Most of the pigments used in textile printing are synthetic organic materials, except for
carbon black, titanium dioxide of the rutile and anatase types (for white pigments),
copper and aluminium alloys (for metallic bronze pigments), and sometimes iron oxide
(for browns) and titanium dioxide coated glimmer (for pearl lustre pigments). When
choosing synthetic pigments, the price, the fastness properties, the brilliance and the
colouring power of the many products available are all taken into consideration.
Among the organic pigments in use, the following are important:
– azo pigments (yellows, oranges, reds)
– naphthalene, perylenetetracarboxylic acid, anthraquinone, dioxazine and
quinacridone pigments (very fast and brilliant oranges, reds and violets)
– halogenated copper phthalocyanine derivatives (blues and greens).
Examples include:
CI Pigment Yellow 81 (disazo, arylamide), bright greenish yellow
CI Pigment Yellow 83 (disazo, arylamide), reddish yellow
CI Pigment Orange 13 (disazo, pyrazolone), orange
CI Pigment Red 7 (monoazo), neutral red
CI Pigment Red 122 (quinacridone), bright bluish red
CI Pigment Green 7 (perchlorinated copper phthalocyanine), bright green
CI Pigment Blue 60 (anthraquinone), navy blue
CI Pigment Violet 23 (dioxazine), bluish violet.
The chosen pigments are treated in a disintegrator or grinding mill in the presence of
suitable surfactants until they have been reduced to the optimum particle size – in the
region of 0.03–0.5 µm. If the pigment is not fine enough, the prints are dull and grey: a
particle size of less than the wavelength of visible light, however, results in a loss of
covering power and colour intensity.
With aqueous pigment pastes, the dispersion medium is of course water.
The surfactants used must have an HLB value (see section 7.4) >10; nonionic dispersing agents based on ethylene oxide condensates with C12 to C20 alcohols are commonly used. When producing a pigment paste starting from a wet press-cake that contains about 25% pigment mixed with water, it is only possible to make about 20% pigment dispersions. Highly concentrated pigment pastes (with 35–45% pigment content) are obtained from dried press-cake; the drying conditions for this must be as mild as possible, in order to avoid formation of hard pigment agglomerates. Water-free pigment pastes (for w/o emulsion pastes) were produced from aqueous pigment press-cakes by using the ‘flushing’ process, which is usually employed in paint production.
The press-cake is worked with a solution of selected polyester resins in suitable hydrocarbons in a container known as a ‘flusher’, equipped with efficient kneading and stirring apparatus, until the pigment is finely dispersed and has completely passed over into the hydrophobic phase. The water in the press-cake is almost completely separated and can be removed. Binding agents, based on the condensation of maleic anhydride, saturated and unsaturated fatty acids and polyols, are added; they also act as dispersing agents.
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| Pigment dispersions |
The surfactants used must have an HLB value (see section 7.4) >10; nonionic dispersing agents based on ethylene oxide condensates with C12 to C20 alcohols are commonly used. When producing a pigment paste starting from a wet press-cake that contains about 25% pigment mixed with water, it is only possible to make about 20% pigment dispersions. Highly concentrated pigment pastes (with 35–45% pigment content) are obtained from dried press-cake; the drying conditions for this must be as mild as possible, in order to avoid formation of hard pigment agglomerates. Water-free pigment pastes (for w/o emulsion pastes) were produced from aqueous pigment press-cakes by using the ‘flushing’ process, which is usually employed in paint production.
The press-cake is worked with a solution of selected polyester resins in suitable hydrocarbons in a container known as a ‘flusher’, equipped with efficient kneading and stirring apparatus, until the pigment is finely dispersed and has completely passed over into the hydrophobic phase. The water in the press-cake is almost completely separated and can be removed. Binding agents, based on the condensation of maleic anhydride, saturated and unsaturated fatty acids and polyols, are added; they also act as dispersing agents.
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