Ink composition comprising optically variable pigments, use of the composition, optically variable pigment and method of treating said pigment
View Patent ↗The invention relates to passivated optically variable pigment, a method of preparing said passivated optically variable pigment, an ink composition comprising said passivated optically variable pigment, the use of said ink composition, and a document carrying a marking made with said ink composition.
1. A method of treating an optically variable pigment having a stack of planar layers, said stack of planar layers comprising at least one reflective layer and at least one dielectric layer, and at least one layer of a semitransparent metal layer made of Cr, said reflective layer and said dielectric layer having at least one chemically exposed edge surface and being corrosion susceptible said method comprising steps of
(a) dissolving a passivating agent selected from the group consisting of organic esters and fluorinated organic esters of phosphoric acid having the structural formula
(Rf—CH2-CH 2 —O) x P(O)(OH) y wherein:
Rf=F—(CF 2 —CF 2 ) z
x=1 or 2
y=2 or 1
x+y=3
Z=1 to 7,
in an organic solvent;
(b) mixing water into the solution provided in step a);
(c) adjusting the pH of the composition obtained in step b) to a value between 7.0 to 9.0; and
(d) dispersing said optically variable pigment into the composition obtained in step c) while keeping the pH value at 7.0 to 9.0.
2. A method according to claim 1 wherein the amount of the said passivating agent comprises 0.5 to 15.0% of the total weight of the optically variable pigment.
3. A method according to claim 1 wherein the amount of the said passivating agent comprises 1.5 to 6.5% of the total weight of the optically variable pigment.
4. A method according to claim 1 wherein the amount of the said passivating agent comprises 2.5 to 5.0% of the total weight of the optically variable pigment.
5. A method according to claim 1 wherein the organic solvent is selected from the group consisting of glycol ethers and glycols.
6. A method according to claim 1 wherein the pH value is adjusted to 7.3 to 8.5.
7. A method according to claim 1 wherein the pH value is adjusted to 7.5 to 8.0.