GB2333100A

GB2333100A - Titanium dioxide particles coated with aluminium phosphate

Info

Publication number: GB2333100A
Application number: GB9909018A
Authority: GB
Inventors: Jonathan Banford; Geoffrey Elliott Maud; Maria Leonida Demosthenous
Original assignee: Tioxide Group Services Ltd
Current assignee: Tioxide Group Services Ltd
Priority date: 1995-07-13
Filing date: 1996-06-12
Publication date: 1999-07-14
Anticipated expiration: 2016-06-12
Also published as: GB9909019D0; GB2333101B; GB2333100B; GB9909018D0; GB2333101A

Abstract

A titanium dioxide pigment particles have a surface coating of aluminium phosphate in which the molar ratio of aluminium to phosphorus lies in the range 1.8:1 to 1:1.4 A1:P and an oxidising agent in an amount effective to improve the lightfastness of the pigment. The oxidising agent is preferably potassium nitrate in amount 0.5-1.5 wt% based on the dry weight of titanium dioxide, or may be potassium iodate or copper sulphate.

Description

TITANIUM DIOXIDE PIGMENTS This invention relates to titanium dioxide pigments of improved light fastness and particularly to pigments for use in laminates.

Laminates consisting of multilayers of, for example, paper, impregnated with melamine-formaldehyde, phenol-formaldehyde or urea-formaldehyde and then compressed, are widely used in industry for purposes such as decorative panels, colouration being effected by incorporating pigments into the top layer or layers of paper.

Titanium dioxide is a pigment which is eminently suitable for this purpose. However, it is known that titanium dioxide, normally a white pigment, tends to develop a grey colouration when exposed to UV radiation (e.g. from sunlight) in the presence of the resin.

Various attempts have been made to solve this problem by coating the surface of the pigment as disclosed, inter alia, in GB Specifications Nos 1 023 590, 1 419 157, 1 479296 and 2042573.

It is an object of this invention to provide a solution to the problem of grey colouration by coating particles of titanium dioxide with a coherent coating of aluminium phosphate.

According to the invention there is provided a titanium dioxide pigment the particles of which have a surface coating of aluminium phosphate in which the molar ratio of aluminium to phosphorus lies in the range 1.8:1 to 1:1.4 Al:P and an oxidising agent in an amount effective to improve the lightfasness of the pigment.

Compounds which are suitable as oxidising agents include potassium iodate and copper sulphate but the preferred compound is potassium nitrate.

A process for the manufacture of the pigments of the invention in which a suspension of titanium dioxide is mixed with a reagent which has been formed by reacting concentrated phosphoric acid with aluminium or an aluminium compound is described and claimed in GB 2303366 from which this application has been divided.

Surprisingly, a particularly coherent coating of aluminium phosphate is formed on the pigment particles when precipitated according to the process of GB 2303366.

Whilst the mechanism by which the precipitation occurs is not fully understood, it is believed that O-P-O-Al-O-P- chains are formed in solution during the reaction between the aluminium or aluminium compound and the phosphoric acid. When the pH is raised these short chains condense into longer chains and networks, (the excess phosphate being ejected) which have a high affinity for the pigment surface and cover it completely.

It is believed that the aluminium phosphate which is deposited as a coating on the particles of titanium dioxide is a form in which aluminium and phosphate radicals are present in approximately equimolar proportions.

The amount of aluminium phosphate which is precipitated on to the particles of titanium dioxide is sufficient to form a coherent and impervious coating on the particles. Normally an amount of aluminium equivalent to from 2 per cent to 6 per cent by weight calculated as Al 203 with respect to TiO2 is precipitated. Preferably, the amount of aluminium is from 3.5 per cent to 4.5 per cent by weight calculated as Al203 with respect to TiO2. The amount of phosphorus in the coating is normally from 2 per cent to 12 per cent by weight calculated as P2Os with respect to TiO2 preferably from 3 per cent to 10 per cent by weight calculated as P205 with respect to TiO2 and, more preferably, from 5 per cent to 6.5 per cent by weight calculated as P20s with respect to TiO2.

Examination of the pigments of the invention by electron microscopy has disclosed that the coating of aluminium phosphate is smooth and coherent. The light fastness of the coated pigments is good.

The invention will be more readily understood from the following examples.

EXAMPLE 1 Coating reagent was prepared by first adding 10.5 litres of demineralised water to 50 litres of 85% phosphoric acid and then adjusting the temperature of the resulting mixture to 32-38"C. 16.7 kg aluminium hydroxide was added and the mixture agitated until the exothermic reaction had ceased. The temperature was then adjusted to 70"C and maintained until a clear solution was obtained which was diluted to 104 litres with demineralised water.

200 kg of rutile titanium dioxide particles manufactured by the sulphate process were sand milled using 0.4% by weight monoisopropanolamine (MIPA) as dispersant and then diluted with demineralised water to form a dispersion containing 250 g per litre titanium dioxide.

The dispersion was heated to 50-600C and the coating reagent added over a period of 15 minutes. After a further 15 minutes a solution containing 220g per litre sodium hydroxide was added slowly over a period of 30 minutes until a pH of 5 was reached (i.e. an amount equivalent to 0.75 litres per litre of reagent). The resulting dispersion was filtered and the filter cake washed with warm demineralised water. 0.75% by weight (based on the dry weight of titanium dioxide) of potassium nitrate added as an approximately 10 wt% solution was mixed into the washed filter cake, which was then dried and milled in a fluid energy mill.

Analysis showed that the titanium dioxide carried a coating having an aluminium to phosphorus ratio of 1:1.05 Al:P in a quantity equivalent to 3.95% Alp03 with respect to TiO2.

EXAMPLE 2 Samples of the products from Example 1 and samples of coated titanium dioxide pigments available commercially as TIOXIDE TR44 (from Tioxide Group Limited) and Tiofine R820 (from Kemira) were incorporated into paper, the amount incorporated being calculated to produce 30 + 1% ash. The resulting papers were impregnated with an aqueous solution of Beetle BL434 melamine-formaldehyde resin, dried and pre-cured.

Laminate test samples were prepared for light fastness and colour measurement by pressing together in a conventional manner a double layer of pigment-containing paper and five kraft core papers.

Samples for opacity and contrast measurement were prepared in a similar way except that half the area of the surface was formed by a single layer of pigment-containing paper.

Exposure was carried out with the samples half masked in a Xenotest 1200LM accelerated weathering machine for 96 hours at 80% power, 50"C and 30% humidity (i.e. an exposure sufficient to fade the Blue Wool Standard No 6 to Grey Scale No 4).

Measurements were carried out using a Pacific Scientific Colorguard System 05 colorimeter.

The results of the tests are shown in Table 1 in which: Retention is the percentage of the original pigment added which is retained in the paper; CR is the ratio of the percentage of the light reflected from the single layer thickness to the light reflected from the soluble layer thickness and is thus a measure of the opacity of the pigment-impregnated paper; L*, a* and b* are the values for brightness, red/green hue and yellow/blue hue respectively according to the modified CIE system (CIELAB);

and is a measure of the initial appearance

and is a measure of the change in appearance.

BWS is the value of the exposed sample on the Society of Dyers and Colourists Blue Wool Scale.

TABLE I Example 1 TR44 R820 Ash (%) 30.50 29.00 30.30 Retention (%) 64.50 68.10 62.10 CR 95.50 95.90 95.60 L* unexposed 94.00 93.80 94.60 a* unexposed -0.90 -0.90 -1.00 b* unexposed 3.80 4.10 3.50 w 86.60 85.80 87.70 L* exposed 93.50 93.30 94.30 a* exposed -1.10 -1.00 -1.20 b* exposed 3.00 3.10 2.60 AE 0.89 1.10 0.96 BWS 6-7 5-6 6-7 EXAMPLE 3 Coating reagent was prepared by adding 20 grams of commercial aluminium foil to 160 millilitres of 85% phosphoric acid, waiting until the spontaneous effervescence had died down and the foil had completely dissolved, and diluting with 250 millilitres of demineralised water.

2000 grams of rutile titanium dioxide particles manufactured by the sulphate process were sand milled using 0.4% by weight monoisopropanolamine (MIPA) as dispersant and then diluted with demineralised water to form a dispersion containing 250g per litre titanium dioxide, half of which, containing 1000 grams of titanium dioxide, was used in this example.

The dispersion was heated to 50-60"C and the above-mentioned coating reagent added over a period of 15 minutes. After a further 15 minutes, a solution containing 110g per litre sodium hydroxide was added slowly over a period of 30 minutes until a pH of 5 was reached. The resulting dispersion was filtered and the filter cake washed with warm demineralised water. 0.75% by weight (based on the dry weight of titanium dioxide) of potassium nitrate in aqueous solution was mixed into the washed filter cake, which was then dried and milled in a fluid energy mill.

Analysis showed that the titanium dioxide carried a coating having an aluminium to phosphorus ratio of 1:1.22 Al:P in a quantity equivalent 2 3 th respect to TiO to 3.8% A1,0, with respect to EXAMPLE 4 Samples of the products from Example 3 and control samples of coated titanium dioxide pigments available commercially as TIOXIDE TR44 (from Tioxide Group Limited) and Tiofine R820 (from Kemira) was incorporated into paper and were tested in paper laminates in an identical manner to Example 3. The ash content of the papers produced with the control samples were found to be slightly outside the target range of 30+1% but this is not believed to have a significant effect on the results.

The results of the tests are shown in Table II.

TABLE II Example 4 TR44 R820 Ash (%) 30.50 31.50 32.60 Retention (%) 50.00 57.50 55.00 CR 95.60 94.40 95.10 L* unexposed 94.50 94.80 95.65 a* unexposed -0.72 -0.91 -1.03 b* unexposed 2.82 3.11 2.39 w 89.50 88.90 91.30 L* exposed 94.40 94.85 95.76 a* exposed -1.54 -0.74 -0.75 b* exposed 2.15 2.05 1.43 E 0.70 1.07 1.01 BWS 6 6 7 EXAMPLE 5 Coating reagent was prepared by slurrying 80 grams of aluminium hydroxide in 25ml of demineralised water, and then adding 1 12ml of 85% phosphoric acid. The slurry was warmed with stirring until a definite exothermic reaction began. Once the reaction had ceased, the temperature was maintained at 90"C for 30 minutes. The resulting paste was let down with 200 mil of 50% phosphoric acid, followed by dilution to 500ml with demineralised water.

The dispersion was heated to 50-60"C and 370ml of the coating reagent added over a period of 15 minutes. After a further 15 minutes, a solution containing 220g per litre sodium hydroxide was added slowly over a period of 30 minutes until a pH of 5 was reached (i.e. an amount approximately equivalent to 0.5 litres per litre of reagent). The resulting dispersion was filtered and the filter cake washed with warm demineralised water. 0.75% by weight (based on the dry weight of titanium dioxide) of potassium nitrate in aqueous solution was mixed into the washed filter cake, which was dried and milled in a fluid energy mill.

Analysis showed that the titanium dioxide carried a coating having an aluminium to phosphorus ratio of 1:1.16 Al:P in a quantity equivalent to 2.9% Al203 with respect to TiO2.

EXAMPLE 6 2000 grams of rutile titanium dioxide particles manufactured by the sulphate process were sand milled using 0.4% by weight monoisopropanolamine (MIPA) as dispersant and then diluted with demineralised water to form a dispersion containing 250g per litre titanium dioxide, half of which, containing 1000 grams of titanium dioxide, was used in this example.

The dispersion was heated 50-60"C and 494ml of the coating reagent used in Example 7 was added over a period of 15 minutes. After a further 15 minutes, a solution containing 220g per litre sodium hydroxide was added slowly over a period of 30 minutes until a pH of 5.1 was reached (i.e. an amount approximately equivalent to 0.5 litres per litre of reagent). The resulting dispersion was filtered and the filter cake washed with warm demineralised water. 0.75% by weight (based on the dry weight of titanium dioxide) of potassium nitrate in aqueous solution was mixed into the washed filter cake, which was dried and milled in a fluid energy mill.

Analysis showed that the titanium dioxide carried a coating having an aluminium to phosphorus ratio of 1:1.15 A1:P in a quantity equivalent to 3.8% A1203 with respect to TiO2.

EXAMPLE 7 2000 grams of rutile titanium dioxide particles manufactured by the sulphate process were sand milled using 0.4% by weight monoisopropanolamine (MIPA) as dispersant and then diluted with demineralised water to form a dispersion containing 250g per litre titanium dioxide, half of which, containing 1000 grams of titanium dioxide, was used in this example.

The dispersion was heated 50-60"C and 617ml of the coating reagent used in Example 7 was added over a period of 15 minutes. After a further 15 minutes, a solution containing 220g per litre sodium hydroxide was added slowly over a period of 30 minutes until a pH of 5.1 was reached (i.e. an amount approximately equivalent to 0.5 litres per litre of reagent). The resulting dispersion was filtered and the filter cake washed with warm demineralised water. 0.75% by weight (based on the dry weight of titanium dioxide) of potassium nitrate in aqueous solution was mixed into the washed filter cake, which was dried and milled in a fluid energy mill.

Analysis showed that the titanium dioxide carried a coating having an aluminium to phosphorus ratio of 1:1.10 Al:P in a quantity equivalent to 4.6% 3 Al203 with respect to TiO z 2 EXAMPLE 8 Samples of the products from Examples 5, 6 and 7 and samples of coated titanium dioxide pigments available commercially as TIOXIDE TR44 (from Tioxide Group Limited) and Tiofine R820 (from Kemira) were incorporated into paper and were tested in paper laminates in an identical manner to Example 2. The results of the tests are shown in Table TABLE III Example Example Example TR44 R820 5 6 7 Ash (%) 29.10 29.00 28.60 30.30 29.10 Retention (%) 44.60 44.40 41.70 63.90 48.90 CR 95.00 94.90 93.60 94.40 94.50 L* unexposed 94.57 94.73 94.88 94.67 95.23 a* unexposed -0.92 -0.90 -1.05 -1.02 -1.12 b* unexposed 3.19 3.13 3.23 3.40 2.93 W 88.50 88.80 88.60 88.00 89.60 L* exposed 93.96 93.91 93.94 93.97 94.58 a* exposed -0.70 -0.70 -0.78 -0.75 -0.94 b* exposed 2.35 2.16 2.34 2.26 1.58 AE 1.06 1.29 1.32 1.36 1.51 BWS 6-7 7 5 6-7 7 EXAMPLE 9 2000 grams of rutile titanium dioxide particles manufactured by the sulphate process were sand milled using 0.4% by weight monoisopropanolamine (MIPA) as dispersant and then diluted with demineralised water to form a dispersion containing 250 g per litre titanium dioxide, half of which, containing 1000 grams of titanium dioxide, was used in this example.

The dispersion was heated to 50-600C and 494ml of coating reagent made as in Example 7 was added over a period of 15 minutes.

After a further 15 minutes, a solution containing 220 g per litre sodium hydroxide was added slowly over a period of 30 minutes until a pH of 5.0 was reached (i.e. an amount approximately equivalent to 0.5 litres per litre of reagent). The resulting dispersion was filtered and the filter cake washed with warm demineralised water. the cake was split in half, and to one half was added 0.75% by weight (based on the dry weight of titanium dioxide) of sodium nitrate in aqueous solution. The treated filter cake was dried and milled in a fluid energy mill.

Analysis showed that the titanium dioxide carried a coating having an aluminium to phosphorus ratio of 1:1.14 A1:P in a quantity equivalent to 4.2% A1203 with respect to TiO2.

EXAMPLE 10 The half of the filter cake from the previous example not treated with sodium nitrate was treated with 0.75% potassium nitrate in aqueous solution, and was then dried and fluid energy milled in the same manner.

EXAMPLE 11 Samples of the products from Examples 9 and 10 and control samples of coated titanium dioxide pigments available commercially as TIOXIDE TR44 (from Tioxide Group Limited) and Tiofine R820 (from Kemira) were incorporated into paper and were tested in paper laminates in an identical manner to Example 2. As in Example 4, the ash contents for the papers containing control samples were found to be outside the target range of 30+ 1%. The results of the tests are shown in Table IV.

TABLE IV Example 9 Example 10 TR44 R820 Ash (%) 29.70 31.80 31.50 32.60 Retention (%) 48.00 53.00 57.50 55.00 CR 94.80 95.60 94.40 95.10 L* unexposed 93.63 93.88 94.80 95.65 a* unexposed -0.76 -0.73 -0.91 -1.03 b* unexposed 2.76 2.57 3.11 2.39 W 89.20 89.80 88.90 91.30 L* exposed 93.61 93.56 94.85 95.76 a* exposed -0.63 -0.64 -0.74 -0.75 b* exposed 1.78 1.64 2.05 1.43 AE 0.99 0.99 1.07 1.01 BWS 7 7 6 7 EXAMPLES 12- 15 Coating reagent was prepared by dissolving with warming and stirring 1 60g aluminium hydroxide in a mixture of 224 ml 85% phosphoric acid and 81 ml demineralised water. After the exothermic reaction had ceased the resulting paste was diluted with 200 ml 25% by weight phosphoric acid followed by 1000 ml demineralised water to give a clear solution containing the equivalent of 120g A1203 per litre.

2000 grams of rutile titanium dioxide particles manufactured by the sulphate process were sand milled using 0.4% by weight monoisopropanolamine (MIPA) as dispersant and then diluted with demineralised water to form a dispersion containing 250 g per litre titanium dioxide.

This dispersion was split into two, each half containing 1000 grams of titanium dioxide. Each half was heated to 50-60"C and 250ml of coating reagent was added over a period of 30 minutes, while a solution containing 222 g per litre of sodium hydroxide was added simultaneously, to keep the solution at a pH value between 4.7 and 5.3 (approximately 140ml was required). After a further 15 minutes, 115 ml of a solution of aluminium sulphate containing the equivalent of 87g per litre of A1203 was added over a period of 15 minutes, while a solution containing 222 g per litre of sodium hydroxide was added simultaneously, to keep the solution at a pH value between 5.2 and 5.8.

The two slurries were then combined prior to filtration. The filter cake was washed with warm demineralised water, and was then split into four equal parts. The first of these was dried and milled in a fluid energy mill without further treatment (Example 12). To the second was added 0.5% by weight potassium nitrate as an aqueous solution (Example 13), to the third was added 1.0% potassium nitrate as an aqueous solution (Example 14), and to the fourth was added 1.5% potassium nitrate as an aqueous solution (Example 15). Each cake was dried and milled in a fluid energy mill.

Analysis showed that in each case the titanium dioxide carried a coating having an aluminium to phosphorus ratio of 1:1.00 A1:P in a quantity equivalent to 3.5% A1203 with respect to TiO2.

EXAMPLE 16 Samples of the products from Examples 12 - 15 and samples of coated titanium dioxide pigments available commercially as TIOXIDE TR44 (from Tioxide Group Limited) and Tiofine R820 (from Kemira) were incorporated into paper and were tested in paper laminates in an identical manner to Example 2. The results of the tests are shown in Table V: TABLE V Example Example Example Example TR44 R820 12 13 14 15 Ash (%) 31.00 29.70 31.00 30.00 30.40 30.40 Retention (%) 62.40 58.70 62.40 59.50 66.20 56.00 CR 94.70 93.90 93.10 94.40 94.80 94.20 L* unexposed 95.10 94.59 94.73 94.82 94.86 95.43 a* unexposed -1.18 -1.17 -118 -1.17 -1.05 -1.14 b* unexposed 3.24 3.15 3.10 3.10 3.24 2.61 W 88.64 88.66 88.87 88.91 89.54 90.57 L* exposed 93.49 93.46 93.25 93.31 94.50 94.88 a* exposed -0.88 -0.87 -0.91 -0.95 -0.78 -1.00 exposed 2.17 1.93 2.16 2.16 2.04 1.55 E 1.96 1.69 1.77 1.79 1.28 1.20 BWS 5 5-6 6 6 6-7 7 EXAMPLE 17 Coating reagent was prepared by slurrying 80 grams of aluminium hydroxide in 25 ml of demineralised water, and then adding 106 ml of 85% phosphoric acid. The slurry was warmed with stirring until a definite exothermic reaction began. Once the reaction had ceased, the temperature was maintained at 900C for 30 minutes. The resulting paste was let down with 200 ml of diluted phosphoric acid prepared by mixing equal weights of phosphoric acid and water, following by dilution to 500 ml with demineralised water. The resulting reagent contained aluminium and phosphorus in the ratio of 1:2.97 A1:P by weight.

A dispersion containing 1000 grams of rutile titanium dioxide particles was prepared by diluting a slurry obtained from the sane mill of a chloride process titanium dioxide plant to 1 80g per litre titanium dioxide.

The dispersion was heated to 50-60"C and 152 ml of the coating reagent added over a period of 15 minutes. After a further 15 minutes, a solution of alkaline sodium aluminate containing the equivalent of 2.4 per cent Alto, by weight with respect to TiO2 was added slowly over a period of 30 minutes. When a pH of 5 was reached hydrochloric acid was added simultaneously at a rate which maintained the pH in the range 5.0 to 5.5.

When all the sodium aluminate had been added the resulting dispersion was stirred for a further 20 minutes and filtered and the filter cake washed with warm demineralised water. 0.75% by weight (based on the dry weight of titanium dioxide) of potassium nitrate in aqueous solution was mixed into the washed filter cake, which was dried and milled in a fluid energy mill.

Analysis showed that the titanium dioxide carried a coating having an aluminium to phosphorus ratio of 1:1.15 in a quantity equivalent to 2 A120; with respect to TiO 4.0%AlO with 2 A sample of this product and samples of coated titanium dioxide pigments available commercially as TIOXIDE TR44 (from Tioxide Group Limited) and Tiofine R820 (from Kemira) were incorporated into paper and were tested in paper laminates in a manner similar to that used in Example 2. The following results were obtained: Example 17 TR44 R820 AE 1.3 1.6 1.5 BWS 7-8 7 7

Claims

CLAIMS 1. A titanium dioxide pigment the particles of which have a surface coating of aluminium phosphate in which the molar ratio of aluminium to phosphorus lies in the range 1.8:1 to 1:1.4 A1:P and an oxidising agent in an amount effective to improve the lightfastness of the pigment.
2. A titanium dioxide pigment according to claim 1 in which the oxidising agent is potassium nitrate in an amount of from 0.5 per cent to 1.5 per cent by weight based on dry weight of titanium dioxide.
3. A titanium dioxide pigment according to claim 1 or 2 in which aluminium phosphate is present in an amount equivalent to from 2 per cent to 6 per cent by weight calculated as Awl2 03 with respect to Tri 02.
4. A titanium dioxide pigment according to claim 3 in which the amount of aluminium phosphate is equivalent to from 3.5 per cent to 4.5 per cent by weight calculated as Al203 with respect to TiO2.
5. A titanium dioxide pigment according to any one of claims 1 to 4 in which the aluminium phosphate is present in an amount equivalent to from 2 per cent to 12 per cent by weight calculated as P205 with respect to TiO2.
6. A titanium dioxide pigment according to claim 5 in which the amount of aluminium phosphate is equivalent to from 3 per cent to 10 per cent by weight calculated as P205 with respect to TiO2.
7. A titanium dioxide pigment according to claim 5 or 6 in which the amount of aluminium phosphate is present in an amount equivalent to from 5 per cent to 6.5 per cent by weight calculated as P205 with respect to Tit2.
8. Titanium dioxide pigments as hereinbefore described with reference to Examples 1, 3, 5, 6, 7, 9, 10, 12 to 15 and 17.