The colloidal polysaccharide thickening agents, such as starch, cellulose ether,
alginates or locust bean gum, have been used successfully throughout the history of
textile printing in printing pastes for various groups of dyes. They are not suitable for
pigment printing, however, because their flow properties are unsuitable and because
the films they form are brittle. The prints produced with such thickeners are dull and
flat, with poor fastness and a harsh handle.
It is necessary for pigment printing pastes to have pseudoplastic (shear-thinning)
flow (see section 7.7.3); they can then be transferred on to the textile material easily,
but their penetration is limited.
Pseudoplastic flow is usually found in heterogeneous
and, physically speaking, coarsely dispersed systems. Flow occurs under shear, but when
the shear is removed the pastes return to the consistency of a solid on the surface of the textile. Unlike pastes with no yield value, they cannot penetrate deep down into the
textile because of their rheology. Since they therefore remain on the fabric surface,
they produce much better colour value, a sharp mark and brilliance of colour. In
addition, because of the superficial coating of the fabric with printing paste, the textile
yarns are not bonded to each other by the binders and crosslinking agents, and this
results in a better handle to the printed goods.
The hardening of the handle of the textiles after printing is mainly caused by the sticking together of the yarns and fibres, and this harshness can be broken down by using a suitable breaking machine. It is important for the fastness to wear and to washing that the same flow characteristics also hold the binder, crosslinking agent and pigments very close to one another. Typical systems, and the ones most suitable for pigment printing, are the emulsion thickeners, of which the white spirit/water emulsions (o/w type) were of greatest practical interest.
They consist of about 70% by mass of hydrocarbons in the disperse phase, 0.5–1% of nonionic emulsifiers with HLB in the region 12–15, and 29% of water in the continuous phase. The droplet size of the emulsified white spirit depends on the emulsifying agent and the efficiency of the high-speed mixer necessary to produce the emulsion. Efforts must be made to achieve a uniform droplet size (homogeneous emulsions), as this gives maximum stability and yield value. For physiological reasons the white spirit should have as little toxicity and aroma as possible, and a flash point of at least 30 °C, a boiling range of between 160 and 200 °C and a density of 0.76–0.80 g cm–3 are desirable.
Emulsion thickeners of the white spirit type are particularly suitable for pigment printing because all the components, except the emulsifying agents, will evaporate completely, leaving no residues. The thickener does not therefore cause any hardening of the handle of the textile and, because the evaporation enthalpy for white spirit is about 25% of that for water, drying of the printed textile is quicker; this improves the printing speed and consequently the production output. This is particularly important in drying chambers with a limited drying capacity. On the other hand, mineral oil products are too valuable to be used just as print paste thickening agents and, for environmental reasons, the emission of organic substances into the atmosphere has met with opposition in many countries and has been prohibited by law in some. Recovering the white spirit from the exhaust gases of drying chambers, by means of cooling apparatus and batteries of charcoal filters, is too complicated and too expensive to provide a solution to this problem.
The burning or catalytic combustion of evaporated white spirit, to comply with emission protection laws, also requires substantial capital expenditure. It is vitally important that the gases in the dryer should contain no more than 0.4% by volume of white spirit, to avoid explosions in the plant. White spirit emulsions for pigment printing are therefore being phased out all over the world; perhaps this may be regarded as a sign of our times. In the 1960s, in anticipation, synthetic-polymer thickening agents were developed for pigment printing, with rheology similar to that of emulsion thickeners and extremely low solids content [7,8].
They are polyanionic compounds derived from monomers such as acrylic acid and maleic anhydride. A small degree of transverse crosslinking of the polymer molecules is built in, so that packets or bundles of chain molecules are held together. These products have very high swelling power, which is at a maximum at pH 9–10 when ionisation increases the degree of solvation in water. The original polymer with undissociated carboxylic acid groups will be in the coiledchain form. Neutralisation leads to a straightening and separation of the polymer chains because the carboxylate ions are fully dissociated and strongly repel one another.
This brings about a hundredfold expansion in size and the solvated particles then have dimensions similar to those of the white spirit droplets in emulsion thickeners. When the water-swollen particles are tightly packed they give a paste with pseudoplastic flow properties. As coarse colloids they are subject to salting-out effects, and small additions of electrolyte reduce the viscosity, which is not the case with white spirit emulsions. Another disadvantage is that, on drying, this type of thickener leaves a residue; depending upon the requirements, 0.5–1% of solid substance remains on the textile. This residue, because of its extremely polar character, produces a hard, inflexible film which can (depending upon its chemical constitution and particularly in the case of carbonamide groups) react with crosslinking agents.
This rigid film has an adverse effect on the handle of the printed goods, although it is considerably less than that of polysaccharide thickeners. The effect on the handle can be satisfactorily balanced by adding suitable softening agents, such as polydimethylsiloxane, mineral oils or synthetic wax. The neutralising agent, usually ammonia, in the synthetic thickener is removed under drying and fixing conditions; the free carboxyl groups in the polyanion then produce the acid medium which the binder and external crosslinking agents require for the crosslinking reaction. Printing pastes based on emulsion thickeners need about 0.5% potential acid, such as diammonium phosphate, in order to obtain the necessary acid medium during fixation.
Some manufacturers have combined softeners, synthetic thickeners neutralised with ammonia and dispersing agents to make application simpler and have marketed these products in paste form (‘printing concentrate’). Emulsion and synthetic thickeners can be grouped under the heading ‘disperse thickeners’. They are to a large extent interchangeable. If, for example, it is not possible to print economically with spirit-free pastes on the grounds of inadequate drying capacity, an emulsion thickener and a synthetic thickener can be used together to give pastes containing perhaps 25% white spirit. The printer knows from experience that ‘short’ flow pastes (see section 7.7.3) are more sensitive to crushing effects (section 1.3.2) than are ‘long’ pastes.
The less mobile the pastes are on the textile, the more easily are they picked up on the following printing rollers, in the case of multicoloured prints. This phenomenon makes itself unpleasantly noticeable, particularly in roller printing, with disperse thickeners having yield values and very steeply decreasing shear resistance; the original colour depth may be reduced by up to 70% after three or four times crushing. In screen printing the decrease in colour depth as a result of crushing is not nearly so pronounced, but for this reason there is the risk that uneven prints will be obtained on smooth, hydrophobic textiles.
In flat-bed screen printing there may, in addition, be screen frame marks. Combinations of disperse thickeners with colloidal thickeners (for example, cellulose ethers of average viscosity) reduce the crushing effect considerably. The pastes are, in the printer’s terms, softer or longer. The larger the proportion of colloidal thickener in the printing paste, the harder is the handle of the printed goods. Auxiliary substances with dilatant flow properties are more suitable and are effective as ‘flow moderators’ in comparatively low concentrations
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| Thickening systems |
The hardening of the handle of the textiles after printing is mainly caused by the sticking together of the yarns and fibres, and this harshness can be broken down by using a suitable breaking machine. It is important for the fastness to wear and to washing that the same flow characteristics also hold the binder, crosslinking agent and pigments very close to one another. Typical systems, and the ones most suitable for pigment printing, are the emulsion thickeners, of which the white spirit/water emulsions (o/w type) were of greatest practical interest.
They consist of about 70% by mass of hydrocarbons in the disperse phase, 0.5–1% of nonionic emulsifiers with HLB in the region 12–15, and 29% of water in the continuous phase. The droplet size of the emulsified white spirit depends on the emulsifying agent and the efficiency of the high-speed mixer necessary to produce the emulsion. Efforts must be made to achieve a uniform droplet size (homogeneous emulsions), as this gives maximum stability and yield value. For physiological reasons the white spirit should have as little toxicity and aroma as possible, and a flash point of at least 30 °C, a boiling range of between 160 and 200 °C and a density of 0.76–0.80 g cm–3 are desirable.
Emulsion thickeners of the white spirit type are particularly suitable for pigment printing because all the components, except the emulsifying agents, will evaporate completely, leaving no residues. The thickener does not therefore cause any hardening of the handle of the textile and, because the evaporation enthalpy for white spirit is about 25% of that for water, drying of the printed textile is quicker; this improves the printing speed and consequently the production output. This is particularly important in drying chambers with a limited drying capacity. On the other hand, mineral oil products are too valuable to be used just as print paste thickening agents and, for environmental reasons, the emission of organic substances into the atmosphere has met with opposition in many countries and has been prohibited by law in some. Recovering the white spirit from the exhaust gases of drying chambers, by means of cooling apparatus and batteries of charcoal filters, is too complicated and too expensive to provide a solution to this problem.
The burning or catalytic combustion of evaporated white spirit, to comply with emission protection laws, also requires substantial capital expenditure. It is vitally important that the gases in the dryer should contain no more than 0.4% by volume of white spirit, to avoid explosions in the plant. White spirit emulsions for pigment printing are therefore being phased out all over the world; perhaps this may be regarded as a sign of our times. In the 1960s, in anticipation, synthetic-polymer thickening agents were developed for pigment printing, with rheology similar to that of emulsion thickeners and extremely low solids content [7,8].
They are polyanionic compounds derived from monomers such as acrylic acid and maleic anhydride. A small degree of transverse crosslinking of the polymer molecules is built in, so that packets or bundles of chain molecules are held together. These products have very high swelling power, which is at a maximum at pH 9–10 when ionisation increases the degree of solvation in water. The original polymer with undissociated carboxylic acid groups will be in the coiledchain form. Neutralisation leads to a straightening and separation of the polymer chains because the carboxylate ions are fully dissociated and strongly repel one another.
This brings about a hundredfold expansion in size and the solvated particles then have dimensions similar to those of the white spirit droplets in emulsion thickeners. When the water-swollen particles are tightly packed they give a paste with pseudoplastic flow properties. As coarse colloids they are subject to salting-out effects, and small additions of electrolyte reduce the viscosity, which is not the case with white spirit emulsions. Another disadvantage is that, on drying, this type of thickener leaves a residue; depending upon the requirements, 0.5–1% of solid substance remains on the textile. This residue, because of its extremely polar character, produces a hard, inflexible film which can (depending upon its chemical constitution and particularly in the case of carbonamide groups) react with crosslinking agents.
This rigid film has an adverse effect on the handle of the printed goods, although it is considerably less than that of polysaccharide thickeners. The effect on the handle can be satisfactorily balanced by adding suitable softening agents, such as polydimethylsiloxane, mineral oils or synthetic wax. The neutralising agent, usually ammonia, in the synthetic thickener is removed under drying and fixing conditions; the free carboxyl groups in the polyanion then produce the acid medium which the binder and external crosslinking agents require for the crosslinking reaction. Printing pastes based on emulsion thickeners need about 0.5% potential acid, such as diammonium phosphate, in order to obtain the necessary acid medium during fixation.
Some manufacturers have combined softeners, synthetic thickeners neutralised with ammonia and dispersing agents to make application simpler and have marketed these products in paste form (‘printing concentrate’). Emulsion and synthetic thickeners can be grouped under the heading ‘disperse thickeners’. They are to a large extent interchangeable. If, for example, it is not possible to print economically with spirit-free pastes on the grounds of inadequate drying capacity, an emulsion thickener and a synthetic thickener can be used together to give pastes containing perhaps 25% white spirit. The printer knows from experience that ‘short’ flow pastes (see section 7.7.3) are more sensitive to crushing effects (section 1.3.2) than are ‘long’ pastes.
The less mobile the pastes are on the textile, the more easily are they picked up on the following printing rollers, in the case of multicoloured prints. This phenomenon makes itself unpleasantly noticeable, particularly in roller printing, with disperse thickeners having yield values and very steeply decreasing shear resistance; the original colour depth may be reduced by up to 70% after three or four times crushing. In screen printing the decrease in colour depth as a result of crushing is not nearly so pronounced, but for this reason there is the risk that uneven prints will be obtained on smooth, hydrophobic textiles.
In flat-bed screen printing there may, in addition, be screen frame marks. Combinations of disperse thickeners with colloidal thickeners (for example, cellulose ethers of average viscosity) reduce the crushing effect considerably. The pastes are, in the printer’s terms, softer or longer. The larger the proportion of colloidal thickener in the printing paste, the harder is the handle of the printed goods. Auxiliary substances with dilatant flow properties are more suitable and are effective as ‘flow moderators’ in comparatively low concentrations
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