lubricating action of organic pigment during the grinding of pigment mixtures

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Progress in Organic Coatings, 4 (1976) 209 - 223 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands 209 LUBRICATING ACTION OF ORGANIC PIGMENT DURING THE GRINDING OF PIGMENT MIXTURES NORIMICHI KAWASHIMA and KENJIRO MEGURO Science University of Tokyo, I-3 Kagurazaka, Shinjuku-ku. Tokyo (Japan) Contents 1 Ir.troduction, 209 2 The effect of organic pigment on the grinding of titanium dioxide, 209 3 The effect of organic pigment as a lubricant on the transformation of calcium carbonate, 214 3.1 &Copper phthalocyanin*cakium carbonate system, 214 3.2 Cromophthal Yellow GR-calcium carbonate system, 219 4 The effect of copper phthalocyanine as a lubricant on the grinding of alumina, 221 References, 223 1. Introduction Solid materials, when subjected to mechanical treatments such as crushing, rolling and impact, suffer a change in particle shape and size, and in some cases, their physical and chemical properties become different from the intrinsic ones of the original crystals [ 1 - 5]_ Particularly on the grinding of crystal powders, such changes as the formation of an oxide layer or an amorphous Iayer on the surface of particles [6], a reduction of crystallinity, a structural disordering [ 7 - 11 J , or polymorphic transformations [ 11 - 161 have been observed. Furthermore, many investigations have been reported on mechanical changes of inorganic-inorganic pigment mixtures 117 - 24 3. In the paint industry, mixtures of several kinds of pigments are used to obtain various colours, and it is very common to use a mixture of an organic with an inorganic pigment. However, few studies have dealt with the grinding of such mixtures. 2. The effect of organic pigment on the grinding of titanium dioxide The phthalocyanine pigments and quinacridone pigments are some of the most important coloured organic pigments due to their outstanding proper- ties. Many detailed reviews have been already published on the phthalocyanine pigments 125,261 and quinacridone pigments [27 - 321 from the practical

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Page 1: Lubricating action of organic pigment during the grinding of pigment mixtures

Progress in Organic Coatings, 4 (1976) 209 - 223 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

209

LUBRICATING ACTION OF ORGANIC PIGMENT DURING THE GRINDING OF PIGMENT MIXTURES

NORIMICHI KAWASHIMA and KENJIRO MEGURO

Science University of Tokyo, I-3 Kagurazaka, Shinjuku-ku. Tokyo (Japan)

Contents

1 Ir.troduction, 209

2 The effect of organic pigment on the grinding of titanium dioxide, 209

3 The effect of organic pigment as a lubricant on the transformation of calcium carbonate, 214 3.1 &Copper phthalocyanin*cakium carbonate system, 214 3.2 Cromophthal Yellow GR-calcium carbonate system, 219

4 The effect of copper phthalocyanine as a lubricant on the grinding of alumina, 221

References, 223

1. Introduction

Solid materials, when subjected to mechanical treatments such as crushing, rolling and impact, suffer a change in particle shape and size, and in some cases, their physical and chemical properties become different from the intrinsic ones of the original crystals [ 1 - 5]_ Particularly on the grinding of crystal powders, such changes as the formation of an oxide layer or an amorphous Iayer on the surface of particles [6], a reduction of crystallinity, a structural disordering [ 7 - 11 J , or polymorphic transformations [ 11 - 161 have been observed. Furthermore, many investigations have been reported on mechanical changes of inorganic-inorganic pigment mixtures 117 - 24 3. In the paint industry, mixtures of several kinds of pigments are used to

obtain various colours, and it is very common to use a mixture of an organic with an inorganic pigment. However, few studies have dealt with the grinding of such mixtures.

2. The effect of organic pigment on the grinding of titanium dioxide

The phthalocyanine pigments and quinacridone pigments are some of the most important coloured organic pigments due to their outstanding proper- ties. Many detailed reviews have been already published on the phthalocyanine pigments 125,261 and quinacridone pigments [27 - 321 from the practical

Page 2: Lubricating action of organic pigment during the grinding of pigment mixtures

210

and crystallographic points of view. In this section, the change of the surface properties of mixtures of a-copper phthalocyanine (abbrev. ar-CuPc) and titanium dioxide, and y-quinacridone and titanium dioxide with grinding is discussed.

When two kinds of pigments are ground by a grinder, the behaviour of the pigment particles in the grinding process can be classified roughly into three types as illustrated in Fig. 1.

Pigment A

Fig. 1. Grinding process of pigment mixtures.

(1) Two kinds of pigment mixtures are mixed without any interaction; the surface of the pigment mixture will ir..dicate the additive nature of the original component.

(2) One pigment becomes covered with another in the grinding process; the surface of the mixture will indicate the properties of the covering pigment.

(3) Two kinds of pigment react to form a new kind of pigment by grinding, such as the mechanochemical change of inorganic materials; the ground mixture will probably indicate quite different surface properties from the original component.

In the case of a mixture of organic pigment and titanium dioxide, the kind of grinding process that proceeds in practice is examined by means of specific surface area measurement, heat of immersion, and X-ray diffraction analysis.

The specific surface areas of ol-CuPc, y-quinacridone, titanium dioxide, the (1 :l) mixture of cr-CuPc and titanium dioxide, and the (1:l) mixture of r-quinacridone and titanium dioxide are plotted against the grinding time in Fig. 2.

The specific surface areas of titanium dioxide increase with grinding, while those of CuPc alone and their mixture decrease. The specific surface of quinacridone alone and their mixture increase in the early stages of grinding and afterwards gradually decrease. Since an inorganic pigment such as titanium dioxide is very hard, in general the particles tend to be finely divided on grinding. In the early stages of grinding, titanium dioxide becomes finely divided, so the specific surface area increases rapidly with grinding,

Page 3: Lubricating action of organic pigment during the grinding of pigment mixtures

211

60

I Ti02 0 .___-‘_.c.y’

_ ___.__-.-.-

0 .I /

1 . 10 30 60 100 180

(a) grinding time (min)

GO- Ti 0, ”

0 _.-.--‘- _-.- -

o/s _._--.---. o

10 30 GO 100

lb) grinding time (min)

1e0

Fig. 2. Change of the specific surface areas of WCUPC, y-quinacridone (Qc), Ti02, (1: 1) mixture df C&UPC and TiO~, and (1:l) mixture of Qc and TiOg with grinding time.

and aftetiards remains constant. Siinilar reports for other metal oxides have been published 133, 343. On the o$her hand, the particles of organic pigment such as CuPc and quinacridone aggregate to form secondary particles on

*- _.7

Page 4: Lubricating action of organic pigment during the grinding of pigment mixtures

212

grinding. This aggregation is considered to be due to sintering, which occurs by absorbing the grinding energy.

The heats of immersion of CuPc, quinacridone, titanium dioxide, the (1: 1) mixture of CuPc and titanium dioxide, and the (1 :l) mixture of quinacridone and titanium dioxide in water at various stages of grinding are given in Fig. 3.

The heat of immersion of titanium dioxide decreases markedly with grinding and afterwards remains constant. On the other hand, the heat of immersion of CuPc alone gradually increases with grinding_ The heat of immersion of the (1 :l) mixture of CuPc and titanium dioxide increases with grinding as in the case of CuPc alone. The heats of immersion of quina- &done and their (1: 1) mixture decrease in the early stages of grinding and afterwards slowly increase, and then remain constant.

In general, the heat of immersion in water is considered to evolve by the following mechanism [ 351.

(1) Interaction between the surface hydroxyl groups on the pigment particles and water molecules.

(2) Interaction between a crystal defect or active site formed by crystal distortion or crystal transformation with grinding and water molecules.

(3) Electron- and charge-transfer interaction between pigment surface and water molecules.

It is well known that hydroxyl groups exist on the surface of metal oxides. The heat of immersion of titanium dioxide is mainly due to these surface hydroxyl groups [36 - 381. Two factors are considered as determin- ing the change in the heat of immersion of a metal oxide by grinding. One is associated with the disappearance of the surface hydroxyl groups, while the other is related to the change in the crystallite or growth of cracks, and the increase in active sites. Consequently, the heat of immersion of titanium dioxide in water is assumed to decrease with grinding because of the disap- pearance of surface hydroxyl groups, i.e. the disappearance of surface hydroxyl groups is responsible for the increase in the hydrophobicity.

On the other hand, the increase in the heat of immersion of CuPc in water is explained by the following mechanism. CuPc is well known as an organic semiconductor. From the fact that the heat of immersion of CuPc in water increases with grinding, the electron- and charge-transfer mechanism is considered to play a role in the change in the heat of immersion.

The change in the specific surface area and heat of immersion of the mixture of organic pigment and titanium dioxide shows the same tendency as that of the organic pigrr.aiii;. This suggests that the surface of titanium dioxide becomes cover&d with organic pigment in the grinding process.

From the results of X-ray diffraction analysis, the reflections character- istic of y-quinacridone near 13.5 - 15" in 28 and 26.4" in 28 become broad during the grinding process, and almost disappear in the specimen ground for 180 min. On the other hand, the reflection characteristic of the anatase form of titanium dioxide in the mixture near 25.5” in 20 does not decrease with grinding. As in the case of the mixture of quinacridone and titanium

Page 5: Lubricating action of organic pigment during the grinding of pigment mixtures

213

\ ‘1.

\ \ 0

CuPctTiO,

200 p CuPc,-‘Q o’. //

--‘~~--._._.I /’

/’ TiOZ

P’ , 100. / / /.

;---v-u- __,G/

c . 10 30 60 100 180

(a) grinding time (min)

‘.\

200 - “.b. Qc

_-_------a __a- _--- jYJ 1.

/ -.-. y.-.o 0, / TiOz / \ ,

.a-/

10 30 60 100 160

(b) grinding time (min)

Fig. 3. Change of the heats of immersion of a-&PC, y-quinacridone (Qc), Ti02, (1: 1) mixture of Cr-CuPc and TiO,, of grinding.

and (1:l) mixture of Qc and TiO2 in water at various stages ._

Page 6: Lubricating action of organic pigment during the grinding of pigment mixtures

214

dioxide, the reflections characteristic of a-CuPc corresponding to (200) and (002) crystal planes near 5 - 8” in 28, and 14 - 17” in 20, become broad and hard to resolve as the grinding proceeds. On the other hand, the reflection intensity of titanium dioxide in the mixture does not decrease during the grinding process.

Accordingly, the titanium dioxide is considered to be covered with organic pigment such as CuPc and quinacridone, and the grinding of titanium dioxide is interfered with by the addition of organic pigment. These facts are also confirmed by electron micrographs and the dispersibility in an organic solvent [ 391.

3. The effect of organic pigment as a lubricant on the transformation of calcium carbonate

3.1 a-Copper ph thalocyanine-calcium carbonate system Organic pigment and calcium carbonate were ground together in various

proportions, and the crystal decay, i.e. the changes in crystal structure, particle size, and form of the components, were investigated with X-ray diffraction analysis, B.E.T. surface area, and electron micrography [40, 411.

Calcium carbonate occurs in several crystal forms, the most commonly known being stable calcite, at ordinary temperature and pressure, and meta- stable aragonite [ 421.

The X-ray diffraction traces of calcium carbonate ground alone are given in Fig. 4. The intensities of all reflections in the X-ray diffraction diagrams decrease and become weaker during the grinding process. The prominent peak of calcite corresponding to the (104) crystal plane decreases more than the others. The aragonite peaks are detected for the first time in the specimen ground for 6 h. After 10 h grinding, aragonite has become the main component.

The X-ray diffraction traces of a-CuPc at various stages of the mechan- ical treatment are given in Fig. 5. The reflections characteristic of cr-CuPc at the (200) and (002) crystal faces near 5 - 8” in 28 and four successive reflec- tions become broad with grinding; they become hard to resolve in the specimen ground for 10 h, very unlike the original situation. The reflection characteristic of p-CuPc corresponding to the (203) crystal face appears in the specimen ground for 2 h. In general, p-CuPc is known to be transformed to a-CuPc when comminution media are used [43]. However, cr-CuPc is considered to be transformed partially to the p-CuPc by absorbing the grinding energy.

The specific surface areas of both calcium carbonate and cr-CuPc alone are plotted against the grinding time in Fig. 6. It can be seen from this figure that the specific surface area of calcium carbonate gradually increases with the treatment, while that of cu-CuPc decreases_ Two factors are considered to be the causes cf the change in the specific surface area which takes place in the process of grinding. One is associated with changes in the crystallite or

Page 7: Lubricating action of organic pigment during the grinding of pigment mixtures

215

3 hr

? hr

12 hr 1 I

20 30 LO 50 Degrees 28 Cub

0 hr.

2 hr.

4 hr.

6 hr.

8 hr.

70 hr.

10 Degrees 2:°CuKn

30

Fig. 4. X-ray diffraction patterns of CaC03 after different grinding times. C, calcite; A, aragoni te.

Fig. 5. X-ray diffraction patterns of CY-CuPc after different grinding times.

s JO- G! CaC03 5 m 0

* zo- .u c E :10- a- CUPC

n a

0 2 4 6 8 IO grinding time ( hr )

Fig. 6. Change of the specific surface areas of CY-CuPc and CaC03 with grinding time.

particle size, the shape of the particles, and the growth of cracks. The other factor is related to the formation of secondary particles, which grow as a result of the aggregation or agglomeration of primary particles [443. From the results of electron micrography, the particles of calcium carbonate are observed to be finely divided and no formation of secondary particles occurs. On the other hand, the particles of ar-&PC, compared with the case of calcium carbonate aione, aggregate considerably in the specimen ground for 10 h; the specific surface area of CY-CuPc is considered to decrease with grinding because of the aggregatidn.

Page 8: Lubricating action of organic pigment during the grinding of pigment mixtures

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Page 9: Lubricating action of organic pigment during the grinding of pigment mixtures

217

The X-ray diffraction patterns of the mixture in the grinding process are given in Fig. 7. The reflections characteristic of cr-CuPc in the (1: 1) mixture of &UPC arid calcium carbonate corresponding to the (200) and (002) crystal planes become broad, and the medium reflections at higher angular ranges become hard to resolve as the grinding proceeds. On the other hand, the (104) reflection of calcium carbonate in the mixture does not decrease during the treatment more than in the case of calcium carbonate alone. The calcite-aragonite transition is not observed in the specimen

ground for 10 h in the (1:l) and (1:9) mixtures; it is detected only in the (1:19) mixture ground for 10 h. The fact that the calcite-aragonite transi- tion does not occur when the calcium carbonate is mixed with cr-CuPc suggests that the calcium carbonate is covered with C&UPC, i.e. the grinding of calcium carbonate is interfered with by the lubricating action of a-CuPc. The strength of the lubricating action is also observed to increase with the amount of cr-CuPc in the mixture system.

The electron micrographs of the specimens ground for 10 h are shown in Fig. 8. In the case of the (1 :l) mixture, the particles of the mixture aggregate with each other, and the growth of secondary particles takes place in the specimen ground for 10 h, as in the case of a-CuPc alone. As is to be

expected from X-ray diffraction analysis, the calcium carbonate is observed to be covered with cy-CuPc and the grinding of calcium carbonate is inter- fered with by the lubricating action of cr-CuPc. In the case of the (1:9) mixture, comparing with the case of (1 :l) mixture, the ground particles of c~-CuPc adhere to calcium carbonate particles, but these do not transform to (spherical) aragonite particles. On the other hand, the calcium carbonate particles in the (1:19) mixture are observed to be ground considerably more than the other mixtures to form (spherical) aragonite crystals. The electron micrograph of this specimen ground for 10 h is similar to that of calcium carbonate alone.

The calcium carbonate in the mixture is hardly ground more than calcium carbonate alone by the lubricating action of WCUPC, judging from the results of X-ray diffraction analysis and the electron micrographs. Conse- quently, though the specific surface area of calcium carbona+& alone increases with grinding, the lubricating action of <r-CuPc is expected to

prevent an increase in the specific surface area of a mixture. Therefore, it is possible to determine the extent to which the lubricating action of cr-CuPc prevents the grinding of calcium carbonate, by comparing the experimental value with the theoretical value obtained by addition of the specific surface areas of a-CuPc and calcium carbonate alone.

The specific surface areas of mixtures are plotted against the grinding time in Fig. 9. It is interesting that the specific surface area of (1:19) mixture gradually increases with the grinding, as with that of calcium carbonate alone, and that of the (1:9) mixture decreases in the early stages of grinding and afterwards slowly increases, while that of the (1:l) mixture decreases, as with that of cz-CuPc. The experimental curve is observed to become lower than the theoretical one. This fact can be explained as follows: as has been

Page 10: Lubricating action of organic pigment during the grinding of pigment mixtures

Fig. 8. Electron micrographs of mixtures ground for 10 h. The bars drawn represent 1 pm. (A): (1:1) mixture of a-CuPc and CaC03, (B): (1:9) mixture, (C): (1:19) mixture.

described above, if the lubricating action of cr-CuPc prevents the grinding of calcium carbonate, the specific surface area of calcium carbonate in the mixture till not increase as the grinding continues; i.e. as the particle surfaces of calcium carbonate are covered with a-CuPc particles, aggregation occurs. The lubricating action of WCUPC is observed in every' mixture, and the strength of the lubricating action increases with the amount .of wCuPc in the mixture. These facts agree with the results of the X-ray diffraction analysis and the electron micrographs.

Page 11: Lubricating action of organic pigment during the grinding of pigment mixtures

219

-501 ER

1 a(1 : llmixture

E40 2 is

Theoretical curve

Experimental curve

0 2 4 6 a 10 grinding time (hr 1

- 50

z at1 : 9)mixture

-40 - z? % u” 30

Sf<>Z

0 2 4 6 a 10 grinding time (hr )

ilu

- i.

ol a(1 : 19 1 mixture

z 40 4

F! a UJ 30

I;:‘*=

0 2 4 6 8 10

grinding time (hr )

Fig. 9. Change of the specific surface areas of mixtures with grinding time.

3.2 Cromophthal Yellow GR-calcium carbonate system The X-ray diffraction traces of Cromophthal Yellow GR alone at various

stages of the mechanical treatment are given in Fig. 10. All reflections of Cromophthal Yellow GR are observed to decrease with grinding, and all details of the patterns have disappeared in the specimen ground for 4 h.

_.

Page 12: Lubricating action of organic pigment during the grinding of pigment mixtures

Fig. 10. X-ray diffraction patterns of Cromophthal Yellow GR after different grinding

times.

Fig. 11. X-ray diffraction patterns of ground mixtures. (A): (1:l) mixture ground for 10 h, (3): (1:9) mixture ground for 10 h, (C): (1:19) mixture ground for 10 h, (D):

(1:3S) mixture ground for 8 h, (E): (1:39) mixture ground for 10 h.

The X-ray diffraction traces of ground mixtures are given in Fig. II. The (104) reflection of calcium carbonate in the mixture does not decrease more than in the case of calcium carbonate alone with the mechanical treatment. The calcite-aragonite transition is not observed in the (1 :l), (1:9) and (1: 19) mixtures; aragonite peaks are first detected in the (1:39) mixture ground for 8 h. This fact suggests that the calcium carbonate becomes covered with Cromophthal Yellow GR, and the grinding of calcium carbonate is inter- fered with increasingly by the lubricating action of increasing amounts of Cromophthal Yellow GR.

From the electron micrographs, the particles of Cromophthal Yellow GR are observed to aggregate considerably to form larger particles, as wit.h cr-CuPc ground alone. In the case of the (1 :l) mixture, the particles of mixture aggregate considerably and growth of secondary particles has taken place in the specimen ground for 10 h, as with Cromophthal Yellow GR alone. In the case of (1:9) and (1:19) mixtures, the ground particles adhere to calcium carbonate particles, and these do not transform to (spherical) aragonite crystals, i.e. the reduction in particle size is considered to be inter- fered with by the lubricating action of Cromophthal Yellow GR. On the other hand, the calcium carbonate in the (1:39) mixture is ground considerably more than that of the other mixtures and transforms to (spherical) aragonite crystals.

‘be specific surface areas of all specimens are plotted against their grindk~g times in Fig. 12. The specific surface area of Cromophthal Yellow GR is considered to decrease with grinding because of the aggregation

Page 13: Lubricating action of organic pigment during the grinding of pigment mixtures

221

Grtndmg time (hr.)

Fig. 12. Change of the specific surface areas of ail specimens with grinding time. (0) CaCO3, (0) Cromophthal Yellow GR, (m) (1:l) mixture, (nj (1:9) mixture, (0) (1:19) mixture, (a) (1: 39) mixture.

of particles. The specific surface area of (1: 1) mixture decreases with grinding as with Cromophthal Yellow GR, and that of (1:9) mixture decreases in the early stages of grinding and afterwards gradually increases, while those of (1:19) and (1:39) mixtures gradually increase, as with calcium carbonate alone. The specific surface areas of (1 :l), (1:9) and (1:19) mixtures become lower than that of calcium carbonate alone. This fact can be explained as follows: in the case of (1:l) mixture, the grinding is not effective for reduc- tion in particle size. Thus, the primary structural changes will not contribute greatly to an increase in the specific surface area. On the other hand, a growth of secondary particles is observed; this could compensate for the primary structural effects and results in the decrease in the specific surface area. In the case of (1:9) mixture, the primary structural effect will contrib- ute to an increase in the specific surface area much more than the opposed effect of the formation of secondary particles. In the case of (1:19) mixture, the aggregation of secondary particles is not observed and, as can be seen from the fact that the calcium carbonate in the mixture does not transform to (spherical) aragonite, the calcium carbonate in the mixture is ground less than calcium carbonate alone, i.e. the decrease in the specific surface area is considered to be caused by calcium carbonate being covered with Cromophthal Yellow CR. On the other hand, the specific surface area curve of the (1:39) mixture climbs above that of calcium carbonate alone. These facts agree with the results of X-ray diffraction analysis and electron micrographs.

4. The effect of copper phthalocyanine as a lubricant on the grinding of aIunliIla

In a previous section, the mechanochemical changes in the surface properties of organic pigment-calcium carbonate mixed system were exam- ined. In this section, the grinding process of the mixture of a-CuPc and alumina is reported [ 45,46 1..

Page 14: Lubricating action of organic pigment during the grinding of pigment mixtures

222

From the results of X-ray diffraction analysis it is observed that the intensities of ali reflections of alumina alone decrease and become weaker during the grinding process, and in particular the small peak near 21.5” in 28 almost disappezzs in the specimen ground for 180 min. In the case of (1:3) and (1:15) mixtures of a-CuPc and alumina, the (200) and (002) reflection intensities of a-CuPc in the mixture is observed to decrease with grinding; however, the reflection intensity of alumina near 21.5” in 28 in the mixture does not decrease during grinding more than in the case of alumina alone. Consequently, the lubricating action of cr-CuPc is observed in these mixtures. On the other hand, the reflection intensity of alumina near 21.5” in 28 markedly decreases with grinding and almost disappears in the (1:39) and (1:79) mixtures ground for 180 min; the lubricating action of cy-CuPc is not observed in these mixtures.

The specific surface area of alumina alone gradually decreases with grinding. The decrease in the specific surface area is caused by the formation of secondary particles due to aggregation or agglomeration of primary particles, as with organic pigment alone. The specific surface areas of the (1:3) and (1:79) mixtures of CuPc and alumina are plotted against their grinding times in Pig. 13.

01 100 x ‘= is

.u 5c z

d-CuPc’Ats& (1:3)

Theoretical curve _

Y I2

010 30 60 180

Grinding time (min.)

0 --

E i50 la

E! m ,100 P

; In 50 .u_ E x

::

.P-CuPc - AlrCl~ (I : 79)

Cl 10 30 60 180

Grinding time (min.)

Fig. 13. Change of the specific surface areas of (1:3) and (1:79) mixtures of WCUPC and alumina with grinding time.

The specific surface areas of these mixtures gradually decrease with grinding. In the case of the (1:3) and (1:15) mixtures, in which the lubri- cating action of CuPc is observed, the experimental value becomes higher than the theoretical. On the other hand, the experimental value becomes lower than the theoretical in the case of (1:39) and (1:79) mixtures. It may be considered that the lubricating action of cr-CuPc prevents the grinding of alumina in the mixture and, consequently, the specific surface area of alumina in the mixture does not decrease more than that of alumina alone.

The main conclusion is as follows. When organic pigment is admixed with inorganic pigment, the surface of the inorganic pigment is covered with that of organic pigment and the grinding of inorganic pigment is interfered

Page 15: Lubricating action of organic pigment during the grinding of pigment mixtures

223

with increasingly by the lubricating action of increasing amounts of organic pigment.

References

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