GB1568710A

GB1568710A - Chrome pigments

Info

Publication number: GB1568710A
Application number: GB3304676A
Authority: GB
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1976-08-09
Filing date: 1976-08-09
Publication date: 1980-06-04
Also published as: DE2735867A1; IT1084397B; CA1096107A; NL7707906A; FR2361448B1; FR2361448A1; BE857366A

Description

(54) IMPROVED CHROME PIGMENTS (71) We, IMPERIAL CHEMICAL INDUS TRIES LIMITED of Imperial Chemical House, Millbank, London SW1P 3JF a British Com- pany do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to chrome yellow pigments of improved sulphur dioxide resistance and high gloss and to a method for their manufacture.

According to the invention there is provided a process for the manufacture of an improved chrome yellow pigment having a superficial coating of cerous metaphosphate which comprises the simultaneous treatment of an aqueous suspension of a chrome yellow pigment with two treating agents which are (i) metaphosphor ic acid or a water-soluble salt thereof and (ii) a water-soluble cerous salt.

By chrome yellow pigments is meant lead chromate yellow pigments, such as primrose chromes, lemon chromes and middle chromes.

As water-soluble cerous salts there may be mentioned for example cerous chloride, cerous sulphate, cerous nitrate and cerous acetate.

As water-soluble metaphosphates there may be mentioned for example sodium and potassium metaphosphates and metaphosphoric acid, but other water soluble salts may be used if desired. It is preferred to use a salt rather than the free acid.

The process of the invention may be carried out conveniently by adding an aqueous solution of the water-soluble cerous salt or water-soluble metaphosphate gradually with stirring to a suspension of the pigment in an aqueous solution of the other water-soluble treating agent and, preferably after stirring for a further short time, collecting the treated pigment by for example filtration, washing with water, and drying, for example by heating in air at 700C. It is, however, preferred to add the two treating agents gradually and simultaneously preferably at substantially equivalent rates, to a stirred aqueous suspension of the pigment.

Pigments of particularly good sulphur dioxide resistance are obtained if the water-soluble cerous salt and the water-soluble metaphosphate are added to a stirred aqueous suspension of chrome pigment at approximately equivalent rates. It is however sometimes advantageous to add a minor portion, such as 10%, of the water soluble metaphosphate before commencing addition of the water-soluble cerous salt since the metaphosphatemay act as a dispersing agent on the pigment.

The addition of the water-soluble cerous salt and of the water-soluble metaphosphate should desirably be carried out over a period of at least 15 minutes. Shorter periods may give nonuniform treatment of the pigment particles.

Periods longer than 30 minutes do not in general offer significant advantage.

The treatment may be carried out on the slurry of the pigment obtained directly from, for example, a solution of sodium dichromate, sodium sulphate and sodium hydroxide and a lead nitrate solution, but a slightly superior product is obtained if the untreated pigment is isolated, washed with water and then reslurried for treatment.

The treatment is preferably carried out at a temperature between 60 and 800C but other temperatures outside this range may be used if desired.

The amount of water-soluble cerous salt and of water-soluble metaphosphate are preferably chosen to be roughly equivalent, that is in the moler ratio 1 : 3, although a moderate excess of either will not normally be deleterious, and to be sufficient to provide cerous metaphosphate in amount between 0.5 and 20%, and more preferably between 3 and 12% of the weight of untreated pigment.

The product of the process of invention is a chrome yellow pigment each particle of which is coated with insoluble cerous metaphosphate which affords protection to the pigment against attack by atmospheric sulphur dioxide. This chrome yellow pigment, having a superficial coating of cerous metaphosphate, is also a feature of the invention.

It is desirable to maintain the pH of the suspension at a suitable value, not only because of the well-known instability of chrome yellow pigments in solutions of very low or very high pH, but because unsatisfactory coatings may be obtained at unsuitable pH values. At low pH precipitation may be incomplete and at high pH the cerium may be precipitated as hydrated oxide instead of as metaphosphate. The preferred pH range is 6-8,5. The pH may be adjusted to this range by the addition as necessary of dilute acid or dilute alkali or of acidic or alkaline salts.

The process of the invention may be used in conjunction with other known methods for improving the properties of chrome pigments. It may, for example, precede, or preferably, follow treatment of the pigment to deposit coatings of oxides of metals such as aluminium, cerium, titanium, tin, antimony, manganese or silicon on the pigment.

The pigments so treated may be isolated and dried by conventional means, including for example drying in vacuo.

By the process of the invention pigments are obtained which are of improved sulphur dioxide resistance when incorporated into paint media, for example alkyd based and acrylic based paints. They also show an advantage in superior gloss over other treatments which provide a comparable improvement in sulphur dioxide resistance.

The following description provides two examples of the present invention and comparisons of these with five pigments that are either uncoated or are coated with salts that are closely related to cerous metaphosphate. In the description all the parts and percentages are by weight. All the examples and pigments are prepared from the same pigment suspension.

Pigment Suspension An untreated lemon yellow shade chrome pigment is prepared by the following procedure.

A solution of 219 parts of sodium dichromate dihydrate, 93.75 parts of anhydrous sodium sulphate, 18.75 parts of sodium chloride and 210 parts of aqueous sodium hydroxide (specific gravity 1.35) in 2700 parts of water is added over about 30 minutes with stirring to 6000 parts of 0.4 molar lead nitrate solution at pH 4 and 20"C until the soluble lead is reduced to 0.014 molar and the pH raised to 5.2 - 5.4.

The pH is then raised to 5.8 by addition of aqueous sodium carbonate and the slurry heat ed at 70"C for one hour, diluted with water to 18,000 parts and allowed to settle for 18 hours.

The supernatant liquid is removed by decanta tion and the residual slurry diluted to 10,000 parts and heated to 700C. 10% Sulphuric acid is added to bring the pH to 2.6 followed by a sol ution of 21 parts of sodium sulphate in 100 parts of water and 20% aqueous sodium car bonate to bring the pH to 5.5. 100 Parts of aqueous sodium silicate (containing 24.8% sili con as Si02 and 9.2% sodium as Na2 0) diluted to 250 parts is added over 15-20 minutes fol lowed by a solution of 105 parts of ammonium sulphate in 500 parts of water over 30 minutes.

A mixture of 21 parts of antimonous oxide, 21 parts of citric acid and 60 parts of concentrated hydrochloric acid is then added over 10 min utes followed by 20% aqueous sodium carbon ate to bring the pH to 7.0. The suspension, which contains about 750 parts of pigment, is stirred for 10 minutes. This suspension is used for providing treated and untreated pigments as described below.

Pigment 1, untreated pigment for comparison purposes, is obtained by filtration and washing with water of a portion of the Pigment Suspen sion which is then dried at 70-90 C.

Pigments 2 to 5 and Examples 1 and 2 are obtained by adding solutions of cerium salts and phosphates as described below at approx imately equivalent rates over about 15 minutes to stirred portions of the Pigment Suspension, prepared as described above, each portion con taining about 190 parts of pigment at pH 7.0 and about 700 C. The treated pigments are isol ated by filtration, washing with water, and drying at 70-900C.

Pigment 2 32.3 Parts of 20% ceric sulphate 'liquor and 8 2 parts of sodium orthophos- phate, each in 250 parts of water.

Pigment 3 18 5 Parts of cerous nitrate hexahy drate and 7 parts of sodium orthophosphate, each in 250 parts of water.

Pigment 4 37.6 Parts of 20% ceric sulphate liquor and 9.0 parts of sodium metaphosphate, each in 250 parts of water.

Pigment 5 14.8 Parts of calcium chloride and 10.4 parts of sodium metaphosphate, each in 250 parts of water.

Example 1 - 11.6 Parts of cerous nitrate hexahydrate and 82 parts of sodium metaphosphate each in 250 parts of water.

Example 2 23.2 Parts of cerous nitrate hexahydrate and 16.4 parts of sodium metaphosphate, each in 250 parts of water.

Example 1 and Pigments 2-5 illustrate pig ments having a coating of about 5% by weight of phosphate, and Example 2 illustrates a pig ment having a coating of about 10% by weight of phosphate. Pigment 1 illustrates a pigment which has not been coated with either cerium or phosphate. Only Examples 1 and 2 illustrate treated pigments of the invention, Pigments 1-5 belong described for the purposes of the following comparisons.

Comparison 1 Treated pigments prepared as described in Examples 1 and 2 and Pigments 2-5 and untreated pigment (Pigment 1) are each incorporated into a paint by the following procedure.

28 g. of pigment, 23.4 g. of a 20% solution of an alkyd resin in xylene and 125 g. of 3 mm.

Ballotini beads are placed in a 4 oz. glass bottle and shaken for 30 minutes. After separation from the beads the mixture is blended with 70.8 g. of a 50% solution of alkyd resin in xylene and 18.8 of a butylated melamine formaldehyde resin. The blend is applied in a coating of thickness 0.01 inch onto a primed metal panel and the coating dried in air for 30 minutes and then heated in an oven at 1200C for 30 minutes.

The coatings of the paints are tested for sulphur dioxide resistance in a Kesternich apparatus by the procedure of the German standard method for paint panels (DIN 50018).

The following results are obtained: Pigment Sample 3 Cycles 5 Cycles Pigment 1 (Control) 2 1-2 Example 1 4 4 Example 2 4-5 4-5 Pigment 2 2 1-2 3 4-5 1-2 4 4 1-2 1 5 5 4-5 4-5 The resistance is expressed on a scale of 1 to 5, 1 representing complete destruction of colour and 5 no change. Only Examples 1 and 5 afford a considerable measure of protection against sulphur dioxide.

Comparison 2 Paint coatings prepared as described in Comparison 1 are tested for gloss by the method of ASTM Spec. D523 using a specular gloss meter at an angle of 20 with a setting for black glass of 88% reflectance.

The difference in reflectance between the samples of paints from treated pigments and from untreated, pigment (Pigment 1), expressed as percentage reflectance are as follows: Example 1 + 12 2 +10 Pigment 2 + 18 Pigment 3 + 18 Pigment 4 + 1 Pigment 5 + 3 Thus although calcium metaphosphate provides a similar improvement to cerous metaphosphate in sulphur dioxide and both cerous and ceric metaphosphates provides improved gloss only cerous metaphosphate provides a reasonable measure of improvement in both gloss and sulphur dioxide resistance.

WHAT WE CLAIM IS: 1. A process for the manufacture of an improved chrome yellow pigment having a superficial coating of cerous metaphosphate which comprises the simultaneous treatment of an aqueous suspension of a chrome yellow pigment with two treating agents which are (i) metaphosphoric acid or a water-soluble salt thereof and (ii) a water-soluble cerous salt.

2. A process according to Claim 1 wherein aqueous solutions of the two treating agents are mixed together in the presence of the aqueous suspension of the chrome yellow pigment.

3. A process according to Claim 2 wherein the aqueous solutions of the two treating agents are added gradually and simultaneously to the aqueous suspension of the chrome yellow pigment.

4. A process according to Claim 3 wherein the two solutions are added at equivalent rates.

5. A process according to any one of Claims 1 to 4 wherein up to 10% of the metaphosphate is added to the aqueous suspension of the chrome yellow pigment before commencing addition of the water soluble cerous salt.

6. A process according to any one of Claims 1 to 5 when carried out at a temperature of from 60 to 800C.

7. A process according to any one of Claims 1 to 6 when carried out at a pH of from 6 to 8.5.

8. A process according to any one of Claims 1 to 7 wherein the molar ratio of metaphosphate ions to cerous ions in the two treating agents is 3.

9. A process according to any one of Claims 1 to 8 wherein the quantities of treating agents used are sufficient to provide the cerous metaphosphate in an amount between 0.5% and 20% of the weight of untreated pigment.

10. A process according to Claim 9 wherein the quantity of cerous metaphosphate provided is between 3% and 12%.

11. A process according to any one of Claims 1 to 10 wherein the treating agent providing metaphosphate ions is a water soluble metaphosphate salt.

12. A process according to Claim 11 wherein the water soluble metaphosphate salt is sodium metaphosphate.

13. A process according to any one of Claims 1 to 12 wherein, subsequent to the treatment process, the pigment is isolated from the suspension after a short period of stirring.

14. A process according to any one of Claims 1 to 13 wherein the untreated pigment has been isolated from the reaction medium in which it is produced and washed with water.

15. A process according to any one of Claims 1 to 14 wherein the pigment has been previously treated with other agents conven

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Claims

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of phosphate. Pigment 1 illustrates a pigment which has not been coated with either cerium or phosphate. Only Examples 1 and 2 illustrate treated pigments of the invention, Pigments 1-5 belong described for the purposes of the following comparisons.

Comparison 1 Treated pigments prepared as described in Examples 1 and 2 and Pigments 2-5 and untreated pigment (Pigment 1) are each incorporated into a paint by the following procedure.

28 g. of pigment, 23.4 g. of a 20% solution of an alkyd resin in xylene and 125 g. of 3 mm.

Ballotini beads are placed in a 4 oz. glass bottle and shaken for 30 minutes. After separation from the beads the mixture is blended with 70.8 g. of a 50% solution of alkyd resin in xylene and 18.8 of a butylated melamine formaldehyde resin. The blend is applied in a coating of thickness 0.01 inch onto a primed metal panel and the coating dried in air for 30 minutes and then heated in an oven at 1200C for 30 minutes.

The coatings of the paints are tested for sulphur dioxide resistance in a Kesternich apparatus by the procedure of the German standard method for paint panels (DIN 50018).

The following results are obtained: Pigment Sample 3 Cycles 5 Cycles Pigment 1 (Control) 2 1-2 Example 1 4 4 Example 2 4-5 4-5 Pigment 2 2 1-2

3 4-5 1-2 4 4 1-2 1

5 5 4-5 4-5 The resistance is expressed on a scale of 1 to 5, 1 representing complete destruction of colour and 5 no change. Only Examples 1 and 5 afford a considerable measure of protection against sulphur dioxide.

Comparison 2 Paint coatings prepared as described in Comparison 1 are tested for gloss by the method of ASTM Spec. D523 using a specular gloss meter at an angle of 20 with a setting for black glass of 88% reflectance.

The difference in reflectance between the samples of paints from treated pigments and from untreated, pigment (Pigment 1), expressed as percentage reflectance are as follows: Example 1 + 12

2 +10 Pigment 2 + 18 Pigment 3 + 18 Pigment 4 + 1 Pigment 5 + 3 Thus although calcium metaphosphate provides a similar improvement to cerous metaphosphate in sulphur dioxide and both cerous and ceric metaphosphates provides improved gloss only cerous metaphosphate provides a reasonable measure of improvement in both gloss and sulphur dioxide resistance.

WHAT WE CLAIM IS: 1. A process for the manufacture of an improved chrome yellow pigment having a superficial coating of cerous metaphosphate which comprises the simultaneous treatment of an aqueous suspension of a chrome yellow pigment with two treating agents which are (i) metaphosphoric acid or a water-soluble salt thereof and (ii) a water-soluble cerous salt.
2. A process according to Claim 1 wherein aqueous solutions of the two treating agents are mixed together in the presence of the aqueous suspension of the chrome yellow pigment.
3. A process according to Claim 2 wherein the aqueous solutions of the two treating agents are added gradually and simultaneously to the aqueous suspension of the chrome yellow pigment.
4. A process according to Claim 3 wherein the two solutions are added at equivalent rates.
5. A process according to any one of Claims

1 to 4 wherein up to 10% of the metaphosphate is added to the aqueous suspension of the chrome yellow pigment before commencing addition of the water soluble cerous salt.
6. A process according to any one of Claims 1 to 5 when carried out at a temperature of from 60 to 800C.
7. A process according to any one of Claims 1 to 6 when carried out at a pH of from 6 to 8.5.
8. A process according to any one of Claims 1 to 7 wherein the molar ratio of metaphosphate ions to cerous ions in the two treating agents is 3.
9. A process according to any one of Claims 1 to 8 wherein the quantities of treating agents used are sufficient to provide the cerous metaphosphate in an amount between 0.5% and 20% of the weight of untreated pigment.
10. A process according to Claim 9 wherein the quantity of cerous metaphosphate provided is between 3% and 12%.
11. A process according to any one of Claims 1 to 10 wherein the treating agent providing metaphosphate ions is a water soluble metaphosphate salt.
12. A process according to Claim 11 wherein the water soluble metaphosphate salt is sodium metaphosphate.
13. A process according to any one of Claims 1 to 12 wherein, subsequent to the treatment process, the pigment is isolated from the suspension after a short period of stirring.
14. A process according to any one of Claims 1 to 13 wherein the untreated pigment has been isolated from the reaction medium in which it is produced and washed with water.
15. A process according to any one of Claims 1 to 14 wherein the pigment has been previously treated with other agents conven

tionally used to improve the properties of chrome pigments.
16. The process described in Example 1.
17. The process described in Example 2.
18. A chrome yellow pigment have a superficial coating of cerous metaphosphate.
19. A chrome yellow pigment having improved properties produced according to the process claimed in any one of Claims 1 to 17.
20. A chrome yellow pigment produced by the process described in Example 1.
21. A chrome yellow pigment produced by the process described in Example 2.