Amorphous silica
An amorphous silica suitable for use in a dental composition has a weight mean particle size in the range 3 to 15 μm with at least 90 per cent by weight of particles having a size below 20 μm, a Radioactive Dentine Abrasion (RDA) determined on an aqueous slurry of the silica powder of 100 to 220, a Pellicle Cleaning Ratio (PCR), when incorporated in a dental composition at 10 per cent by weight, greater than 85, the ratio of PCR to RDA being in the range 0.4:1 to less than 1:1 and having a Plastics Abrasion Value (PAV) in the range 11 to 19. A silica having the above properties is prepared by a precipitation route. The silica made available by the invention is also useful as an anti-blocking agent in plastics.
1. An amorphous silica suitable for use in a dental composition having a weight mean particle size in the range 3 to 15 μm with at least 90 per cent by weight of particles having a size below 20 μm, a Radioactive Dentine Abrasion (RDA) determined on an aqueous slurry of the silica powder of 100 to 220, a Pellicle Cleaning Ratio (PCR), when incorporated in a dental composition at 10 per cent by weight, greater than 85, the ratio of PCR to RDA being in the range 0.4:1 to less than 1:1 and having a Plastics Abrasion Value in the range 11 to 19.
2. An amorphous silica according to claim 1 having an oil absorption, using linseed oil, in the range 70 to 150 cm 3 /100 g.
3. An amorphous silica according to claim 1 having a BET surface area in the range 10 to 450 m 2 g −1 .
4. An amorphous silica according to claim 1 having a particle size distribution such that at least 90 per cent by weight of the particles have a size below 17 μm.
5. An amorphous silica according to claim 1 having a weight mean particle size in the range 3 to 7 μm and a particle size distribution such that at least 90 per cent by weight of the particles have a size below 16 μm.
6. An amorphous silica according to claim 1 having a pH value, measured on a 5 per cent by weight suspension, in the range 5 to 8.
7. An amorphous silica according to claim 1 having a water content, as determined by ignition at 1000° C., of up to 25 per cent by weight.
8. An amorphous silica according to claim 1 having a bound water content in the range 3.8 to 5.8 per cent by weight.
9. An amorphous silica according to claim 1 having a loose bulk density in the range 200 to 400 g/dm 3 .
10. An amorphous silica according to claim 1 having a Pore Volume in the range 0.1 to 1.0 cm 3 g −1 .
11. An amorphous silica according to claim 1 having a mean pore diameter in the range 5to 45 nm.
12. A dental composition comprising an amorphous silica having a weight mean particle size in the range 3 to 15 μm with at least 90 per cent by weight of particles having a size below 20 μm, a Radioactive Dentine Abrasion (RDA) determined on an aqueous slurry of the silica powder of 100 to 220, a Pellicle Cleaning Ratio (PCR) when incorporated in a dental composition at 10 per cent by weight greater than 85, the ratio of PCR to RDA being in the range 0.4:1 to less than 1:1 and having a Plastics Abrasion Value (PAV) in the range 11 to 19 and an orally acceptable carrier.
13. A dental composition according to claim 12 containing from 0.1 to 25 per cent by weight of the amorphous silica.
14. A dental composition according to claim 12 containing from 0.1 to 6 per cent by weight of the amorphous silica.
15. A dental composition according to claim 12 further comprising a second amorphous silica having an RDA value lower than the amorphous silica.
16. A dental composition according to claim 15 containing an amount of amorphous silica in the range 1 to 15 per cent by weight and an amount of a second amorphous silica in the range 4 to 20 per cent by weight.
17. A dental composition according to claim 14 further comprising a principal silica in an amount in the range 4 to 25 per cent by weight.
18. A dental composition according to claim 12 wherein the composition is a clear composition and has a maximum transparency with a light transmission of at least 70 per cent at a refractive index in the range 1.435 to 1.445.