Solvent-resistant ink for printing on non-absorbent surfaces

Abstract

Ink for marking objects with non-absorbent surfaces, wherein the ink comprises formic acid and at least one dye, and characterized in that the ink is free of humectants, viscosity modifiers, surface tension modifiers, preservatives, pH modifiers, and UV stabilizers, characterized in that the dye is selected from chromium complex dyes, such as in particular Solvent Black 29 or Solvent Black 27, or nigrosine dyes.

Description

The present invention relates to a solvent-resistant ink for printing on non-absorbent surfaces. The ink according to the invention is therefore resistant to common solvents, such as alcohols, esters, ethers, white spirit, and the like. The application of inkjet digital printing for marking, labeling, and coding on non-absorbent plastic surfaces is well-established. This process utilizes solvent-based inks containing a binder. Upon drying, the solvent forms a film that fixes the ink to the surface. Common binders include styrene acrylates. The resulting marking or coding is generally water-resistant, as the binders typically used are not water-soluble, and offers limited abrasion resistance. However, the binder film can be easily removed with relatively mild solvents such as alcohols, esters, white spirit, and similar substances. The unprotected marking is then easily removed from the surface, as even minimal pressure is sufficient to wipe or scrape off the ink.Increased contact alone can cause the lettering to be completely or partially removed, as sweat and skin oils can attack the binder that protects the dye. Additionally, the binders used in the formulation generally make inkjet printing more difficult, as they can lead to drying out on the printhead and thus reduce the so-called decap time (open time at the printhead inkjet nozzles during a printing break). Another well-known method for marking non-absorbent plastic surfaces is thermal transfer printing. However, this method cannot be used to print on uneven surfaces. JP H10-88 053 ​​A discloses an inkjet ink composition comprising at least one anionic dye component which is colored, according to the general formula (I): where R 1 , R 2 and R 3 , which can be the same or different, are a hydrogen atom or an alkyl group with a carbon number of 1 or 2, R 4 and R 5 are the same or different and are a hydrogen atom or a hydroxyalkyl group with 1 to 3 alkyl groups or carbon atoms with 1 to 3 carbon atoms. CN 111471353 A discloses a pressurized, steam-sterilised, water-based organic ink produced from raw materials, which contains the following components: a compound material 20 wt.% to 50 wt.%; an organic dye 0.1 wt.% to 2 wt.%; 5 wt.% to 20 wt.% polysaccharides; an acidity regulator 5 wt.% to 20 wt.%; 30 wt.% to 50 wt.% water-based solvent; wherein the aqueous solvent consists of water and an organic solvent, and the organic solvent is selected from one or more of the following substances: methanol, ethanol, n-propanol, isopropanol, and n-butanol. US 2020 / 0013521 A1 discloses a dispersing element comprising: a copper oxide, a dispersant and a reducing agent, wherein the content of the reducing agent is in a range of the following formula (1) and the content of the dispersant is in a range of the following formula (2): WO 2009 / 096491 A1 discloses a flocculent containing a metal oxide as its main component, comprising: an oxide matrix formed from the metal oxide; an organic dye dispersed in the oxide matrix (excluding a fluorescent dye); and an organic acid. Therefore, there is a need for an ink that can be used to print on non-absorbent surfaces, such as plastic surfaces, while avoiding the disadvantages of the prior art. Surprisingly, it has been shown that the disadvantages of the prior art can be avoided by using formic acid in the ink. The present invention is described by independent claim 1. Preferred embodiments of the present invention are described as dependent claims. The problem underlying the present invention is therefore solved by an ink for marking objects with non-absorbent surfaces, wherein the ink comprises formic acid and at least one dye, and is characterized in that the ink is free of humectants, viscosity modifiers, surface tension modifiers, preservatives, pH modifiers, and UV stabilizers, and characterized in that the dye is selected from chromium complex dyes, such as in particular Solvent Black 29 or Solvent Black 27, or nigrosine dyes. Formic acid is a carboxylic acid with the molecular formula HCOOH (methanoic acid). The present invention describes an ink formulation that does not require conventional binders and uses at least one carboxylic acid and at least one dye, which penetrates the non-absorbent surface of the object to be printed, thus providing a permanent encoding that is not only water-resistant but also cannot be removed with conventional solvents. Further advantages include excellent abrasion and rub resistance, as well as increased decap time and thus improved processability. The ink according to the invention also exhibits resistance to perspiration and skin oils, so that even with repeated contact, the print is not removed and remains legible. In a further embodiment, the present invention relates to the use of methanoic acid (formic acid, HCOOH) as a solvent for dyes for use in the production of an ink for marking non-absorbent surfaces. The present invention is further explained below, wherein the features disclosed below can be combined with one another in any way. Moreover, the features are applicable to both embodiments, i.e., to both the ink and the claimed use. Surprisingly, it has been found that the ink according to the invention provides a fast-drying ink suitable for printing on non-absorbent surfaces. At the same time, the printed surfaces can be used again quickly without having to worry about the presence of solvents, etc. The lettering or marking is resistant to abrasion and wear. While prior art inks require binders to ensure lightfastness and limited abrasion resistance, the ink according to the invention achieves this without binders. The formic acid used in the invention is able to dissolve the dye. Due to its chemical properties, the formic acid slightly attacks the surface to be printed. The dye can then penetrate the surface. However, because of the small amount of ink applied, the surface to be printed is not destroyed, but only slightly etched. This ensures that the ink dries quickly and is smudge-proof and abrasion-resistant after just a few seconds. Furthermore, chemical resistance is achieved. The dye cannot be wiped off the surface. The amount of formic acid is also not harmful to humans. In inks used in the prior art, a binder typically ensures that the dye is fixed to the surface. If the binder is removed by a suitable solvent, the dye can also be easily removed from the ink. This is prevented by the ink according to the invention. Because the dye penetrates the surface areas of the material being printed, the ink cannot simply be scratched off, which is another reason why it has particularly good durability. The present ink is preferably binder-free. In a preferred embodiment, it consists of formic acid and at least one dye. In an alternative and equally preferred embodiment, the ink according to the invention comprises, in addition to formic acid and at least one dye, at least one organic solvent, wherein the organic solvent is selected such that it does not undergo a chemical reaction with the carboxylic acid. In particular, the organic solvent is therefore selected from ketones, cyclic hydrocarbons, and mixtures thereof; in particular, the solvent is selected from methyl ethyl ketone, acetone, and / or cyclohexanone. The solvent-resistant ink according to the invention enables the marking of non-absorbent surfaces. According to the invention, non-absorbent surfaces are those surfaces on which conventional inks do not produce a permanently stable printed image because the inks cannot penetrate the surface. The entire object can be made of the surface material, or the surface to be marked can consist of the material in the area of ​​marking, i.e., the object can be fully or partially coated on the surface. In particular, the surface to be printed is selected from PVC (polyvinyl chloride) and / or PC (polycarbonate) and / or PA (polyamide) and / or PET (polyethylene terephthalate) and / or PUR (polyurethane) and / or ABS (acrylonitrile butadiene copolymer).Surprisingly, it has been found that these surfaces, which cannot be printed with sufficient quality using commercially available inks, can be printed with the ink according to the invention. This results in a sharp printed image. The ink does not smudge, but is smudge-proof after just a few seconds. The printed image is comparable to laser printers, where pigments already present in the plastic are colored black by the action of a laser. In contrast, the present method has the advantage that any plastic or other non-absorbing surface can be printed on without the need for specific pigments or other dyes to be present in the surface, especially the plastic. According to the invention, different dyes can be applied to surfaces with the ink, provided they are soluble or stably dispersible in the solvent according to the invention. In particular, the dye is a pigment, especially a black pigment, or a dye, especially a black dye. According to the invention, the dye is selected from Solvent Black 29, Solvent Black 27, or nigrosine dyes. Chromium complex dyes such as Solvent Black 29 exhibit the highest UV resistance and are therefore particularly preferred. The ink preferably has a dye content of 0.5 wt.% to 15 wt.%, in particular 1 wt.% to 10 wt.%, and preferably 1.5 wt.% to 8 wt.%, such as 2 wt.%, 3 wt.%, 4 wt.%, 5 wt.%, 6 wt.%, or 7 wt.%. The total weight of the ink is 100 wt.%. This dye content is sufficient to ensure intense color reproduction regardless of the precise design of the surface to be printed. Higher dye contents do not result in improved print quality. Rather, they negatively affect the ink's processability. The ink according to the invention is characterized in that, in addition to the dye, it can contain further components as required, such as auxiliaries for UV stabilization of the dye. However, it is particularly preferred that the ink contains no other additives. Therefore, the ink according to the invention is particularly preferably free of humectants, viscosity modifiers, surface tension modifiers (e.g., surfactants), preservatives, pH adjusters, and UV stabilizers.

Claims

Ink for marking objects with non-absorbent surfaces, wherein the ink comprises formic acid and at least one dye, and characterized in that the ink is free of humectants, viscosity modifiers, surface tension modifiers, preservatives, pH modifiers, and UV stabilizers, characterized in that the dye is selected from chromium complex dyes, such as in particular Solvent Black 29 or Solvent Black 27, or nigrosine dyes. Ink according to claim 1, characterized in that it further comprises at least one organic solvent. Ink according to claim 2, characterized in that the at least one organic solvent is selected from ketones, cyclic hydrocarbons and mixtures thereof, in particular the solvent is selected from methyl ethyl ketone, acetone and / or cyclohexanone. Ink according to one or more of claims 1 to 3, characterized in that the object consists of the material of the surface to be printed or at least one surface of the object is fully or partially coated with the non-absorbent material. Ink according to one or more of claims 1 to 4, characterized in that the surface of the object to be printed consists of PVC (polyvinyl chloride) and / or PC (polycarbonate) and / or PA (polyamide) and / or PET (polyethylene terephthalate) and / or PUR (polyurethane) and / or ABS (acrylonitrile butadiene copolymer). Ink according to one or more of claims 1 to 5, characterized in that the ink has a dye content of 0.5 wt.% to 15 wt.%, in particular of 1 wt.% to 10 wt.%, preferably of 1.5 wt.% to 8 wt.%. Ink according to one or more of claims 1 to 6, characterized in that the ink consists of the solvent and the dye.