GB2253839A

GB2253839A - Particulate inorganic titanate with a double coating of oxides

Info

Publication number: GB2253839A
Application number: GB9125909A
Authority: GB
Inventors: Paul Inchley
Original assignee: Tioxide Group Services Ltd
Current assignee: Tioxide Group Services Ltd
Priority date: 1991-03-21
Filing date: 1991-12-05
Publication date: 1992-09-23
Anticipated expiration: 2011-12-05
Also published as: GB9105979D0; GB9125909D0; GB2253839B

Abstract

An electroconductive powder comprises a particulate inorganic titanate having an inner coating on the particles of dense amorphous silica and a coating of tin oxide containing antimony oxide on said dense amorphous silica. Titanates of Mg, Zn, Ba, Sr and Ca are typical inorganic titanates and can have an average size of from 0.1 to 0.5 micron. Dense silica coatings in amounts of up to 50 weight per cent are typical but preferably from 12 to 25% by weight are applied. Amounts of the outer coating of tin oxide are typically 10 to 50% by weight of the titanate and can contain preferably 0.1 to 20% by weight of antimony oxide.

Description

TREATED POWDER This invention relates to a treated powder and particularly to an electroconductive pigment powder.

According to the present invention an electroconductive powder comprises a free flowing particulate inorganic titanate having an inner -coating on the particles thereof of dense amorphous silica and a coating of tin oxide containing antimony oxide on said dense amorphous silica.

The inorganic titanates include, but are not limited to, magnesium titanate, zinc titanate, barium titanate, strontium titanate, calcium titanate.

In its most preferred form the powder of the invention is pigmentary commercial inorganic titanate of normal purity, i.e. not having been specially purified, having the inner coating of dense amorphous silica and an outer coating of tin oxide containing antimony. The presence of the inner coating of dense amorphous silica acts as a barrier to the effect of elements or compounds present in the inorganic titanate core particle which in the absence of the inner coating often seriously reduce the electroconductive effect of the outer coating layer. Deliberate additions of agents which affect the properties of the inorganic titanate such as its optical properties, can be made without any substantial deleterious effect on the overall electroconductive properties of the powder.

Pigmentary inorganic titanate, such as those above have an average particle size of from 0.1 to 0.5 micron and preferably from 0.2 to 0.4 micron.

The particles of the present invention can be coated with widely differing amounts of dense amorphous silica but usually the amount of dense amorphous silica is at least 10 per cent by weight (expressed as SiO2) on weight of inorganic titanate and can be up to 50 per cent by weight (as SiO2) on weight of inorganic titanate.

Preferably the amount of dense amorphous silica is from 12 per cent to 25 per cent by weight (expressed as SiO2) on weight of inorganic titanate.

The coating of dense silica is substantially non-porous, amorphous and continuous over the particle. The coating of dense silica is formed from an alkaline solution and preferably from a solution of a soluble silicate at a pH greater than 8, most preferably at a pH of from 9 to 11.

The deposition of the dense silica results from the addition of a mineral acid such as sulphuric acid or hydrochloric acid to an alkaline solution of the soluble silicate containing the dispersed inorganic titanate to hydrolyse the silicate in solution to dense amorphous silica. For instance a solution of a soluble silicate can be mixed with an alkaline slurry or dispersion of the particles of inorganic titanate to be coated and then slowly acidified to deposit dense amorphous silica.

Alternatively there can be added to the slurry or dispersion of the particles of inorganic titanate an alkaline solution of a water soluble silicate and simultaneously a mineral acid to maintain the pH of the slurry at a value greater than 8, say 9 to 10.5 to form and deposit the required dense amorphous silica coating. Generally the temperature of the slurry is maintained at from 60"C to 100"C, preferably from 70"C to 90"C during deposition of dense amorphous silica and the slurry will be stirred to maintain effective coating.

Any suitable water soluble silicate can be used as the source of dense amorphous silica although preferably an alkali metal silicate is employed. Particularly useful are sodium and potassium silicates and also the solution of the silicate is freshly prepared.

After coating with the dense amorphous silica the product is usually washed and dried. However drying is not strictly necessary and the wet, coated inorganic titanate can be dispersed in water and coated with a hydrous oxide of tin and a hydrous oxide of antimony.

The amount of the outer coating can be within wide limits and typical amounts of the tin oxide expressed as SnO2 are in the range 10% to 50% of the weight of inorganic titanate. Usually the amount of antimony oxide in the coating layer is in the preferable range of 0.1% to 20two, most preferably 1% to 15% of the weight of the coating layer.

The hydrous oxides of tin and antimony are usually precipitated from hydrolysable salts of tin and antimony and whilst any water-soluble salt can be used, the chlorides are preferred although the sulphates, nitrates, oxalates and acetates are usable.

Precipitation can be effected by heating, e.g. at a temperature of from 60"C to 100 C an aqueous dispersion of the inorganic titanate having the inner coating of dense amorphous silica to which the hydrolysable salts are added. Slow addition of the salts in aqueous solution is preferred and the addition to the aqueous solution of hydrochloric acid is also preferred to reduce the rate of hydrolysis of the salts and achieve controlled deposition of the coating.

Precipitation of the hydrous oxides at a pH of from 1.5 to 10 can be effected with the pH being maintained at a fixed level, if desired, through the addition of an alkali.

After coating has been completed the product is washed and dried prior to heating at a temperature, usually of from 300"C to 900"C, preferably from 400"C to 7500 to dehydrate the hydrous oxides and produce the necessary mixed oxide conductive coating.

Products of the present invention find use in paints, plastics in forming reprographic toners and in electroconductive layers on papers for use in reproducing or duplication of documents and also as additives to resins to form antistatic resins.

The invention is illustrated in the following Examples.

EXAMPLE 1 A magnesium titanate powder with the following specification was used: MgO : 30.0% TiO2 : 64.5% Specific surface area : Approximately 6 m2/g Crystal size : Approximately 0.3 micron This powder was coated using the following procedure.

35g of magnesium titanate powder were dispersed in 700 cm3 of demineralised water by stirring, and the mixture heated to maintain the temperature at 90"C throughout the coating process. 33 cm3 of sodium silicate solution containing 158 g/l SiO2 was added to the stirred mixture over 1 hour, with a sufficient quantity a 10% w/v solution of sulphuric acid being added simultaneously to maintain the pH of the mixture in the range 9-9.5. The mixture was then stirred for a further 15 minutes before the pH was adjusted to 7-7.5 with 10% w/v sulphuric acid over 10 minutes. Finally the mixture was stirred for a further 10 minutes before being filtered, washed in demineralised water, reslurried in demineralised water, re-filtered and dried at llO"C.

The dense silica coated magnesium titanate was then coated with antimony-doped tin oxide as follows: 25g of the dried powder were redispersed in demineralised water by stirring, and the mixture heated to reflux. A solution made up from 16.6g of aqueous tin tetrachloride containing 430 g/l tin as SnO2, 2.36 cm3 of aqueous antimony pentachloride containing 200 g/l antimony as Sb203, and 10 cm3 of concentrated hydrochloric acid, was added dropwise to the refluxing mixture over 1 hour. The mixture was refluxed for a further 15 minutes, cooled and filtered, the solids washed and then reslurried in demineralised water, re-filtered and dried at 110 C, The resulting powder was then calcined at 600"C for 2 hours to crystallise the antimony doped tin oxide coating.

The product was a very pale blue-grey powder, with an electrical resistivity of 3 ohm cm.

EXAMPLE 2 The same magnesium titanate powder as in Example 1 was coated with silica using the following procedure.

1 Kg of magnesium titanate powder was dispersed in approximately 1400 cm3 of demineralised water by adding 63 cm3 of sodium silicate solution containing 158 g/l SiO2 and sand-milling this mixture for 1 hour. The sand was removed to leave a magnesium titanate slurry by washing on a coarse filter, and then diluted to a concentration of approximately 50 g/l. This was then stirred and heated to maintain a temperature of 90"C throughout the subsequent coating process. 887 ctii3 of the same sodium silicate solution were then added to the stirred mixture over 1 hour, with a sufficient quantity of a 10% w/v solution of sulphuric acid being added simultaneously to maintain the pH in the range 9-9.5.The mixture was then stirred for a further 15 minutes before the pH was adjusted to 7-7.5 with 10% w/v sulphuric acid over 10 minutes, and then stirred for a further 10 minutes.

The antimony tin oxide coating was applied immediately as follows.

The stirred mixture was maintained at 90"C by heating and a solution of antimony and tin chlorides, prepared in the same manner as in Example 1 and containing 180g of tin as SnO2 and 19g of antimony as Sb203, was added over a period of 1 hour, with a sufficient quantity of 300 g/l sodium hydroxide solution being added simultaneously to maintain the pH in the range 6-8. The mixture was then stirred for a further 15 minutes, cooled and filtered, the solids washed and then reslurried in demineralised water, refiltered and dried at 110 C. The resulting powder was then calcined at 600"C for 2 hours to crystallise the antimony-doped tin oxide coating.

The product was then hammer-milled to give a fine, very pale blue-grey powder with a resistivity of 16 ohm cm.

EXAMPLE 3 (Comparative Example) 50g of the same magnesium titanate powder as in Example 1 was coated with antimony tin oxide using the procedure given in Example 1, but without first applying a silica coating.

The resulting powder was very pale blue-grey in colour and had a resistivity of > 105 ohm cm and was not conducting.

Claims

1. An electroconductive powder comprising a free flowing particulate inorganic titanate having an inner coating on the particles thereof of dense amorphous silica and a coating of tin oxide containing antimony oxide on said dense amorphous silica.

2. A powder according to claim 1 in which said particles have an average size of from 0.1 to 0.5 micron.

3. A powder according to claim 2 in which the average size is from 0.2 to 0.4 micron.

4. A powder according to claim 1, 2 or 3 in which the amount of said dense amorphous silica is at least 10 per cent by weight as SiO2 on weight of inorganic titanate.

5. A powder according to claim 4 in which said amount is up to 50 per cent by weight as SiO2 on weight of inorganic titanate.

6. A powder according to claim 5 in which said amount is from 12 to 25 per cent by weight as SiO2 on weight of inorganic titanate.

7. A powder according to any of the preceding claims in which the amount of the tin oxide as SnO2 is from 10 to 50 per cent by weight of the inorganic titanate.

8. A powder according to claim 7 in which the amount of antimony oxide is from 0.1 to 20 per cent by weight of the weight of the coating layer containing tin oxide and antimony oxide.

9. A powder according to claim 8 in which the amount of antimony oxide is from 1 to 15 per cent by weight of the weight of said coating layer.

10. A powder according to any one of the preceding claims in which the inorganic titanate is magnesium titanate, zinc titanate, barium titanate, strontium titanate or calcium titanate.

11. A method for the manufacture of an electroconductive powder which comprises depositing an inner coating of dense amorphous silica on particulate inorganic titanate, depositing a coating on said particulate inorganic titanate of hydrous tin oxide containing hydrous antimony oxide and heating said coated particulate inorganic titanate to dehydrate said hydrous tin oxide and said hydrous antimony oxide.

12. A method for the manufacture of an electroconductive powder substantially as described in the foregoing ,examples 1 and 2.

13. An electroconductive powder when obtained by a method according to claim 11 or 12.