Indigoid & Thioindigoid Pigments
Indigoid & Thioindigoid Pigments
Indigoid & Thioindigoid Pigments
PIGMENTS
INDIGOID PIGMENTS
Indigo itself (C.I. Vat Blue 1, C.I. 73000) has a
certain amount of interest as a pigment
because of its insolubility in most solvents and
its very good fastness to light and heat (in spite
of its tendency to sublimation). That is why it is
still used, notably in rubber and for mass coloration of rayon in a navy
blue shade, although it has the drawback of
Figure 1: C.I. Vat Blue 1
being sensitive to the reducing action of
viscose.
6 halogenated
5
but today they are obsolete.
On the contrary, several
7 valuable pigments.
4
derivatives of thioindigo have
been recognized as
Figure 2: Thioindigo
They are reds and violets, three of which are also textile vat dyes of
importance. These pigments are shown below:
CI Constitution No.
Thioindigo derivatives
Pigment Red 86
73375
6,6-Dibromo-4,4-dimethyl
Pigment Red 87
73310
7,7-Dichloro
Pigment Red 88
73312
4,4,7,7-Tetrachloro
Vat Red 1
73360
6,6-Dichloro-4,4-dimethyl
73390
4,4-Dichloro-7,7-dimethyl
Vat Violet 2
73385
5,5-Dichloro-7,7-dimethyl
Vat Violet 3
73395
5,5-Dichloro-4,4,7,7-tetramethyl
They are not, however, the best, since they bleed slightly in solvents and
have only moderate fastness to light in pale tints.
The fastest are the derivatives substituted in the 4,4,7,7 positions:
i.
ii.
Rn
S
O
OH
O
HOC
CH2
S
SO2Cl
Cl
Red.
Cl
ClSO3H
Cl
Cl
KSCOC2H5
SCOC2H5
N
Cl
Hydrolysis
SH
Cl
Cl
2,5-dichlorothiophenol
Na2S2
Red.
Cl
Cl
S
Cl
S
Cl
Cl
Cl
ClSO3H, 35oC
ClCH2COOH
CH2
S
SCH2COOH
SH
Cl
Cl
Cl
Cl
Cl
Cl
O
S
C
S
Cl
Cl
AFTER-TREATMENT
In most cases, it is necessary to modify the crude thioindigo derivative by
appropriate after-treatment in order to develop the desired pigment
properties.
The list of options include milling the crude product with salt or dispersing
agents or reprecipitation from H2SO4 orClSO3H, followed by after-treatment
with organic solvents.
Tinctorially strong transparent pigment grades, for instance, are obtained
by milling the crude pigment suspension in the presence of an aqueous
base.
The same result is achieved by oxidizing the leuco form of
tetrachlorothioindigo in the presence of sodium dithionite with air while
applying shearing forces (for instance in a pearl mill).
PROPERTIES
Thioindigo pigments provide a range of hues from red violet and maroon
to brown shades.
ii.
R4
R7'
R5
R6'
S
S
R6
R5'
O
R7
R4'
No.
73312
73360
R4
R5
R6
R7
Shade
Cl
CH3
H
H
H
Cl
Cl
H
red-violet
bluish red
INDIVIDUAL PIGMENTS:
Pigment Red 88
REFERENCES: