Thin aluminum flakes
Described are thin plane-parallel aluminum flakes illustrated in FIG. 1 having a thickness of up to 200 nm and comprising an inner layer of oxidized aluminium having a thickness of 0.5-30 nm, a process for the manufacture thereof and the use thereof, e.g. in formulations, like paints, electrostatic coatings, printing inks, plastics materials, and cosmetics. Surprisingly, due to the inner layer of oxidized aluminum the aluminum flakes have an improved shear stability as evidenced e.g. by the difference in lightness before and after shear stress.
1. A plane-parallel aluminium flake having a thickness of up to 200 nm comprising an inner layer of oxidized aluminium having a thickness of from 0.5-30 nm, adjacent aluminum layers, and one or more outer layers of oxidized aluminium, each having a thickness of from 0.25-25 nm.
2. The aluminium flake according to claim 1 , wherein the thickness of the flake is less than 100 nm, and a diameter of the flake is from 5-50 μm.
3. The aluminium flake according to claim 2 , wherein the inner layer of oxidized aluminum has a thickness of from 2-30 nm.
4. The aluminium flake according to claim 2 , wherein the thickness of each of the adjacent aluminum layers is from 5-100 nm and wherein the adjacent aluminum layers can have the same or different thickness.
5. The aluminium flake according to claim 1 , wherein said aluminium flake has been subjected to an additional surface treatment.
6. The aluminium flake according to claim 1 ,
wherein said aluminium flake has been subjected to an additional surface treatment, said surface treatment is selected from chromate treatment, phosphate treatment with phosphoric acids or esters thereof, polymer encapsulations with acrylates, silica treatment, and the application of silane layers containing one or more silanes or polymerized silanes optionally in combination with silica treatments.
7. A method for the manufacture of aluminium flake according to claim 1 , comprising
a) coating a polyester film with an acrylate based release layer,
b) metallizing the precoated polyester film with an aluminum layer in a vacuum via physical vapor deposition,
c) removing the vacuum,
d) oxidizing the aluminium surface by contact with air,
e) metallizing the oxidized aluminum surface with another aluminum layer in a vacuum via physical vapor deposition,
f) dissolving the release layer,
g) sizing down the obtained coarse aluminum flakes to the desired size,
h) removing the dissolved polyacrylate stemming from the release layer, and
i) separating the aluminum flakes.
8. A paint, an electrostatic coating, a printing ink, a plastic material, and a cosmetic comprising the aluminium flake according to claim 1 .
9. The aluminium flake according to claim 1 , having a thickness of less than 50 nm, and a diameter from 8-20 μm.
10. The aluminium flake according to claim 2 , wherein the thickness of each of the adjacent aluminum layers is from 5-40 nm and wherein the adjacent aluminum layers can have the same or different thickness.
11. The aluminium flake according to claim 2 , wherein the inner layer of oxidized aluminum has a thickness of from 3-10 nm.
12. The method of claim 7 , wherein a), b), c), d), and e) are performed in sequence two or more times before f), g), h) and i).
13. The method of claim 7 , wherein said method is performed on both sides of the polyester film.