Microcapsules used for producing rubber and method for their production
The invention relates to microcapsules made of a core which contains at least one rubber additive and of a shell made of a first polymer. At least one coating made of a second polymer, which differs from the first polymer, and/or of a low molecular inorganic or organic compound, is deposited on the surface of the microcapsules as sliding or wearing layer in order to reduce the static friction.
1. Microcapsules comprising:
a) a core containing at least one rubber additive,
b) at least two shells made from an amino resin or phenol formaldehyde resin, wherein the shells together have a thickness between 30 and 100 nm; and,
c) at least one sliding or wearing layer applied to the surface of the microcapsules selected from the group of polyacrylates, polyacrylonitriles, polyethylene glycols, ethyl celluloses, starch fatty acid esters and starch carbamates of long-chain isocyanates, or from low-molecular inorganic or organic compounds selected from the group of waxes, fatty acid derivatives, silicones, siloxanes and silicates,
wherein the shells and the sliding or wearing layer together have a thickness between 40 and 200 nm.
2. Microcapsules according to claim 1 , wherein the shells are mechanically stable and thermally stable up to at least 120° C.
3. Microcapsules according to claim 1 , wherein the rubber additive can be released in a controlled manner under vulcanisation conditions.
4. Microcapsules according to claim 1 , wherein the rubber additive is ground or liquid sulphur.
5. Microcapsules according to claim 1 , wherein the proportion of sulphur in the microcapsules is more than 50% by weight.
6. Microcapsules according to claim 1 , wherein the proportion of sulphur in the microcapsules is between 80 and 95% by weight.
7. Microcapsules according to claim 1 , wherein the amino resin is selected from dicyandiamide formaldehyde resin, or melamine formaldehyde resin.
8. Microcapsules according to claim 1 , wherein the average particle diameter of the microcapsules is between 1 and 50 μm.
9. Microcapsules according to claim 1 , wherein the average particle diameter of the microcapsules is between 5 and 20 μm.
10. Method for producing microcapsules made of a core which contains at least one rubber additive, and at least two shells and at least one sliding or wearing layer as in claim 1 comprising the following steps:
a) dispersing the rubber additive in a prepolymer solution that forms the first shell;
b) curing the microcapsules chemically by the addition of a catalyst and/or by increasing the temperature,
c) depositing the second shell from a prepolymer solution that forms the second shell; and
d) depositing at least one sliding or wearing layer on the surface of the microcapsules.
11. Method according to claim 10 , wherein ground or liquid sulphur is used as rubber additive.
12. Method according to claim 10 , wherein a reactive resin selected from the group of melamine formaldehyde resin or phenol formaldehyde resin is used as first polymer that form the shells.
13. Method according to claim 10 , wherein after the curing in step b), the microcapsules are separated from the prepolymer solution.
14. Method according to claim 10 , wherein the sliding or wearing layer is deposited by means of coacervation, solvent evaporation, salting-out or spray-drying.
15. Method according to claim 10 , wherein the sliding or wearing layer is formed from low molecular inorganic or organic compounds deposited from organic solution or aqueous dispersion.
16. Method according to claim 10 , wherein the sliding or wearing layer is deposited by spraying processes.
17. Method according to claim 10 , wherein the microcapsules, during deposition in step d), are granulated by means of the sliding or wearing layer.
18. Method according to claim 10 , wherein the microcapsules, after deposition in step d), are granulated by means of a granulation aid.
19. Method for rubber vulcanisation comprising using the microcapsules according to claim 1 .