Textile Printing Properties-Preparation of the Print Paste ---TIANYU Rotary Screen Printing Solution
Edit time: 2009-11-02Preparation of the Print Paste
The specifics of print paste formulation depend on the fiber content of the fabric, the colorant system used, and to some extent, the type of printing machine employed. However, the typical ingredients found in most paste formulations include the following: dyes or pigments, thickeners, sequestering agents, dispersing or suspending agents (surfactants), water-retaining agents (humectants), defoamers, catalysts, and hand modifiers. In addition to the ingredients, pigments require a binder or resin system to fix the pigment and may include adhesion promoters. The most important ingredients of any print paste formulation are the colorants and the thickener system. As mentioned, dyes are fiber specific. The dyes for cellulose fibers – specifically cotton, rayon, and lyocell (Tencel®) – that are used for printing are reactives, vats, naphthols, and directs. Reactives dominate the dyes used for printing these fibers, because of their wide shade range, bright colors, good washfastness, and good availability. Vat dyes are also quite popular for textile printing. They usually have very good overall colorfastness properties, but have a limited shade range and are available in mainly deep colors such as violets, blues, and greens. Naphthols or azoic coupling components are unique in that the dye is actually made through a reaction of two separate chemicals inside the fiber. The typical method uses a stabilized naphthol and coupling component print paste mixture printed onto the fabric then exposed to an acid steaming to develop the color. These are known as the rapid fast or rapidogen colors. The use of naphthols is limited due mostly to application complexity. For all of these dye systems for cotton, thorough afterwashing is essential for good crockfastness and washfastness.
If the printed fabric is a blend, then a combination of different dye types in the print paste will be necessary. For example, a cotton/polyester blend would require reactive dyes for the cotton and disperse dyes for the polyester. These would also require different color fixation conditions.
Therefore, the dominant type of colorant for blended fabrics is pigment systems. Pigments are not dyes, but are colored particles glued to the surface of the fabric. They can color all fibers in the blend the same shade with a single colorant. Once applied, fixation of a pigment color just requires dry heat for a defined amount of time. The colorfastness of pigments directly depends on the binder system employed. Binders are chemicals, which have the ability of forming a three-dimensional film used to hold the pigment particles in place on the surface of a textile substrate. Binders can be water-based (latex) or solvent-based and vary widely in their stiffness.
Adhesion promoters (low crock additives) are chemicals added to increase the adhesion of the binder to the fabric. The major drawbacks of pigment prints include poor crockfastness, especially on deep shades, and stiffening of the fabric so that it may feel somewhat boardy.
Their wide shade range as well as the flexibility and simplicity of processing make pigments an extremely popular choice for both blended and 100% fiber fabrics.
The thickener system is the next crucial component of print paste. The purpose of the thickener system is twofold. First, the thickener gives the print paste the proper viscosity or flow characteristics, so the color can be applied uniformly and evenly. Second, it holds the color in place so that one color paste can be applied adjacent to another without the color bleeding onto
the other. With dyes, the thickener also holds the color in place after drying until the printed fabric goes through the fixation process where the dye is released from the thickener and is diffused into the fiber. Thickeners used with dyes are then washed off the fabric before any chemical or mechanical finishing is performed. However, the thickener applied with a pigment system will remain with the print, as no afterwashing is required. There is a wide range of thickener materials available including alginates, natural vegetable gums, synthetic polymers, or even foams. These materials show sensitivity to factors such as temperature, pH, and salt content.
The following form a group of optional, but often used additives to print paste formulations. Sequestering agents are compounds which complex (bond) with metallic contaminants to prevent interference with the print color or necessary auxiliary chemicals. Calgon is the most well known of this type of additive. Surfactants are additives, which allow chemicals of dissimilar nature to mix. They are used in print paste as dispersing agents, suspending agents, and/or wetting agents. Water-retaining agents or humectants are additives, which prevent premature water evaporation or “skim-over” from print paste. Additionally, they often absorb moisture from the air to keep dried print paste from cracking and shedding off the fabric before fixation.
Defoamers are materials added to the print paste to eliminate unwanted bubble or foam formation during the mechanical action of the printing process. Unwanted foaming leads to uneven or light print color. Defoamer additives must be rechecked to ensure against adverse effects on final print quality. Hand modifiers, most specifically softeners, are often incorporated with pigment print formulations, because print binders tend to overly stiffen the fabric. While these additives may improve certain aspects of the print, they may also interfere with the binder and should be used with caution.