GB2115394A

GB2115394A - Titanium dioxide pigment

Info

Publication number: GB2115394A
Application number: GB08235673A
Authority: GB
Inventors: Peter Barry Howard; Brian Barnard; Donald Urwin
Original assignee: Tioxide Group Ltd
Current assignee: Tioxide Group Ltd
Priority date: 1982-02-13
Filing date: 1982-12-15
Publication date: 1983-09-07
Also published as: GB2115394B

Abstract

A process for the manufacture of titanium dioxide pigment suitable for use in a paper laminate comprises forming an aqueous dispersion of uncoated rutile titanium dioxide which dispersion contains a source of fluoride, and a solution of cerium ions, phosphate ions and a strong acid in amounts sufficient to produce a pH not greater than 0.5, and then adding to the dispersion a water-soluble aluminium compound and changing the pH to a value within the range 4 to 9 to produce the desired coating on the pigment.

Description

SPECIFICATION Titanium dioxide pigment This invention relates to a titanium dioxide pigment of improved light stability and particularly to a pigment for use in laminates.

In our British Patent Application No. 2 042 573Awe have described and claimed a titanium dioxide pigment which has an inner coating comprising cerium and phosphate radicals and an outer coating covering the inner coating and comprising aluminium and phosphate radicals. A process for the manufacture of the pigment is described and claimed which comprises adding to an aqueous dispersion of pigment rutile titanium dioxide a water-soluble cerium compound followed by a water-soluble phosphate or phosphoric acid followed by a water-soluble aluminium compound and changing the pH of the mixture to a value from 5 to 7.5.

The pigment of British Application No. 2 042 573A was designed for use in laminates, particularly paper laminates. Very surprisingly it has now been found possible to improve further the light-fastness of such pigments when so used through an important advance in pigment coating technology.

According to the invention a process for the manufacture of a titanium dioxide pigment comprisyes forming an aqueous dispersion of uncoated particulate pigmentary rutile titanium dioxide containing a source of fluoride, a water-soluble compound of cerium, a water-soluble phosphate or phosphoric acid and a strong acid different from phosphoric acid in an amount sufficient to produce an aqueous dispersion having a pH value of not greater than 0.5, adding to the dispersion a watersoluble aluminium compound and changing the pH of the dispersion to a value in the range 4 to 9 to effect deposition of a desired coating.

The uncoated titanium dioxide pigment which is to be coated in accordance with the process of the present invention may be that obtained by the "sulphate" process or that obtained by the "chloride" process. The "sulphate" process for the manufacture of titanium dioxide involves the digestion of a titaniferous ore with concentrated sulphuric acid and the subsequent hydrolysis ofthetitanyl sulphate solution obtained by the dissolution of the digestion cake. Subsequently the hydrous titanium dioxide obtained by hydrolysis is calcined at an elevated temperature.

The "chloride" process involves the oxidation in the vapour phase of a titanium halide, usually titanium tetrachloride, to produce titanium dioxide directly in pigmentaryform.

The titanium dioxide to be coated in accordance with the present invention is rutile titanium dioxide and should preferably contain at least 95% by weight of the titanium dioxide in the rutile form. Most preferably the pigment contains at least 98% by weight of the titanium dioxide in the rutile form.

Pigments of particular usefulness contain in excess of 99% by weight of the titanium dioxide in the rutile form.

Pigments according to the present invention are prepared by forming an aqueous dispersion of the rutile "sulphate" or "chloride" titanium dioxide to be coated and then adding to the dispersion the required coating reagents to form the desired coatings on the particles of pigment. In the case of "sulphate" pigment usually the dispersion is milled, for example in a sand mill prior to the coating reagents being added. Usually the pigment is dispersed with the assistance of a dispersing agent and depending on the particular form of the pigment suitable dispersing agents are inorganic or organic compounds such as sodium hexametaphosphate or amines.In those cases when a "sulphate" based titanium dioxide pigment free of zinc is to be coated than it is desirable to effect the dispersion of the pigment through the use of an organic dispersing agent such as an alkanolamine for example monoisopropanolamine.

In any event, where a dispersing agent is used the amount of the dispersing agent should preferably be low and is only that which is sufficient to effect the required degree of dispersion. For instance when monoisopropanolamine is employed as a dispersant an amount of less than 0.2% based on the weight of TiO2 is recommended.

To the aqueous dispersion of the uncoated pigmentary rutile titanium dioxide there is added a water-soluble salt of cerium such as cerium sulphate. The water-soluble salt of cerium is added usually in the form of an aqueous solution and in an amount sufficient to provide on the surface of the pigment the desired amount of cerium phosphate.

The amount of the water-soluble compound of cerium usually in such as to be equivalent to 0.05% to 1% (expressed as CeO2) of the weight of TiO2 in the pigment. Preferably the amount is from 0.1% to 0.4% by weight (expressed as CeO2) on the weight of TiO2 in the pigment.

To the dispersion containing the water-soluble salt of cerium there is then added a water soluble phosphate or orthophosphoric acid in an amount at least equal to that required to precipitate the whole of the cerium as phosphate. A typical water-soluble phosphate is an alkali metal orthophosphate or ammonium orthophosphate.

When the dispersion contains both the cerium salt and the phosphate or orthophosphoric acid it is believed that the cerium phosphate is deposited as a discrete coating upon the surface of the pigment particles.

To the aqueous dispersion is added a strong acid in an amount such that the pH of the dispersion is not greater than 0.5. The strong acid usually will be an inorganic acid other than phosphoric acid and the most convenient acid is sulphuric acid. Hydrochloric acid can be used if desired. The acid can be added to the aqueous dispersion prior to the water-soluble compound of cerium and prior to the water-soluble phosphate or phosphoric acid. The subsequent addition of these coating reagents will not reduce the pH much below the value produced by the addition of the strong acid. Alternatively, if desired the acid can be added during the addition of the coating reagents or thereafter.

To the aqueous suspension there is then added a water-soluble aluminium compound, usually a water-soluble alkali metal aluminate but aluminium salts, such as aluminium sulphate may be used if desired. Usually the water-soluble aluminium compound is added in the form of an aqueous solution in an amount sufficient to provide a required amount of aluminium phosphate in the outer coating. Typically the amount of the water-soluble compound of aluminium will be such as to provide from 0.05% to 5% by weight expressed as Al203 on the weight of TiO2 in the pigment. Preferably the amount is from 0.1% to 3% as Al203 by weight of the TiO2.Most preferably the amount of the water-soluble compound of aluminium is in excess of that required to provide the desired amount of aluminium phosphate in order that there shall also be present in the coating a quantity of a hydrous oxide of aluminium.

If the amount of water-soluble phosphate and/or phosphoric acid added originally is insufficient to provide both the cerium phosphate and the aluminium phosphate then a further amount of watersoluble phosphate and/or phosphoric acid is added to the dispersion. Usually this further amount is added prior to the addition of the water-soluble aluminium compound.

After or during the addition of the water-soluble aluminium compound, the pH of the aqueous dispersion is changed to a value within the range 4 to 9, preferably 5 to 7.5.

The coated pigment is then separated from the solution byfiltering and is washed, dried and preferably milled in a fluid energy mill.

The pigment produced by the method of the invention also contains a source of fluoride. Preferably this is calcium fluoride which can be added as the pure chemical but preferably is added in the naturally occurring form fluorspar. The fluoride can be added to the aqueous dispersion at any stage but preferably is added after the addition of the source of cerium. Typical amounts of fluoride range from 0.05% to 15% by weight expressed as the fluoride and preferably from 0.25% to 5% by weight of TiO2.

The pigment also preferably contains a second surface stabiliser which has the effect of increasing further the resistance of a composition containing the pigment to discolouration by light. Examples of suitable surface stabilisers are the other halides, halates and perhalates such as metal chlorates, bromates, iodates, metaperiodates and paraperiodates. The most preferred other surface stabiliser is an antimony oxide. Most preferably the pigment contains a mixture of calcium fluoride and antimony oxide. Typical amounts of antimony oxide which can be used range from 0.01% to 1.0% by weight as Sub203, and preferably 0.05% to 0.5% by weight as Sb203 on TiO2. Preferably the source of antimony oxide is added prior to the addition of the source of cerium to the aqueous dispersion.

The pigments of the present invention are of particular use in the formation of pigmented aminoplastic resinous materials and especially when these are used to form white or coloured laminates which are required to be resistant to discolouration by light. Such laminates are products in which the resin acts not only as reinforcement for one or more layers or masses of such materials as wood, glass fibre and paper or other fabric but also to import strength and durability to the finished product. A typical decorative paper laminate consists of pigmented paper sheets impregnated with resin and cured under pressure and at elevated temperature. The resin may alternatively contain filler such as fibre glass, wood flour etc and be used in safety helmets etc.

Typical aminoplastic resinous materials which may be used with rutile titanium dioxide of the present invention are melamine-formaldehyde, urea-formaldehyde and phenol-formaldehyde resins.

Accordingly the present invention also provides a process for the manufacture of a pigmented aminoplastic resinous material in which a coated titanium dioxide pigment in accordance with the invention is added, if desired with a carrier material comprising a fibrous base, to an aminoplastic resinous material, which is then heated to effect formation of a cross-linked state.

Whilst it is believed that the pigment of the invention is coated with the stated materials this is not restrictive in anyway and should be understood to include pigments in which the stated materials have been precipitated into association with the titanium dioxide.

The present invention is illustrated in the following Examples in which the light fastness of the pigment when employed in a decorative laminate was determined by the following general method.

Alpha pulp was lightly processed at 2.5% consistench in a laboratory scale, Hollanderbeater. By this means the fibre was dispersed without undergoing significant refining or hydration. After processing the stock was diluted to 1.25% consistency.

Using the British Standard Pulp Evaluation Apparatus, a 2 litre aliquot of stock was disintegrated for 10 minutes. 1 2.0g pigment to be tested were then added and disintegration continued for 5 minutes.

0.5% Al2(SO4)3 (calculated on fibre) was added and disintegration was continued for a further 5 minutes.

The disintegrated stock was then diluted to 0.5% consistency.

380 ml dilute stock were placed in the sheet forming machine. After forming the sheet it was dried on a felt covered rotary drier, weighed and ashed. Having ensured that the desired specification i.e. 125 g/m2 and 25% pigment content (equivalent to ash) had been achieved, further papers were made for subsequent stages in the test procedure.

Samples of paper were immersed in a bath of melamine formaldehyde resin solution, at 47.5% concentration. After 60 seconds the sheet was removed and allowed to drain for 15 seconds. The sheet was then inverted and hung on a suitable rack.

When sufficient papers had been impregnated, they were transferred to an oven at 11 00C and they remained there for 10 minutes.

4 commercial core papers impregnated with phenol formaldehyde resin were used for the body of the laminate. The pigmented, impregnated paper to be tested was placed on top. The assembly of papers was placed between mirror finished, stainless steel plates and placed in a hydraulic press equipped with steam heated/water cooled platens. Hydraulic pressurn was applied at 1400 pounds/square inch (on the surface of the laminate) and the temperature was raised to 1 400C and maintained for 30 minutes. The press was cooled to ambient temperature, pressure released and the laminate removed.

The laminate was tested for light fastness in accordance with the method described in BS3794: 1973 Appendix G. The experimental pigments described in this application gave light fastness results superior to Blue Wool Scale Standard No.6 (ref BS1006:1953).

Example 1 1000 grams of titanium dioxide pigment in the form of a reactor discharge obtained from the chloride process for the preparation of the pigment was dispersed in water by mixing in the presence of sodium hexametaphosphate and sodium hydroxide in amounts sufficient to produce on dispersion a pH value of 10 and an amount of hexametaphophate equivalent to 0.10% P205 by weight of titanium dioxide pigment. The concentration of the pigment in the aqueous dispersion obtained was adjusted to a value of 240 grams per litre titanium dioxide pigment and the temperature raised to 50"C and maintained at this value throughout the subsequent coating process stages.

Solid antimony trioxide in an amount equivalent to 0.1% by weight on pigment was added to the slurry followed by aqueous cerium sulphate solution (52 g.p.l. CeO2) in an amount equivalent to 0.3% by weight CeO2 on weight of pigment added over a period of 5 minutes and then the dispersion mixed for a further 10 minutes prior to the addition of solid finely ground fluospar (CaF2) in an amount equivalent to 2% by weight CaF2 on pigment. The aqueous dispersion was then mixed for a further 5 minutes.

Aqueous monoammonium phosphate solution in a concentration of 100 g.p.l. P205 was then added over a period of 10 minutes in an amount sufficient to introduce into the aqueous dispersion the equivalent of 0.2% by weight P205 on pigment. Aluminium sulphate solution (90 g.p.l. Al203) in an amount equivalent to 1% by weight Al203 on pigment was then added over a period of 10 minutes and the dispersion mixed for 5 mins.

Aqueous sodium aluminate solution containing 88 g.p.I.Al2O3 was then added to the aqueous dispersion over a period of 20 minutes in an amount sufficient to introduce the equivalent of 3.0% by weight Al203 on pigment. The pH of the slurry was then adjusted to a value of 10 by the addition of alkali and the dispersion mixed for a further 30 minutes.

Dilute sulphuric acid was then added to the dispersion in an amount sufficient to reduce the pH to a value of 6.

The pigment obtained was tested for its lightfastness and a value of 6 was obtained.

This Example is a control example.

Example 2 The procedure described in Example 1 was repeated except that after the dilution of the initial aqueous dispersion to a pigment concentration of 240 g.p.l. the pH of the aqueous dispersion was reduced by the addition of dilute sulphuric acid solution (10%) in an amountsufficientto reduce the pH to a value of not greater than 0.5.

The pigment was coated by the procedure described in Example 1 and tested for its lighffastness.

The light fastness of the pigment had a value of 2.

Example 3 Titanium dioxide which had been prepared by the sulphate process as calciner discharge was milled in a sand mill in the presence of 0.18% monoisopropanolamine (MIPA) on weight of titanium dioxide to give an aqueous milled dispersion containing 220 grams per litreTiO2. The temperature of the aqueous dispersion so obtained was maintained at 500C throughout the coating procedure. The pH of the aqueous dispersion was reduced by the addition of dilute sulphuric acid solution (10%) in an amount sufficient to reduce the pH to a value not greater than 0.5.

The pigment was then coated by the procedure described in Example 1 and tested for its lightfastness. The lightfastness of the pigment had a value of 3.

It will be seen that comparing the properties of pigment prepared by the procedures of Example 2 and 3 with that obtained by the procedure of Example 1 a very surprising improvement in the lightfastness value is obtained.

Claims

1. A process for the manufacture of a titanium dioxide pigment which comprises forming an aqueous dispersion of uncoated particulate pigmentary rutile titanium dioxide containing a source of fluoride, a water-soluble compound of cerium, a water-soluble phosphate or phosphoric acid and a strong acid different from phosphoric acid in an amount sufficient to produce an aqueous dispersion having a pH value of not greater than 0.5, adding to the dispersion a water-soluble aluminium compound and changing the pH of the dispersion to a value in the range 4 to 9 to effect deposition of a desired coating.

2. A process according to claim 1 in which the strong acid different from phosphoric acid is added to an aqueous dispersion of said titanium dioxide prior to the addition of the water-soluble compound of cerium and the water-soluble phosphate or phosphoric acid.

3. A process according to claim 1 in which the strong acid different from phosphoric acid is added to an aqueous dispersion of titanium dioxide during the addition of the water-soluble compound of cerium and the water-soluble phosphate or phosphoric acid.

4. A process according to claim 1 in which the strong acid different from phosphoric acid is added to an aqueous dispersion of titanium dioxide after the addition thereto of the water-soluble compound of cerium and the water-soluble phosphate or phosphoric acid.

5. A process according to any one of the preceding claims in which the amount of the water-soluble compound of cerium in the aqueous dispersion is equivalent to 0.05% to 1% by weight expressed as CeO2 on the weight of TiO2 in the pigment.

6. A process according to claim 5 in which the amount of the water-soluble compound of cerium is from 0.1% to 0.4% by weight expressed as CeO2 on weight of TiO2 in the pigment.

7. A process according to any one of the preceding claims in which the amount of the water soluble phosphate or phosphoric acid in the aqueous dispersion is at lesst equal to that required to precipitate the whole of the cerium as phosphate.

8. A process according to any one of the preceding claims in which the amount of the water-soluble aluminium compound is from 0.05% to 5% by weight expressed as Al203 on the weight of the TiO2 in the pigment.

9. A process according to claim 8 in which the amount of the water-soluble aluminium compound is from 0.1% to 3% as Awl203 by weight of the TiO2 in the pigment.

10. A process according to any one of the preceding claims in which the strong acid is sulphuric acid.

11. A process according to any one ofthe preceding claims in which the pH is changed to a value of 5 to 7.5 after the deposition of the coating.

12. A process according to claim 1 substantially as described in the foregoing Examples.

13. A pigment when prepared buy a process in accordance with any one of the preceding claims.