A water-in-oil emulsion ink comprises a water phase and an oil phase, wherein the water phase comprises a carbon black pigment having a pH of less than 8 dispersed therein by a water-soluble resin, and wherein the water phase also comprises a pH-adjusting agent to achieve a pH value for the water phase of 8 or greater. The ink maintains stable rheology on ageing, and provides a stable and consistent ink detection signal even after periods of non-use.
1. A water-in-oil emulsion ink comprising a water phase and an oil phase, wherein the water phase comprises a carbon black pigment having a pH of less than 8 dispersed therein by a water-soluble resin and a pH-adjusting agent to achieve a pH value for the water phase of 8 or greater, and wherein the water phase is substantially free of surfactants and emulsion resins.
2. An ink according to claim 1 , wherein the pH of the water phase is 9 or greater.
3. An ink according to claim 2 , wherein the water phase is totally free of surfactant.
4. An ink according to claim 2 , which further comprises, in the oil phase, an ABA block copolymer prepared from 12-hydroxystearic acid and polyethylene oxide.
5. An ink according to claim 1 , wherein the water-soluble resin is a non-ionic resin.
6. An ink according to claim 5 , wherein the pH of the water phase is 9 or greater.
7. An ink according to claim 5 , wherein the water phase is totally free of surfactant.
8. An ink according to claim 5 , which further comprises, in the oil phase, an ABA block copolymer prepared from 12-hydroxystearic acid and polyethylene oxide.
9. An ink according to claim 1 , wherein the water-soluble resin is a homopolymer or copolymer of N-vinyl pyrrolidone.
10. An ink according to claim 9 , wherein the pH of the water phase is 9 or greater.
11. An ink according to claim 9 , wherein the water phase is totally free of surfactant.
12. An ink according to claim 9 , which further comprises, in the oil phase, an ABA block copolymer prepared from 12-hydroxystearic acid and polyethylene oxide.
13. An ink according to claim 9 , wherein the water-soluble resin is a non-ionic resin.
14. An ink according to claim 1 , wherein the water phase is totally free of surfactant.
15. An ink according to claim 14 , which further comprises, in the oil phase, an ABA block copolymer prepared from 12-hydroxystearic acid and polyethylene oxide.
16. An ink according to claim 1 , which further comprises, in the oil phase, an ABA block copolymer prepared from 12-hydroxystearic acid and polyethylene oxide.
17. An ink according to claim 1 , wherein the pH of the water phase is greater than 8.
18. An ink according to claim 1 , wherein the water phase is totally free of emulsion resins.