-Benzoisothiazolin-3-one 0.2 mass% Example

[0038] An ink composition according to the present invention was prepared by mixing the following ingredients Pigment Black 7 5.0 mass% Water-soluble acrylic resin 4.5% by mass Ethylmethylimidazole BF4 (emimbf4) 5.5% by mass Water 82.0% by mass Triethanolamine (Johneryl 7001) 1.0 mass% Acrylic emulsion (Johneryl 7001) 1.5% by mass Wetting agent (Byk 349) 0.8% by mass 1,2-Benzoisothiazolin-3-one 0.2 mass%

[0039] The composition according to the invention (example) differs from the composition according to the prior art (comparative example) in that the organic solvent used according to the prior art (isopropyl alcohol) was completely replaced by an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate).

[0040] The ink compositions according to the example and the comparative example were applied to a plastic film by inkjet printing and then tested for their adhesion and chemical resistance.

[0041] The assessment is based on the following scale: 5 = 100% = no damage to the coating 4 = 75% = minor damage to the coating 3 = 50% = significant damage to the coating 2 = 25% = severe damage to the coating 1 = 0% = complete removal of the coating

[0042] The following results were obtained for the various tests performed. Table 1: Comparison example Example Tesa test 3 4-5 Scratch test 5 5 Ethanol 0 3-4 saliva 2 3 Distilled water 3 2-3

[0043] As shown in Table 1, replacing VOCs with ionic liquids in digital printing ink formulations leads to significant adhesion improvements of up to 40%. Improved chemical resistance can also be achieved.

[0044] In addition, the inventors found a reduction in drying energy consumption of about 15%.

Claims

1. Method for printing on a substrate using a digital printing method comprising: a) providing the substrate; and b) printing on the substrate in the digital printing method using a composition which comprises: i) 50-98 wt.% water; ii) 1-49 wt.% of an ionic liquid; and iii) 1-30 wt.% of a pigment, a colorant or a mixture thereof; each relative to the total weight of the composition, wherein the substrate is a plastic film; the digital printing method is an inkjet printing method; the composition further comprises a polymer, a precursor thereof or a mixture of the polymer and the precursor; the polymer is a polyacrylate; and the composition further comprises a base.

2. Method according to claim 1, wherein the ionic liquid is an imidazolium salt.

3. Method according to claim 1 or 2, wherein the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate.

4. Method according to any one of the preceding claims, wherein the composition further comprises one or more additives, selected independently from the group consisting of binders, fillers, rheology additives, stabilizers, surfactants, preservatives and humectants.

5. Method according to any one of the preceding claims, wherein the composition does not comprise an organic solvent.

6. Use of a composition as defined in any one of claims 1 to 5 as ink for printing on a plastic film in an inkjet printing method.