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Developments in the Field of Inorganic Pigments.

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Developments in the Field of Inorganic Pigments[**'
By Knut Bittler and Werner Ostertag1"
Dedicated to Professor Matthias Seefelder on the occasion of his 60th birthday
The specific properties of inorganic pigments result from an interplay of solid-state properties,
particle size, and particle shape. However, phenomena occurring at the pigment interfaces also
have implications for the use of the pigment. The interrelations for individual classes of pigments are indicated, and two selected examples, namely transparent colored pigments and
magnetic pigments, are subsequently discussed with particular attention being paid to industrial developments.
1. Introduction
Inorganic pigments have been known for a very long time.
They are found in prehistoric cave drawings, for example the
drawings in the Pech-Merle caves in the South of France.
Fig. 1. a) Detail from a cave drawing in Pech-Merle (France). b) Antique Grecian vase, from the 5th century BC.
Figure l a shows a detail from these cave drawings, believed to be about 17OOO years old.
Another example of the use of inorganic colored pigments
is to be found in antique pottery, such as the antique Grecian
vase in Figure l b which dates from the 5th century BC.
An appreciation of the artistic value of the pigments was
soon followed by an appreciation of their commercial importance, and in this respect inorganic pigments have meanwhile lost none of their attractiveness. Today, with world
sales running at about 10 billion DM per annum they are an
important economic factor.
The beginnings of the pigment industry date back to the
18th century when colored pigments such as Berlin blue,
Fe[Fe,(CN)&, cobalt blue, CoA1204, Scheele's green,
Cu(AsO&. CU(OH)~,and chrome yellow, PbCrO,, were
discovered and produced on a large scale.
The real commercial upswing, however, only occurred in
the present century thanks to more modern means of produc-
tion and more refined methods of scientific investigation.
New applications were also discovered.
Nowadays, pigments are classified under the following
headings:
Colorants; magnetic pigments; rustproofing pigments; and
fillers.
Pigments are small solid particles roughly within the size
range from 0.1 to 1 pm['], and of greatly varying shape. They
furthermore have a narrow particle size distribution and conform to a number of stability criteria, such as heat resistance,
insolubility in solvents and binders, fastness to weathering,
and stability to ultraviolet light.
The specific properties of a pigment, which determine its
use as a colorant, magnetic pigment, rustproofing pigment or
filler, essentially result from the interplay of the solid state
properties, particle size, and particle morphology of the pigment. In addition, there are numerous phase boundary phenomena occurring between the pigment and the binder.
Typical solid state properties, i. e. properties resulting from
the chemical composition and structure of the solid, include
the color, the refractive index, and the magnetic order.
Pigment properties which are determined by, or affected
by, the particle size and particle shape include the hue, the
light scattering and the magnetic coercive field strength.
Phase boundary phenomena occurring between the pigment and the binder in the main play a part in the dispersibility of a pigment and the stability of the dispersion, but
also in other specific effects, which will be referred to later.
Pigments are always employed in dispersed form, because
only in this form do the properties other than purely solids
properties of the pigments come into play. At the same time,
dispersions are technically easy to apply.
2. Properties of Pigment Categories
2.1. Colorants
Colorants are classified into white pigments and colored
pigments[**'.
White pigments show no absorption in the visible range;
colored pigments absorb certain wavelengths of visible light.
The reasons for the color reside in the molecule itself or in
['I
Dr. Knut Bittler, Dr. Werner Ostertag
BASF Aktiengesellschaft,Hauptlaboratorium
D-6700 Ludwigshafen (Germany)
Based on a plenary lecture read at the General Meeting of the GDCh ai
Berlin on September 12, 1979.
r*]
190
0 Verlag Chemie, GmbH, 6940 Wernheim, 1980
['I The size range is not precisely defined there are pigments ofO.O1 Fm particle
size and also pigments of 10 Fm particle size.
I**]
In this classification, black pigments are regarded as colored pigments.
0570-0833/80/0303-0190
$02.50/0
Angew. Chem. I n t . Ed. Engl. 19. 190-196 (1980)
the type of lattice. Various absorption mechanisms may be
involved['1.
The following are some characteristic examples: The color
of yellow lead chromate PbCr04 is attributable to charge
transfer in the chromate anion. This electronic transition
from oxygen to chromium is caused by the blue region of visible light.
The color of the blue cobalt aluminum spinel CoA1204is
due to the ligand field of the 0'- tetrahedra of the spinel lattice. This ligand field splits the originally equivalent d-levels
of the Co'+ and permits electron transitions between the
split energy levels as a result of light absorption in the visible
range.
In the case of the red pigment Cd(SeS), the color is caused
by the absorption of light of short wavelengths, which triggers electron transitions between the valency band and the
conduction band of the solid.
Finally, the color of pigments can also be due to the presence of different valencies of one and the same element in
the lattice, and to non-stoichiometric compositions. Examples are black iron oxide Fe304, and molybdenum blue
Moo3- I , a non-stoichiometric molybdenum oxide.
Whilst the color of a colored pigment is due to the light
absorption per se, the shape of the absorption spectrum is responsible for the purity of hue.
The hiding power of a colorant depends primarily on the
light scattering which occurs at the pigment particles in the
dispersion. This phenomenon is shown diagrammatically in
Figure 3.
Fig. 3. Diagrammatic representation of light scattering at pigment particles in a
dispersion;particles of high refractive index on the left; particles of low refractive
index on the right. The gray background represents the binder.
The light scattering is seen to be more pronounced, the
greater the difference in refractive indices of the pigment and
the binder.
However, the scattering power also depends on the particle
size of the pigment; this dependence is shown in Figure 4 for
a rutile dispersion in 0ilI'l.
Fig. 4. Dependence of the relative scattering power of a rutile/oil dispersion on
the type of light and the particle size [3].
Fig. 2 Absorption spectra of chrome yellow, PbCrO,, and yellow iron oxide,
FeOOH, in the 400-660 pm range [2].
Figure 2 shows the absorption spectra of chrome yellow
and yellow iron oxide'']. The sharper, i. e. the steeper, the absorption edge, and the fewer additional absorption bands
present in the visible range, the purer the hue of a colorant.
The sharper rise in absorption in the case of chrome yellow is
responsible for its exceptional brilliance. In the case of yellow iron oxide, the absorption curve is overall not so steep
and therefore the color is distinctly duller.
Color and purity of hue are, however, not the only requirements which a colorant has to meet; it must also possess good
tinctorial strength"' and hiding power.
['I In practice, the tinctoriai strength is determined as the relative tinctorial
strength according to DIN 53254 and DIN 55943; it results from the particular
absorption characteristics and scattering characteristics of the pigment in question.
Angew Chem. Inf.Ed Engl. 19, 190-196 (198#)
It will be seen that optimum light scattering requires a
very specific particle size for each wavelength. For the white
pigment TiOz, the particle sizes corresponding to this optimum are in the range from 0.15 to 0.25 Fm. This is because,
for Ti02, the scattering maxima for the wavelengths of white
light lie in this range.
2.2. Magnetic Pigments
The most important requirement for a magnetic pigment is
ferromagnetism or femmagnetism. As is well known, this
magnetism depends on the crystalline state of the material
and is due to the parallel orientation of the spin moments of
greater or lesser domains of the crystal lattice.
Examples of ferromagnetic materials are iron and CrOz,
and examples of femmagnetic materials are certain iron oxides, e. g. Y-Fe203,or femtes, e. g. CoFez04.Both categories
of materials are equally suitable for the production of magnetic pigments.
191
The first field of use for magnetic pigments was in soundrecording tapes. At the present time, magnetic tapes are used
extensively not only in the entertainments industry but in information and data processing. Figure 5 shows, in a greatly
simplified form, the principle of magnetic recording141. The
magnetic tape is led past the gap of a horseshoe-shaped iron
or ferrite core, which is wound with a coil and referred to as
the magnetic head. The information to be stored is first converted into an electrical alternating voltage. This alternating
voltage is used to generate, via the coil and the iron or ferrite
core, a magnetic leakage field at the gap. This leakage field
orients the initially random magnetic domains of the pigment coating on the tape. This orientation is retained after
the magnetization, i. e. after leaving the leakage field.
Magnetmtion
Magnetization
Fig. 6. a) Magnetization behavior of a magnetic pigment. Ordinate: Magnetization of the pigment. Abscissa: Field strength of an externally applied magnetic
field. M , = saturation magnetization, M , = residual induction, H, = coercive field
strength. b) Effect of the orientation of acicular pigment particles. Broken curve
without orientation; solid curve with orientation. AM. = gain In residual induction, AH,=gain In coercive field strength.
acteristics. This effect was recognized at an early stage, and it
was found that acicular particles give higher coercive field
strengths than compact particles. Moreover, acicular pigment
particles also prove advantageous in other respects. This is
because, the pigment particles in the flowing dispersion can
be oriented parallel to the tape surface and tape direction by
means of a permanent magnet. This orientation widens the
hysteresis curve and thus further increases the coercive field
strength and produces a not insubstantial gain in residual induction15](see Fig. 6b).
2.3. Rustproofmg Pigments
Fig. 5. Principle of magnetic recording. The arrows repreKnt the magnetic domains of the pigment coating. The core may be iron or ferrite.
The intensity and durability of the magnetic recording is
essentially dependent on three parameters: the Curie temperature, the residual induction, and the coercive field strength.
The Curie temperature indicates the temperature at which
the pigment loses its magnetization due to thermal motion of
the spins, and becomes paramagnetic. Hence, for practical
applications the Curie temperature should be as high as possible. The residual induction and coercive field strength are
characteristic parameters of the magnetization behavior of a
pigment. The magnetization behavior is represented in the
form of a hysteresis loop (Fig. 6a). The residual induction M ,
is the magnetization which remains after magnetically saturating the pigment by means of the external magnetic field
(Ms= saturation magnetization) and then switching off the
field. The coercive field strength H , of a pigment is the magnetic field needed in order completely to demagnetize the
magnetic pigment again. The coercive field strength can also
be regarded as a measure of the ability of the pigment to resist demagnetization, i e. a measure of its ability to retain information once accepted. The hysteresis curve has a characteristic appearance for each pigment.
In general, pigments of high residual induction are desirable, since they exhibit powerful recording signals and hence
permit production of very thin magnetic coatings. On the
other hand, the coercive field strength must not be too low or
too high. If it is too low, the tape can be demagnetized too
easily, and loses its information. If it is too high, the tape can
no longer be erased.
However, the residual induction and coercive field
strength of a magnetic tape are not merely characteristics of
the particular solid-state material used. The shape of the pigment particles also greatly influences the magnetic tape char-
192
The effect of a rustproofing pigment is attributable to diverse physical and chemical phenomena occurring at the surface of the pigment particle. The physical protective action
may be illustrated with reference to micaceous iron ore,
which is a rustproofing pigmenti6](Fig. 7).
Fig. 7. Diagrammatic representation of physical rustproofing action illustrated
for micaceous iron ore [6].
The black bars in the figure represent the flake-like micaceous iron ore particles. The binder -indicated in gray -is
Fig. 8. Diagrammatic representation of chemical rustproofing action.
Angew. Chem. Int. Ed. Engl. 19, 190-196 (1980)
located between the particles. It is found in practice that pigmented finishes show better adhesion to the substrate than
non-pigmented finishes. This adhesion is symbolized by
brackets. In addition, the overlapping flakes form a kind of
labyrinth for any penetrating air and moisture, and thus
lengthen their diffusion path to the surface to be protected.
Further protection results from the fact that the pigment
screens the organic binder against UV rays, both by absorption and by reflection.
As is shown diagrammatically in Figure 8, the chemical
protective action of pigments is based on chemical interactions in the interfacial region between the pigment and the
substrate, the pigment and the binder, and the pigment and
any moisture which has penetrated. For example, redox reactions occur, new protective compounds are formed or a certain pH results in the vicinity of the pigment particles.
that the redox reaction
In the case of red lead it is
between tetravalent lead and the iron substrate results in the
formation of insoluble protective layers containing Pb2 and
Fe3+ ions. In addition, the reaction of the red lead with the
fatty acids of the binder leads to the formation of “lead
soaps”, which also act as a protective film. Hydrolysis finally
gives Pb(OH),, which produces a basic pH in the vicinity of
the particles and hence additionally retards corrosion.
It follows from what has been said above that the ideal
rustproofing pigment must perform a large number of functions. The problem of rusting would in principle be solved if
it were possible to produce coatings which cannot be damaged mechanically, adhere firmly, and are impermeable to
water and to air. However, this has not hitherto proved possible. Hence, pigments remain the best solution to the problem
of corrosion.
shows a photograph covered with two lacquer film strips pigmented with iron oxide. Both strips contain the same amount
of iron oxide and are of equal thickness. Nevertheless, the
left-hand strip is opaque and the right-hand strip transparent. The phenomenon is readily explained when the particle
size of the particular pigments is considered. The opaque dispersion layer has a particle size of about 0.5 km whilst the
particles in the transparent layer are substantially smaller.
The hiding power of a pigmented lacquer film dependsas indicated in Section 2.1 -on the particle size of the pigment used. The larger particles show high light scattering
and hence good hiding power. On the other hand, the substantially smaller particles have largely lost the ability to
scatter light; the corresponding lacquer film is transparent.
However, the light absorption, and hence the color, persists
also in the case of the small particles. As a result, interesting
effects can be achieved with transparent pigments. For example, it is possible to pigment articles without masking and
hence losing the structure of the substrate. This effect is
shown in Figure 9b on three samples of wood treated with
transparent pigment dispersions.
What pigments are suitable for transparent lacquers and
glazes? In principle, all colored pigments, provided their
process of manufacture allows them to be obtained in a sufficiently finely divided form.
2.4. Fillers[*’
Table 1 shows compounds with which it has hitherto
proved possible to produce transparent colorations. Essentially, they are transition metal oxides.
Of these pigments, it is especially the iron oxide pigments
which are commercially important. The reasons include their
good pigmenting power and the ability of iron oxide to absorb UV radiation and hence to protect the organic dispersants against rapid degradation. Among the transparent iron
oxides, iron oxide red, which is particularly heat-stable, has
found the widest use. Transparent iron oxide red pigments
are manufactured industrially by two processes, a wet chemical precipitation process and a pyrolytic process.
The precipitation process[9]involves three steps:
+
Fillers serve as diluents, for extending and improving pigment dispersions[*].For example, fillers can be used to adjust
the flow characteristics of finishes and dispersions, and to
improve their processability. Moreover, the hardness, and
adhesion to the substrate, of a pigment dispersion can also
often be optimized by adding fillers. Fillers can also be used
to render surfaces hydrophilic or hydrophobic. In some
cases, fillers are used in order to fix dyes. Well-known fillers
include pyrogenic silica, S O 2 , and Blanc fixe, BaSO,.
3. Recent Developments
3.1. Transparent Colored Pigments
The relatively recent field of transparent colored pigments
has become commercially interesting in the last 10 years, because they make possible particular optical effects. Figure 9a
[*I The classification of fillers and pigments overlapped for a long time. According to the definition in a German standards specification (DIN 55943) and an international standards specification ( I S 0 standard 3262), white powders with refractive indices greater than 1.7 are regarded as pigments and those with refractive indices of less than 1.7 as fillers.
Angew. Chem. Inf. Ed. Engl. 19, 190-196 (1980)
Table 1. Transparent inorganic colorants
Iron oxide yellow
Iron oxide red
Hydrated chromium oxide green
Chromium oxide green
Cobalt blue
Precipitation Fe2++ 2 0 H - +Fe(OH)2
Oxidation Fe(OH)2+air+FeOOH
Dehydration FeOOH+Fe20,
Pigments prepared by this method have the typical acicular appearance shown in Figure 10a. The average particle
size is about 0.1 pm. Due to the method of preparation, iron
oxide pigments obtained by precipitation are difficult to disperse. The user must frequently subject the finely divided oxide, which tends to agglomerate, to a very lengthy deagglomeration treatment because the optical effect of agglomerates
is similar to that of larger particles, i. e. agglomerates reduce
the transparency.
193
tor particularly important in practice is the excellent dispersibility of the pyrolytically prepared pigment, as shown by the
short dispersing time needed to develop the full depth of color.
Transparent iron oxides are in the main used in highgrade metallic-effect finishes in the automotive industry.
Other fields of use are in tinting wood, in printing of packaging materials, and in coloring plastics and leather.
Fig. 9. a) Photograph with two lacquer strips pigmented with iron oxide (the
amounts of iron oxide and the thickness of the lacquer film being the same for
both strips). The left-hand lacquer film is opaque and the right-hand strip transparent. b) Samples of wood treated with various transparent pigment dispersions.
A more easily dispersible finely divided pigment is obtained by the pyrolytic process['']:
Fe(C0)5+air
bW 'C
Fig. 11. Microscopic section through a metallic-effect finish. Bottom: primer
layer. Top: pigmented finish containing aluminum flakes: 12.5 mm correspond to
25 pm.
Fe203+ CO,
The essential point in this process is to avoid sintering. The
particles of pyrogenic iron oxide red pigments are smaller by
about a factor of ten than the particles of the products obtained by precipitation. This can be seen very clearly in the
electron micrographs in Figure 10. In contrast to the acicular
precipitated product, the pyrolytically produced particles
have a round shape.
Figure 11 shows a microscopic section through a metalliceffect finish. The lacquer layer, pigmented with transparent
iron oxide red, lies on top of the dark primer layer. This lacquer layer additionally contains aluminum flakes, about 1050 Lm in size, which in the electron micrograph appear as
thin black lines in the lacquer layer. These aluminum flakes
act as miniature mirrors and produce the well-known metallic effect. They also allow the surface of the automobile to
show a pronounced light-dark visual effect (cf. Fig. 12a).
Such light-dark effects can only be achieved with particularly finely divided pigments.
Fig. 10. Electron micrographs of transparent iron oxide red pigments. a) Pigment
obtained by the wet precipitation process. 15 mm in the figure correspond to
1 pm. b) Pigment obtained by the pyrolytic process.
Table 2 compares further properties of the pyrolysis product with the corresponding properties of the precipitated
product. The very finely divided nature of the pyrolysis
product manifests itself in its larger specific surface area and
in a more yellowish hue. The smaller particles give better
transparency but as a result a lower tinctorial strength. A facTable 2. Comparison of some properties of transparent FezO, pigments.
Product
Pyrolysis
product
Precipitation
product
194
Surface
area,
BET
method
Im2/gl
Hue
Relative
tinctorial
strength
Dispersing
time
[min]
1x1
160
yellowish
red
100
red
85
5
100
40
Fig. 12. a ) Light-dark effect in metal-effect finishes. b) Light-dark effect as a
function of the particle size of the pigment used. Left: lacquer film strip containing high-hiding pigment (larger particles); right: lacquer film strip containing
transparent pigment (very small particles); between these: two lacquer film strips
containing semi-transparent pigments.
This effect is demonstrated for a slightly curved aluminum
foil in Figure 12b. Four different lacquer film strips are applied to this foil; one containing a high-hiding pigment is
shown on the extreme left, followed by two containing semitransparent pigments and, finally, one containing a transparent pigment on the extreme right. The light-dark change in
brightness is very pronounced only in the case of the righthand strip. In the case of the semi-transparent strips the
change in brightness is just still discernible, while in the case
of the high-hiding film the brightness differences have disappeared.
Angew. Chem. Int. Ed. Engl. 19, 190-196 (1980)
3.2. Advances in Magnetic Pigments
The very rapid development of radio, television, and not
least electronic data processing is ultimately attributable to
magnetic pigments. This development started in the early
1930's and resulted from the cooperation between AEG and
the Ludwigshafen Research Department of what was at that
time IG Farben.
Subsequently, four important types of pigment have come
to the forefront. Table 3 shows their characteristic parameters'"].
Table 3. Types of magnetic pigment and some of their characteristic data. Curie
temperature, coercive field strength ( H c ) , saturation magnetization (MJ, and
residual induction (Mr).The values (average values) shown apply to acicular
pigments.
Pigment
y-FeKh
Cr02
Co,Fe3 r 0 4 [a]
Fe (Co, Ni) [b]
Curie
temp.
["CI
H,
M,
Pel
[gauss cm'/g]
600
120
580
340
570
630
lo00
800
M.
76
60
100
75
90
70
160
217
[a]Average values for a pigment containmg 10%of cobalt. [b] The magnetic values apply to an iron pigment.
The classical product among these pigments is y-Fe203.
The manufacture of acicular y-Fez03 pigments requires a
complicated multi-stage process leading via iron oxide hydrate, FeOOH, and magnetite, Fe30415.121.
The key product is
FeOOH which is easily obtained in an acicular form and to
which the shape of the pigment ultimately obtained is attributable.
the audio and video industry started and the age of the cassette began.
In order to achieve the desired playing time of several
hours in small handy cassettes it is necessary for space reasons to make better use of the tapes in these cassettes, i. e. to
write more closely on the tapes. The high recording density
of the cassette tapes is nowadays achieved, inter alia, by a
drastic reduction in the speed at which the tape is led past
the recording head or playback head. The high recording
densities required for this technology can, however, not be
achieved with conventional y-Fez03pigments without sacrificing some quality; pigments with higher coercive field
strengths are required.
As may be seen from Table 3, the other three pigments
show the requisite higher coercive field strengths.
Chromium dioxide pigment was first introduced in the
early 1970's. Chromium dioxide is chemically metastable; it
contains tetravalent chromium. Nevertheless, it can be handled industrially since it releases substantial amounts of oxygen only at elevated temperatures.
Chromium dioxide magnetic pigments can be prepared by
two methods[131:
Cr03--t Cr02+ 0.5 O2
Cr03+ Cr20,4 3 CrO,
Both reactions must be carried out under hydrothermal conditions.
Fig. 14. Electron micrograph of a CrOl pigment. 14 mm in the figure corresponds to 1 wn.
Fig. 13. Electron micrographs of iron oxide pigments. a) an FeOOH pigment; b)
the y-Fe,03 pigment prepared therefrom. 16 mm in the figure correspond to 1
w.
Figure 13a shows an electron micrograph of FeOOH needles. The needles shown in Figure 13b are the y-Fe,03 pigment prepared from the FeOOH needles. It will be seen that
whilst the acicular shape suffers considerably as a result of
the intermediate conversion stages, it is nevertheless basically retained, and this is the vital factor.
The particular advantage of y-Fe203 pigments resides in
their chemical stability and heat stability, and in the low
temperature-dependence of the magnetic properties. The
magnetic properties of y-Fe203 shown in Table 3 allow yFe203pigments to be used for all applications, for example
computer tapes, video tapes, and radio tapes. Hence, it is not
surprising that this pigment enjoyed a monopoly position for
a long time. However, in the mid-1960's miniaturization in
Angew. Chem. i n [ . Ed. Engl. 19, 190-196 (1980)
Figure 14 shows an electron micrograph of a chromium
dioxide pigment. The extreme uniformity of the particles is
discernible; they are essentially single crystals.
A conspicuous feature is the low Curie temperature of
CrOz, which is about 120°C. However, this has not proved
of disadvantage in practice. On the contrary, a particularly
interesting use for CrOz pigments results from this very prop-
Fig. 15. Diagram illustrating thermoremanent copying.
195
erty: the low Curie temperature of CrOz and the substantial
stability of the magnetization up to the Curie temperature
make thermoremanent copying feasible. This process, which
is only feasible with chromium dioxide pigments, is illustrated in greatly simplified form in Figure 15.
The parent (or mother) tape corresponds to a printing
plate, carrying information in mirror image form. The parent
tape is brought into intimate contact with the daughter (or
slave) tape, which has been heated to just above the Curie
temperature. On cooling, the magnetic domains of the pigment of the daughter tape undergo orientation corresponding to the parent tape, and pick up the information of the latter.
In audio cassettes and video cassettes, however, chromium
dioxide soon encountered competition from cobalt-modified
iron oxides, which show magnetic properties similar to those
of Cr02. It has been known since about the mid-1950’s that
the coercive field strength of Fe304 pigments is greatly increased by modifying the pigments with
Originally, the modification was effected by replacing part of the
Fe2+ in the octahedral lattice positions in the magnetite randomly by Co2+ so that the modification extends over the entire lattice.
With this type of modification, however, the heat stability
of the magnetic recording diminishes considerably with increasing cobalt content. Hence, the magnetic tapes produced
with such modified iron oxide pigments did not prove satisfactory.
A breakthrough was only achieved when a different type
of modification was used, namely surface modification[’51.
The principle in shown in Figure 16.
However, the preparation of metallic iron
yields pyrophoric particles, which raises substantial problems. Not only must the particles be protected against oxidation during the manufacturing process; it is also necessary, in
particular, to ensure that the pigment subsequently remains
stable in the tape.
Looking at the magnetic properties of metallic pigments
(Table 3), it is clear that they are the pigments of the future.
However, how rapidly metallic pigment tapes will be able to
gain a foothold in magnetic recording technology, and how
wide their use will be, depends not least on the equipment
manufacturers and on the success of their endeavors to modify the equipment to give optimum results with such pigments.
The tape manufacturers have already offered the first metallic pigment tapes to the public-for the first time in 1978
at the “Consumers Electronic Show” in Chicago, and more
recently at the 1979 Radio Exhibition in Berlin.
4. Future Prospects
The present-day importance of inorganic pigments is attributable, to a not insignificant degree, to the constantly refined methods of investigation and preparative techniques
available in the field of solid-state compounds. The likely future advances in the physics and chemistry of surfaces and
solids may therefore be expected also to lead to new fields of
use, and to maintain the momentum of the inorganic pigment sector.
Received December 19, 1979 [A 311 IE]
German version: Angew. Chem. 92, 187 (1980)
Fig. 16. Principle of surface modification
In this process, a coating of, say, cobalt femte is applied,
e.g., to acicular y-Fe203 pigments. In this way, the advan-
tages of both types of pigment are exploited: the y-Fe203
core gives heat-stable magnetic properties while the Co-containing coating at the same time confers a high coercive field
strength upon the pigment.
The ultimate objective of magnetic pigment research has
always been to obtain suitable metallic pigments, since metals have attractive magnetic properties. The fact that metal
pigments have not long been in use is due to the difficulty of
preparing stable, elongate -preferably acicular -metallic
pigment particles. Only very recently has this been
achieved.
The preferred element for the production of metallic pigments is iron, mainly because of its residual induction.
196
[I] W. Feitknechti The Theory of the Color of Inorganic Substances. Pigments -An Introduction to their Physical Chemistry. Elsevier, Amsterdam
1967.
121 P. Hauser, BASF Aktiengesellxhaft, unpublished data.
131 Ullmanns Encyklopadie der technischen Chemie, 3rd. Edit., Vol. 13. Urban
& Schwarzenberg, Munich 1962, p. 740; 4th Edit., Val. 18, Verlag Chemie,
Weinheim 1979, p. 554; R J. Briihlman, L. W . Thomas, E. Gonick, Off. Dig.
Fed. Paint Varn. Prod. Clubs 43, 252 (1961).
E. A. Sobotta: Die Hysterese und andere Gmdbegriffe des Magnetismus
eum Verstandnis der magnetischen Speicherung. Die BASF- A publication reporting the work of Badische Anilin- & Soda-Fabrik AG 21 (October
1977). p. 65; E. Koster, Grundlagen der magnetischen Datenaufzeichnung.
NTG-Fachberichte, Vol. 58, VDE-Verlag, Berlin 1977.
BASF brochure: Tonband, Herstellung und Eigenschaften, pp. 5-7 (Order
No. M. 0582682).
G.u. Pokorny, DEFAZET Dtsch. Farben-Z. 12, 588 (1973).
J. Rufi Korrosion/Schutz durch Lacke + Pigmente. Verlag W. A. Colomb,
Stuttgart 1912, pp. 76 et seq.
D. Hunkar, DEFAZET Dtsch. Farben-Z. 20, 39 (1966).
T. C. Palton, Pigment Handbook, Val. I. Wiley & Sons, New York 1973, p.
343; G. C. Morcot. W. J. Cauwenberg, S. A . Lamanna, U. S. Pat. 2558302
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