Printing pastes

The process of textile print coloration can be divided into three steps. First, the colorant is appHed as pigment dispersion, dye dispersion, or dye solution from a vehicle caUed print paste or printing ink, containing in addition to the colorant such solutions or dispersions of chemicals as may be required by the colorant or textile substrate to improve and assist in dye solubUity, dispersion stabUity, pH, lubricity, hygroscopicity, rate of dye fixation to the substrate, and colorant-fiber bonding. The required viscosity characteristics of a print paste are achieved by addition of natural or synthetic thickening agents or by use of emulsions. 

Disperse Dyes. Disperse dyes are used in powder or paste form, or ready-to-prepare aqueous dispersions for incorporation into a thickener solution. Eor disperse dyes that show sensitivity to alkaline hydrolysis or reduction during fixation, an acid donor or acid, and, if necessary, a mild oxidizing agent, are added to the print paste. 

Eor printing on polyester, the fixation conditions are more rigorous than on other disperse dyeable fibers, owing to the slower diffusion of disperse dyes in polyester. Eor continuous fixation the prints are exposed at atmospheric pressure to superheated steam of 170—180°C for 6—8 min. A carrier may be added to the print paste for accelerated and fliU fixation. Dry-heat fixation conditions of 170—215°C for 1—8 min are less popular for printed fabrics, but are sometimes employed because of lack of other equipment. 

In batch-type fixation, the prints are steamed in a pressurized autoclave with saturated or nearly saturated steam at 14—18 kPa (105—135 mm Hg) and 125—130°C for 45—60 min. In this case, the amount of carrier can be reduced or completely omitted. Generally, a small amount of urea (5 wt % of print paste) is added to the print paste to aid in fixation. 

Printing of wool or silk with acid dyes is of minor importance. For these fibers the print paste is made with dye solvent, humectant (glycerol and urea), a suitable thickener, and dilute organic acid. An oxidising agent is also added. Fixation follows the procedure for polyamide with fully saturated steam. 

Acid dyes can be ptinted on acetate, produciag prints with very good wetfastness and exceptional brightness. The print paste contains a solvent, urea, and ammonium thiocyanate, as a fiber swelling agent to aid ia diffusion of the dye. Again, fixation and scouting foUow the procedures for polyamide. 

Azoic Dyes. These are used to produce cost-effective heavy yellow, orange, red, maroon, navy blue, brown, and black shades and are ptinted alongside other dye classes to extend the coloristic possibiUties for the designer. Two approaches are adopted. The common method ia the United States is to use both a naphthol derivative and a stabilized color base, usually in the form of a diazo imino compound in the same print paste. This mixture is soluble in dilute caustic soda and no coupling takes place at this stage. The dried prints are passed through steam at 100—105°C that contains acetic and/or formic acid vapor. As neutralization takes place on the print, the coupling occurs rapidly and the insoluble azoic dye is formed. 

Phthalocyanine Dyes. These days are synthesized as the metal complex on the textile fiber from, eg, phthalonittile and metal salts. A print paste typicaUy contains phthalonittile dissolved in a suitable solvent and nickel or copper salts. During a heat or steam fixation of 3—5 min, the dye is formed. The color range is restricted to blue and green shades and can be influenced to some extent by the choice of metal salt. A hot acid bath during afterscouting completes the process. 

Dye Combinations. In certain cases it is desirable to print fiber blends with combiaations of the appropriate dye classes, rather than with pigments. Only polyester—ceUulose blends are of commercial importance and the foUowing dye systems have been developed for them. The dyes of the different classes are contained in the same print paste and, therefore, are appUed simultaneously in one print operation. 

Direct Printing. This is the simplest technique and accounts for the majority of printed fabrics. In this style the print paste is apphed directiy onto the fabric which can either be white or already dyed with a pale ground shade. Each color requires its own print paste and appHcation and each color is printed immediately after each other without intermediate drying. 

Resist Printing. In resist printing, print pastes are used that can inhibit the development or fixation of different dyes that are apphed to the textile prior to or after printing. These resists can be of a chemical or mechanical nature, or combine both methods. For example, fiber-reactive dyes, which require alkaU for their fixation, can be made resistant by printing a nonvolatile organic acid, such as tartaric acid, on the textile. Colored resists are obtained by printing pigments with a nonvolatile acid. 

To confine the print paste to the engraved parts of the print roUer, any excess that may have been appUed by the furnishing roUer is scraped off the... 

Reserve, /. reserve (Calico) reserve, resist, also resist paste, -artikel, m. — Reservage-artikel. -druck, m. Reservagedruck. -druckfarbe,/. resist printing paste, -mittel,... 

Alkyl phosphates are also proposed as a humectant for printing pastes [14]. 

In direct printing of acrylic fabrics a typical stock print paste [29] may contain the following components by mass ... 

Table 12.11 Conductivity and print paste viscosity data for various salts of lignosulphonate dispersant [91]...

Lignosulphonate cationic salt Conductance (m.mhos) at 5% dispersant concentration Print paste viscosity (cps) at 25 °C... 

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