Erasable inks, writing instruments, and methods
Erasable inks are provided for use in a writing instruments. In one aspect, the inks include a solvent, and, dispersed in the solvent as a colorant, a pigment having a flake-like morphology, the ink being substantially free of other colorants.
1. An erasable ink for use in a writing instrument, comprising:
a solvent,
a shear-thinning additive, and
a graphite flake pigment having an average thickness of less than about 0.25 microns dispersed in the solvent, wherein said ink has a shear-thinning index of between about 0.01 and 0.8.
2. The ink of claim 1 , wherein said ink is substantially free of colorants other than said graphite flake pigment.
3. The ink of claim 1 , wherein said ink exhibits an erasability of at least about 80%.
4. The ink of claim 1 , wherein said ink has a shear-thinning index of between about 0.05 and about 0.60.
5. The ink of claim 1 , wherein said ink has a shear-thinning index of between about 0.1 and about 0.3.
6. The ink of claim 1 , wherein said ink has a viscosity between 2000 mPa·sec and 20,000 mPa·sec at a shear rate of about 1.0 sec −1 , and a viscosity between 10 mPa·sec and 1000 mPa·sec at a shear rate of about 100 sec −1 .
7. The ink of claim 1 , wherein said solvent comprises an aqueous solvent system.
8. The ink of claim 7 , wherein said aqueous solvent system comprises water and a water-soluble organic solvent.
9. The ink of claim 1 , wherein said shear-thinning additive is selected from the group consisting of clays, organoclays, water-dispersible gums, and acrylic acid-based polymers.
10. The ink of claim 9 , wherein said shear-thinning additive is a polysaccharide.
11. The ink of claim 1 , wherein said graphite flake pigment has an average diameter from about 1 micron to about 25 microns.
12. The ink of claim 1 , wherein said graphite flake pigment has an aspect ratio of between about 1 and about 8.
13. A writing instrument, comprising:
a reservoir containing an erasable ink comprising a solvent, a shear-thinning additive, and a graphite flake pigment having an average thickness of less than about 0.25 microns dispersed in the solvent, wherein said ink has a shear-thinning index of between about 0.01 and 0.8.
14. The writing instrument of claim 13 , wherein said solvent comprises an aqueous solvent system.
15. The writing instrument of claim 14 , wherein said aqueous solvent system comprises water and a water-soluble organic solvent.
16. The writing instrument of claim 13 , wherein said shear-thinning additive is a polysaccharide.
17. The writing instrument of claim 13 , wherein said graphite flake pigment has an aspect ratio of between about 1 and about 8.
18. A method of forming an erasable marking on a paper substrate, comprising:
making a marking on the substrate with a writing instrument containing an erasable ink comprising a solvent, a shear-thinning additive, and a graphite flake pigment having an average thickness of less than about 0.25 microns dispersed in the solvent, wherein said ink has a shear-thinning index of between about 0.01 and 0.8.
19. The method of claim 18 , wherein said solvent comprises an aqueous solvent system.
20. The method of claim 19 , wherein said aqueous solvent system comprises water and a water-soluble organic solvent.
21. The method of claim 18 , wherein said shear-thinning additive is a polysaccharide.
22. The method of claim 18 , wherein said graphite flake pigment has an aspect ratio of between about 1 and about 8.