CA1220005A - Monoclinic lead chromate pigments - Google Patents

Abstract

3-14540/TEN/1+2/+

Monoclinic lead chromate pigments Abstract Monoclinic lead chromate pigments containing lead chromate and lead sulfate in a weight ratio of 100:0 to 90:10, wherein the pigment particles have a median value of 0.3 0.4 µm and 60-80% by weight of all the particles have a Stokes diameter of 0.15-0.5 µm.
The pigments obtained are distinguished on appli-cation by low particle sizes, uniform particle size dis-tribution, high tinctoriai strength, great hiding power, good rheological properties and high saturation.

Description

3-14540/TEN~1~2/-~

Monoclinic Lead chro~ate pigments .
The invent;on relates to monoclinic lead chro~
mate pi~ments which are distinguished by a particularly high fineness of the pi~ent particles and a narrow par-ticLe ~ize distribution.
Le~d chromate pi~ments have been ~;dely used for many years for colouring pLastics mater;als and paints.
Preferred pi~ments consist of small particles of little difference in size. US Paten~ 2,212,917 describes lead chromate p;gments which, although d;stin~uished by ~maLl average particle size (for example 0~45 ym~, contain indi-vidual particLes which can be up to 3.5 ~m long. German Offenlegungsschrift 1,807,891 describes coated lead chro-mate pigments where at least 50X of the pigment particles have a partisLe size of at most 1.4 um and 10% have a par-ticle size of less than 4~1 um or more. Even these pig-ments Leave a great deal to be desired in terms of the fineness of the particles and the uniformity of particle size.
The present invention provides monoclinic lead chromate pigments containing lead chroma~e and lead sul-fate in a weight ratio of 100:0 to 90:10, wherein the pig-ment particles have a median value of 0.3-0.4 ~m and 60-80X
of all the particles have a Stokes diameter of 0.15-0.5 ~mu In preferred monocl;nic lead chromate pigments the pigment particles have a medial1 value of 0.3-0.4 ~m and 65-75X of all the particles have a Stokes diameter of 0.15-0~5 ~m.

s The lead chromate pigments according to the inven~
tion are obtained by mixing an aqueous solution of a lead salt, for example lead acetate, in part;cular lead nitra~e~
with an aqueous solution of a chromate in part;cular sodium or potassium chromate and if desired of a sulfate, for example sodium or potas~
sium sulfate or ammonium sulfate, under condit;ons of h;gh turbulence. The chroma-te solut;on is advantageously pro-duced by addin~ alkali to a b;chromate solution before or dur;ng the reaction w;th the lead saLt.
H;gh turbulence can be produced by var;ous means, for example by continuously combining the solutions to be ~;xed ;n a mix;ng nozzle. M;xing nozzle is to be under~
stood as mean;ng a device where the solut;ons to be mixed are combined with one another within a relatively small space to wh;ch at least one of the solutions ;s suppl;ed by means of a nozzle and preferably under elevated pres~
sure. The m;xing nozzle can be constructed for example in accordance with the pr;nc;ple of the water jet pump, the way one of the liqu;ds i5 fed into the mixing noz~le corresponding to the way the water ;s fed into the water jet pump and the way the other l;quid ;s fed into the mix;ng nozzle corresponding to the connect;on between the water jet pump and the flask to be evacuated and if des;red this latter way of supplying liqu;d ~ay also be effected under elevated pressure.
The precip;tation ;n the m;x;ng nozzle ;s advan-tageously carr;ed out by cont;nuously comb;ning, prefer-ably at room temperature, the aqueous solut;on conta;n;ng the chromate and if des;red a sulfate at a flow rate of at least 4.9 m/sec w;th the aqueous solution of the lead salt at a flow rate of at least O~û8 m/sec~ The precipitation is advantageously carried out in the presence of an excess of O~û07-û.û7, preferably O û07-û~017 mole of chro-mate per l;tre over the stoichiometr;c amount and at pll Another way of producing high turbulence at the point where the solutions are combined is available in the ~ZZ~ 3.~i form of commercially available h;gh-performance stirrers, such as~ for example, the Ultra-Turrax stirrer from Janke und Kunkel KG~ Staufen, Wes~ GermanyO the Ystral stirrer from Ystral GmbH, ~allrechten-Do-ttingen, West Germany~ the Poly-tron from Kinematica, Kriens-Lucerne~ Switzerland~ the Silverson stirrer ~rom Silverson Mach Ltd~, Chesham/
United Kingdom, or the Chemcol mixer from Chemiecolor AG
Kilchberg~Zurich~ Switzerland Other types of high-per-formance stirrers which can likewise be used include inter aLia the Pendraulik stirrer from Pendraul;k Maschinen und Apparate GmbH, 8ad Munder am Deister, West Germany, and continuous mixers such as those supplied by Gronfa Process Technik BV/Rozendaal~ Netherlands. It is important here that the lead salt and the chroMate solutions and if pre sent the sulfate solution are added as close as possible to the shaf~ of the rotor of the high-performance stirrer~
The two ma;n components can be mixed in the zone of turbu-lence either by feeding them in together or by adding one to the other. In the first case, the solutions are passed into the immediate vicinity of thP shaft of the rotor by two separate lines, with a chromate solu~
tion and ;f present also a sulfate soLution advan-tageously in one l;ne and a lead salt solution in the other. In the second case, for example, the chromate solution and ;f used the sulfate solut;on are put ;nto the flask first and the lead salt solution is added as close as poss;ble to the shaft of the rotor of the h;gh-performance stirrer through a tube.
The precipitated pigment is distinguished by an extremely low part;cle size. It has been found that bet ter crystal structures are obtained if the precipitation is followed by a maturing process~ for example in the form of allowing the precipitate to stand at room temperature or by heatingr The pigment obtained can be treated with texture-improving agents~ for example long-chain aliphatic alcoholsO
esters, acids or their salts~ amines, amides, waxes or :~2~ S

~, res;nous substances, such as abietic acid, its hydrogen-ation products, esters or salts, and also non;onic, an;on;c or cationic surface-active agents.
To improve the stability to heat~ light and chemical attack it is advantageous to coat the pigment particles with an inorganic coating agent during the pre-cipitation or in the course of an aftertreatment in accor-dance with known processes described, for example, in US Patents 3,370,9719 3,639,133 and 4,046,588~
For th;s purpose, an inorganic compound, for example an aluminium, silicon, antimony~ tinD cerium, titanium, or zirconium compound or comb;nations thereof, is precipitated on ~he pigment~ It has been found to be advantageous to use a cerium-aluminium-silicate lay~r wh;ch is formed on the p;gment~ for example by addin~ an aqueous solution of cerium nitrate, of an alkali metal silicate and of aluminium sulfate to the aqueous pigment suspension.
The level of coating agent is advantageously

2-4D, preferably 2-20 and ;n part;cular 3-10X based on the total weight of the pigmen~.
The pigment is worked up in conventional manner~ for exa~ple by filtering it off, washing the filter cake with water to remove soluble salts~ drying and pulver;sing.
The median value Dz ~see DIN 53,206 sheet 1~
August 19729 page o) and the par~icle size distribution can be determined by known methodsO for example by means of a d;sc centrifuge ~see The Particle Si~e Determination of Pigments with the Disc Centrifuge, K. Brugger, Powder Technology 13 t1976)~ 215-221J. Median value and par-t;cle size distribution can be determ;ned in a particu-larly simple manner using the centrifugal particle si~e distr;but;on analyser (model CAPA 500) put on the mar-ket by the f;rm of Horiba, Kyoto, Japan, which outputs Stokes diameters (see DIN 53,206) and weight distr;but;ons.
The determined values also hold for pigments containing up to 10X of coating material.

~Z~ 5 The t;nctor;al strength was determ;ned using a pro-gram based on DIN 53,235 and an 8/d angle of measure-ment.
Electron m;crographs of the p;~ments obta;ned clearly show the great advantages of these pigments~ Two character;st;c main features stick out~ The particles prepared are smaller and more uniform than those of tra-ditional products. The pigments obtained are dis-t;ngu;sbed on appl;cat;on by h;gher tinctor;al strength~
a more greenish shade, great hid;ng power, good rheological properties and higher sa~urat;on. Noteworthy is also ~he lower proport;on of lead salts soluble in dilute acids~ such as carbonate and sulfate salts, compared with known products of the same hueO
In the case of p;gments con~aining more than 10%
of coatin~ material, the median value and the particle size distribution can change so much that they are no longer w;thin the range defined above. Even such pigments have said application advantages of higher tinctorial strength and sa~uration, a more greenish hue and greater hiding power over conventionally ob~ained pigments~ as described ;n~ for example~ US Patent 4,046,588, conta;ning the same proportion of coating materialu The pigments accord;ng to the invention can be used alone or m;xed w;th one another or with other pigments, for example phthalocyanine bLue,molybdate orange or Berlin blue, for pig-ment;ng hi~h molecular we;ght organic materiaL, for example ceLlulose ethers and esters~ acetylcellulose, nitrocellu~
lose~ natural resins or synthetic resins, such as polymer-isation or condensation resins, for example am;noplasts, in partirular urea- and melamine-formaldehyde resins~
alkyd res;ns, phenoplasts, polycarbona~es, polyolef;ns, such as polyethylene or polypropylene, as well as polysty-rene, polyvinyl chloride, polyacrylonitrile, polyacrylates, rubber, case;n, silicone and sil;cone res;ns.
Said high molecular weight compounds can be not only ;n the form of amorphous mater;als or melts but also ~L,f~2~a~

in the form of spinning solutions, lacquers or printiny inks.
Depending on the intended use, it has been found to be advantageous to use -the new pigments as -toners or in the form of preparations.
In the followi.ng examples and preceding description the parts and percentages are by weight unless otherwise stated.
The median values and the particle size distribution were measured with the CAPA-500 analyser at 3000 rpm. The amount of pigment must be so adjusted, that the absorption of the beam is between 0.5 and 1Ø
The pigments were dispersed as follows: 15 mg of pigment are thoroughly wetted by means of 100 mg of Teepol~ HB6 (34g6 strength solution of the sodium salt of a sulfated primary alcohol having an average molecular weight of 267, supplied by Shell) in a mortar. To the so obtained dispersion is added in a 250 ml conical flask a total amount of 100 ml of distilled waterO
Then the flask is put at room temperature for 10 min on the bottom of an ultrasonic bath (Bransonic 48 kHz from Bransonic s.V., Soest, Netherlands) filled with 1 litre of water.
In the following Examples reference is made to the attached drawings in which:
Figure 1 represents a suitable mixing nozzle;
Figure 2 represents an electron micrograph (20,000x) of the pigment particles of Example 2; and Figure 3 represents an electron micrograph (20,000x) of the pigment particles of Example 3.
Example 1: In a mixing nozzle (see Figure 1), an aqueous solution containing, per 1,000 parts by volume, 49.25 ~z~
- 6~ -parts of sodium bichromate x 2H2O and 13.5 par-ts o:E 100% strength sodium hydroxide, is passed at room temperature through pipe a at a flow rate of 4.9 m/sec and is continuously brought together at the same time with an aqueous solution containing, per l,000 parts by volume, 96 parts of lead nitrate and passing through tube b at a flow ra-te of 0.08 m/sec. The supply of the solutions is controlled in such a way that, during the precipitation, there is always present an excess of sodium chromate of 0.0168 mole per litre over the stoichiometric amount. The resulting pigment suspension leaves the nozzle via tube c. After the precipitation the suspension is brought to pH 5.5 by adding nitric acid. The precipitate is heated to .,., ,, ~

80C, is filtered off, washed with ~ater to remove soluble salts~ and dried at a temperature of 80-90C.
The pigment is 100% lead chromate.
The values determined with the CAPA-5ûO analyser are: median value: 0O37 and particle s;ze distribution:
69% between 0.15 and 0.5 ~u.
The relative tinctorial strength compared to a pigment of identical composition obtained by a conventional process was found to be 115X using the method of DIN 53,235.
Example 2: In a mixing nozzle (see Figure 1), an =. . . = ~
aqueous soLution containingO per 1,000 parts by volume, 49~25 parts of sodium bichroma~e x 2H20 and 13.5 parts of 100% strength sodium hydroxide~ is passed at room tem-pera~ure ~hrough pipe a at a flo~ rate of 4.9 m/sec and is continuously brought together at the same time with an aqueous solution containing, per 1,000 parts by volume, 96 parts of lead nitrate and passing through tube b at a flow rate of a.oB m/sec~ The supply of the solutions is controlled in such a way that, during the precipitation, there is always present an excess of sodium chromate of 0.0168 mole of Na2CrO4 per l;tre over the sto;chiometric amount. The resulting pigment suspension leaves the nozzLe v;a tube c. After the precipitation the suspension is brought to pH 5.5 by adding nitric acid~ The precipitate is heated to ~0C and is stirred for one hour~ The ex-cess sodium chromate or bichromate ;s precipitated w;th a small excess o~ lead nitrate at the same pH.
The pigment particles are then coated by add;ng at a temperature of 60-70 w;th st;rr;ng to the aqueous p;gment suspens;on a solution of 11.2 parts of sodium silicate ~28% of S;02) in 70 parts of waterD followed by a solution of 17.8 parts of aluminium sulfate x 18H~0 and 11.8 parts of 52% stren~th HN03 in 100 parts of water. 2.5 parts of lead nitrate in 8 parts of water and 1 part of cerium hydrox;de in 3.2 parts of 52%
HN03 are then added at room t~mperature with stirring.

The end pH is brought to 5.0 by adding 13.3 par~s of sodium carbonate in 150 parts of water. The prec;p;tate is filtered off, washed with water to remove soluble salts~
and dried at 3 temperature of 80-90C.
Ratio of the lead salts: PbCrO~ 97.9%
PbS04 Z~1%
The values determined with the CAPA 500 analyser are as follows: median value: 0.33 ~m particle size distribution: 69.4% between 0.15 and 0.5 ~m~
The relative tinctorial strength was found to be 145X by the method described in DIN 53,235~ The coating accounts for 6% of the total ~e;ght of the pigment.
The hue in paints, as measured by DIN 53~235~ is distinctly greenish.
Fi~ure 2 shows an electron micrograph taken with 20,000-fold enlargement of the pigment particles ultrasoni-caLly d;spersed in an alcohoL-~ater mixture.
~ e~ o An aqueous solution containing per 1,000 parts by volume, 49.25 parts of sodium bichromate x 2H20 and 13.5 parts of 100% sodium hydrox;de and an aqueous solution containing, per 1,000 parts by ~olume9 9~ parts of lead n;trate are s;multaneously fed in separate tubes into a reaction vessel equ;pped with a high-performance stirrer ~diameter of ~he rotor: 4 cm~
circumferent;al speed of the rotor: 4 ~/sec) and are con-tinuously brought together in the immediate vicinity of the shaft of the rotor of the high-performance st;rrer.
The supply of the solut;ons ;s controlled in such a way that dur;ng the precipitation an excess of sodium chromate of 0.0168 mole of Na2CrO4 per l;tre over the sto;ch;o-metr;c amount is always present. After ~he precipitation the pH is brought to 5.5 by adding nitric acid. The prec;pitate ;s heated to 80C and st;rred for one hour.
The excess sodium chromate or bichromate is precipitated with a small excess of lead n;trate at the same pH.
The p;gment particles are then coated by adding at a tem-9 ~perature of 60-70C w;th st;rring to the aqueous suspen-sion a solution of 11.2 parts of sod;um s;l;cate (Z8% of SiO2) ;n 70 parts of water followed by a solut;on of 17.8 parts of alum;n;um sulfate x 18H20 and 11.8 parts of 52% HN03 ;n 100 parts of water. 2.5 parts of Lead n;trate ;n 8 parts of water and 1 part of cer;um hydroxide ;n 3.2 parts of 52% HN03 are then added to the p;gment suspens;on. The end pH ;s brought to 5.0 by add;ng 13.3 parts of sodium carbonate in 150 parts of water.
The prec;pitate is filtered off~ is ~ashed with water to remove soluble salts, and dried at a tempera-ture of 8D-90C.
Ratio of the lead salts in the pigment: PbCrO~ 9709X
PbS04 2 n 1 %
Relative tinctorial strength: 138%
Median value: 0~34 ~m Particle size d;stribution: 69% bet~een 0~15 and 0.5 ~m Proportion of the coating in the total weight of the p;gment: 6X
The hue in pa;nts as m~asured by DIN 53~235 is dis-tinctly green;sh.
Figure 3 shows an electron micrograph taken with 20,000 fold enlargement of the pigment particles ultra-son;cally d;spersed in an alcohol-water m;xture~
Example ~: In a mixing nozzle ~see Figure 1)9 an aqueous solution containing~ per 1,000 parts by volume, 49u25 parts of sodium b;chromate x 2H20 and 13~5 parts of 100% sodium hydro~ide is passed through pipe a at a flow rate of 4~9 m/sec and is continuously brought together at the same t;me with an aqueous solution con-taining, per 1,000 parts by volume, 96 parts of lead nitrate and pass;ng through tube b~ The supply of the solutions is controlled in such a way that, during the precipitation, there is always present an excess of Na2CrO4 of 0.0168 mole per litre over the stoichiometric amount. After the precipitation the suspension is brought to pH 5.5 by adding nitr;c acid. The precipitate is heated to 8ûC and is stirred for one hour. The excess sodium chro~ate or b;chromate is precipitated with a small excess of lead nitrate at the same pH. The pigment particles are then coated by adding at a temperature of 60-70C with stir-r;~g to the pigment suspension a solution of 77 parts of sodium silicate ~28% of SiO2) in 200 parts of water, fol~owed by a solu~ion of 17.8 parts of aluminium sulfate x 18H20 and 32 parts of 52% HN03 in 200 parts of water. A solution of 2.5 parts of lead nitrate and 1 part of cer;um hydroxide in 3.2 parts of 52X HN03 ;s then added with stirring. The end pH is brought to 5~0 by add;ng 13~3 parts of sodium carbonate in 150 parts of water. The precipitate is filtered off~ the f;lter cake is washed with water to remove soluble salts, and dried at a temperature of 80-90C.
Rat;o of the lead salts in the piyment: PbCrO4 97~9%
PbS04 2.1X
Proportion of the coat;ng in the total weight of the pigment: 18X
The relative tinctor;al strength compared w;th a pigment of the same composition obtained conventionally is 1 28%o Example 5~ 1,003 ml of an aqueous solution con-I ~
taining 49.25 9 of sodium bichromate x 2H~0, 5.4 parts of sodium sulfate and 13.5 9 of 100% sodium hydrox-ide are ;ntroduced into an empty 5 Litre reaction vessel equipped with a high-performance stirrer tdiameter of the rotor: 4 cm; circumferential speed of the rotor: 4 m/sec).
1,D00 ml of an aqueous solution containing 111 g of lead nitrate are then added at room temperature in the immedi~
ate vic;n;ty of the shaft of the rotor of the high-perfor-nance stirrer by means of a glass tube in the course of 2 Mi nutes~
After the precipitation the excess of sodium chro~
mate is 0.0168 mole per litre. The pH is then adjusted to 5~5 by adding nitric acid. The prec;pitate is heated to 80C and is stirred ~or one hour~ The excess sodium ~26)(7~

~ 11 -chromate or bichromate is precip;tated with a small ex~ess of ~ead n;trate at the same pH.
The p;gment part;cles are then coated by adding at a temperature of 60-70C with stirring to the aqueous suspension obta;ned in paragraph 1 a solution of 20 9 of t;tanium oxychloride and 22~l~ 9 of alum;nium sulfate x 18H20 in 125 ml of water. This is followed by 2.2 9 of cer;um hydrox;de ;n 7.2 9 of 52% HN03. The end p~ ;s adjusted to 5.0 by add;ng 20 9 of sodium carbonate in 200 ml of water. The precipitate is f;ltered off, washed with water to remove soluble salts and dried at a temperature of 80-90C.
Ratio of the lead salts in the pigment: PbCrO4 92X
PbS04 8%
The relative tinctorial strength compared with a conven-t;onally produced pîgment is 133~ The values determined w;th the CAPA-S00 analyser are as follows:
Median value- 0.4/um Particle size distribution: 72X between 0.15 and O.S ~m~
Propor~ion of the coating in the total weight of the p;gment: 9X
The hue in paints as measured by DIN 53,235 is distinctly greenish.
~ 1,000 ml of an aqueous solution con-taining 4~.25 9 of sodium bichromate x 2~20 and 13.5 g of 100X sodium hydroxide are ;ntroduced ;nto an empty S litre reaction vessel equipped with a high-performance stirrer (diameter of the rotor: 4 cm;
circumferential speed of the rotor: 4 m/sec). 1,000 ml of an aqueous solution containing 111 g of lead ni~ra~e are then added at room temperature in the immediate v;cinity of the shaft of the rotor of the high-perfor-mance stirrer by means of a glass tubè in the course of 2 m;nutes~
After the precipitation the excess of sodium chro-mate is 0.01h8 mole per litre~ The pH is then adjusted - ~2~

to 5.5 by adding nitric acid. The precipitate is heated to 80C and is stirred for one hour. The excess sod;um chromate or bichromate is precipitated with a small excess of lead nitrate at the same pH.
The pigment particles are then coated by adding at a temperature of 70C with stirring to the aqueous sus-pension a solution of 13.6 9 of sodium silicate (28% of SiO2) in 120 ml of water~ This is followed by 4.2 g of antimony trioxide, 4.2 9 of sodium fluoride and 11.3 g of 52~ HN03 ;n 120 ml of ~ater. The end pH
is adjusted to 6~5 by adding 20 g of sodium carbonate in 200 ml of water~ The precipitate is filtered off~
~ashed with ~ater to remove solubLe salts and dried at a temperature of 90C.
Ratio of the lead salts in the pigment: PbCrO4 98X
PbS04 2%
The relative ~inc~orial strength compared with a conven~
tionally produced pigment ;s 130.5%~ The values determined with the CAPA;~S00 analyser are as follows:
Median valueO 0~37 Par~icLe size distribution: 71.8% be~ween 0715 and 0.5 ~m.
Proportion of the coating in the total weight of the p;gment: 7.4%
The hue in paints as measured by DIN 53,235 is distinctly greenish.
~ : 0.6 part of the coated pigment obtained as in Example 2 is mixed with 7~ parts of polyvinyl chlor ide, 33 parts of dioctyl phthalate, 2 parts of dibutyl tin dilaurate and 2 parts of titanium dioxide~ and the mixture is processed at ~60C in a roll mill into a thin film ;n the course of 15 m;nutes. The green;sh yellow colour-ation thus produced is intense and fast to migration and lightO
~ : 0.05 part of the coated pigment ob-tained as in Example 2 is mixed dry with 100 parts of poly-styrene. The mixture is kneaded at temperatures of between ~2~S

- 13 ~
180 and 220C until it is homogeneously coLoured. The coloured material is allo~ed to cool down and is ground in a mill down to a particle size of about 2 4 mm. The granula~e thus ob~ained is processed a~ temperatures be-t~een 220 and 300C in an injection moulding machine into mouldings. The result is reddish yellow materials of good light fastness and thermal stability.
Example 9: 60 parts of a 60X solution of a non-drying alkyd resin in xylene (supplied by the firm of Reichold-Albert-Chemie, West Germany, under the tradename of 0eckosol~ 27-320), 36 parts of a 50X
s~rength solution of a ~elamine formaldehyde resin in an alcohol-aromatics mixture (supplied by the firm of Reichold~
Albert-Chemie under the tradename of Super~eckamin~ 13-501)~ 2 parts of xylene and 2 parts of methylcellosolve are Mixed, and 100 parts of this mixture are stirred by means of a stirrer to give a homogeneous lacquer solution.
95 parts of the transparent lacquer thus obtained and 5 parts of the coated pi~ment as per Example 2 are ball-milled for 72 hours. The coloured lacquer is then applied onto sheet metal by 3 conventional spraying method and is baked thereon at 120C for 30 minu~esl The result is a yellow coating of good light f2stness~

Claims (12)

Claims

1. Monoclinic lead chromate pigments containing lead chromate and lead sulfate in a weight ratio of 100:0 to 90:10, wherein the pigment particles have a median value o-f 0.3-0.4 µm and 60-80% by weight of all the particles have a Stokes diameter of 0.15-0.5 um.

2. Lead chromate pigments according to claim 1, wherein the pigment particles have a median value of 0.3 0.4 µm and 65-75% by weight of all the particles have a Stokes diameter of 0.15-0.5 um.

3. Lead chromate pigments according to claim 1, which additionally contain texture-improving and/or surface-active organic agents.

4. Lead chromate pigments obtainable by producing during the precipitation or in the course of an aftertreat-ment a coating of an inorganic coating agent on the lead chromate pigments according to claim 1.

5. Lead chromate pigments according to claim 4, which contain 2-40% of an inorganic coating agent.

6. Lead chromate pigments according to claim 4, which contain 2-20% of an inorganic coating agent.

7. Lead chromate pigments according to claim 4, which contain 3-10% of an inorganic coating agent.

8. Process for preparing monoclinic lead chromate pigments according to claim 1, which comprises mixing an aqueous solution of a lead salt with an aqueous solution of a chromate or of a mixture of a chromate with a sulfate under conditions of high turbulence.

9. Process according to claim 8, wherein the turbu-lence is created by high flow rates or mechanical stirr-ing.

10. Process according to claim 8, wherein an excess of 0.007-0.07 mole of chromate per litre over the stoichio-metric amount is present during the precipitation.

11. Process according to claim 8, wherein the pigments obtained are coated with an inorganic protective coating.

12. High molecular weight organic material which con-tains a lead chromate pigment according to claim 1.