Ink composition and method of determining a degree of curing of the ink composition
A silver paste ink composition comprises a plurality of first particles comprising silver; a polymer binder; a carrier solvent; and a plurality of second particles comprising silver. The second particles are nanoparticles that are different than the first silver particles, the amount of second particles in the ink composition being sufficient to impart a first color to the uncured ink composition, the first color being different than the color of the same ink composition without the nanoparticles. The silver nanoparticles have a property of causing a change in the color of the ink composition when the ink composition is cured.
1. An uncured silver paste ink composition, comprising:
a plurality of first particles comprising silver, the first particles being flakes and having a largest dimension with an average length ranging from about 2 microns to about 10 microns, the first particles being present in the silver paste ink composition in an amount ranging from 70% by weight to about 95% by weight, based on the total weight of the silver paste ink composition;
a polymer binder comprising a thermoplastic material selected from the group consisting of a polyvinylbutyral terpolymer of a formula I and a mixture of the polyvinylbutyral terpolymer of formula I and polyvinylpyrrolidone,
where R 1 is a chemical bond or a divalent hydrocarbon linkage having from about 1 to about 20 carbons; R 2 and R 3 are independently an alkyl group, an aromatic group or a substituted aromatic group having from about 1 to about 20 carbon atoms; x, y and z represent the proportion of the corresponding repeat units respectively expressed as a weight percent, and the sum of x, y and z is about 100 weight percent; x is independently from about 3 weight percent to about 50 weight percent; y is independently from about 50 weight percent to about 95 weight percent; and z is independently from about 0.1 weight percent to about 15 weight percent, the polymer binder being in an amount ranging from 0.5 to 5 weight percent, based on the total weight of the silver paste ink composition;
a carrier solvent; and
a plurality of second particles comprising silver and having an average diameter ranging from about 3 nanometers to about 10 nanometers, the second particles being nanoparticles that are different than the first silver particles, the second particles in the ink composition being in an amount effective to impart a first color to the uncured silver paste ink composition, the first color being different than the color of the same ink composition without the nanoparticles, wherein the silver nanoparticles have a property of causing a change in the color of the ink composition when the ink composition is cured, the amount of second particles ranging from 3% by weight to about 20% by weight, relative to the total weight of the silver paste ink composition, wherein the plurality of second particles comprising silver are coated with an organic molecule stabilizer selected from the group consisting of octylamine, decylamine, dodecylamine, stearyl amine, octanoic acid, decanoic acid and stearic acid.
2. The composition of claim 1 , wherein the uncured silver paste ink composition without the nanoparticles would be a color having a first lightness, L 1 ; the nanoparticles impart a color having a second lightness, L 2 , to the uncured silver paste ink composition; and wherein if the ink composition were cured it would have a third lightness, L 3 , where L 1 , L 2 and L 3 are lightness values as defined in CIELAB color space, and further wherein L 2 is less than both L 1 and L 3 .
3. The composition of claim 1 , wherein the concentration of second particles ranges from about 3% to about 10% by weight, relative to the total weight of the composition.
4. The composition of claim 1 , wherein the concentration of carrier solvent ranges from about 5% to about 25% by weight, relative to the total weight of the composition.
5. The composition of claim 1 , wherein R 1 is a bond, R 2 is a 3 carbon alkyl group, and R 3 is a 1 carbon atom alkyl group.
6. The composition of claim 5 , wherein the solvent is Di(ethylene glycol) monobutyl ether.
7. The composition of claim 5 , wherein the solvent is a glycol ether.
8. The composition of claim 1 , wherein the polyvinylbutyral terpolymer has a weight average molecular weight of about 10,000 to about 600,000 Daltons.
9. The composition of claim 1 , wherein the polyvinylbutyral terpolymer has a glass transition temperature (“T g ”) of from about 60° C. to about 100° C.
10. The composition of claim 1 , wherein the polymer binder is the mixture of polyvinylbutyral terpolymer of formula I and polyvinylpyrrolidone.
11. The composition of claim 10 , wherein the polyvinylpyrrolidone has a weight average molecular weight of about 5,000 to about 80,000 Daltons.
12. The composition of claim 10 , wherein the polyvinylpyrrolidone has a glass transition temperature (“T g ”) of from 125° C. to 180° C.
13. A method of determining a degree of curing of a silver paste ink, the method comprising:
providing the uncured silver paste ink composition of claim 1 ;
coating the silver paste ink composition onto a substrate;
heating the silver paste ink composition to a temperature and for a length of time sufficient to cure the composition, wherein the ink has a first color prior to heating and has a second color when curing is complete, the second color being different than the first color; and
visually inspecting the silver paste ink composition to determine whether the color change indicates that curing is complete.
14. The method of claim 13 , wherein the coating comprises screen printing the silver paste ink.
15. The method of claim 13 , wherein the first color has a lightness, L 2 , and the second color has a lightness, L 3 , wherein L 2 and L 3 are lightness values as defined in CIELAB color space, and further wherein L 2 is less than L 3 .
16. The method of claim 13 , wherein the curing temperature ranges from about 80° to about 200° C.