Water-Based Screen Printing Ink
A water-based screen printing ink composition which can be used for flexo or gravure printing includes an acrylic or urethane-acrylic copolymer emulsion, a crosslinker, a pigment selected from the group consisting of luminescent pigments, magnetic pigments, optical effect pigments, optically variable pigments, and a catalyst. The crosslinker has at least two different chemical functionalities within the same molecule, a first of which forms a covalent link with the polymer prior to printing and a second of which effects crosslinking of the polymer to cure the printed ink.
1 . A water-based screen printing ink composition which can be used for flexo or gravure printing, having
(a) an acrylic or urethane-acrylic copolymer emulsion in an amount between 30 and 70% by weight with respect to the total weight of the composition;
(b) a crosslinker in an amount between 0.25 and 3% by weight with respect to the total weight of the composition;
(c) a pigment selected from the group consisting of luminescent pigments, magnetic pigments, optical effect pigments, optically variable pigments, and mixtures thereof, wherein the pigments content is between 12% and 20% of the total weight of the composition; and
(d) a catalyst in an amount between 0.05% and 0.6% by weight of the total weight of the ink composition;
wherein said crosslinker comprises at least two different chemical functionalities within the same molecule, wherein a first of said functionalities is chosen such as to form a covalent link with said polymer prior to printing and a second of said functionalities is selected to effect crosslinking of said polymer to cure the printed ink and that the acrylic or urethane-acrylic copolymer emulsion is selected from the group consisting of those copolymer emulsions having self-crosslinking properties, and wherein the composition has a pH value between 7.0 and 8.5.
2 . A water-based screen printing ink composition according to claim 1 wherein the acrylic or urethane-acrylic copolymer emulsion is alkali soluble.
3 . A water-based screen printing ink composition according to claim 1 wherein the acrylic or urethane-acrylic copolymer has a T s value in the temperature range between −10° and 50° C.
4 . A water-based screen printing ink composition according to claim 1 wherein the acrylic or urethane-acrylic copolymer emulsion has an acid number between 10 and 90 mg KOH/g.
5 . A water-based screen printing ink composition according to claim 1 wherein the acrylic or urethane-acrylic copolymer emulsion has an acid number between 15 and 75 mg KOH/g.
6 . A water-based screen printing ink composition according to claim 1 wherein the acrylic or urethane-acrylic copolymer emulsion has a solid content of 35 to 55% by weight of the emulsion.
7 . A water-based screen printing ink composition according to claim 1 wherein the said crosslinker is chosen from the group consisting of substituted alkoxy silanes (R 1 ) y (R 2 O) z Si, wherein R 1 , R 2 are different substituents, and y+z=4.
8 . A water-based screen printing ink composition according to claim 1 wherein the said crosslinker is chosen from the group consisting of monosubstituted trialkoxy silanes (R 1 ) y (R 2 O) z Si, wherein y=1 and z=3.
9 . A water-based screen printing ink composition according to claim 1 wherein the said crosslinker is chosen from the group consisting of monosubstituted triethoxysilanes.
10 . A water-based screen printing ink composition according to claim 1 wherein the said crosslinker is chosen from the group consisting of epoxy-cycloaliphatic triethoxysilanes and glycidyl-triethoxysilanes.
11 . A water-based screen printing ink composition according to claim 1 further comprising an imidazole catalyst.
12 . A water-based screen printing ink composition according to claim 1 comprising between 35 and 60% by weight of self-crosslinking acrylic or urethane acrylic copolymer emulsion with respect to the total weight of the composition.
13 . A water-based screen printing ink composition according to claim 1 comprising between 40 and 55% by weight of self-crosslinking acrylic or urethane acrylic copolymer emulsion with respect to the total weight of the composition.
14 . A water-based screen printing ink composition according to claim 1 comprising between 0.5 and 2% by weight of said crosslinker, with respect to the total weight of the composition.
15 . A water-based screen printing ink composition according to claim 1 comprising between 1 and 2% by weight of said crosslinker, with respect to the total weight of the composition.
16 . A water-based screen printing ink composition according to claim 1 wherein the catalyst constitutes between 0.1 and 0.4% by weight of the total weight of the composition.
17 . A water-based screen printing ink composition according to claim 1 wherein the catalyst constitutes between 0.2 and 0.4% by weight of the total weight of the composition.
18 . A water-based screen printing ink composition according to claim 1 wherein said pH value is between 7.5 and 8.3.
19 . A water-based screen printing ink composition according to claim 1 wherein said pH value is between 7.5 and 8.0.
20 . A water-based screen printing ink composition according to claim 1 further comprising pigments making up between 15 and 18% of the total weight of the composition.
21 . A water-based screen printing ink composition according to claim 1 further comprising a flake pigment having first and second parallel planar surfaces and an aspect ratio greater than two.
22 . A water-based screen printing ink composition according to claim 1 further comprising a flake pigment having first and second parallel planar surfaces and an aspect ratio greater than five.
23 . A method of manufacturing a water-based screen printing ink composition according to claim 1 , comprising the steps of
(a) providing the acrylic or urethane-acrylic copolymer emulsion in an amount between 30 and 70% by weight with respect to the total weight of the composition;
(b) providing the crosslinker which is capable of forming a covalent link under first conditions with the polymer provided in (a), in an amount between 0.25 and 3% by weight with respect to the total weight of the composition, and crosslinking the resulting polymer under second conditions;
(c) providing a pigment selected from the group consisting of luminescent pigments, magnetic pigments, optical effect pigments, optically variable pigments, and mixtures thereof wherein the pigments content is between 12% and 20% by weight of the total weight of the ink composition;
(d) providing a catalyst in an amount between 0.05% and 0.6% by weight of the total weight of the ink composition;
(e) intimately mixing the components provided in (a), (b) and (c) and allowing the polymer provided in (a) to react with the crosslinker provided in (b) under said first conditions; and
(f) adjusting the pH value of the composition to a value between 7.0 and 8.5.
24 . A value document carrying indicia printed by using a water-based screen, flexo or gravure printing ink composition according to claim 1 .
25 . A method of using a water-based ink composition comprising acrylic or urethane-acrylic copolymer emulsions in an amount between 30 and 70% by weight with respect to the total weight of the composition and a crosslinker in an amount between 0.25 and 3% by weight with respect to the total weight of the composition, and a catalyst in an amount between 0.05% and 0.6% of the total weight of the ink composition, wherein said crosslinker is covalently linked with said acrylic or urethane-acrylic copolymer and wherein said acrylic or urethane-acrylic copolymer emulsion is selected from the group consisting of copolymer emulsions having self-crosslinking properties, and wherein the composition has a pH value between 7.0 and 8.5, as a screen, flexo or gravure printing ink, comprising the steps of providing a water-based ink composition comprising the above ingredients, and performing a screen, flexo or gravure printing process with said water-based ink composition.