4

4’

5’

6’

7’

5

6

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INDIGOID & THIOINDIGOID 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 being sensitive to the reducing action of viscose.

Halogenated derivatives of indigo have also been used as blue pigments,

but today they are obsolete. On the contrary, several halogenated derivatives of thioindigo have been recognized as valuable pigments.

They are reds and violets, three of which are also textile vat dyes of importance. These pigments are shown below:

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Figure 1: C.I. Vat Blue 1

Figure 2: Thioindigo

Figure 3: Pigment Red 86 Figure 4: Pigment Red 87 Figure 5: Pigment Red 88

Figure 6: Vat Red 1

Figure 8: Vat Violet 2

Figure 7: Vat Red 198

Figure 9: Vat Violet 3

Table 1: Thioindigo Pigments

CI name 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

Vat Red 198 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

These pigments are all appreciated for their brilliance of shade.

The most used at the present time are:

i. Thioindigo Red Violet RH (C.I. Vat Violet 2), and ii. Thioindigo Pink (C.I. Vat Red 1)

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. Thioindigo Red Violet Y or Maroon (C.I. 73390), and especiallyii. Thioindigo Red Violet B or Bordeaux (C.I. Pigment Red 88).

The latter is shown as non-migrating and exceptionally fast to light, even in pastel shades.

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4,4’,7,7’-Tetrachlorothioindigo with a reddish violet shade reigns supreme as a pigment among the derivatives of this indigo.

It can be used for Bordeaux shades in automotive refinishes.

Thioindigo pigments are generally used in industrial coatings and plastics for their good lightfastness and weather-fastness in deeper shades.

CHEMISTRY & MANUFACTURE

Thioindigo pigments have the following general structure:

In commercially important products,

R typically represents a Cl atom or a CH3 group, and n stands for 2 or 3. The compounds assume the trans-structure.

The synthesis is performed in two steps:

i. attaching the 5-membered ring to the benzene ring, andii. oxidatively linking two molecules of the resulting thionaphthenone-

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The most important route involves cyclization of appropriately substituted phenylthioglycolic acids:

Suitable agents are ClSO3H, monohydrate, or conc. H2SO4.

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Other methods include cyclization of phenylthioglycolic acid chlorides, which is afforded by Friedel-Crafts reaction with AlCl3, possibly in the presence of H2SO4/NaCl. The acid chloride is obtained from phenylthioglycolic acid with SOCl2 or PCl3.

There are more routes to cyclization.

The synthetic route to tetrachlorothioindigo may serve as an example for an industrial scale synthesis:

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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).

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PROPERTIES

Thioindigo pigments provide a range of hues from red violet and maroon to brown shades.

The type and position of the substituents affects the shade:

i. Electron donating substituents (methyl groups) in 5-position cause the most drastic bathochromic shift, which decreases in the following order:

4- > 7- > 6- position

ii. The reverse is also true in the case of electron acceptors (such as chlorine): they cause more of a bathochromic shift in 6-position than in 5-position.

As a class, tetrachlorothioindigo pigments exhibit good to excellent lightfastness and weather-fastness as well as solvent stability and migration fastness.

4,4’,7,7’-substituted derivatives demonstrate highest fastness to solvents.

Cl substitution has a better effect in this respect than CH3 substitution.

Shifting only one of the substituents on 4,4’,7,7’-tetrachlorothioindigo, for instance, will adversely affect the migration resistance of the parent compound (fastness to bleeding).

COMMERCIALLY AVAILABLE TYPES & THEIR APPLICATION

GENERAL:

Of the two remaining thioindigo pigments, only Pigment Red 88, the 4,4’,7,7’-tetrachlorothioindigo, has been able to maintain interest throughout the pigments industry. It is broad in scope. The other type, Pigment Red 181, is a special-purpose type for a limited number of applications. The following table lists the chemical constitutions of both pigments.

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Figure 10: Vat Red 41

Table 2: Commercially available thioindigo pigments

CI nameCI Constitution

No.R4 R5 R6 R7 Shade

P.R. 88 73312 Cl H H Cl red-violet

P.R. 181 73360 CH3 H Cl H bluish red

Unsubstituted thioindigo, which is marketed as Vat Red 41, 73 300, is used in rigid PVC, polystyrene, and a number of other plastics. Dissolved in its medium, the pigment affords fluorescent bluish red shades.

Unsubstituted indigo is marketed as Pigment Blue 66, 73 000. It is a suitable pigment for spin dyeing of synthetic viscose rayon and spun rayon, to which it lends navy blue shades. The pigment provides good lightfastness. However, Pigment Blue 66 performs poorly regarding a number of textile fastnesses, especially in terms of bleaching with chlorite, and vat resistance.

INDIVIDUAL PIGMENTS:

Pigment Red 88

It is a suitable colorant especially for industrial coatings, including automotive (O.E.M.) finishes and automotive refinishes.

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Figure 11: Pigment Blue 66

Figure 12: Pigment Red 88

Its red-violet shade is used frequently to produce opaque dark shades of red, bordeaux, and maroon, for which purpose the pigment is typically used in combination with inorganic pigments, such as Iron Oxide Red or Molybdate Red.

P.R.88 is both very lightfast and weather-fast. P.R.88 shows good stability to organic solvents. Its fastness to over-coating is good, but not reliable at baking

temperatures above 140°C. Paints containing P.R.88 are acid and alkali resistant. P.R.88 is one of the standard red-violet pigments for use in plastics. It is particularly suitable for use in PVC, including PVC plastisols and

PUR coatings. Transparent polyolefin systems containing P.R.88 are stable up to 260

to 300°C, depending on the type and on the pigment content. P.R.88 is recommended for use in PP spin dyeing, special-purpose

grades are used for polyacrylonitrile spin dyeing. The resulting fibres are completely fast to perspiration, dry and wet

crocking, perchloroethylene, and other important solvents, as well as to dry heat fixation.

Used in the spin dyeing of polyester, the pigment dissolves in the fibre. Both sublimation and migration fastness are excellent in this medium.

Depending on the temperature, the pigment dissolves in this polymer and accordingly changes its colour and fastness properties. This presents major difficulties in trying to match a defined shade.

Incorporated in methacrylate and unsaturated polyester cast resins, P.R.88 not only withstands several hours of thermal exposure during processing but is also resistant to the peroxides which are used as catalysts.

P.R.88 is also used for printing inks. Its red-violet shade is especially used for printing inks which are to be

targeted for packaging, posters, and other special purposes. The prints are entirely fast to organic solvents, plasticizers, and

packaged goods, such as butter and soap. P.R.88 prints are fast to alkali and acid, heat stable up to 200°C, and

fast to sterilization. P.R.88 is also used in wood stains, for instance in combination with

yellow pigments, such as P.Y.83, or carbon black to produce shades of brown.

PIGMENT RED 181

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P.R.181 is a special-purpose product for polystyrene and similar polymers.

P.R.181 largely dissolves in its medium at the high temperatures at which these polymers are processed.

The pigment affords brilliant, bluish shades of red. It possesses excellent lightfastness and satisfies the application requirements.

Another important market is in toothpaste, for which the pigment has been approved worldwide.

It also lends colour to lipstick and other decorative cosmetics. In the US, it is registered by FDA as D&C Red 30, in Germany as C-Rot 28 according to the DFG Catalogue for Cosmetic Colorants, in Japan as Red No. 226.

REFERENCES:

▪ The Chemistry of Synthetic Dyes Volume II by K. Venkataraman

▪ Industrial Organic Pigments by W. Herbst, K. Hunger

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Figure 13: Pigment Red 181