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Pretreatment of Fabric Before Printing

The printing process is the process of printing patterns on textiles with dyes or pigments. Printing can be divided into fabric printing, top printing, and yarn printing, and fabric printing is the main type. Top printing is used to make mixed colored tweed; Yarn printing is used to weave special style colorful pattern fabrics. Textile printing has a long history. In the Warring States period, China already used hollow-out printing. India already had wood stencil printing in the 4th century BC. Continuous concave roller printing began in the 18th century. Screen printing is developed from a hollowed-out version, which is suitable for small batch and multi-variety printing of easily deformed fabrics.

The fabric must be pretreated before printing to make it have good wettability. The dyes used in printing are basically the same as those used in dyeing. Some patterns with small areas can be painted (pigments). In addition, there are fast pigments, fast amines, fast sulfones, and other dyes for printing. Different kinds of dyes can be selected to print various patterns on the same fabric. When printing, dye or pigment is mixed into a color paste. After printing and drying, steaming, color developing, or color fixing treatment is usually carried out, followed by soaping and water washing to fully remove the paste, chemicals, and floating color in the color paste. Printing paste is composed of dye (or pigment), hygroscopic agent, cosolvent, etc., and original paste. The function of printing paste is to make the color paste have certain viscosity and fluidity. It is prepared from hydrophilic polymer paste. The commonly used pastes include starch, starch degradation products (white dextrin and yellow dextrin), starch aldehyde derivatives, sodium alginate (or ammonium), and hydroxyethyl soap glue, dragon glue, cellulose, synthetic polymer electrolyte, etc. The emulsion paste made of water, kerosene, and emulsifier is sometimes used as the original paste for printing. The printing paste should have good stability to the chemicals in the color paste, not interact with the dyes, have certain adhesion to the fibers, and be easy to wash away from the fabric. The viscosity of printing paste depends on the nature of the original paste. When printing, if the paste viscosity drops too much, it is difficult to print fine lines. If the viscosity is too high, the paste is not easy to pass through the fine holes of the screen.  The fabric printed with color paste is steamed after drying. The steam condenses on the fabric, raising the temperature of the fabric, swelling the fiber and paste, dissolving the dyes, and dyeing. Some dyes also have chemical reactions during the steaming process. Due to the existence of paste, the dyeing process of printing dyes is more complex, and the steaming time is longer than that of pad dyeing. There are three types of steaming equipment: ① steaming box: hang the fabric on the support and push it into the box for steaming; ② Continuous steamer: hang the fabric in a ring on the roller, slowly move forward, and continuously steam; ③ Various rapid steamers. Steam steamer and hanging continuous steamer are suitable for silk fabrics, knitted fabrics, and fiber fabrics. Steam steamer steams under closed conditions, but cannot be produced continuously. The steaming conditions vary with the properties of dyes and fibers. When the printing fabric with vat dyes is steamed, the air in the steaming chamber should be removed. Polyester printed fabrics with dispersed dyes can be steamed in a closed steamer at about 130 ℃, or in superheated steam at about 175 ℃ under normal pressure, or baked at about 200 ℃.

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