Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
- 1 ~ J~ 7 1 ~
The invention relates ~o monoclinic lead chro-
mate pigments wh;ch are dist;ng~ished by a part;cularly
high fineness of the pigment particLes and a narrow par-
ticle size distribution.
; Lead chromate pigments have been widely used for
many years for colouring plastics materials and paints.
Preferred pigments consist of small particles of little
difference in size. US Patent 2,212,917 describes lead
chromate pigments which, although distinguished by small
average part;cLe s;ze ~for example 0.45 ~ m)~ conta;n ;nd;-
vidual particles which can be up to 3.5 ~m long. German
Offenlegungsschrift 1,807~8~1 describes coated Lead chro-
mate pigments where at least 50X of the pigment particles
have a particle size of at most 1.4 ~m and 10X have a par-
t;cle ~ize of less than 4.1 /um or more. Even these pig- -
ments leave a great deal to be desired in terms of the fine~
ness of the particles and the uniformity of particLe size~
The present invention providss monoclinic lead
; chromate pigments containiny lead chromate and lead sul-
fate ;n a weight ratio of 89~9:1001 to 60:40, wherein the pig-
ment particles have a median value of 0~35-0.45~m and 50-75X
of all the particLes have a Stokes diame~er of 0.15-0.5 ~m.
In preferred monoclinic lead chromate pi~ments the
pigment particles have a median value of 0.35~0.45 ~m and 55-
70% of all the particles have a Stokes diameter of 0.15-0~5Jum.
In particularly preferred monoclinic lead chromate
pigment~ the pigment particles have a median valu of 0.3 0~4
~m and 65-70æ of all the particles have a Stokes di~meter of
.
~,
~L2~
0.15-0.5 ~m~
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 particular lead nitrate,
with an aqueous solution of a chromate
in particular sodium or potassium chromate
and ;f desired of a sulfate, for example sodium or potas-
sium sulfate or ammonium sulfate~ under conditions of high
turbulencea The chromate solution is advantageously pro-
duced by adding alkali to a bichromate solution before or
during the reaction with the lead salt.
The high turbulence can be produced ~y various means~
for example by cont;nuously comb;ning the solut;ons to be
mixed in a mixing nozzle. Mixing nozzle is to be under-
stood as meaning a device where the solutions to be mixed
are combined with one another within a relatively small
space to which at least one of the solutions is supplied
by means o~f a nozzle and preferably under elevated pres-
sure. The mixing nozzle can be constructed for example
in accordance with the principle of the water jet pump,
the way one of the liquids is fed into the mixing nozzle
corresponding to the way the water is fed into the water
jet pump and the way the other liqu;d is fed into the
mixing nozzle corresponding to the connection between the
water jet pump and the flask to be evacuated and if
des;red this latter way of supplying liquid may also be
effected under elevated pressure
The precipitat;on ;n the m;x;ng nozzle ;s advan
tageously carried out by continuousLy combining~ prefer-
ab(y at room temperature, the aqueous solution contain;ng
the chromate and the sulfate at a flow rate of at least
4.9 m/sec ~ith the aqueous solution of the lead salt at
a flow rate of at le~st 0.08 m/sec. The prec;pitation is
advantageously carried out in the presence of an excess of
lead ions of 0.003- 0.06 mole per litre over the stoichio-
metr;c amount, preferably at room temperature and at pH 3-5.
Another way of producing high turbulence at the
po;nt where the solutions are combined is available in the
.~
~2~ f
-- 3 --
form of commercially available high-performance stirrers,
such as, for example, the Ultra-Turrax stirrer from Janke und
Kunkel KG, Staufen, West Germany, the ~stral stirrer from
Ystral GmbH, Ballrechten~Do~tingen, West Germany, the Poly-
tron from ~inematica~ Kriens-Lucerne, Switzerland, the
Silverson stirrer from S;lverson Mach Ltd~, Chesham~
United Kingdom, or the Chemcol~mixer from Chemiecolor AG
Kilchberg-Z~r;ch, Switzerland. Other types of high-per
formance stirrers which can likewise be used include in~er
al;a the Pendraul;k~st;rrer from Pendraul;k Masch;nen und
Apparate GmbH~ Bad M~nder am Deister, West Germany, and
cont;nuous mixers such as those supplied by Gronfa Process
Technik BV/Rozendaal, Netherlands~ It is important here that
the lead salt solution on the one hand and the chromate and
suLfate solution on the other are added as close as possiblP
to the shaft of the rotor of the high~performance stirrer.
The two main 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 immed;ate vicinity of the shaft of the rotor by
two separate lines, with a chromate solu-
tion and if desired also a sulfate solution advan-
tageously in one line and a lead salt solution in the
other. In the second case, for exarnple, the chromate
solution and if used the sulfate solution are put
into the flask first and the lead salt solution is added
as close as possible to the shaft of the rotor of the high-
performance stirrer through a tube.
The prec;pitated pigment is distinguished by an
extremely low particle 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 allo~ing the precipitate to stand at room temperature
or by heating.
The pigment obtainPd can be treated with texture-
improv;ng agents, for example long cha;n aliphatic alcohols,
esters, acids or their salts, amines, amides, waxes or
~ I Aaf~C 1~'~
- 4 -
resinous subs~an~es, such as abi~tic acid, hydrogenation
products, esters or salts thereof, and also non;on;c,
an;onic or cationic surface-active agents.
To improve the stability to heat, light and
chemi~al attack it is adva~tageous to coat the pigmen~
part;cles with an ;norganic coating agent dur;ng the pre-
c;p;tat;on or ;n the course of an aftertreatmer,t in accor-
dance with known processes descr;bed, for example, in
US Patents 3,370,971, 3,639~133 and 4,046,588.
For th;s purpose, an ;norganic compound, for
example an aluminium, sil;con, ant;mony, t;n, cerium,
t;tan;um~ or zircon;um compound or comb;nat;ons thereof,
;s precip;tated on the p;gment. It has been found to be
advantageous to use a cer;um-alumin;um-silicate layer
wh;ch ;s formed on the pigmentO for example by add;ng an
aqueous solution of cerium nitrate, of an alkali metal
silicate and of aluminium sulfate to the aqueous pigment
suspens;on.
The level of coating agent is advantageously
2-~0, preferably 2-20 and in particular 3-10% based on the
total weight of the pigment~
The pigment is ~orked up in conventional manner, for
example by filtering it off, wash;ng the filter cake with
water to remove soluble salts, drying and pulverising.
The median value Dz (see DIN 53,206 sheet 1,
August 1972, page b) and the particle size distribution
can be determined by known methods, for example by means of
a disc centrifuge Csee The Particle Size Determination of
P;gments with the Disc Centrifuge~ K. Brugger, Powder
Technology 13 S1976), 215~221~u Median value and par-
ticle s;ze d;stribution can be determined in a particu
larly simple manner using the centrifugal particle size
distribution analyser (model CAPA 500~ put on the mar-
ket by the firm of Horiba, Kyoto, Japan, which outputs
Stokes diameters ~see DIN 53,206) and weight distributions~
The determined values also hold for p;gments containing up
to 10X of coating material.
~ 5 --
The tinctorial strength was determined using a pro-
gram based on DIN 53,235 and an 8/d angle of measure~
ment.
Electron m;crographs of the p;gm2nts obtained
clearly show the great advantages of these pigments~ Two
characterist;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;shed on application by higher tinctorial strength,
a more greenish shade, great hid;ng power, good rheological
properties and higher saturation~ Noteworthy is also the
lower proport;on of lead sal~s soluble in dilute acids, such
as carbonate and sulfate salts9 compared with known
products of ~he same hue.
In the case of pigments containing more than 10X
of coating ma~erial, the median value and the particle size
distribution can change so much that they are no longer
within the r3nge defined above. Even such pigments have
said applicat;on advantages of higher tinctoriaL strength
and saturat;on, a more greenish hue and greater hiding
power over conventionally obtained pigments, as described
;n, for example, US Patent 4,046,58~9 containing the same
proportion of coating ma~erial.
The pigments according to the invention can be
used alone or mixed uith one another or with other pigments~
~or e~ample phthalocyanine blue, molybdate orange or Berlin blue, for pig-
ment;ng h;gh molecular weight organic material, for example
celluLose ethers and esters~ acetylcellulose, ni~rocellu-
lose, natural resins or synthetic resins~ such as polymer
isation or condensation resins~ for example aminoplasts,
in particular urea- and melamine-formaldehyde resins,
alkyd resins, phenoplasts, polycarbonates, polyolefins~
such as polyethylene or polypropylene, as well as polysty-
rene, polyvinyl chloride, polyacrylonitrile~ polyacrylates,
rubber, case;n, silicone and silicone resins~
Said high molecular weight compounds can be not
only in the form of amorphous materials or melts but also
- 6 --
in the form oE spinning solutions, ]acquers or prin-ting inks.
Depending on the intended use, it has been found to be advantag-
eous to use the new pigments as toners or in the form of prepar-
ations.
In the following 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 Teepo ~ HB5
(34~ strength solution of the sodium salt of a sulEated 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 water.
Then the flask is put at room temperature for 10 min on the
bottom of an ultrasonic bath (Bransonic 48 kHz from Bransonic
B.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;
Figure 3 represents an electron micrograph (20,000x)
of the pigment particles of Example 4; and
Figure 4 represents an electron micrograph (20,000x)
of the pigment particles of Example 6.
~22~
- 6a -
Example 1: In a mixiny nozzle (see Figure 1), an
aqueous solution containing, per 1,000 parts by volume, 40.2 parts
oE sodium bichromate x 2H2O, 19.35 parts of sodium sulfate and
11.25 parts of 100% sodium hydroxide, is passed at room
temperature through tube a at a flow rate of 4.9 m/sec and is
continuously brought together a-t the same time with an aqueous
solution containing, per 1,000 parts by volume, l~L6 parts of lead
nitrate and 2 parts of sodium carbonate passing through tube b
at a flow rate of 0.08 m/sec. After the precipitation a solution
of 22~5 parts of sodium chloride in 120 parts of water is added
in order to control the crystal size, and the pH is adjusted to
5.8 by adding an aqueous sodium carbonate solution. The precipi-
tate is filtered off, washed with water to remove soluble salts,
and dried at a temperature of 80-90C. Ratio of the lead salts
in the mixed
-- 7 --
crystal: 66% PbCrO~
34% PbS04
The values determined with the CAPA-500 analyser are:
median value: 0.37~um;
Particle size distribution: o6% between 0.15 and 0~5 ~m.
The relative tinctorial strength compared to
pigment of identical composition obtained by a conven-
tional process was found to be 115X using the method
of DIN 53,Z35.
Example 2: In a mixing nozzle (see Figure 1),
an aqueous solu~ion containing~ per 1~000 parts by ~o-
lume, 40.2 parts of sodium bichroma~e x 2H~0, 19~35
parts of sodium sulfate and 11.2S parts of 100%
sod;um hydrox;de, ;s passed a~ room temperature through
tube a at a flow rate of 4.9 m/sec and ;s cont;nuously
brought together at ~he same t;me w;th an aqueous solu-
t;on conta;n;ng, per 1,000 parts by volume~ 146 parts of
lead nitrate and 2 parts of sodium carbonate and passing
through tube b at a flow rate of 0.08 m/sec. The supply
of the soluticns is controlled in such a way that,
during the precipitation, there is always present an ex-
cess of lead ions of 0~003 mole per litre over the
stoichiometric amount. The resultin~ pigment suspension
leaves the nozzle via tube CA After the precipitation
a solution of 22~5 parts of sodium chloride in 12D parts
of water 1s added in order to control the crystal size~
and the suspensisn is brought to pH 5 by adding an
aqueous sodium carbonate solution. To ~ature the crystal
structure the precipitate is alLowed to settle for 8
hours~ The supernatant liquid above the precipi~a~e is
decanted off~ and then~ ~o coat the pigment particles of
the pigment suspens;on left behind, the follow;ng are
added at room temperature w;th st;rring: a solution of
13 parts of sodium s;l;cate (Z8X of S;02~ in 120 parts
of water, followed by a solution of 20 parts of alu-
minium sulfate x 18H20 and 10 parts of 52%
n;tric acid in 300 parts of water, and then a solution
o~
-- 8 --
of 1.~ parts of cer;um hydrox;de ;n 3.7 parts
52% nitric acid. The end pH is adjusted to 4a5
by adding 12~5 parts of sod;um carbonate in 120 parts of
water.
The pigment obtained is isolated in the custo
mary manner by filtering, washed with water to remove
soluble saLts~ and dried at 80-90C.
Rat;o of the lead salts in the
mixed crystal: 66% PbCrO4
34% PbS04
The values determined with the CAPA-500 analyser
are:
med;an value: 0~42 ~m
particle size distr;bution 56X between 0.15 and 0.5 ~ m.
Proportion of the coating in the total wei~ht of the
p;gment: 5.5%.
The rela~ive tinctorial strength in paints com-
pared w;th a conventionally obtained pigment of the same
composition was found to be 123% us;ng the method given
in DIN 53,235.
Figure 2 sho~s an electron micrograph taken with
Z0,000-fold enlargement of the pigment particles ultra~
sonically d;spersed in an alcohol~water mixture.
The hue in pa;nts as measured by DIN 53,235 is
distinctly greenish.
~ : Example 1 is repeated to produce a
pigment conta;ning 73% of PbCrO4 and 27% of PbS0
affording a pigment having a relative ~inctorial
strength of 14D% compared with a conventionally obta;ned
pigment of the same hue.
median value: 0.41 ~m
partic~e size distribution: 59% between 0.15 and 0~5 ~m.
~ : In a mix;ng nozzle (see Figure 1),
an aqueous solution containing, per 1~000 parts by vo-
lumeD 40.2 parts of sod;um b;chromate x 2H20, 19.35
parts of sodium sulfate and 11.25 parts of 100X
sodium hydroxide, is passed at room temperature through
tube _ at a flow rate of 4.9 ~/sec and is continuously
brought together at the same time with an aqueous solu-
:,
z~
tion containillg, per 1,000 parts by volumeO 157.6 partsof lead nitrate and 2 parts of sodium carbonate and
passing through tube b at a flow rate of 0.08 m/sec.
The supply of the solutions is controlled in such a way
that, during ~he precipitation, there is always present
an excess of lead ions of 0~014 mole per litre over the
stoichiometric amount.
After the precipitation a solution of 22.5 parts of
sodium chloride in 1~0 parts of water is added in order to
control the crystal size, and the suspension is brought to
pH 5n~ by adding an aqueous sodium carbonate solutionn
To mature the crystal structure the precipitate îs allowed
to settle for 8 hours~
The supernatant l;quid above the precipitate is
decanted off, and ~hen, to coat ~he pigment particles of
the p;gment suspension left behind~ the folLowing are
added at room temperature with stirring: a solution of
13 parts of sodium silicate ~8% of siO23 in 120
parts of water, followed by a solution of 20 parts of
alumin;um sulfate x 18H20 and 10 parts of 52%
n;tr;c ac;d and 300 parts of water, and then a solution
of 1L4 parts of cerium hydroxide in 3.7 parts of 52%
strength nitric acid~ The end p~ is adjusted to 4.5 by
addin~ 12.5 parts of sod;um carbonate in 120 parts of
water~ The pigment obtained is isolated in the customary
manner by filtering, washed with ~ater to remove
soluble salts, and dried at 80-9DC~
Ratio of the lead salts in the
mixed crystal 64X PbCrO4
36X PbS04
Median value: 0.38 ~m
Particle size distr;but;on: 65~4% between 0u15 and
0.5 ~m.
Proport;on of the coating in the total weight of the
pigment: 5.4%.
- 10 -
The relative tinctorial strength is 110X com-
pared with a conventionally produced pigment of the same
composition.
The hue in paints as measured by DIN 53,235 is
s;gnilf;cantly more greenish than that of known pigments
of the same composition~
Figure 3 shows an electron micrograph of the
pigment dispers;on ;n 20,000 fold enlargament.
Example 5: In a mixing nozzle (see F;gure 1) an
aqueous soLution conta;n;ng per 1,000 parts by volume
40.2 parts of sodium bichromate tNa2Cr207 x 2H20)~
19~35 parts of anhydrous sodium sulfate and 11~25 parts
of 100% sodium hydrox;de ;s passed at room tem
pera~ure through tube 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 1~000 parts by volume
14h par~s of lead nitrate in 2 par~s of sodium car-
bonate and pass;ng through tube b at a flow rate of 0~08
mlsec. The supply of the soLutions is controlled in
such a way that, during the precip;tation, there is al-
ways present an excess of lead ions of 0.003 mole per
litre over the stoichiometric amount. After the preci-
pitat;on a solution o; 22.5 parts of sodium carbon chlo
ride in 12Q parts of water is added in order to control
the crystal size, ~nd the pH ;s then adjusted to 5.~
by adding an aqueous sod;um carbonate solution. To ma-
ture the crystal structure the precipiate is allowed to
settle for 8 hours.
To coat the pigmen~ particles the supernatant
liquid above the precipitate is decanted oFf and 95
parts of sodium sil;cate (28X of SiO2~ in 200 parts
of water are then added at room temperature with stir-
ring to the suspension left behind~ This is followed~
likewise at room temperature w;th stirring, by a solu-
t~on of 20 parts of aluminium sulfate x 18H20 and 35
parts of 52% HN03 in 300 parts o~f water and
then by a solution of 1~4 parts of cerium hydroxide in
~Z~
3.7 parts of 52% strength HN03.
The end pH is adjusted to 4.S by add;ng 12.S
parts of sodium carbonate in 120 parts of water, and the
result;ng precip;tate is filtered off, washed w;th
water to remove soluble salts, and dr;ed at 80-90C.
Ratio of the lead salts in the
m;xed crystal: 68X PbCrO4
32% PbS0~
Proport;on of the coat;ng in the total we;ght of the
pigment: 19X.
The relative tinctorial strength is 14~% com-
pared with a conventionaLly produced pigment of the same
composition.
~ 1,000 ml of an aqueous solution con-
ta;ning 40.2 9 of sodium bichromate x 2H20, 19.35 9 of
ssdium sulfate and 11025 9 of 100X sodium hy-
droxide and 1,000 mL of an aqueous solution containing
146 9 of lead n;trate and 2 9 of sodium carbonate are
con~lnuously and simultaneously brought together at room
temperature in separate tubes and in the immediate vici-
nity of the shaft of the rotor of a high-performance
stirrer (diameter of the rotor 4 cm, circumferential
speed of the rotor 4 m/sec~ ;n a 5 l;tre reaction vessel
in the course of 2 minutes~ The supply of the solution is
controlled ;n such a way that, dur;ng the precipitat;on,
there is always present an excess of lead ions of o~nO3
mole per litre over the stoichiometric amountO After the
prec;p;tation a solution of 22.5 g of sodium chloride in
120 ml of water is added in order to control the crystal
size~ and the pH is adjusted to 5.8 by adding aqueous
sod;um carbonate solution. To mature the crystal structure
the precipitate is allowed to settle for 8 hours.
To coat the pigment particles the supernatant
l;qu;d above the precipitate is decanted off and the
followin~ are added at room temperature with stirring to
the suspension left behind~ a solution of 13 9 of so-
dium silicate (28% of SiO2) in 120 ml of water, fol-
~2~
lowed by a solution of 20 9 of aluminium sulfate x18H20 and 10 9 of 52% nitric acid in 300 ml
of water~ and then a solution of 1.4 9 of cerium hy~
droxide in 3.7 g of 52% nitric acid~ The end
pH ;s adjusted ~o 4.5 by adding 12.5 g of sod;um carbo~
nate in 120 ml of waterr
The pigment obtained is isolated in conYentional
manner by filtrat;on, is washed w;th water to remove
soluble salts, and is dried at 80-90C.
Ratio of the lead salts in the
mixed crystals: 66~ PbCrO4
34% PbS0
Median value: 0.37 ~m
Particle size distribution: 66% between Oa15 and Do5 ~m
Proport;on of the coating in the total weight of the
pi3ment~ 501%.
The relative tinctorial strength is 147X com~
pared with a conventionally produced pigment of the same
composit;on.
Figure 4 shows an electron Inicrograph of ~he
pigments in 20~000-fold enlargement.
The hue in paints as measured by DIN 53,235 is
significantly more greenish than in the case of known
pigments having the same lead chromate content~
~ : 1,000 ml of an aqueous solution
containing 139 ~ of lead n;trate and 1~7 g of sodium
carbonate are introduced into an empty 5 litre reaction
vessel equipped with a high performance stirrer (diame-
ter of the rotor: 4 cm; circumferential speed of the
rotor: 4 m/sec). 1,000 ml of an aqueous solution con-
ta;n;ng 63.6 g of sodium bichromate x 2H20, 3.4 9 of
anhydrous sodium sulfa'ce and 17.1 9 of 10D%
sodium hydroxide are then added at room temperature in
the immed;ate vicinity of the shaft of the rotor of the
h;gh-performance stirrer by means of a glass tube in the
course of 2 minutes. After the precipitation lead ions
are present in an excess of O.Oû9 mole per litre~ In
- 13 -
order to control ~he crystal si~e a solut;on of Z5 g of
sodium chloride in 120 ml of water is added. The pH
is then adjusted to 5.8 by adding aqueous sodium carbo-
nate solution. To mature the crystal structure the pre-
cipitate is allowed to settle for 8 hours.
To coat the pigment particles the supernatant
liquid above the precipitate is decanted off and a solu-
t;on of 35.7 g of aluminium sulfate x 18H20 and 30 g
of titanium oxychloride in 200 ml of water ;s added at
room temperature with stirring ~co the suspension left
beh;nd. 2.6 9 of cer;um hydroxide in 7.8 g of 52%
strength HN03 are then added. The end pH is then ad-
justed to 5.0 by adding ~0 9 of ~od;um carbonate in
2UO ml of water. The precipitate is filtered o~f~
washed with water to remove soluble salts~ and dried
at a temperature of 80-90C.
Ratio of the lead salts in the
mixed crystals: B5~ PbCrO4
15% PbS04
Proportion of the coating in the total weight of the
pigment: 9X.
The relative tinctorial strength is 134X com
pared with a conventionally produced p;gment.
The values determined with the CAPA-500 analyser
are:
Median value 0.33 ~m
Particle size distribution: 58% between 0.15 and 0.5 ~m.
The hue in paints as measured by DIN 53,235 is
distinctly ~reen;sh~
Example 8: 1,000 ml of an aqueous solution con-
taining 273 g of lead nitrate and 2.5 9 of sodium carbo
nate are introduced in'co an empty 5 litre reaction ves-
sel equipped with a high-performance stirrer (d;ameter
of the rotor 4 cm, circumferential speed of the rotor
4 mlsec). 1,000 ml of an aqueous soLution containing
96.5 9 of sodium bichromate x 2H20, 20~2 9 of anhy-
drous sodium sulfate and 26.2 9 of 1DOX sodium~
- 14 _
hydroxide are then added in the ;mmediate v;cinity of
the shaft of the rotor of the high performance stirrer
by means of a gLass tube in the course of 2 minutes.
After the precipitation the lead ion excess is 0.008
mole of Pb2~ per litre.
To coat the pigment particles the precipiate is
then heated to 65C and a solution of S g of anhydrous
sodium sulfate in 120 ml of water is added. ~he pH is
then adjusted to 2.5 with about 13 9 of 52%
HN03. A solution o~ 32.5 g of sodium silicate (28X of
SiO2) and 10 9 of sod;um carbonate in 120 ml of water
is then added~ followed by a solution of 10 g of an~i-
mono ~rioxide, 10 9 of sodium fluoride and 27 9 of 52X
strength HN03 in 120 ml of water. The end pH is ad-
justed to b.5 by adding about 8 ~ o~ 100X strength so~
dium hydroxide in 120 ml of water. The precipitate is
filtered off, washed with water to remove soLuble
salts~ and dried at a temperature of 90Ea
Rat;o of the lead salts ;n the
mixed crystals~ 83% PbCrO~
17% PbS04
The values determined w;th the CAPA-500 analyser
are:
Med;an value 0.39 ~m
Part;cle s;ze d;stribution: 66% between 0.15 and 0.5 ~m.
The relative tinctoriaL strength compared with
a convent;onally produced p;gment of the same composi-
tion was determined as 137~ using the method given in
DIN 53,235.
The proportion of the coating in the total
wei~ht of the pig~ent is 7.4%,
The hue in paints is measured by DIN 53,235 is
distinctly green;sh.
Exam~e 9: 0.6 part of the coated pigment obtained
as in Example 2 ;s m;xed w;th 76 parts of polyv;nyl chlor
ide~ 33 parts of dioctyl phthalate, 2 parts of dibu~yltin
- 15 -
dilaurate and 2 parts of titanium dioxide, and the mixture
;s processed at 160C ;n a roll mill ;nto a thin f1lm
in the course of 15 m;nutes. The greenish yellow colour-
ation thus produced is intense and fast to migration and
light.
xample 10: û.05 part of the coated pigment ob-
tained as in Example 2 is mixed dry with 100 parts of poly-
styreneO The mixture is kneaded at temperatures of between
180 and 220C un~il ;t is homogeneously coloured. The
coloured material is allowed to cool down and is ground
in a mill down to a particle size of about 2-4 mm. The
granulate thus obtained is processed at temperatures be-
tween 220 and 300C in an injection moulding machine
into mouldings. The result is reddish yello~ materials
of good light fastness and thermaL stability.
Example 11: 60 parts of a 60% solution
of a non drying alkyd resin in xylene ~supplied by the
firm of Reichold-Albert-Chemie, West Germany, under the
tradename of Ee~-koso ~ 27-320)o 36 parts of a 50%
solution of a melamine-formaldehyde res;n in an
alcohol-aromatics mixture tsupplied by the firm of Re;chold~
Albert-Chemie under the tradename of Super-8eckami ~ 13-
501)0 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.
9S parts of the transparent lacquer thus obtained and S
parts of the coated pigment as per Example 2 are ball-
milled for 72 hours~ The coloured lacquer is then applied
to sheet metal by a convent;onal spraying method and is
baked thereon at 120~C for 30 minutes. The result i5 a
yello~ coating of good light fastness.
